Contents

Executive summary

Preface

	Introduction 1.1 Rationale of groundwater protection 1.2 Cause and consequences of irrational and excessive exploitation 1.3 Problems and effects of groundwater pollution 1.4 Administrative arrangements and legal provisions
	Control of groundwater exploitation 2.1 Administrative and legal framework 2.2 Evaluation of state of groundwater resource development 2.3 Control of water-well drilling and construction 2.4 Control of groundwater abstraction 2.5 Problem of abandoned wells
	Land surface zoning for pollution protection 3.1 General strategy 3.2 Mapping of aquifer pollution vulnerability 3.3 Groundwater source protection areas
	Control of groundwater pollution sources 4.1 Classification of polluting activities 4.2 Strategy for potential new sources 4.3 Control measures for common installations 4.4.1 Industrial plants 4.4.2 Intensive livestock rearing 4.4.3 Municipal and industrial effluent lagoons 4.4.4 Solid waste disposal by landfill 4.5 Control of diffuse pollution 4.5.1 In-Situ sanitation of residential areas 4.5.2 Drainage soakaways for urban areas 4.5.3 Agricultural land-use 4.6 Impact of mining and petroleum develpment 4.7 Approaches to contaminated land legacy
	Selected bibliography

Executive summary

Scope of Manual

1. The major use and strategic importance of groundwater, especially for municipal water-supply, in the Latin American-Caribbean region is such that it merits urgent protection against deterioration from the effects of resource overexploitation and anthropogenic contamination. At present there are few locations at which human economic activity is being constrained in the interests of protecting groundwater supplies.
2. This document is a general guide to the development and implementation of groundwater protection strategies (Figure Oa). It is not feasible to provide a rigorous manual since each nation or region needs to evolve its policy to the appropriate level given local circumstances. This guide relates closely to and draws heavily upon previous reports in the same series; the relationship is shown in Figure Ob.
   
   Control of Groundwater Exploitation
   
3. The risk of near-irreversible deterioration of aquifers and premature loss of capital investment associated with unrestricted groundwater exploitation are such as to urge nations to place some controls on aquifer exploitation as a first step in positive (as opposed to passive) groundwater resources management.
4. The unrestricted exploitation of groundwater resources, free from all control, can only be considered a tolerable option for (i) extensive aquifers with very large storage reserves in situations where, (ii) the consequences of temporary overdevelopment are not irreversible, (iii) no account needs to be taken of reductions in spring discharge and river baseflow, and (iv) it will not cause further social inequity between water-users.
5. Regulation of groundwater exploitation is more readily achieved through controlling the drilling of waterwells, including their depth, diameter and screen intakes, rather than subsequently licensing their pumping, although both elements will be present in a balanced policy.
6. A widespread failure to invest adequate resources in waterwell maintenance has led to a tendency to overcapitalise aquifer development by drilling excessive numbers of wells in relation to total yield achieved. More attention needs to be placed on operational monitoring to diagnose maintenance requirements and routine pump servicing and intermittent well cleaning and rehabilitation.

Figure Ob
Elements in the evaluation, management and protection of groundwater resources

the position of the stretegies present in this guide is shown in relation to the
overall scheme and to earlies manuals

7. Groundwater resource estimates are invariably rather imprecise and this is a complicating factor for
    regulatory agencies. However it must be recognised that well-monitored operational pumping is also
    the most cost-effective method of refining resource estimates and should be used more widely.

11.In view of the time-lag in the response of many aquifers to imposed contaminant load, the common
      inadequacy of monitoring networks and the widespread limitations in analytical capacity and
      reliability for many pollutants, it is not appropriate to wait for proof of pollution before acting to
      control contaminant loads.

12.If logical progress is to be made with groundwater protection against anthropogenic pollution it will
     be essential, however, to assign priorities. This in effect requires the zoning of the land surface on
     simple but consistent criteria, and can be approached by mapping aquifer pollution vulnerability.
     Vulnerability is defined in terms of the intrinsic characteristics of the strata separating the saturated
     aquifer from the land surface, which provide a degree of natural protection against imposed
     contaminant load.

18. Although individual situations have to be evaluated in their local circumstances, a general guide to
      the main sources of potential subsurface contaminant load and roundwater pollution risk is given,
      together with the extent to which it is feasible to use the soil and unsaturated zone for
      effluent/leachate treatment. An outline scheme is presented of the types of control measures that
      can be introduced. Actions can include effluent treatment, reducing effluent generation by recycling
      or process modification, improving the integrity and security of chemical storage tanks, pipelines,
      sewers, etc.

22. As far as unsewered sanitation in residential areas is concerned two principal problems arise: (a)
      chemical contamination of groundwater under a wide variety of hydrogeological conditions if the
      density of population served is too high; (b) more localised pathogenic microbiological
      contamination of groundwater if hydrogeological conditions are unsuitable. The only effective
      pollution control measure in the case of the former is installation of mains sewerage with
      subsequent effluent treatment and safe disposal.

23. The intensification of agricultural cultivation, especially where this involves regular soil ploughing
       and large applications of inorganic fertilisers and/or synthetic pesticides, can give rise to insidious
       groundwater contamination in a fairly wide range of hydrogeological and agroclimatic conditions. It
       is difficult to control but has to be addressed through modifying recommendations on best
       agricultural practice to include consideration of leaching losses. Stronger measures, including the
       prohibition of use of some chemicals or obligatory change in land use in special protection areas,
       may also have to be considered.

Preface

(d) The potentially insidious health effects associated with the pollution of groundwater supplies,
      consequent upon slow but persistent increases in the concentration of contaminants with uncertain
      toxicology.

(d) Alleviation of diffuse source groundwater pollution.

In view of the fact that this manual is designed as a selfcontained guide, no bibliographic references are cited in the text. Many of the works that might have been referred to are, in any case, not widely available in the Latin American-Caribbean Region. A short list of selected references and further reading is, however, provided.

Because of the diversity of administrative and institutional arrangements and legal provisions in individual nations, it is not possible to present a rigid model for groundwater protection. it is more appropriate to provide a guide which can be adapted to local circumstances, which may vary as a result of centralised or regionalised administration, the number of agencies involved in water and environmental management, and so on.

This guide has been reviewed and improved by the Technical Committee guiding the PAHO-CEPIS Regional Groundwater Protection Program, which has included representatives of institutions in Argentina, Bolivia, Brasil, Colombia, Costa Rica, Cuba, El Salvador, Honduras, Mexico, Paraguay, Peru, Puerto Rico, Republica Dominica, Jamaica, and Venezuela. This committee has celebrated seven reunions: I-Lima, Peru/November 1984; II-II/Mexico City/February 1986; III-Sao Paulo, Brasil/May 1987; IV-San Juan de Puerto Rico/June 1988; V-Lima, Peru/February 1989; VI-Santa Cruz de 1a Sierra, Bolivia/October 1990:VII-Merida/Venezuela.

The authors are grateful to Eng. Alberto Florez Mufoz (CEPIS Director) and Eng. Henry Salas (CEPIS Water Pollution Adviser) for their initial interest and continued encouragement in the preparation of this guide The authors are indebted to various colleagues for helpful discussion and provision of relevant material, and especially to Ing Katherine Batista and Bio1. Milagros Rodriguez (EQB of Puerto Rico), Eng Omar Ascuntar (CVC of Colombia) and Eng. Geol John Chilton (BGS of United Kingdom).

1. Introduction

1.1 Rationale of Groundwater Protection

1.1.1 What do we mean by the expression "groundwater protection"? It is generally taken to imply the conservation of groundwater, in terms of both quantity and quality, so as to permit efficient long-term exploitation of aquifers, especially as a secure and reliable source of human watersupply. The same could apply to their use for agricultural irrigation or industrial production, but in this case the quality requirements are generally less onerous. It must be emphasised that the protection of groundwater does not mean its absolute conservation against legitimate wellplanned use.

1.1.2 What are we endeavouring to protect against? The answer to this question can be more specific since the principal problems arise because of:

(a) Irrational development initially, which results in illogical constraints later when trying to optimise groundwater exploitation.

(b) Excessive exploitation, which leads to a reduction in long-term resource availability, as a result of water-table lowering, the inflow of groundwater of inferior quality, and to costly environmental side effects and other undesirable or unpredicted phenomena.

(c) Deterioration of groundwater quality as a result of contamination from the land surface.

1.1.3 Groundwater protection policies have their price, both at the implementation stage and during enforcement in the longer term, but they are necessary to avoid even larger costs, in terms of loss of investment in water resource development, public health risks due to lack of water-supply or to inadequate water quality and irreparable damage to aquifers and/or to the environment.

1.1.4 Significant difficulties arise when attempting both to draw-up and to implement groundwater protection policies because of a number of technical and administrative considerations:

(a) Doubts about the actual magnitude of groundwater resources, because of inevitable
      inaccuracy in estimates of both aquifer recharge and storage.

(b) Uncertainty about the scale of risk of groundwater pollution and problems predicting
      pollutant transport.

(c) Legal problems relating to groundwater exploitation or pollution sources existing before
      the introduction of an aquifer protection policy.

1.1.5 While groundwater lends itself to exploitation close to demand, thus reducing the capital cost of development, it has to be recognised that an uncontrolled proliferation of private wells in urban areas can greatly complicate attempts to implement the protection of groundwater resources and of groundwater quality.

1.1.6 Groundwater flow tends to be very slow and residence times very long (Figure 1). Aquifers have very large storage and generally react only slowly to overdevelopment or to applied contaminant load. This very fact provides a margin for experimentation in resource management and pollution control. However, it all too often simply leads to complacency in taking positive steps, often until it is too late for effective action to be taken.

1.2.1 Any groundwater abstraction results in some lowering of aquifer water-levels and a degree of interference with neighbouring boreholes/wells and springs (Figure 2).

1.2.2 Irrational exploitation of groundwater resources is unfortunately a relatively frequent occurrence in many urban and periurban areas, as a consequence of rapidly-growing demand for watersupply in a number of different and uncoordinated sectors. It includes inefficient and uneconomic use of groundwater resources as a result of competitive development with indiscriminate drilling of excessive numbers of water-supply boreholes in relation to their potential total yield.

1.2.3 One example is large numbers of springhead captures and shallow wells in an aquifer discharge area without clear definition of individual abstraction rights, which subsequently precludes full and logical exploitation of groundwater storage because of the impossibility of determining adequate compensation for such users.

Figure 1
Hypothetical hydrogeological sections to illustrate typical
Groundwater flow regimes under (A) humid and (B) Semi-Arid
Climatic conditions

the residence times indicated are order-of-magnitude values
for the period between recharge and discharge

1.2.4 Another example is the uncontrolled exploitation of a shallow water-table aquifer by large numbers of small boreholes, which will render the exploitation of a deeper, more reliable, semi-confined aquifer all the more difficult in subsequent years.

Figure 2
Hypothetical section of confined aquifer to show drawdown
interferences between production boreholes

abstraction control policy should be aimed to restrict such interferences to tolerable levels
and to prevent marked reduction in individual borehole yields

1.2.5 Control over private groundwater abstraction for residential and industrial uses can be especially important in urban areas if wastewater discharges are to mains sewerage, since it provides the only rational basis for raising revenue for sewer maintenance and wastewater treatment.

1.2.6 Unplanned overdevelopment of groundwater resources is a further example of irrational exploitation and often the most serious. It is convenient to define aquifer overexploitation as the condition in which the abstraction of groundwater, in a given catchment, is greater than the recharge. The definition of the area under consideration is important. It is more logical to consider an entire aquifer or hydrogeological unit. At the same time it must be recognised that local overdevelopment may occur if abstraction is concentrated in a small area such that the interference between pumping boreholes becomes counterproductive, because the regional groundwater flow of the aquifer system has been entirely intercepted.

1.2.7 The time over which the aquifer water balance is considered is also critical. It is logical to consider a long enough period to allow for the cyclicity of recharge events, and thus the period should always be a number of years. However, in arid regions where major recharge events may have a period of decades this may become impractical in the case of small aquifers whose storage could be effectively exhausted by a given level of exploitation before significant recharge is experienced. When attempting to define over development therefore, it is essential to take account of the cyclicity of recharge events, the groundwater flow regime and the magnitude of aquifer storage.

1.2.8 It is normal to extend the definition of overexploitation so as to include any condition where the abstraction of groundwater exceeds the recharge over a given period such that serious disequilibrium in the hydrological balance of the aquifer system could occur.

1.2.9 While all groundwater development must have some negative side effects, we are mainly concerned with those conditions which have serious consequences such as:

(a) Reduction in the yield of neighbouring wells (Figure 3) and/or major increases in their pumping costs
      (Table 1).

(b) The in-flow, through lateral intrusion or upconing, of saline water of coastal or other origin (Figure 4).

(c)  Inducing the infiltration of poor quality groundwater from shallower aquifers.

(d) Unacceptable impact on surface water bodies, such as lakes and rivers, due to induced infiltration.

(e) and subsidence resulting in problems with building stability, land drainage and sewer flows.

1.2.10 The extent to which a given overexploited aquifer will suffer adverse side-effects will depend upon its geohydraulic characteristics and its hydrogeological setting. Since priority areas for control on exploitation will need to be identified, an indication of susceptibility to reversible interference and/or irreversible deterioration is given in Table 2. It is recognised that reliable data on all the relevant parameters may not in practice be available. There is also the possibility of irreversible aquifer compaction and transmissivity reduction in some circumstances.

1.2.11 Groundwater over development is usually corrected by exercising control over pumping rates, either closing certain wells or reducing their permitted abstraction rates.

1.2.12 It is important to mention that not all overexploitation needs to be considered an irrational use of groundwater resources. If it is planned with specific ends,and as long as the negative consequences have been technically evaluated and are economically acceptable, it may represent a logical resource exp;oitation policy.

1.3 Problems and Effects of Groundwater Pollution

1.3.1 A comprehensive list of activities which can potentially generate significant subsurface contaminant load and cause groundwater pollution risk is presented and classified (Table 3). The difference between contamination from a readily identifiable point source (Figure 5) and essentially diffuse sources (Figure 6) is fundamental in the consideration of pollution prevention and control measures.

Figure 3
Dramatic reduction in production performance of municipal water-supply
borehole in overexploited alluvial aquifer of Lima, Peru

the behaviour also reflects the fact that the most permeable and productive
strata of the aquifer in this area occur at relatively shallow depth

Table 1

Operation history of groundwater abstraction by sedapal (the
Lima Water & Sewerage Service)
From an overxploited alluvial aquifer

*average well depth also had to increase substantially during period under consideration

1.3.2 The pollution of groundwater occurs when contaminants infiltrate the groundwater system and this is often caused by uncontrolled discharge at the land surface. Natural soil profiles have capacity to attenuate many, but not all, water pollutants. The processes involved in contaminant attenuation continue, but to lesser degree with increasing depth, in the unsaturated zone and in the saturated zone of aquifers. In the latter hydrodynamic dispersion accompanying groundwater flow will also bring about dilution of persistent and mobile pollutants.

1.3.3 However not all soil profiles, and underlying hydrogeological environments, are equally effective in pollutant attenuation. Moreover, the degree of attenuation will vary widely with type of pollutant and polluting process in any given environment. Human activity at the land surface also modifies infiltration mechanisms and introduces new ones, changing the rate, frequency and quality of groundwater recharge, especially in the more arid climates. Understanding these mechanisms and diagnosing such changes are critical in the estimation of groundwater pollution risk and the implementation of appropriate pollution control measures.

Figure 4
Schematic section of the hermosillo coast, Mexico illustrating overexploitation
and saline intrusion in an alluvial aquifer

the aquifer has been seriously affected by encroachment of saline water

Table 3
Summary of activities potentially generating a subsurface contaminant load

(those considered of greatest importance in Latin America and the
Caribbean are indicated in capital letters)

Figure 5
Characteristics of pont-source groundwater pollution

maximun carbon tetrachloride concentrations during October 1982 - March - 1983 in a deep water-table karstic aquifer are illustrated; the pollution plume developed following the rupture of an underground storage tank at a pharmaceutical plant from which 50,000 litres of process chemicals were lost in September 1982; highest groundwater concentrations exceeded 3000 µ CC14/1, 15 water-supply boreholes producing 20 Ml/d were affected and considerably more than US$10 million was spent in immediate pollution containment, on alternative water supplies and for aquifer clean-up over a period of some years.

Figure 6
Characteristics of diffuse groundwater pollution

this illustrates extensive salinisation of the upper part of a thick coastal alluvial aquifer in an area with rainfall less than 200 mm/a as a result of saline irrigation return water; a large-scale irrigation scheme jbased on conjunctive use of surface water and groundwater resources was initiated during the 1960s

1.3.4 Water movement and pollutant transport from the land surface to groundwater tends to be a slow process in most aquifers. This means that it can take many years, even decades, before the impact of a pollution episode by a persistent contaminant becomes fully apparent in the water supplies drawn from an aquifer, by which time irreversible damage may have been done to the aquifer system.

1.3.5 Evidently pollution control measures need to be prioritised and this can be done through consideration of:

(a) The relative vulnerability of the hydrogeological environment.

(b) The magnitude and character of the subsurface contaminant load generated by a given activity.

(c) The utilisation of local groundwater resources.

The zonification of the land surface by to vulnerability to groundwater pollution is also a key element in planning for the land disposal of solid waste.

1.3.6 Failure to implement adequate pollution control measures can lead to serious consequences such
         as:

(a) The abandonment of water-wells and loss of major investment in groundwater resource development.

(b) The introduction of costly treatment processes at groundwater sources, at least where adequate treatment technology is available.

(c) Exceptionally costly long-term aquifer restoration programmes (Figure 5), although it must be recognised that pollution clean-up may even prove problematic and effects can be irreversible, leading to permanent loss of water resources.

1.4 Administrative Arrangements and Legal Provisions

1.4.1 Administrative arrangements, reinforced by legal provisions, are necessary to regulate groundwater exploitation and to control groundwater pollution, and thereby to avoid or to minimise the deterioration of groundwater resources.

1.4.2 The preferred legal arrangement is that in which a national government has implemented legislation declaring groundwater "anatural resource available for public use in a controlled fashion".

1.4.3 Such legislation will normally be backed by a series of more detailed regulations and codes of practice governing the way in which the resource can be exploited and protected from contamination. An agency will be empowered to implement the legal provision.

1.4.4 In some cases such laws that are in existence give absolute rights to the landowner over the exploitation of groundwater and the use of the ground for disposal of effluents and residues, without impediment or control. In others, no legislation will exist at all. In these instances it will be much more difficult to protect groundwater, but there is still normally the option of some form of special declaration at local government level "for conservation of groundwater resources in the public interest" in or around a major urban area.

1.4.5 Various institutional structures are possible. The advantages and disadvantages of each are compared in Table 4. The preferred administrative arrangement is where a regulatory agency at regional or catchment level, close to water users and pollutants, is charged and empowered to manage and protect groundwater resources. Some form of central coordination will be necessary by a national institution or ministry, although the latter has the disadvantage of favouring one user sector (agriculture, industry, mining or public works) against others.

1.4.6 Other options are possible, and can be equally efficient, given the will and the commitment of adequate human resources. For example, resource protection can be centrally controlled by a ministry or institution with regional offices. Alternatively legal powers can be transferred to local municipal authorities or regional development corporations. Whatever arrangements are made, there are many arguments in favour of decentralisation of groundwater resources management, except in the case of small-island nations.

1.4.7 Whichever administrative arrangement is in operation, the relevant agencies will need to establish clear working relationships with institutions responsible for other sectors. It is common, for example, for

Table 4
Comparison of different institutional arrangements for ground protection

agency for corresponding country operates in sector and at scale indicated by letter below; two entries in different sectors indicates split responsibility for Groundwater Resource Management and Groundwater Pollution Control

PE = Peru, Co = Colombia, BA = Barbados,

PR = Puerto Rico, EW = England & Wales

* indicates autonomous agency, not directly within ministry,

s territory of small area not justifying regional offices

health authorities to have an interest in groundwater quality and for agricultural institutions to be concerned with groundwater resources. Whatever the situation, it is vital that there are effective communication and consultation mechanisms set up between the responsible and interested agencies such that overall control can be achieved.

1.4.8 The regulatory agency will need some capacity for technical investigation in the evaluation of groundwater resources and pollution risk, but the level to which this is developed in-house will vary. In certain cases it will be more effective for such work to be commissioned through national technical institutes, local universities, or private sector consultants. However, it is particularly important for the regulatory agency to have technical capacity to up-date its corresponding database.

1.4.9 The most cost effective way to improve the evaluation of groundwater resources and aquifer pollution vulnerability will be for the regulatory agency to be active in the corresponding hydrogeological data collection, compilation and analysis.

1.4.10 Hydrogeological information on aquifer recharge, flow and discharge, construction details for abstraction boreholes, assessments of aquifer vulnerability, surveys of potential pollution sources will be regularly required for the evaluation of abstraction licences and of planning applications for activities potentially-generating groundwater pollution.

2. Control of grounwater exploitation

2.1 Administrative and Legal Framework

2.1.1 The normal legal requirement is that any individual or limited company who wishes to drill a borehole, dig a well or capture a spring, to exploit groundwater resources requires a permit to construct the proposed water-supply installation (Table 5). The regulatory agency must be empowered to approve or decline such permits. If permitted, a technical design for this installation has to be established which the applicant is legally bound to adopt.

2.1.2 The second stage (Table 5) is the issuing of a licence for abstraction once the new installation is constructed. While it is desirable for all water-supply installations to require a drilling permit, most agencies do not consider it practical to insist upon abstraction licences in the case of small users. Spring capture is especially difficult to control, and in many cases is not attempted, although some form of register is highly desirable. For the control of groundwater exploitation to be effective, some form of legalised sanctions or penalties against those who construct water-wells without permit or exceed licence abstraction rights are required by the regulatory agency.

2.1.3 It is customary to levy a charge in respect of the provision of an abstraction licence. This is often essential to raise revenue for the regulatory agency, although the concept of such agencies being highly dependent upon resource exploitation for their financial solvency is open to question. Charging for abstraction licences is more effective than charging for drilling permits, however, since it results in regular income, which can be increased to cover inflation. It is good practice for abstraction licences to be scaled in relation to the volume of abstraction required and also according to the use which is made of the water-supply.

2.1.4 The final technical report of the new well must be submitted to the regulatory agency. Such reports contain valuable, and in some cases, vital information to aid the evaluation of aquifer exploitation potential.

Table 5
Summary of administrative stages in the control of grounwater abstraction

2.1.5 Public relations are extremely important. Groundwater exploitation controls should be consistently applied. Certain actions can aid good relationships with groundwater users, drilling contractors and the general public:

(a) Undertaking public awareness campaigns about the benefits of sound resource administration.

(b) Providing technical advice and information to drilling contractors.

(c) Keeping the public informed on regional studies of the state of groundwater resource exploitation.

2.2 Evaluation of State of Groundwater Resource Development

2.2.1 Groundwater resource development is normally a progressive process over a period of many years and sometimes decades (Figure 7), sometimes much longer. Formulating a sound policy for the control of groundwater exploitation requires knowledge of the size of the resource (especially the average aquifer recharge rate) so that a reasonable ceiling can be placed on development.

2.2.2 The nature of groundwater and the high cost of hydrogeological investigation, together with problems over definition of aquifer recharge areas and mechanisms and the temporal and spatial variability of recharge rates, lead to inevitable imprecision of resource estimates. They may also arise because of lack of investment in regional hydrogeological studies, insufficient pumping tests and inadequate operational monitoring of aquifer response to abstraction. It is thus not generally realistic to mount a rigid policy from the outset based on the initial resource estimate.

2.2.3 The objective of abstraction control policy should be to reduce the likelihood of suffering the more serious consequences associated with irrational and/or excessive exploitation, but to avoid overregulation, which will have high bureaucratic cost and will discourage economic development.

2.2.4 Levels of groundwater development that to threaten over-exploitation will normally only be reached where there is concentrated urban or industrial demands and/or intensive use of groundwater for agricultural irrigation (although very small aquifers may be an exception to the rule).

Figure 7
Typical evolution of groundwater exploitation
in a semi-arid region

the aim of any abstraction control policy should be not only to prevent excessive but also to avoid unnecessary constraints during the initial development phase Moreover, many aquifers have very large storage (in relation to their average annual recharge) which will act as a buffer against some of the more immediate effects of localised overdevelopment.

2.2.5 In areas where demand for groundwater development is heavy, it thus preferable to undertake piezometric level monitoring to indicate the state of groundwater development and to evaluate new abstraction licence applications in the light of aquifer response to existing abstraction.

2.2.6 Production borehole water-levels are difficult to interpret because of such factors as variability in pumping cycles and uncertainties about recovery period prior to water-level measurement. Thus an observation borehole network is far more sensitive for this purpose, especially if equipped with continuous water-level recorders.

2.2.7 The objectives of monthly or continuous monitoring of aquifer piezometric levels should include:

(a) Determination of groundwater flow directions.

(b) Quantification of the depression in piezometric levels caused by pumping and identification of areas with continuously falling levels, indicating possible resource overdevelopment and hydraulic disequilibrium.

(c) Estimation of recharge volumes during wet periods and the draft on aquifer storage during drought.

2.2.8 It is also important to monitor, at least on a monthly basis, the position of the freshwatersaline water interface in coastal areas subject to significant groundwater development.

2.2.9 Continuous monitoring of aquifer response to abstraction and regular up-date of groundwater resource estimates using the data so collected is generally the most secure and cost-effective method of improving groundwater resource estimates. As the body of data increases in size this will normally be achieved through the use of numerical aquifer simulation models and operational monitoring data are essential to the adequate calibration and proper use of such models in resource evaluation. (Figure 8).

Figure 8
Operational approach to improvement of grounwater resource estimates
using numerical aquifer modelling

aquifer storage parameters and recharge rates are the main calibration variables against observed variations in the groundwater piezometric surface

2.2.10 Nevertheless, limitations exist in this approach in the following circumstances:

(a) An interface of poor quality water is present in the neighbourhood of the area of exploitation.

(b) The aquifer under exploitation is shallow and thin and in consequence of limited storage.

(c) The proposed use of groundwater is sensitive economically to pumping costs.

(d) The location of the water-demand area is distant from the aquifer identified for development, requiring a large investment in external pipeline from the outset. In all these situations it is essential that the preliminary investigations are comprehensive.

2.3 Control of Water-Well Drilling and Construction

2.3.1 The normal legal requirement is that any individual or limited company wishing to drill a borehole (or dig a well) to exploit groundwater resources requires a permit from the regulatory agency to proceed with construction of the proposed water-supply installation (Table 5). The detailed procedure will vary to some degree with the state-of-knowledge of groundwater resources in any given area, but the type of information normally required by the regulatory agency is given in Table 6.

2.3.2 In areas for which the regulatory agency has good data to base decisions, it is generally possible (and advisable) for them to undertake a feasibility study at the request of the applicant and, if favourable, to provide a technical design for the water-supply installation (Table 6), based on data from existing wells in the vicinity.

2.33 In cases where the regulatory agency has inadequate data, they may require the applicant to arrange for a hydrogeological study to be undertaken. This study will include the review of all relevant data and may also require surface geophysical survey and perhaps also some exploratory drilling. The study would be used to make estimates of the probable cone of influence of the proposed abstraction borehole, the quality of groundwater, and as the basis of the technical design for the water-supply installation (Table 6).

2.3.4 The applicant is legally bound to adopt the approved technical design, using a water-well contractor of his choice and to maintain the works open to inspection by the regulatory agency. In some instances the agency itself may be prepared to undertake-the required work.

2.3.5 It is important for the regulatory agency to be in a position to offer technical advice to the applicant, since this will build better relations and will ensure the return of reliable data on the well drilled. This technical advice should include opinions on:

(a) The maximum yield obtainable from the given aquifer in the area concerned.

(b) An estimate of required well depth.

(c) The preferred separation of the new well from existing wells.

(d) The length and type of well-screen required, the preferred construction materials and the
      design of the wellhead sanitary protection.

2.3.6 The provision of an adequate sanitary seal for production boreholes to prevent direct contamination from the wellhead is of primary importance. This is still the commonest mechanism of groundwater supply pollution and there is urgent need to ensure that waterwell drillers achieve higher, and more consistent, standards in this respect.

2.3.7 It is highly desirable for the regulatory agency to hold a register of recognised drilling contractors operating within its area of jurisdiction. It can then require such contractors to provide periodically the programme for each of its drilling machines in some form of operational calendar. It is then in a better position to maintain contact with operations, maximise data collection (well logs and pumping test records), especially where boreholes are being drilled in unexplored or critical areas.

2.3.8 This gives the best chance of ensuring that all new production boreholes are registered and that they can conform with design specifications. On completion, the applicant must provide a technical report on the new well, including the details listed in Table 6. Sanctions or fines might be considered against drilling companies who do not meet such requirements.

Table 6
Summary of information provided or required at various
administrative stages in production well construction

2.4 Control of Groundwater Abstraction

2.4.1 once the new production borehole is constructed, the applicant must apply for an abstraction licence from the regulatory agency (Table 5). For this purpose the technical report on the watersupply installation with full data on a pumping test of adequate duration must be submitted, together with indications of the proposed use of the water abstracted and the desired yield (Table 6).

2.4.2 The regulatory agency would normally reach a decision on the permissible yield on the basis of:

(a) The state of groundwater exploitation in the area.

(b) The results of the pumping test.

(c) The proposed use - domestic, urban, industrial or agricultural (including for the latter case
      the area and method of proposed irrigation).

(d) The quality requirements for the proposed use.

2.4.3 While it is desirable for water-supply installations to require a drilling permit, since this is the only way in which the regulatory agency can effectively control exploitation and avoid irrational development, most regulatory agencies exonerate certain types of user from the need to obtain and pay for an abstraction licence. Such exonerations are normally granted for smallscale abstraction by domestic and agricultural users (Table 5). In these cases controls are normally based upon limiting production borehole diameter and depth.

2.4.4 Once the licensing is established it is still necessary to determine the way in which the yield is controlled (Table 5). The possibilities in this respect include restrictions on:

(a)The penetration of the borehole in the saturated zone of the aquifer (at the pre-design
     stage).

(b) The type of pump installed.

(c) The diameter of the borehole (at pre-design stage), and in effect the size of pump that can
      be installed.

(d) The hours of pumping per day, and annual or monthly abstraction rates, by direct metering
      or occasional inspection.

Some form of periodic inspection in the case of large abstractions, and occasional spot-checks in case of smaller ones, is necessary to enforce the control policy.

2.4.5 For any policy to be effective some form of legal sanctions or penalties against those who construct water-wells without permit or exceed the licensed abstraction are required. These normally include such actions as temporary or even permanent prohibition on the use of the well, depending on the scale of the offence and its effect on third parties or on the aquifer resource itself. Monetary fines may be considered but these are not normally considered appropriate in relation to the control of exploitation.

2.4.6 Much more attention needs to be put into operational monitoring of production well performance to diagnose well and pump maintenance requirements, and to act promptly to increase the useful life and overall efficiency of such assets. The widespread neglect of waterwell maintenance, including the routine servicing of pumping plant and the cleanup or rehabilitation of wells, has led to inefficient groundwater exploitation, the abandonment of many wells and the tendency to drill excessive numbers of wells in relation to the total yield achieved.

2.4.7 In the case of already overexploited aquifers, abstraction control measures will often need to include efforts to actually reduce abstraction, either by reducing pumping periods or selective closure of wells. This is much easier to achieve in situations where an alternative (imported) water-supply can be offered. In situations where this is technically or economically unfeasible, success is more likely if the policy is implemented through some form of water-user group organised within a community or municipal framework.

2.4.8 Special measures will need to be taken in coastal, and other, areas experiencing, or being susceptible to saline intrusion into water-supply aquifers. Understanding the groundwater flow regime and distribution of saline water in space and depth is the foundation of any control policy. The normal strategy will be:

(a) To ensure that dynamic groundwater levels above mean sea-level.

(b) To control, and even to reduce, the depth of wells and the capacity of pumping plant
      installed.

(c) As far as feasible, to redistribute abstraction inland, or away from saline interfaces,
     especially at times of drought.

(d) To develop a borehole network for regular monitoring of groundwater levels and
      salinity/conductivity depth logs, to provide a more reliable basis for aquifer management
      decisions.

2.5 Problem of abandoned wells

2.5.1 It has been widely observed that abandoned or disused water-supply wells are sometimes illegally used for effluent disposal, particularly on industrial or domestic premises in areas that do not have a mains sewerage system.

2.5.2 This practice has been the cause of serious and costly groundwater pollution incidents, either by microbiological pathogens to toxic chemicals, and is a cause of the gravest concern because the pollutants are in effect injected into the most permeable (productive) parts of the aquifer with minimal opportunity for attenuation.

2.5.3 This serious problem has to be addressed very positively by regulatory agencies or municipal authorities, despite the difficulties, and the following approach is recommended:

(a) Maintain comprehensive well inventories and, through a rolling program of field  
      inspection,identify those disused.    

(b) Emplace sealed caps on disused wells, leaving a 20mm diameter access hole with cover
      to permit monitoring, and in cases of abuse backfill.

(c) Initiate public awareness campaigns amongst waterwell owners and operators on the
     risks associated with discharging effluents into disused wells.

3. Land surface zoning for pollution protection

3.1 General Strategy

3.1.1 Improving the protection of groundwater against serious pollution is a difficult task, involving complex and not widely understood concepts.

3.1.2 Two interrelated but independent strategies are recognised (Figure 9), namely protection of:

(a) Groundwater resources or aquifers as a whole.

(b) Groundwater sources, that is those parts of aquifers where the resource is exploited for a
      specific type of water-supply.

The latter is normally considered as an additional special protection, supplementary to the former. A realistic balance between the two needs to be struck, according to local circumstances.

3.1.3 Aquifers are naturally protected against pollution of their groundwater by the unsaturated zone or the confining beds which overlie them, in the case of unconfined (or water-table) aquifers and confined (or artesian) aquifers respectively. It has already been noted that the degree of protection against a given contaminant varies with the type of soil profile and aquifer cover material. It also varies with type of contaminant and its mode of discharge.

3.1.4 The need to achieve maximum aquifer protection will also vary with the utilisation, actual or designated, of groundwater resources. Source protection policies are normally concentrated around major abstraction sources and attempt to protect their groundwater by exercising controls on human activities over various distances from the source calculated from estimates of horizontal flow of groundwater in the saturated zone of the aquifer (Figure 9).

3.1.5 For groundwater protection policy not to be unnecessarily restrictive on human economic activity, the land surface has to be zoned in relation to aquifer pollution vulnerability (Figure 9). This will enable priorities for pollution control to be logically assigned. The overall strategy is that control would be sought

Figure 9
Conceptual framework for groundwater protection

this indicates the principal elements used for zoning the land surface
in relation to the implementation of groundwater pollution control measures

over existing activities and restrictions enforcedon new developments involving potential sources of groundwater pollution, according to their location in relation to such zones.

3.1.6 In areas already highly urbanised and industrialised, or with existing intensive agricultural development, the zones would serve to define priority areas for surveying of subsurface contaminant load, inventory or audit of hazardous chemicals and establishment of an aquifer monitoring network. Such actions would probably be required before rational decisions and justified actions could be taken in relation to the implementation of pollution control measures at existing installations.

In the formulation of groundwater protection policy, therefore, a basic prerequisite is:

(a) The ranking or mapping of aquifer pollution vulnerability.

(b) The definition of various special groundwater source protection areas.

These tasks are discussed in detail and methodological recommendations given in succeeding sections.

3.1.8 While both approaches to groundwater control and pollution prevention - resource protection and source protection - are complementary, the emphasis placed on one or other will depend on the resource development situation and prevailing hydrogeological conditions.

3.1.9 Source-oriented strategies are best suited to the more uniform, unconsolidated, aquifers exploited only by a relatively small and fixed number of high-yielding municipal water-supply boreholes with stable pumping regimes. They are particularly appropriate in sparsely populated regions where their definition could be fairly conservative without producing serious conflict with other interests.

3.1.10 They cannot be so readily applied where there are very large and rapidly growing numbers of individual abstractions, which render consideration of individual sources and the establishment of fixed zones impracticable. Moreover, data deficiencies and scientific uncertainties, especially in heterogenous aquifers, can render the estimation of the required dimension of protection zones inadequate, without costly fieldwork. However, it should be noted that this strategy is indirectly incorporated in regulations governing the minimum separation (according to hydrogeological conditions) between groundwater sources and insitu sanitation installations.

3.1.11 Aquifer-oriented strategies are more universally applicable, since they endeavour to achieve a degree of protection for the entire groundwater resource and for all groundwater users.

3.1.12 It has to be recognised that there may be limited parts of aquifers which do not justify protection because their water quality is naturally too poor or has already suffered excessive deterioration. In such areas a possible management strategy is to prohibit the exploitation of groundwater for potable or sensitive uses, and to promote the use of the ground for low-cost effluent disposal. However, such a policy also needs careful planning and control if serious problems are not to be encountered due to:

(a) Use of local wells, with serious public health hazard at times of drought, as a result of high
      demand  and supply constraints.

(b) Changes in groundwater flow direction threatening sources outside the area concerned.

(c) Contamination of water-supply mains as a result of rising levels of contaminated
      groundwater.

3.2 Mapping of Aquifer Pollution Vulnerability

3.2.1 In view of the complexity of factors affecting pollutant migration into groundwater systems and the uniqueness of each field situation, it may appear more logical to treat each polluting activity in a hydrogeological environment on individual merit and undertake independent assessments of pollution control requirements. However, this type of approach requires considerable human resources, major investment in field investigations and can present administrative problems where institutional responsibility is split.

3.2.2 An overall framework within which to operate protection policy and prioritise protection measures is thus required to give a chance of achieving universal coverage and avoiding inconsistent decisions. A general aquifer pollution vulnerability ranking provides the central element of this framework.

3.2.3 Aquifer pollution vulnerability has already been defined (for the purpose of groundwater pollution risk evaluation) as a set of intrinsic characteristics of the strata separating the saturated aquifer from the land surface which determine the sensitivity of the aquifer to be adversely affected by an imposed contaminant load. This definition is considered adequate and consistent for the present purpose.

3.2.4 Aquifer pollution vulnerability has to be conceived of interactively with the contaminant load that is, will be or might be, applied on the subsurface environment as a result of human activity, causing a groundwater pollution risk. (The term groundwater pollution risk being defined as the probability that the aquifer, but not necessarily a pumping source, will become contaminated to unacceptable levels). Adopting such a scheme, we can have high vulnerability but no pollution risk, because of the absence of significant contaminant load and vice versa. Both are perfectly consistent in practice.

3.2.5 Since contaminant load can be controlled, groundwater protection policy focuses on achieving such control as is necessary in relation to the aquifer vulnerability and, therefore, to the natural capacity for pollution attenuation. Table 7 gives a possible scheme for variable pollution controls according to aquifer vulnerability.

3.2.6 However, if groundwater resource protection is thus based, it must be remembered that the concept of general vulnerability to a universal pollutant in a typical pollution scenario is untenable in rigorous scientific terms. All aquifers are vulnerable to persistent contaminants in the long run. Moreover aquifers which would generally be regarded as less vulnerable to contamination in these terms tend to be the most difficult to clean-up if they become polluted.

3.2.7 Nevertheless, it is considered aquifer pollution vulnerability that general is a useful concept on concept on which to base the implementation of aquifer protection policy (Figure 9). It is primarily a function of:

(a) The inaccessibility of the saturated aquifer to the penetration of pollutants in a hydraulic
      sense.

(b) The relative attenuation capacity of the strata overlying the saturated aquifer, as a result of
      physical retention of, and chemical reaction with, contaminants (Figure 10). This zone is
      favourable for pollution attenuation or elimination because water movement is normally
      slow and restricted to the smaller pores with larger specific surface.

Table 7
Acceptability matrix of common potentially-polluting
activities and installations withinn land surfaces zones
for groundwater protection

3.2.8 On this basis it is reasonable to characterise aquifer pollution vulnerability on the basis of the following (generally available or readily determined) parameters:

(a) Degree of groundwater confinement.

(b) Overall characteristics of the strata overlying the saturated zone, in terms of lithological
      type and grade of consolidation (Figure 11).

(c) Depth to groundwater table or groundwater strike.

Separate colours would be used for major lithological divisions of the strata overlying the saturated zone with different densities of colour for each subdivision of depth to groundwater. It is recognised that in situations of growing aquifer overexploitation, both (a) and (c) will vary with time, complicating the assessment of aquifer vulnerability.

3.2.9 This scheme of ranking and mapping aquifer pollution vulnerability does not include any consideration of the biologically-active soil zone. It is accepted that many processes causing subsurface pollutant elimination and attenuation occur at their maximum rates in this zone. (Figure 10), as a result of its higher clay and organic content and very much larger bacterial populations. In many point sources of contamination, however, the subsurface contaminant load is applied below the soil zone at the base of excavations (such as pits, trenches, lagoons, soakaways and quarries) and so the attenuation capacity of this zone does not contribute to reducing aquifer vulnerability, except in the case of diffuse pollution from agricultural cultivation practices (Figure 9).

3.2.10 These main elements which afford the natural protection of aquifers and, in effect, determine their vulnerability to pollution can be individually mapped (Figure 11).
further step sometimes used is to index individual parameters and combine them into an overall aquifer vulnerability index which can then be mapped.

Figure 10
Summary of processes promoting contaminant attenuation in groundwater systems

the thickness of the corresponding vertical line indicates typically the relative importance of the
corresponding process int the soil, above, at and below the groundwater table

Figure 11
Basic classification of geological strata for the purpose
of aquifer pollution vulnerability mapping

the strata separating the saturated aquifer from the land surface (the unsaturated zone or the confining beds) are considered and the number of lithological subdivisions can be varied somewhat to suit local circumstances

3.2.11 A complication occurs in ascribing uniformly low vulnerability to all areas of relatively impermeable superficial cover. Those portions of such areas which generate flow to rivers which are influent downstream, either naturally or as a result of riverside pumping, should be specifically identified.

3.2.12 Finally it must be stressed that aquifer vulnerability maps are designed to provide a general framework within which to base groundwater protection policy. The two, however, are distinct in both concept and function. The former should represent a simplified, but factual, representation of the best available scientific data on the hydrogeological environment, no more or no less. Policy implementation areas may include more than one vulnerability class depending of their objective. This general framework is not intended to eliminate the necessity to consider in detail the design of actual potentially-polluting activities before reaching policy decisions.

3.3 Groundwater Source Protection Areas

3.3.1 The objective of source protection areas (SPAs) is to provide a special additional element of protection for selected groundwater sources (boreholes or springs). This is achieved by placing tighter controls on activities within all or part of their recharge area. Table 7 gives a possible scheme for pollution controls in source protection areas also but inevitably these will vary with local conditions and needs. It is important to point out that groundwater source protection areas become less appropriate in heavily-exploited aquifers since they tend to interact and combine. In effect the entire recharge area then needs to be protected.

3.2.2 The outermost protection area that can be defined for an individual source is its recharge capture area. This is the area within which all aquifer recharge, whether derived from precipitation or surface watercourses, will be captured at the source concerned. This area should not be confused with the area of hydraulic interference caused by a pumping borehole, which is larger.

3.3.3 In practice the definition requires further specification and it is customary to use the maximum licensed and not the actual) abstraction rate together with the long-term average recharge rate, when calculating such areas. It is accepted that, on this basis, in extreme drought the actual capture area will be larger than that protected.

3.3.4 The recharge capture zones of sources are significant not only for quality protection but also in resource management terms. In situations of intensive groundwater use they could be used for aquifer exploitation control also.

3.3.5 In order to eliminate completely the risk of unacceptable source contamination, all potentially-polluting activities would have to be prohibited or controlled to the required level within the entire capture zone. This will often be untenable, due to socioeconomic pressure for development. Thus, some division of the recharge capture zone is required, so that the more severe constraints will only be applied in the areas closest to the source.

3.3.6 This sub-division could be based on a variety of criteria, depending on the perceived pollution threat, including: horizontal distance, horizontal flow time, proportion of recharge area, saturated zone dilution and/or attenuation capacity, but for general application it is considered that a combination of horizontal flow time and distance criteria are the most appropriate.

3.3.7 The dilution and attenuation capacity of the saturated aquifer are, in practice, difficult to quantify and predict, although the latter will in a general sense increase with increasing horizontal flow distance and flow time.

3.3.8 Intuitively, dilution might appear to be a useful criterion to delimit source protection perimeters within the overall capture zone of the source. However, consideration of two simple examples will give an indication that this is not necessarily so:

(a) In the case of continuous discharge of a persistent pollutant into an aquifer (e.g. from a
      leaking storage tank) at a fixed rate (q), the dilution at a continuously pumping borehole (at rate Q), will be
      q/Q irrespective of the distance of the pollutant source from the borehole.

(b) For instantaneous sources (e.g. a transport accident) consideration of a simple model in
      which radial flow to a borehole is superimposed on uniform regional flow with no recharge
      gives a further indication of the unsuitability of dilution as a criterion. With increasing
      distance upstream from the borehole, groundwater flow becomes essentially rectilinear.
      The dilution upon arrival at the borehole of a fixed volume of pollutant introduced between
       two specific flow lines in this region of rectilinear flow will be independent of distance, if
       we ignore hydrodynamic dispersion which is difficult to quantify and predict.

3.3.9 Special protection of a proportion of the recharge area may be the preferred solution to alleviate diffuse agricultural pollution of groundwater under certain circumstances, but even then the question of which part of the recharge capture zone to protect inevitably arises.

3.3.10 In practice more than 2 or 3 sub-divisions of the total capture zone will generally be practical: the operational courtyard area, an inner protection zone related to the control of pathogenic contamination, and perhaps an outer protection zone to allow differential control of point source or diffuse source pollution in the remaining area.

3.3.11The innermost protection area is the operational courtyard, which comprises a small area of land around the source itself. It is highly preferable for this area to be under ownership and control of the groundwater abstractor. In this area no activities should be permitted which are not related to water abstraction itself, and even these activities need to be carefully assessed and controlled to avoid the possibility of pollutants reaching the source either at the wellhead directly or via adjacent disturbed ground. Specification of the dimension of this area is necessarily somewhat arbitrary. It will be dependent to some degree on the nature of the geological formations present, but a radius of 30 m is desirable.

3.3.12 An inner protection zone, based on the distance equivalent to a specified horizontal flow-time, has been widely recommended for the prevention of pathogenic contamination of groundwater sources.The time used has varied significantly between regulatory agencies in different countries and regions, from 10-400 days.

3.3.13 Review of all published case histories of groundwater -contamination by pathogens concluded that the horizontal travel distance of bacteria and viruses in the unsaturated zone is governed principally by groundwater flow velocity. In reported pollution incidents the horizontal distance between the borehole or spring and the proven source of pollution was equivalent to no more than the distance travelled by groundwater in 20 days, despite the fact that pathogens are capable of surviving in the subsurface for up to 400 days. A value of 50 days is thus considered a reasonably conservative basis with which to define the inner protection zone, and conforms with existing practice in many cases.

3.3.14 An outer protection zone may be necessary to allow differential control of point source or diffuse source pollution in the remaining area. However, current scientific uncertainty about the rates of subsurface degradation of other types of contaminant, together with the complexity of subsurface dispersion and dilution processes, mean that the criteria used for its definition will inevitably be somewhat arbitrary. It can either be a fixed percentage of the area of the capture zone or a fixed time of horizontal flow to the groundwater source. A possible compromise, bearing in mind that both point source and diffuse source pollution and degradable and persistent contaminants need to be controlled, is to use the horizontal flow time one order of magnitude greater than that used for the inner protection zone (500 days), but to set a minimum limit of say 25$ on the proportion of the overall recharge capture zone protected.

3.3.15 The larger the area defined, the greater will be the chance of dilution, attenuation and elimination of degradable toxic contaminants and consequently the less will be the risk of unacceptable pollution of the groundwater source. Additionally, the larger the area defined, the longer will be the time available to take remedial action to control the spread of pollution, at least in cases where the polluting incident is immediately recognised and notified.

3.3.16 An indication of the variation in size and shape of source protection areas under differing hydrogeological conditions is given in Figure . It should be noted that the definition of recharge capture area is mainly dependent on the recharge. Potential problems arise where the around the source is confined, area will in fact be detached it is partially-confined and about the rates of recharge bed.

3.3.17 In every instance it will be essential to reconcile the definition of source protection areas, whether computed by analytical or numerical models, with the available knowledge of local hydrogeological conditions.

3.3.18 Special problems arise, especially with the definition of recharge capture areas, in situations where the groundwater divide is at a great distance, the regional hydraulic gradient is very low and/or there are surface watercourses flowing across unconfined aquifers. It is evident that where surface watercourses are influent within a capture zone to a source, any potentially-polluting activity in the surface water catchment upstream of the recharge capture area could affect groundwater quality and should be designated for special protection, albeit with less rigorous controls than those applying within the protection zones proper.

3.3.19 Aquifer properties (especially 'active' porosity and 'effective' thickness, and in many conditions transmissivity also) exercise the dominant control over the size of the protection areas defined by horizontal flow time (as illustrated by isochrons in Figure 12). Sensitivity to variation of these parameters will be immediately evident and it is essential that realistic estimates are used. The estimation of realistic values for porosity and thickness may present significant problems in the case of fissured porous aquifers and layered aquifer systems respectively. The dangers of overestimating these parameters will also be apparent from Figure 12. The computation of worst case scenarios should also be considered.

Figure 12
Comparison of source protection areas for different
aquifer types and recharge conditions

the recharge capture zones have a total area equal to the average abstraction rate divided by the long-term average recharge rate; the effect of the size of the 50 and 500 day isochrons of variation (or error) in aquifer characteristics can be seen by comparing (a) (b) and (c) for which the recharge condition and thus capture zone are constant; the effect of decreasing recharge and of the source being located in a fully-confined aquifer can be seen by comparing (a), (d) and (e) for which the hydraulic characteristics of the aquifer are constant

3.3.20 A further practical complication with all source protection areas is that they interact and vary position and have complex shapes if numerous sources are developed in close proximity. They are thus best suited to aquifers with stable exploitation regimes. In the case of heavilydeveloped and overexploited aquifers, it is more practical to coalesce individual source protection zones into a larger multisource protection area. However, if a significant proportion of the abstraction is for non-potable uses (especially irrigation) a further complication arises.

3.3.21 The construction of source protection areas can be achieved by using suitable computer models. There are several which have been fairly widely used (Table 8) and are therefore reasonably tested (Table 8). Thus, provided these models are used properly they should give reliable results, within the limits parameter uncertainty. Ideally threedimensional groundwater flow modelling would be used to delineate source protection areas; however, in practice there is rarely, if ever, adequate information on aquifer vertical permeability and hydraulic head variations, and two dimensional formulations with cautious parameter selection are the best that can be achieved.

3.3.22 Computer models invariably determine zones by tracking the movement of particles in a reverse direction upstream from the source in small steps, using the flow-velocity distribution to determine the time to pass from one point to the next and hence the total travel time to the source. Any such sequence of points leading away from a source is called a pathline. By constructing a number of pathlines of equal travel time, but emanating from the source in different directions, an outline for a given isochron can be built-up. For the total recharge capture zone the pathlines must continue to a point of zero velocity or the edge of the region under study.

3.3.23 The velocity distribution required in the tracking process will normally itself have been determined from a head distribution and that head distribution will in most cases be best determined with the aid of a groundwater flow model. General purpose solute transport models (normally incorporating dispersion, retardation and decay simulation) and those dealing with immiscible phase pollutants are considered far too complex for use in this application.

Table 8
Summary of selected coputer software for modelling
groundwater source protection areas

3.3.24 There is already useful experience with groundwater source protection areas in the Latin American-Caribbean Region. As long ago as 1963, concerned about the risk of groundwater pollution, the Barbados authorities divided the island into 5 zones with varying levels of development control (Figure 13). The policy has served Barbados well in protecting its groundwater supplies which are the sole source of water for a population of more than 250,000, together with a major influx of tourists seasonally. Recently groundwater quality has come under pressure in some ares as a result of urban, industrial and highway development and from changing and intensifying agricultural cultivation which is not subject to control.

3.3.25 A valuable first phase in the implementation of source protection areas is to estimate their extension and consider their implications based on calculations using existing hydrogeological data. An example is shown in Figure 14, and it is strongly recommended that this planning exercise is undertaken by all municipal water companies and undertakings as a matter of priority. The feasibility of implementing such protection areas in practice will depend on the level of existing urbanisation and industrial development, and on the level of overall exploitation of the aquifer involved.

Figure 13
Development control zones for groundwater source
protection on barbados

the entire island is divided into zones 1 to 5 around existing and designated public water-supply sources with diminishing levels of development control as follows

Figure 14
Distribution of public groundwater solvents around El Tigre-Venezuela
indicating possible source protection areas

protection areas for individual production boreholes in wellfields I-IV effectively interact and are merged together as appropriate; aquifer is a 50-metre thick semi-unconfined intergranular formation of high transmissivity and storage, recharge rates average about 250 mm/a and groundwater levels are some 30 m below surface except in vicinity of rivers; the wellfields are well protected by both the unsaturated and saturated aquifer but significant threat of chemical groundwater pollution in wellfields II, III and (to much lesser degree) IV arise from unsewered urban areas within their chatchment, there is also a general problem related to the drilling/operation of petroleum wells throughout the area

4. Control of groundwater pollution sources

4.1 Classification of Polluting Activities

4.1.1 Human activities at the land surface generating a subsurface contaminant load may be classified on a number of different bases.

4.1.2 The most familiar classifications are generic ones dividing activities into categories such as residential, industrial, agricultural, and mining with varying levels of subdivision (Table 3). Other criteria including areal distribution (point source, diffuse source, etc) contaminant type, volume and depth of pollutant discharge with respect to land surface, are frequently used in hydrological work (Table 3).

4.1.3 It is, however, necessary when considering strategies for pollution control also to think in terms of the timing of the pollution incident and the attitude of the polluter. It is important to recognise the important difference between:

(a) Future pollution risk arising as a result of a proposed new activity.

(b) Current pollution resulting from activities initiated after the introduction of the groundwater protection
     legislation.

(c) Pollution currently occurring as a result of activities established prior to the introduction of this
      legislation and which have continued unchanged.

(d) Pollution that occurred prior to the existence of this legislation, which is often the legacy of earlier
      industrial activity. Approaches to this, the so-called contaminated land problem, are discussed at the
      end of this chapter.

4.1.4 Environmental legislation can normally be, to some degree, retrospective or retroactive but cannot readily prescribed for the lattermost of the above situations.

4.1.5 Another classification based on the philosophical attitude of the polluter is also of relevance, and recognises four categories in relation to current and recent pollution (Table 9):

Table 9
Classification of potential sources of groundwater contamination
by philosophical attitude of polluter

(a) Intentional. This category includes all authorised activities and systems designed to use the subsoil for
     waste treatment and/or disposal, such as latrines, septic tank soakaways, etc.

(b) Incidental. This category includes planned activities, which lead to an uncontrolled subsurface
     contaminant discharge, such as most sanitary landfills, many effluent lagoons and all agricultural
     cultivation.

(c) Accidental. In this category are included systems designed to avoid subsurface discharge, for
     example chemical storage tanks but which lead to risk of such discharge occurring in case of leakage
     or spillage.

(d) Clandestine. This category constitutes all unauthorised illegal activities that cause or may cause the
     generation of a subsurface contaminant load.

4.1.6 These classifications, however, are not static. The same activity could pass from one category to another, if design changes were incorporated. Moreover, examples may occur of activities which are initially designed to avoid all subsurface contaminant discharge except in the case of accident, which subsequently cause pollution because of operational carelessness or negligence.

4.1.7 For the purpose of this guide to the control of activities causing risk of groundwater pollution, it is convenient to distinguish at the outset between known or potential point sources and diffuse sources of pollution, and this is the subdivision used in this chapter.

4.2 Strategy for Potential New Sources

On the assumption that administrative arrangements and legal powers exist for the regulation of potential new sources of groundwater pollution, a control strategy will need to be developed. Control is normally achieved through the requirement that the regulatory agency be consulted at the planning stage. Problems may arise when strong economic interests have to be balanced or if the powers of the regulatory agency are not mandatory. However, they can still usually play a powerful persuasive role.

4.2.2  In selected areas it will be necessary to prohibit a proposed activity insisting on relocation or to restrict the activity by insisting on alternative technology to protect groundwater (e.g. substituting an industrial effluent treatment plant in place of an infiltration lagoon). Further examples of such alternatives are given in Table 10. In many instances improvements to proposed designs, aimed at reducing the risk of contaminant infiltration or the hydraulic loading associated with contaminants, may be sufficient. Improvements of this type to the more common point pollution sources are presented in a later section of this guide.

4.2.3 In other cases it may be possible to allow the activity unmodified but stipulate a requirement for "offensive" monitoring to detect early any significant impact on groundwater quality so that timely remedial action can be taken. Techniques of groundwater sampling and the design of monitoring networks have been discussed in detail in an earlier manual.

4.2.4 The decision (between prohibit, restrict or permit) will normally be taken on the basis of location in relation to aquifer pollution vulnerability and to interests in groundwater for potable supply (represented by the existence of source protection areas). These concepts have been discussed in the previous chapter, but it must be stressed that attempting to relate specific control measures for a certain activity to a specific zone is not an easy task and will depend significantly on the experience of the regulatory agency.

4.2.5 Failure to comply with pollution control requirements has to be addressed through monetary fines and/or revoking of operational permits or discharge licences. In much environmental legislation two distinct positions are recognised:

(a) Civil law action to recover the full cost of pollution, including not only that of treatment of the
      affected water-supply or provision of an alternative water-supply, but also that of aquifer
      restoration (clean-up). This involves a long-term financial commitment and is invariably is
      very costly. Legal action is usually only successful when evidence of causing pollution is
      "beyond all reasonable doubt". Given the complexity and slow response of many aquifer
      systems, the burden-of-proof is often very onerous and successful cases are rare.

(b) Administrative quasi-legal action involving smaller, often nominal, fines rather than
      recuperating the full cost of the consequences of pollution. Such actions are normally
      carried if the "balance of evidence" proves subsurface pollution, but not necessarily
      contamination of a given groundwater supply, occurred.

Table 10
Options for the control of potential point sources of groundwater containation

4.2.6 The polluter-pays-principle, in its most direct and simple sense, is ineffective in the case of groundwater, compared for example to the control of river pollution, because too much aquifer normally has to become polluted before the problem is clearly recognised. Moreover, the burden-ofproof in respect of the precise source and/or the exact timing of pollution is normally too onerous.

4.2.7 A better approach to the application of this principle is that the potential polluter should pay for aquifer protection, in terms of control measures and offensive monitoring.

4.3 Strategy for Existing Sources

4.3.1 In the context of groundwater pollution control very different problems are posed by. potential sources of pollution existing prior to the implementation of protection policy and the enactment of the associated legislation.

4.3.2 The initial problem in controlling such sources of groundwater pollution is simply identif