National Federation of Group Water Schemes

Preventing Pollution of Wells

The Advice Note sets out the guidelines on how a water supply borehole should be constructed given the nature of Irish aquifers and the nature of common potential risks to groundwater quality in Ireland.

EPA Drinking Water Advice Note No14b

Hazard Pathway Receptor target as a Concept
Groundwater Protection Schemes
Summary of Components of Groundwater Protection Scheme
Response Matrix for a Hypothetical Activity
Contamination of Wells
Main Sources of Well Pollution in Ireland
Vulnerability Assessments
Hazard Surveys
Proper Siting of Wells and Hazards
Performance Criteria for Hazards
Monitoring of Groundwater Quality
Assessment of Water Quality Data
Well Construction and Sanitary Protection
Well Construction Standards
Role of treatment
Taking Responsibility Public Awareness and Common Sense

  1. Introduction    top of pageWells and springs are an important source of drinking water in rural Ireland. Also wells and springs, and particularly 'holy wells', have contributed to the folklore and tradition of rural areas. However, for many people, groundwater is 'out of sight' and frequently 'out of mind', at least until a problem arises. Unfortunately, many problems are arising, with more than 50% of wells in some areas containing faecal bacteria. There are many reasons for this, but a primary one is the lack of understanding and knowledge concerning groundwater, which is often exacerbated by the use of diviners. As a result, the inherent 'common sense' of many well owners is absent, resulting in a lack of care in locating, constructing and maintaining wells, which in turn is leading to frequent pollution of householders' own and/or their neighbours source of water. Even local authorities do not always have adequate knowledge of information on their wells, which are an important part of the infrastructure of an area.This paper is intended to remove some of the mystery associated with wells and enable a better understanding of groundwater and groundwater pollution. The current groundwater quality situation, the main pollutants and sources of pollution are outlined. The reasons for the pollution are described.Practical approaches to preventing pollution are recommended.
  2. Background    top of page
    1. Groundwater An Important Resource   top of page

Ground water is a major natural resource in Ireland providing between 20% and 25% of drinking water supplies (DoELG/EPA/GSI, 1999). In certain counties, particularly in the Midlands, the proportion is greater than 50%. e.g. North Cork, Roscommon, Offaly, Laois and Kilkenny. Many industries, especially food processing industries such as creameries and meat factories, have their own water supply, often from groundwater. There are vast groundwater resources available and unused for water supply at present.

    1. Wells: Ireland's Commonest Water Abstraction Point     top of pageWells and springs have been used for water supply from time immemorial, and wells are by far the most common abstraction point in Ireland. Many thousands of wells ' at least 100,000 and perhaps over 200,000 (Wright, 1999) ' have been drilled, and several hundred new wells are drilled each year.
    1. Groundwater Quality Situation in Ireland      top of page
      • Human activities have not yet caused the same degree of pollution problems to groundwater in Ireland as in most other EU countries. However, there are an increasing number of localised problems where wells are polluted by point sources such as septic tank systems and farmyards. In many areas, at least 30% of private domestic and farm wells are contaminated; in some highly vulnerable areas more than 50% are contaminated at some time during their use. Most of these wells are not just contaminated chemically but are polluted by faecal bacteria.
      • With the exception of firstly, karst (cavernous) limestone areas and secondly, shallow, open and vulnerable sources such as dug wells and springs, high yielding groundwater sources tend to have good quality ground water.
      • In karst limestone areas, intermittent contamination is more widespread and many high yielding wells and springs are polluted at certain times.
      • Pollution of groundwater in Ireland tends to be mainly microbiological rather than chemical ' in fact wells and springs with levels of chemicals greater than the EC Maximum Admissible Concentration (MAC) and arising from human activities are not common. However, human activities have caused chemicals to enter groundwater and contaminate it, even if the concentrations are less than the EC MAC. The main chemical contaminants are nitrate, ammonia, potassium, chloride, iron and manganese. Trace organics derived from refined oils and pesticides are causing occasional pollution problems and it is probable that solvents are causing groundwater pollution in some urban areas, as is the situation in the other European countries. However there are insufficient data available to properly assess the impact of pesticides and other trace organics (as far as we in GSI are aware), although the situation is changing due to the work of the EPA.
      • The EPA County Nitrate Reports provide data set that enable nitrates to be assessed. Out of 1037 public groundwater sources, 28 (2.2%) had a maximum value greater than 50mg/l (the drinking water MAC), with a mean of 40mg/l. Further 7.5% had mean values over 20mg/l, peak values over 40mg/l and occasional exceedences of the MAC. In conclusion, about 10% have nitrate concentrations that give rise to concern.
    1. Health Implications of Water Quality Problems      top of pageThe parameters of most concern in groundwater are microbial pathogens and nitrate. Parameters such as pesticides and other trace organics, trace metals and ammonia may also pose a threat to health, but they are far less common in groundwater than microbial pathogens and nitrate.
      1. Microbial Pathogens    top of page

        A large number of microbial pathogens are known to contaminate groundwater (Macler and Merkle, 2000), including more than 100 viral and several bacterial pathogens, together with protozoa such as Cryptospiridium and Giardia. Most of these organisms are of faecal origin and are transmissible via a faecal-oral route of exposure. The possible microbial illnesses that result from infection vary with the organism and very markedly in their severity. The predominant recognised illness is generalised Acute Gastrointestinal Illness (AGI), resulting in fever, nausea, diarrhoea, and/or vomiting (Macler and Merkle, 2000). Most cases of AGI are of short duration and may not be of major consequence to otherwise healthy individuals. However, for others, such as the elderly, infants and pregnant women, this may not hold true, and the consequences may be chronic, severe and fatal. Other illnesses include gastro-enteritis, giardiases, cryptospiridiosis and hepatitis. An estimated 750,000 to 5.9 million illnesses per year result from contaminated groundwater in the US (Macler and Merkle, 2000). Mortality from these illnesses may be 1400-9400 deaths per year. If these statistics were computed for a population of 5 million (similar to Ireland), the numbers of illness would be ~15,000-120,000 and deaths would be 28-190.

        It was originally thought that protozoa, such as Cryptospiridium, should not occur in groundwater because their relatively large size makes them more subject to natural filtration by soils and subsoils than bacteria and viruses. However, they have been found in groundwater in the US, Britain and Ireland. In the US, they are found mainly in springs. In Britain, Crytospiridium is the fourth most common cause of water borne related diarrhoea. Cryptospiridia oocysts are found in faecal material. Consequently, protozoa are a threat, not only to surface water sources, but also to ground water, particularly in karst areas and areas of extreme vulnerability. As groundwater is seldom filtered as part of the treatment process, they may pose a greater health hazard in groundwater, particularly in springs in karstic and vulnerable areas, than in surface water.

        For assessment of the microbiological quality of water, it is the faecal coliform count which is the primary indicator of pollution of faecal origin in the water. While there is no absolute correlation between the coliform presence and other bacterial pathogens due to the variable and unpredictable behaviour of pathogens, the underlying principle of the test for faecal coliforms is that its presence in waters indicates the potential presence of pathogens. The usefulness of the test as indicators of protozoan or viral contamination is limited. The EU drinking water limit for E.coli is 0. Establishing water quality criteria for viruses and protozoan pathogens is difficult as the infective dose for all strains is generally unknown. In addition, they can persist longer in natural waters than faecal coliforms and are more resistant to water treatment processes. Viruses are a particular cause for concern as they survive longer in groundwater than indicator bacteria (Gerba and Bitton, 1984).

2.4.2-Nitrate  top of page 

Nitrate is one of the most common contaminants identified in groundwater and increasing concentrations have been recorded in many developed countries. The consumption of nitrate rich water by young children may give rise to a condition known as methaemoglobinaeamia (blue baby syndrome). The formation of carcinogenic nitrosamines is also a possible health hazard and epidemiological studies have indicated a positive correlation between nitrate consumption in drinking water and the incidence of gastric cancer. However, the correlation is not proven according to some experts. The EC MAC for drinking water is 50mg/l.

2.5-Hazard-Pathway-Receptor/target as a Concept     top of page

Visualising the interaction between human activities, groundwater flow, contaminant transport and water abstraction from wells helps create awareness of possible problems that may arise for wells, enables preventative measures to be taken and helps solve problems when they arise. The hazard-pathway-target/receptor model for environmental management integrates many relevant factors, which are described in later sections, and provides a useful conceptual framework.

The pathway for contaminants from hazards to wells may be by two mean: (i) through the soils, subsoils and/or bedrock; or (ii) surface water in the immediate vicinity of the well having direct access to the well. In considering well water quality, both possible pathways should be kept in mind.

  1. Groundwater Protection Schemes   top of pageA practical and effective means of protecting groundwater and preventing pollution is through the use of a Groundwater Protection Scheme. The Geological Survey of Ireland (GSI), the Department of Environment and Local Government (DoELG) and the Environmental Protection Agency (EPA) have jointly developed and published (DoELG/EPA/GSI, 1999a) a methodology for the preparation of Groundwater Protection Schemes (GWPSs).A GWPS provides guidelines for the planning and licensing authorities in carrying out their functions, and a framework to assist decision-making on the location, nature and control of developments and activities in order to protect groundwater. Use of the Scheme will help to ensure that within the planning and licensing processes due regard is taken of the need to maintain the beneficial use of groundwater.Two main components are integrated to produce the Groundwater Protection Scheme: (a) land surface zoning and (b) groundwater protection responses for potentially polluting activities.Summary of Components of Groundwater Protection Scheme  top of pageThe land surface zoning is presented on a Groundwater Protection Map, which delineates land areas in terms of groundwater vulnerability to pollution and groundwater potential and is compiled by combining an Aquifer Map and a Groundwater Vulnerability Map. These, in turn, are derived from a series of primary maps; bedrock and subsoil geology, depth to bedrock, and hydrogeological data. These zones are shown in the matrix in Table 1.Table 1- -Matrix of Groundwater Resource Protection Zones   top of page
    Regionally Important Aquifers (R) Locally Important Aquifers (L) Poor Aquifers


    Rk Rf/Rg Lm/Lg Ll Pi Pu
    Extreme (E) Rk/E Rf/E Lm/E Ll/E Pl/E Pu/E
    High (H) Rk/H Rf/H Lm/H Ll/H Pl/H Pu/H
    Moderate (M) Rk/M Rf/M Lm/M Ll/M Pl/M Pu/M
    Low (L) Rk/L Rf/L Lm/L Ll/L Pl/L Pu/L

    A Scheme also provides for the delineation of Source Protection Areas around significant groundwater supply sources. These areas are subdivided into Inner and Outer protection areas, based on the 100 day time of travel and the catchment area respectively, and the associated vulnerability is superimposed on these sub-divisions to give groundwater protection zones (Table 2).

    Table 2 Matrix of Source Protection Zones   top of page

    Inner (SI) Outer (SO)
    Extreme (E) Sl/E SO/E
    High (H) Sl/H SO/H
    Moderate (M) SL/M SO/M
    Low (L) Sl/L SO/L

    Groundwater protection responses for the different zones indicate the acceptability of a particular activity with respect to the potential hazard, aquifer category of source protection area, and groundwater vulnerability. The responses outline the design and construction conditions and investigation requirements which may be appropriate. Responses have been developed for potential hazards such as landfills, on-site wastewater treatment systems (septic tanks) and landspreading of organic wastes. These responses are published separately (DoELG/EPA/GSI, 1999b,c). The approach is illustrated for a hypothetical 'potentially polluting' activity in the matrix in Table 3 below:

Table 3-Groundwater Protection Response Matrix for a Hypothetical Activity   top of page

Regionally Imp. Locally Imp. Poor Aquifers
Inner Outer Rk Rf/Rg LmLg Ll Pl Pu
Extreme (E) R4 R4 R4 R4 R3m R2d R2c R2b
High (H) R4 R4 R4 R3m R3n R2c R2b R2a
Moderate (M) R4 R3m R3m R2d R2c R2b R2a R1
Low (L) R3m R3o R2d R2c R2b R2a R1 R1

R1--Acceptable subject to normal good practice.

R2a.b.c'-Acceptable in principle, subject to conditions in note a,b,c etc. (The number and content of the notes may vary depending on the zone and the activity).

R3m,n,o'-Not acceptable in principle; some exceptions may be allowed subject to the conditions in note m,n,o, etc.

R4--Not acceptable

The Scheme can also be used pro-actively: for example, to identify suitable sites for potentially polluting developments by avoiding, where possible, the main aquifers and vulnerable areas; or to locate water supply sources by identifying the best aquifers and avoiding the most vulnerable areas.

GWPSs are completed or nearing completion by the GSI for 9 counties 'Offaly, Waterford, Tipperary (SR), Limerick, Meath, Wicklow, Clare, Laois and South Cork ' and mapping has commenced in Kilkenny, Tipperary (NR), Monaghan and Roscommon.

While the use of GWPSs will make a major reduction in the threat to groundwater from human activities, they will not be sufficient on their own to prevent pollution of wells. The various elements of GWPSs that assist in reducing pollution of wells, together with other relevant factors, are described in Section 5.


  1. Reasons for Contamination of Wells   top of page 
    1. Introduction

Contamination of well water clearly requires the presence of a hazard in the ZOC of the well. If hazards are present, the probability of contamination arising depends on one or a combination of the following:

  • vulnerability of groundwater;
  • poor design, construction and management of hazards;
  • inappropriate location of hazards and wells;
  • poor well construction and sanitary protection.
    1. Hazards: the Sources of Contamination

There is no part of Ireland without hazards, varying from bird droppings and sheet faeces in remote areas to sewers, industries and petrol stations in urban areas. The potential sources of groundwater contamination or hazards can be classified into two groups: point sources or diffuse, and these are listed in Table 4. Brief comments on the threat posed by the two most important sources ' agricultural activities and on-site wastewater treatment systems ' are given in the following sections.

Table 4

Point Sources Diffuse Sources
  1. Farmyards
  1. Manure and slurry
  2. Soiled water
  3. Silage effluent
1. Organic wastes, landspread or deposited by grazing animals and birds.
2. Septic tank systems and other on-site wastewater treatment systems. 2. Inorganic fertilizers.
3. Spent sheep dip. 3. Spraying of pesticides
4. Landfill sites 4. Urban areas
5. Spillages and leakages (from industrial sites mainly) 5. Rainfall
6. Contaminated surface water
7. Road drainage
      1. Manure and Slurry in Farmyards
  • Not a significant hazard to groundwater as it is in a fairly solid form and cannot get into the ground easily.
      1. Soiled (or dirty) water
  • This is the yard runoff from animal holding yards, drained liquor from dungsteads, wash water from dairies, etc.
  • Often produced in large volumes, particularly on dairy farms and on farms and on farms with open, self-feed silage systems: and on most farms cannot be stored until the spring.
  • Of little fertiliser value and consequently a major nuisance and expense for farmers but has sufficient nutrients to cause contamination.
  • When disposed of by rain guns in free-draining areas, there is a high risk of nitrate problems as the rain guns are used in the non-growing period and are often not shifted regularly.
  • Typical groundwater contaminants are E.coli, ammonia (NH3), nitrate (NO3), potassium (K), and chloride (C1). A high K:Na ratio (.0.4) is often a good indicator of contamination by soiled water and other farmyard wastes.
      1. Silage Effluent
  • Has a high BOD of 70,000 ' 90,000 mg/l
  • Entry of effluent into the ground causes reducing conditions and mobilisation of iron (Fe) and particularly manganese (Mn). Consequently, Mn is often a good indicator of contamination.
      1. Land Spreading of Organic Wastes
  • Organic wastes, by their nature, are composed mainly of the nutrients nitrogen (N), phosphorus (P) and potassium (K). Of these only nitrogen in the form of nitrate poses a significant risk to groundwater. These wastes also contain bacteria, viruses and protozoa, which can pose a significant risk to groundwater in areas where the bedrock is at or close to the ground surface. Other potential contaminants include: chloride (C1), sulphate (SO4), iron (Fe) and manganese (Mn).
  • P and K do not generally pose a risk to groundwater as they are relatively immobile in topsoil and subsoil. However, in extremely vulnerable areas, leaching of P to groundwater may pose a threat to surface water, particularly lakes, by providing the pathway from the land surface to targets such as lakes, streams and wetlands.
  • Land spreading in wet weather close to sinking streams can cause contamination of groundwater.
  • On most farms in Ireland, except where the groundwater is highly vulnerable, the nutrients from farm waters are recycled without entering water, and so do not pose a significant risk to groundwater. In highly vulnerable areas, pollution by faecal bacteria may occur.
  • Organic wastes from piggeries and poultry farms, in contrast, pose a significant risk to groundwater, particularly from NO3 and faecal bacteria and to a lesser extent Fe and Mn, as spreading often occurs at rates greater than the crops can use.
      1. Inorganic Fertilisers
  • Nitrate is one of the most common contaminants identified in groundwater world-wide. It is highly mobile and easily leached out of the rooting zone. Inorganic fertiliser is the commonest source of nitrate in groundwater although land spreading of organic wastes and ploughing of grassland can have the same effect.
      1. On-site Wastewater Treatment Systems
  • Conventional septic tanks produce and effluent which is highly polluting if it enters directly into water. It contains high levels of bacteria (up to 107 ' 108/100ml), nitrogen (50mg/l), phosphorus (10mg/l) and BOD (300mg/l) (EPA, 1998), together with sodium, chloride and other constituents.
  • The main groundwater contaminants from septic tans systems are ammonia (NH3), nitrate (NO3), chloride (C1) and faecal bacteria and viruses.
  • The main treatment of the effluent does not occur in the septic tans system itself, but in the ground (subsoil and bedrock) into which the effluent is discharged.
  • While the pollutant loading may be significantly reduced by alternative on-site wastewater treatment of proprietary systems, effluent from them nevertheless still pose a potential threat to groundwater, and the effluent quantity is not reduced.

4.3-Main Sources of Well Pollution in Ireland   top of page

Farming is by far the most important potential source of contamination of groundwater in Ireland because:

  1. It produces large volumes of organic wastes ' about 80% of organic wastes generated in Ireland comes from agriculture and these wastes are equivalent to 10 people; consequently, a 40 ha dairy or cattle farm produces wastes equivalent to a small town, which would or should have a sewage treatment plant.
  2. It uses large amounts of inorganic fertilisers and pesticides.
  3. As an activity, it takes place above most of our aquifers, irrespective of the vulnerability of the groundwater.

Septic tank effluent is a major source of pollution to wells, particularly domestic wells, in Ireland (Daly et al..1993). There are over 300,000 septic tank systems serving a population of about 1.2 million people and discharging approximately 80 million m3 of effluent into the ground annually (or 50 million gallons each day). Although farmyard wastes have a greater potential to pollute than septic tank systems in view of the relative volumes of waste, much of the farmyard wastes are recycled onto land, whereas septic tank effluent bypasses the topsoil and some of the subsoil, thereby increasing the risk of pollution. Also, there are a greater number of septic tank systems than farmyards. Consequently, it is probable that septic tank systems pollute as many wells as farmyards wastes, although the levels of pollution are probably less and the pollution is less obvious.

Microbiological pollution is caused mainly by wastes in farmyards and septic tank effluent. Other sources are likely to include land spreading of organic wastes in extremely vulnerable areas and polluted surface water sinking underground in karst areas. Some wells become contaminated because they are located too close to potential pollution sources and/or because the well construction is poor.

The sources of nitrate contamination tend to vary from area to area and well to well. Many of the wells with high nitrates are affected by organic wastes, particularly from nearby farmyards and septic tank systems. However, in intensive agricultural areas, background levels have increased significantly in the last 10 years due to the spreading of organic and inorganic fertilisers. Consequently, in the near future the Department of Environment and Local Environment will require the delineation of nitrate vulnerable zones (NVZs) around aquifers and specific groundwater supplies of concern (Fitzsimons et al.. 2000).

In the late 1980's many farm wells and some local authority and group scheme wells were polluted by silage effluent, resulting in high manganese levels. There are far fewer reports of problems now, probably because of the improvements in the design, construction and operation of silage pits.

4.4-Vulnerability   top of page

The ability of the subsoil and bedrock to treat pollutants adequately depends on the geological and hydrogeological characteristics of the site, particularly the permeability (or percolation property), the thickness of subsoil and the depth to the water table. It is the subsoils that provide the most effective protection of groundwater from pollution. Clayey sands/gravels and free-draining sandy or gravelly CLAY or SILT (i.e. combinations of sand, silt and clay) are the most suitable subsoils for septic tank effluent disposal and treatment. In contrast, once pollutants enter bedrock there is little purification. Groundwater is most vulnerable and at risk from pollution where bedrock, particularly limestone, is at or close to the surface: where clean, permeable sand/gravel underlies the site; and in sand/gravel where the water table is close to the surface. In areas with these conditions, pollution of wells is common.

These geological and hydrogeological characteristics can be examined and mapped, thereby providing a groundwater vulnerability assessment for any area or site. Four vulnerability categories are used in Ireland ' extreme (E), high (H0, moderate (M) and low (L) (DoELG/EPA/GSI. 1999a). The hydrogeological basis for these categories is summarised in Table 5.

Table 5 Geological and Hydrogeological Conditions Determining Vulnerability Mapping


Subsoil Thickness Hydrogeological Requirements
Diffuse recharge Point Recharge Unsaturated Zone
Subsoil permeability and type
High Permeability (sand/gravel) Moderate Permeability (sand subsoil) Low permeability (clayey, subsoil, clay, peat) (swallow holes, losing streams) (sand & gravel aquifers only)
0-3 m Extreme Extreme Extreme Extreme (30m radius) Extreme
3-5 m High High High N/A High
5-10 m High High Moderate N/A High
>10 m High Moderate Low N/A High
Notes: (i) N/A = not applicable.

    1. Permeability classifications relate tot he material characteristics as described by BS5930.
    2. Release point of contaminants is assumed to be 1-2 m below ground surface.

(from Deakin and Daly, 2000 and adapted from DoELG/EPA/GSI, 1999)

4.5-Poor Design, Construction, Maintenance and Management of Hazards   top of page

Hazards and potentially polluting activities are designed to either:

  • contain and control all contaminants, e.g. slurry pits, septic tanks, petrol/diesel/oil tanks, silage pits, lined landfills.
  • release contaminants to the ground for treatment, e.g. percolation areas, landspreading of organic wastes.

However, the containment systems often fail, due to one or a combination of poor design, construction and/or maintenance. Examples include: leaking septic tanks; rusting oil tanks; inadequate bonding around storage facilities; leaking silage slabs 'corroded' by silage effluent. Also, there are many instances where the treatment in the soil and subsoil is insufficient to prevent contamination. Examples include: landspreading where the bedrock is at or close (<2m) to the ground surface; percolation areas which do not disperse the effluent: on-site wastewater treatment systems that are not maintained adequately.

      1. Inappropriate Location of Hazards and Wells

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The probability of well pollution is increased when hazards are located:

        • in the ZOC of the well:
        • too close to wells:
        • where the groundwater is extremely or highly vulnerable i.e. where the bedrock is at or close to the round surface.

Many farm wells are located close to or even in the middle of farmyards: private wells are often located (frequently with the advice of diviners) about 30m from soakage pits in vulnerable areas; private wells are often located down-gradient of hazards. All these circumstances have resulted in pollution of wells.

      1. Poor Well Construction and Sanitary Protection

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One of the pathways for contaminants to enter wells is where surface water or shallow groundwater have direct access to the well, usually down the outside of the well casing, but occasionally down the inside of the casing. Any annular space that is produced as a result of the installation of well casing provides a channel for vertical movement of water and/or contaminants, unless the space is sealed. Therefore, a poorly constructed well can result in the bacterial and/or chemical contamination of the well water. It allows the natural protection provided by the soil and subsoil to be by-passed.

  1. Practical Approaches to Pollution Prevention   top of page

Practical pollution prevention approaches depend on a variety of factors, which are often interrelated, and no one approach on its own is likely to be sufficient. These approaches are influenced, to some degree at least, by whether the well is privately owned or is a public supply well. The following is a comprehensive list of approaches, which can be used as appropriate.

  • 'Knowing your well'!
  • Vulnerability assessments;
  • Hazard surveys:
  • Proper siting of wells and hazards;
  • Performance criteria for hazards;
  • Monitoring of well water quality;
  • Assessments of water quality data;
  • Well construction and sanitary protection
  • Disinfection
  • Public awareness.
      1. 'Knowing Your Well'

   top of pageWhether you are a local authority engineer or a householder, a well is a vital engineering structure, providing an essential requirement for living ' water! Also, they are a relatively large capital investment. Therefore it is advisable for well owners to understand and know about their source of water and to record relevant details.

      1. Understanding Groundwater Flow to Wells

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Prior to the start of pumping, water will have been flowing to and by the well from the up-gradient side, perhaps from a long distance ' several kilometres in some instances, but usually at least tens of metres and frequently hundreds of metres. When water is pumped from a well, the water level in the well and the area surrounding the well is lowered. A hydraulic gradient is set up towards the well from all around in the aquifer. During pumping, groundwater flowing b the well is drawn into the well as the nearby flowlines are directed towards the well. Pumping of the well causes some of the flowlines on the down-gradient side to reverse their direction and flow back towards the well. The entire land surface area contributing water to a well is called the zone of contribution (ZOC)

The size of the area depends on the abstraction rate and the recharge (the amount of rainfall that infiltrates through soils, subsoils and bedrock to the water table). The shape depends on the local hydrogeology of an area. An idealised diagram as a ZOC is shown in Figure 1. Contamination of a well can only occur from potentially polluting activities or hazards in the ZOC; consequently, a knowledge and understanding of this area is a vital part of pollution prevention.

The factors to take into account in locating the ZOC are:

      • likely size of ZOC;
      • groundwater flow direction;
      • shape of ZOC;

The likely size of the ZOC will depend on the pumping rate and the recharge. Taking conservative estimates for both, the recharge area required for domestic supplies will be in the range 1000-2500m2. In permeable rocks, this area will usually be a considerable underestimate for the ZOC because ground water may be flowing in the direction of the well for a long distance, but is only intercepted for the short period each day when the pump is turned on. For public supply and group scheme wells, the ZOC will be much more extensive ' for large supplies over 5km2.

Figure 1. -Conceptual drawing of the Zone of Contribution (ZOC) at a pumping well.

Most of the flow to a well comes from up-gradient. Therefore, the groundwater flow direction must be known if the ZOC is to be located. The water table can be visualised as a planar surface that is a subdued reflection of topography. Groundwater flow is usually to the nearest permanent stream. Therefore, the topography and nearby streams can provide a general indicator of groundwater flow direction. However, in permeable areas small changes in topography will not influence the water table, and in karst (cavernous) limestone areas flow directions are unpredictable.

For domestic wells, the ZOC can be visualised as a narrow zone ' 15-30m wide' starting at most 10m down-gradient of the well and extending back up-gradient for at least 150m in poorly permeable bedrock and more than 500m in more permeable bedrock.

For higher yielding wells, the ZOC's are highly variable depending on pumping rate, recharge and the specific hydrogeology in the vicinity of the well. Consequently, it is not possible to estimate these without specialist help. In some instances, the ZOC will have already have been delineated ' the GSI have completed source protection zones around about 50 public supply wells. To give an idea of the possible dimensions, public supply sources yielding between 200-800m3/d have down-gradient boundaries 75-400m away and up-gradient boundaries usually within 2.0km.

It is recommended that every well owner standing at the well site should be able to visualise in 3-D groundwater flow to the well, the potential hazards in the ZOC and the possible threats to the well posed by the hazards. The information given here will assist in this; however, it is general and simplified, and more detailed and specific information would be required for public supply wells.

      1. Recording Well Details

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The GSI receives at least 200 queries every year from householders and engineers concerning problems with wells, e.g. pollution, high iron, silting, caving-in, pump stuck. Assisting with these queries is frequently made more difficult by the lack of information available on the well.

For householders, it is recommended that the following information should be collected and filed:

      • Depth of well
      • Diameter
      • Depth of lining
      • Diameter of lining
      • Details on sealing/grouting
      • Depth of bedrock
      • Type of subsoil
      • Type of bedrock
      • Water entry levels
      • Depth to any cavities met in drilling
      • Static water level below ground
      • Measured pumping rate
      • Drawdown during pumping
      • Estimated maximum safe yield
      • Chemical and bacteriological analyses
      • Drilling Contractor
      • Date of drilling
      • Drilling method

While this may seem a very comprehensive list to a householder at first glance, in fact the driller will provide most of the information, and in any case it is basic information that will assist if a problem arises. Engineers and hydrogeologists will use a dipper (an electric probe with a twin-core cable incorporated into a measuring tape on a revolving drum) to measure the static water level and the drawdown (the change in water level caused by pumping); householders could use a measuring tape and a weight (such as a bolt) tied to bailer twine.

If a householder is buying a house with and existing well, it is worthwhile checking on the precise location of the well, the well details and water quality information. If the existing water quality information is inadequate, insist on having the well tested, chemically and microbiologically.

      1. Vulnerability Assessments

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A vulnerability assessment of an area provides:

        • information on the hydrogeological setting, in particular the permeability and thickness of the subsoils;
        • information needed to evaluate of the degree of risk posed by any hazard;
        • information that can be used in decision-making on the location of hazards, the site investigation requirements and the type of engineering methods needed to prevent contamination.

For example, a well located in an area with ' 1m of soil subsoil over bedrock, i.e. an extremely vulnerable area, is at risk from nearby septic tank systems, landspreading, etc. In contrast, a well in and area with >10m of low permeability subsoil, i.e. low vulnerability, is well protected from contamination, and hazards such as septic tank systems and landspreading are unlikely to contaminate the well (provided surface water does not enter directly into the well).

It is advisable for householders to check for outcropping bedrock and enquire about the depth to bedrock prior to buying a site, if they need to drill a well. Local authorities are advised to carry our a preliminary assessment of vulnerability prior to choosing sites for public supplies.

Vulnerability maps have been completed for 9 counties ' Offaly, Waterford, Tipperary (SR), Limerick, Meath, Wicklow, Clare, Laois, South Cork ' and mapping has commenced in Kilkenny, Tipperary (NR), Monaghan and Roscommon. However, a vulnerability assessment can be carries out, using the national guidelines (DoELG/EPA/GSI. 1999a) on any site or area. The GSI can provide advice on this, if requested.

      1. Hazard Surveys

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      1. Local authorities are advised to identify and map existing hazards in the higher in the higher risk areas, particularly in zones of contribution of significant water supply sources (DoELG/EPA/GSI, 1999a). This would involve conducting a survey of the area and preparing an inventory of hazards. This may be followed by further site inspections, monitoring and a requirement for operational modifications, mitigation measures and perhaps even closure, as deemed necessary. New potential sources of contamination can be controlled at the planning or licensing stage, with monitoring required in some instances. In all cases, the risk, control measures and response category depend on the potential contaminant loading, the groundwater vulnerability and the groundwater value.
    1. Prior to purchasing a site or a house, it is advisable for householders to look at existing developments and potentially polluting activities nearby and, together with consideration of the vulnerability, assess the likely risk to the well. The treatment system on the site may be located a good distance downhill of the well, but the neighbours system might be over the hedge, 10m away!
      1. Proper Siting of Wells and Hazards

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The location of wells and hazards, in a way that minimises the probability of contamination, depends on the following:

        • hazard contaminant loading (for example, the contaminant loading from a single house on-site wastewater treatment system is less than from a landfill);
        • vulnerability of the site/area;
        • hydrogeology of the site/area;
        • whether the hazard is inside of outside the ZOC of the well;
        • distances (often called 'setback' distances) between wells and hazards;
        • the density if hazards;
        • the planning control measures for hazards;
        • risk assessment and management.

For regulatory bodies, such as local authorities, all these aspects are encompassed in the source protection component of groundwater protection schemes and the groundwater protection responses (refer to Section 2). Four examples are given below.

        1. Landfills: in view of the contaminant loading, landfills are not acceptable in the ZOC of public supply wells (DoELG/APR/GSI, 1999b).
        2. Landspreading of organic wastes from intensive farming enterprises: this is 'not acceptable' in the inner and outer protection areas, where the vulnerability is extreme (i.e. SI/E and SO/E); is 'not acceptable' in the inner protection area, where the vulnerability is moderate of low (i.e. SI/M and SI/L): and is 'acceptable subject a maximum organic nitrogen load not exceeding 170kg/ha/yr' in the outer protection area, where the vulnerability is high, moderate or low (i.e. SO/H, SO/M and SO/L) (DoELG/EPA/GSI, 1999c).
        3. Setback distances for on-site wastewater treatment systems: Private wells down-gradient must be 30-60 away, depending on the groundwater vulnerability, i.e. the permeability and thickness of subsoils (DoELG/EPA/GSI, 2000).
        4. Setback distances for landspreading: No spreading is allowable within 50m of public supply wells or 30m of karst features (DoELG/EPA/GSI, 1999c). Farmers are advised not to spread within 50m of their own or their neighbours well.

However, groundwater protection responses are only available at present for three hazards ' landfills, landspreading an on-site systems. For the location of other hazards, the factors listed above need to be considered on a site by site basis, preferably using a general risk assessment approach. The level of investigation and evaluation prior to decision-making will depend on the degree of risk involved.

For everyone involved, use of 'common sense', based on some knowledge of groundwater flow, in locating wells is essential. So, choose the site carefully. Be wary of using diviners. As a general rule, a well should be located up-slope and as far as possible from potential pollution sources, such as farmyards and on-site wastewater treatment systems. If hazards are located outside the 100-day time of travel area (se Figure 1), faecal bacteria should not be present in the well water, provided sanitary protection is adequate. In addition, keep away from streams as pumping may draw in contaminated water. Avoid areas with shallow bedrock. If in doubt, get advice from the GSI.

Local authorities are advised to buy large sites when planning and undertaking development of groundwater

      1. Performance Criteria for Hazards
      2. Monitoring of Groundwater Quality
      3. Assessment of Water Quality Data
      4. Well Construction and Sanitary Protection

   top of pagePerformance criteria include requirements on the design, construction, operation, maintenance and/or inspection of hazards. Examples include the EPA Landfill Manuals, the EPA Wastewater Treatment Manuals and the DoE/DAFF Code of Good Agricultural Practice to protect Waters from Pollution by Nitrates (DoE/DAFF, 1996). These are particularly effective for new developments, but can also be used in dealing with existing developments.

   top of pageMonitoring for contaminants provides direct information on contamination, and a lot of emphasis and perhaps even over-emphasis is given to monitoring of groundwater in EU countries. However, monitoring, while desirable and useful, only provides an indication of the presence or absence of contamination, and is inadequate by itself to ensure protection. Monitoring results are useful in a reactive mode, in that they are generally available after exposure to the contamination has occurred. Monitoring results may be equivocal (Macler and Merkle, 2000): (1) if a well is positive for a pathogen or faecal indicator at a given time, uncertainties remain in the frequency and magnitude of this contamination, as well as in the types and health significance of other organisms that might co-occur; (2) if a system is found negative for indicators, it may in fact be contaminated, but the limitations of monitoring frequency, sample size, and level of quantitative analysis may not show this (for example, well water may not have E.coli ' the indicator bacteria ' but may contain viruses which are smaller than bacteria and some of which live longer); (3) a system that is negative may be without contamination now but not in the future. Also, in comparison to surface water, contamination of groundwater lasts far longer ' for weeks, months and even years ' because groundwater moves slowly. Therefore, emphasis must be on prevention rather than monitoring and remediation.

   top of pageWater quality data, whether from a monitoring programme or once-off sampling, enable not only a check on the presence of contamination but also an assessment of the likely source of contamination.

In assessing groundwater quality, the approach taken in the GSI is to distinguish between the terms 'contamination' and 'pollution'. Groundwater becomes 'contaminated' when substances enter it as a result of human activity. The term 'pollution' is reserved for situations where contaminant concentrations are sufficiently high to be objectionable i.e. above the EU maximum admissible concentration (MAC).

As human activities have had some impact on a high proportion of groundwater in Ireland, there are few areas where the groundwater is in pristine condition. Consequently most groundwater is contaminated to some degree although it is not necessarily polluted. In assessing groundwater quality there is often a tendency to focus only on the EU maximum admissible concentration (MAC). In the view of the GSI, there is a need for assessment of the degree of contamination of groundwater as well as showing whether the water is polluted or not. This type of assessment can indicate where sources of contamination before major incidents occur. Consequently, thresholds for certain parameters can be used to help indicate situations where significant contamination but not pollution is occurring. The thresholds for assessing water quality are given below.

Parameter Threshold mg/l EU MAC mg/l
Nitrate 25 50
Potassium 4 12
Chloride* 25-50 250
Ammonia 0.15 0.3
Faecal bacteria 0 0
K/Na ratio ** 0.4

* depending on distance from the sea.

** in some areas a value of 0.3 may be more appropriate.

In the GSI, if we are examining an area with potential groundwater contamination problems, we assess the analyses as follows:

E. coli present ' organic waste source nearby (except in karst areas), usually either a septic tank system or farmyard.

E. coli absent ' either not polluted by organic waste or bacteria have not survived due to attenuation or time of travel to well greater than 100 days.

Nitrate > 25mg/l ' either inorganic fertiliser or organic waste source; check other parameters.

Ammonia > 0.15mg/l ' source is nearby organic waste; fertiliser is not an issue.

Potassium (K) > 5.0mg/l ' source is probably organic waste.

K/Na ratio > 0.4 (0.3, in many areas) ' Farmyard waste rather than septic tank effluent is the source. If <0.3, no conclusion is possible.

Chloride > 30mg/l (25mg/l in some areas) ' organic waste source. However this does not apply in the vicinity of the coast (within 20km at least).

In conclusion, faecal bacteria, nitrate, ammonia, high K/Na ratio and chloride indicate contamination by organic waste. However, only the high K/Na helps distinguish between septic tank effluent and farmyard wastes. So, while the analyses can show potential problems and indicate likely sources, other information is needed to complete the assessment, Also, this approach is not conclusive. Obviously, details on nearby hazards help in the assessment.

The EPA are currently developing guidelines and intervention values for the protection of groundwater (Keegan, M., et al., 2000), which will expand the advice given above.

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        1. Role

   top of pageThis is a means of preventing pollution that is within the control of the well owner. Even if the well has already been drilled, improvements can usually be made.

        1. Recommendations

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  1. The annular space outside the causing should be filled with a suitable sealant, such as cement or cement/bentonite grout to prevent surface water runoff or shallow groundwater seeping directly into the well. It is preferable to the casing should be grouted for a minimum of 3m into bedrock.
  2. No unsealed openings should exist in the wall or along the joints of the casing.
  3. A concrete slab, 150mm thick, should be keyed in around the casing to a distance of a least 0.5m from the casing.
  4. The casing should protrude above the surface of the concrete slab.
  5. A secure, watertight well cap should be fitted to prevent foreign matter or small animals from falling into the well.
  6. The wellhead should be housed, preferably in a manhole with a manhole cover.
  7. If the general land surface around the well id depressed or susceptible to flooding, it should be raised and regarded so that it slopes away form the well.
  8. If the well is in a field used by farm animals, it should be enclosed so that animals cannot get close ' a distance of 10m from the well is recommended. It is suggested that a relatively high wall should be used rather than a fence, which tends to attract farm animals with the consequent concentration of faeces and urine in the area least suitable!
  9. Inspect the well at regular intervals.

Good advice on well construction is given in Briody (1995), Ball (1995) and NSAI (1992).

        1. Well Construction Standards

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      1. Treatment
        1. Role of treatment

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Disinfection is an important means of protecting human health, but it should not be considered as a solution to groundwater contamination;

  • it is not a sustainable strategy for managing groundwater in that it is merely treating the symptom of the problem;
  • the proposed EU Water Framework Directive aims to reduce dependence on purification and pre-treatment of drinking water;
  • disinfection systems fail on occasions;
  • chlorination may not be adequate to treat protozoa such as Cryptospiridum.

However, disinfection is recommended in certain situations.

        1. Recommendations

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    1. All new wells should be disinfected after drilling.
    2. Existing wells should be disinfected on a regular basis (each Autumn is suggested). A GSI information sheet on disinfection of private wells is given in Table 5.
    3. Drilling equipment should be cleaned and sterilised prior to drilling.
    4. Householders with private wells in karst areas and extremely vulnerable areas should install a disinfection system such as an ultra violet (UV) light to reduce the likelihood of consuming microbial pathogens.
      1. Taking Responsibility, Public Awareness and 'Common Sense'

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'The primary responsibility for groundwater protection rests with any person who is carrying out an activity that poses a threat to groundwater'. This is the first sentence in 'Groundwater Protection Schemes' (DoELG/EPA/GSI, 1999(a)); it is a powerful principle that needs to be more widely accepted and followed.

"Command and Control" regulations are unlikely on their own to ensure successful prevention of contamination. Therefore other alternatives, such as public education, must supplement food planning regulations and enforcement. Greater awareness of the significance of hazards, such as septic tank systems and farmyards, as a source of water pollution and as a health risk is required. Also, a greater appreciation and understanding of groundwater, which is 'out of sight, out of mind' for many people, is needed. Improved awareness would lead to a more responsible approach to environmental issues and the use of 'common sense' in dealing with wells and the possible impacts of human activities.

    1. Acknowledgements   top of page
    2. References   top of page

The author thanks numerous colleagues, inside and outside the GSI this paper is published with the permission of Dr. Peadar McArdle, Director, Geological Survey of Ireland.

Ball, D., 2000. The need for a national well standard and suggested content. Proceedings of the Portlaoise Seminar 'Groundeater and the Law; Directives, Standards & Regulations'. International Association of Hydrogeologists (Irish Group).

Ball, D., 1995. Sustainable groundwater sources. Proceedings of the Portlaoise Seminar 'the Role of Groundwater in Sustainable Development'. International Association of Hydrogeologists (Irish Group).

Briody, A., 1995. Protecting private domestic boreholes. The GSI Groundwater Newsletter, No. 27, p3-4.

Daly, D. Thorn, R. and Henry, H., 1993. Septic tank systems and groundwater in Ireland. Geological Survey Report Series RS93/1, 30pp.

Deakin, J. and Daly, D., 2000. County Clare groundwater protection scheme. Joint GSI/Clare County Council report. 67pp.

DoE/DAFF, 1996. Code of good agricultural practice to protect waters form pollution by nitrates. Department of the Environment and Department of Agriculture, Fisheries and Food. 57pp.

DoELG/EPA/GSI, 1999a. Groundwater protection schemes. A joint publication by the Department of the Environment and Local Government, Environmental Protection Agency and Geological Survey of Ireland, 26pp.

DoELG/APA/GSI, 1999b. Groundwater protection responses for landfills. A joint publication by the Department of the Environment and Local Government, Environmental Protection Agency and Geological Survey of Ireland, 4pp.

DoELG/APA/GSI, 1999c. Groundwater protection responses to the landspreading of organic wastes. A joint publication by the Department of the Environment and Local Government, Environmental Protection Agency and Geological Survey of Ireland, 4pp.

EPA, 1998. Treatment systems for single houses. Draft Wastewater Treatment manual. Environmental Protection Agency.

Fitsimons, V., Wright G.R. and Daly, D 2000. Groundwater aspects of the Nitrates Directive. Proceedings of the Portlaoise Seminar 'Groundwater and the Law: Directives, standards & Regulations'. International Association of Hydrogeologists (Irish Group).

Keeg, M., O'Leary, G. and Carty, G. The development of guideline and intervention values for the protection of groundwaters in Ireland. Proceedings of the Portlaoise Seminar 'Groundwater and the Law; Directives, Standards & Regulation'. International Association of Hydrogeologists (Irish Group).

Gerbe, C.P. and Bitton, G., 1984. Microbial pollutants: their survival and transport pattern to groundwater. In: G. Bitton and C.P. Gerba (Editors), Groundwater Pollution Microbiology, Whiley ' Intersciences Publishers. P65-88.

Macler, B.A. and Merkle, J.C. 2000. Current knowledge on groundwater microbial pathogens and their control. Hydrogeology Journal, Volume 8, p29-40.

NSAI, 1992. Bottles water. Irish Standard I.S. 432: 1992. National Standards Authority of Ireland 54pp.

Wright, G.R. The GSI Groundwater Newsletter, No. 36.

Table 5 Disinfecting Wells
Method 1 Using Bleach (sodium hypochlorite, 3-5% available Chlorine)

  1. Obtain 2 gallons (9 litres) of 3% strength1 gallon (4.5 litres) of 5% strength (e.g. Parazone)
  2. Make up to 5 gallons by adding water and mix thoroughly

3. If sampling during a pumping test, on the day before the test starts pour half of the solution into

the well, start the pump and let it run briefly until water with a distinct smell of chlorine pours

from the outlet pipe. Turn off the pump immediately. Add the remainder of the solution and

leave overnight. Then pump to waste until the smell of chlorine disappears before taking a

sample for analysis.

4. If sampling from a well that is connected to a house, pour half of the solution into the well, start

the pump and open all taps until water from each tap has a distinct smell of chlorine. Stop the

pump and add the rest of the solution. Allow to stand for 12-24 hours, then pump to waste until

the smell of chlorine disappears.

Method 2 Using chloros (12% available chlorine)

  1. Obtain 0.5 gallon (2-3 litres) chloros

2, 3 and 4 as above.