Department of the Environment,
Transport and the Regions

Cryptosporidium in Water Supplies

4 Assessment of contamination risk for groundwater resources 1

4.1 Groundwater use and protection

Introduction

4.1.1 The potential for contamination of groundwater by Cryptosporidium was addressed in the first and second reports of the Group of Experts (DOE/DH 1990 and 1995). These reports, however, only considered direct contamination at the wellhead via faulty, or poorly maintained, well or borehole construction. The likelihood that the outbreak of cryptosporidiosis in March 1997 in North London was associated with a groundwater supply raised questions about Cryptosporidium as a possible groundwater contaminant. The terms of reference for the re-convened Expert Group therefore included consideration of the need for protection of groundwater resources and sources.

4.1.2 For the purposes of this report, groundwater includes water abstracted from aquifers via springs, wells, boreholes and adits. There is a glossary of terms at the end of the chapter.

Use of groundwater

4.1.3 Groundwater provides over 30% of all water abstracted for public water supplies in England and Wales (DETR 1997), 8% in Northern Ireland and 5% in Scotland (Bell et al 1997). The regional differences reflect the distribution of aquifers and the more favourable geological conditions for surface water resource development in Northern Ireland and Scotland. It should be noted that in upland Britain while the proportion of the total supply derived from groundwater is low, the number of individual sources involved is large.

4.1.4 Over 80% of the total public supply in south-east England is derived from groundwater, while in the Severn and Trent basins, eastern England, the Thames Valley and the Wessex region the figure is between 30 and 50%. Extensive blending of groundwater and surface water further increases the extent of groundwater use. Industry and the agricultural community rely on groundwater in many areas and it is the predominant source for private water supplies. The total abstraction of groundwater in the UK, including that used by industry and agriculture, is some 2400 million m3/year. About 85% is pumped from the two major aquifers, the Chalk and the Permo-Triassic sandstones which provide 60% and 25% respectively.

Protection of groundwater

4.1.5 Groundwater source catchments are more difficult to define than the surface water equivalents as they cannot be as easily delineated by topography. Information from the Environment Agency's (EA) groundwater protection zone database shows that 875 of the 2,200 sources so far defined have a total catchment area exceeding 11,000 km 2 . It therefore seems likely that as much as 15% of the land area of England and Wales may comprise catchment of an important potable groundwater supply source. The majority of groundwater-derived supplies is drawn from six major aquifer systems:

4.1.6 More than one hundred other formations, notably fluvio-glacial gravels, provide locally important public and private potable supplies. The majority of important public supply groundwater sources are deep wells or boreholes, but there are several hundred licensed public spring supplies. Over a hundred of the wells have adit systems, mostly located in the Chalk of southern and eastern England. Many of these are highly productive: the average yield per source for 72 of these adited systems is over 10 Ml/d.

4.1.7 A technical and policy framework for groundwater protection in England and Wales is provided by the EA Policy and Practice for the Protection of Groundwater (NRA 1992). A similar strategy for Scotland was published subsequently (ADRIS 1995). A draft policy and practice document is under review for Northern Ireland. Central to each of these protection strategies is the principle of land surface zoning to protect aquifers as a whole and to safeguard specific sources of water supply.

4.1.8 The methodologies employed comprise:

This chapter was prepared by a sub-committee consisting of:

Mr Tony Lloyd (Drinking Water Inspectorate)
Professor Stephen Foster (British Geological Survey)
Mr Brian Morris (British Geological Survey)
Mr Keith Hall (Water UK)
Dr Paul West (Water UK)
Mr Rob Robinson (Environment Agency)
Dr Philip Aldous (Thames Water Utilities)
Mr Alwyn Hart (Environment Agency)
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4.2 Microbiological contamination risk

Vulnerability of groundwater to waterborne microbiological contamination

4.2.1 Most infectious agents in human and animal wastes are potentially. Faecal matter contains very large numbers of non-pathogenic bacteria but may also contain helminth eggs, protozoa, bacteria and viruses which are capable of causing infectious diseases. Most protozoa and helminths are likely to be removed by filtration during passage through soil, or by predation by other micro-organisms. The protozoan Cryptosporidium may be atypical in that the 4-6µm diameter of the environmentally-resistant oocyst stage is smaller than many other protozoa.

4.2.2 While most microbial contaminants die off in passage through the soil, they may enter the subsurface directly via structures which by-pass the soil zone such as septic tanks, latrines or soakaway pits. As these structures are designed to dispose of liquid wastes quickly, they will almost invariably impose a higher hydraulic loading than pollutants entering at the top of the soil zone. This implies less potential for pathogen removal during subsurface flow. In agriculture, new or substantially modified farm waste storage facilities must comply with the Control of Pollution (Silage, Slurry and Agricultural Fuel) Regulations 1991, as amended, which requires them to be impermeable. An analogous infiltration process can occur along losing reaches of watercourses, especially where the flow regime and underlying drift deposits result in a silt-free permeable stream-bed.

4.2.3 Where by-passing of the sub-soil is common, the role of the unsaturated zone is especially important. Thus in rural communities using on-site domestic or agricultural wastewater disposal, the unsaturated zone offers an effective barrier for pathogen removal. Like the soil profile, the unsaturated zone exhibits aerobic conditions. As aerobic degradation is more effective than anaerobic decomposition for many organic compounds, this will favour pollutant removal as part of a natural biological process. However, soil and unsaturated zones can be overloaded. Furthermore, microorganisms may percolate most soils and rock pores, except in fine-grained strata where pore diameters are small. The proximity of highly permeable fluvio-glacial gravels to rivers creates a particular contamination risk for sources in such groundwater settings.

4.2.4 Cryptosporidium oocysts are larger than the typical 1µm pore size of the Chalk aquifer but they are within the pore size range of arenaceous aquifers like the Permo-Triassic sandstones. Oocysts are almost certainly smaller than the fissure and micro-fissure/bedding plane aperture systems which dominate groundwater transmission in most important UK aquifers. Of the most productive UK formations referred to above, only the Lower Greensand is now regarded as a predominantly intergranular-flow aquifer. Thus the capacity of many UK aquifers to physically detain oocysts is limited. Intergranular unsaturated zone vertical flow is also typically slower than its saturated zone equivalent. However it should be noted that the effects of fissures can be even more important in the unsaturated zone than below the water table, because significant by-pass flow can occur along vertical fractures activated after major recharge events. This by-pass flow can be very fast compared to typical intergranular flow rates.

4.2.5 The fate of micro-organisms in the subsurface is also determined by the interaction of flow travel time with survival rates. At 20°C, a 90% reduction in bacterial counts may be expected within about 10 days, although a few may persist for 200 days or more (Lewis et al 1982). Groundwater temperatures in England are typically about 10°C and at that temperature, survival of micro-organisms may be at least twice as long. Viability of Cryptosporidium in the deep subsurface has not been studied but oocysts have been reported to survive dormant for months in moist soil or up to a year in clean water (Badenoch 1990). Oocysts may be expected to survive in the subsurface for periods that are at least two orders of magnitude longer than that of most bacterial cells (including pathogens) of faecal origin.

4.2.6 The groundwater vulnerability mapping and protection zone programmes undertaken by the environmental regulators provide the sound basis for assessing which British public groundwater supply catchments are likely to be most susceptible to contamination but there are some limitations when assessing risk from Cryptosporidium.

4.2.7 These limitations could be overcome by incorporating a fourth class of 'extreme vulnerability' into the existing schemes operated by the EA and equivalent bodies in Scotland and Northern Ireland. This would apply to suspected rapid-access points such as solution features,sinkholes, karst or pseudo-karst features, as well as known areas of ground disturbed by human activities, such as mining and aggregate extraction. This zone, together with the presence of influent (losing) surface watercourses and water bodies could be mapped and would, in effect, define those areas in which groundwater is, or may be, under the rapid influence of surface water.

Reported groundwater-related Cryptosporidium incidents

4.2.8 Evidence is accumulating worldwide for the potential for contamination of groundwater by Cryptosporidium (Harvey 1992; National Cryptosporidium Survey Group 1992; Hancock et al 1998). Table 4.2 lists events associated with water supplies in England since 1990. The range of aquifer types varies but chalk, which is the UKs most productive and widely used aquifer, predominates. This may be more a consequence of its extensive exploitation as a water resource. Fissure flow, dual porosity and intergranular flow are represented. However, intergranular flow is only affected in settings where the residence time in the aquifer is likely to be very short, eg in river gravels close to a surface watercourse. Rural and part-rural/part-urban catchments are involved, although occurrences in the former are more commonly reported. Wells with adits, springs with galleries or former mines with adits appear to be particularly vulnerable settings, accounting for seven of the known events. This feature appears to be more significant than mere proximity to a watercourse.

Table 4.1 Suspected Cryptosporidium groundwater contamination events in England since 1990

Year of occurence

Aquifer lithology

Aquifer flow type

Supply source type

Comments

1990-1991River gravels over chalkIntergranular/dual porosityWell with aditsAdjacent to river
1992, 1995River gravelsIntergranularCollector well used conjunctively with surface waterAdjacent to river
1992SandstoneDual porosityWell with aditsContaminated grazed field runoff to wellhead, possible septic tank leakage in wellhead area
1995ChalkDual porosityWell with adits-
1995, 1996SandstoneFissureAdited spring-
1997ChalkDual porositySingle boreholeGrazed catchment, losing stream with sewage effluent discharge close to borehole
1997Sandstane, karstic limestoneFissureAdit of former minePossible slurry pit leakage
1997ChalkDual porosityWell with AditsAdjacent to river
1997GravelsIntergranularCollector wellVery shallow well in thin gravel on flood plain, seasonal flooding and gravel pits adjacent

4.2.9 Table 4.2 lists factors and prioritises risk factors that water utilities should consider for modelling or when assessing the risk of groundwater contamination by Cryptosporidium. A more detailed treatment of groundwater risk assessment models is included in Lawrence et al 1996 and in a water industry research report (UKWIR 1998).

Table 4.2 Factors for consideration in the risk assessment of groundwater contamination

PREDISPOSING GROUNDWATER TO CRYPTOSPORIDIUM RISKPOSSIBLE VERIFICATION TECHNIQUES
Well/raw water source factors:
Supply source tapping shallow flow systems eg adits, springs, mine galleriesCheck site plans, tracing
Adits with upbores or construction-stage ventilation shaftsCheck site plans, site inspection
Poor casing integrityCCTV, geophyscal logging
Masonry linings above pumping water level without additional sanitary sealCCTV, check site plans
Sewer/septic tank/slurry pit systems near wellhead or above aditsSite inspection
Inadequately fenced source especially around spring boxes, catchpits, galleriesSite inspection
Old, poorly documented well constructionSite plans/BGS National Well Record Archive
Hydrogeological factors:
Known or suspected river aquifer connection nearbyFlow gauging, modelling, hydrochemistry
Unconfined conditions with shallow water tableWell water-level monitoring
Karst or known rapid macro-fissure flow conditions, especially in shallow groundwaterField mapping, farm surveys
Patchy drift cover associated with highly contrasting aquifer intrinsic vulnerabilitiesField mapping, shallow drilling
Solution features observed or inferred in catchmentField mapping
Shallow flow cycles to springsTracing hydrochemistry, water temperature logging
Fissure-dominant flow (as suggested by high transmissivity or specific capacity)Downhole fluid/flow logging, pumping test analysis
Catchment factors:
High wastewater returns, including sewage effluent to losing river reaches, especially under baseflow conditionsHydrochemistry, microbiology, hydrometry
Livestock rearing in inner catchment, especially if intensiveFarm survey
Likely Cryptosporidium - generating activities in catchment eg abattoirsEconomic activity survey
Urbanising catchmentCadastral survey
Livestock grazed/housed near wellhead patiocourtyardSite inspection

4.3 Catchment control to minimise groundwater contamination

Introduction

4.3.1 Catchment control measures can be broadly based on:

(i) avoiding the development or establishment of a hazardous activity within the catchment or zone (prevention);

(ii) removing, shutting down or banning hazardous activities (elimination);and

(iii) adopting measures which reduce the risk associated with hazardous activities (mitigation).

4.3.2 Prevention measures are available through statutory mechanisms involving the Town and Country Planning legislation and pollution control legislation. The possibilities for elimination of hazardous activities may be limited if the owner or occupier of affected land or property can claim for compensation. Possible approaches to mitigating a hazardous activity include: statutory controls; codes of practice; and raising of awareness through education and promotion of good practice.

Regulatory controls

4.3.3 The Town and Country Planning legislation allows water utilities and the EA to comment on planning applications involving change of land use. However, water utilities are not statutory consultees and therefore rely on the goodwill of councils and the EA to bring relevant applications to their attention. While there is no obligation for the planning authorities to act on these comments, in practice they are taken seriously and can result in an application being refused, or approved subject to conditions. However, certain types ofdevelopment, including many agricultural developments are exempt from planning requirements. Therefore, existing activities, many of which could be regarded as undesirable, fall outside the scope of planning controls, which are triggered mainly by applications to change land use.

4.3.4 The Water Resources Act 1991 requires the EA to consent all trade and sewage effluent discharges to controlled waters, which include inland surface and groundwater. The position for groundwater is complex as indirect discharges, ie those to the ground itself, may not need consent. The Control of Pollution (Silage, Slurry and Agricultural Fuel) Regulations 1991 require that new or substantially altered silage or slurry storage structures be built to a minimum standard, which must be notified to the EA before being brought into use. Legislation also provides powers for the EA to serve a notice requiring improvements where there is a risk of pollution. The European Communitys Water Framework Directive may provide additional indirect controls. Ammonia is designated as a List II substance and as such its discharge into groundwater should be minimised. This offers the possibility of reducing the impact of organic effluents on groundwaters. The designation of Nitrate Vulnerable Zones may also provide some reduction in the risk of groundwater contamination from Cryptosporidium by obliging farmers to comply with a Programme which is based on the Code of Good Agricultural Practice for the Protection of Water (COGAP-Water) (MAFF 1991; 1998).

4.3.5 Groundwater contamination arising from waste disposal is controlled in a number of ways. Landfill sites require a waste management licence from the EA. For new sites, this will normally involve lining of the site and a comprehensive leachate management programme. The spreading of wastes 'for agricultural benefit' is permitted subject to the requirements of COGAP-Water. This practice is exempt from the licensing requirements but the EA must be notified. In practice this provides an opportunity for the EA to examine proposals and control those that do not meet the required criteria.

4.3.6 The conditions for application of sewage sludge to land are set out in the Sludge (Use in Agriculture) Regulations 1989. These set out sludge treatment requirements and application rates to protect public health and soil condition. The Regulations are supported by a code of practice. Sewage sludge spreading is expected to increase significantly when disposal at sea stops in 1998 in line with the European Community's Urban Wastewater Treatment Directive (EEC 91/271). Following recommendations by the Royal Commission on Environmental Pollution, the House of Commons Environment Committee and a report by independent consultants, the Government has agreed to amend the Regulations in order to phase out the use of untreated sludge on agricultural land by the end of 2001. Discussions between the British Retail Consortium and the Water Industry are close to finalising an agreement, which will introduce additional controls on sludge use and cropping to further reduce perceived risks of pathogen transmission.

Codes of Practice

4.3.7 COGAP-Water, which was revised in 1998, includes guidance on avoiding water pollution from agricultural activity. The Code covers faecal contamination of water and Cryptosporidium in particular. The Code advises that special precautions, for example, on slurry and manure storage be taken on farms where cryptosporidiosis has been identified in livestock, in order to contain the spread of Cryptosporidium in the farm environment. The Code has a statutory designation code under section 97 of the Water Resources Act 1991. As such, any contravention can be taken into account if legal action follows a pollution incident. Raising awareness of this important Code among farmers remains vital for the continued protection of water resources (See Appendix A2).

Awareness, education and partnerships

4.3.8 Regional and national initiatives to promote good agricultural practice have achieved locally increased use of the Code by the farming industry. In respect of Cryptosporidium, a pilot initiative 'Wise Ways With Waste' (Water Services Association 1995) set out to promote safe slurry application to protect watercourses.

4.3.9 There is scope for innovative practice such as development of further incentiveschemes within the framework of the EC Agri-Environment Regulation under the Common Agricultural Policy. This Regulation promotes schemes which aim to encourage farmers to undertake positive measures to conserve and enhance the rural environment. The latter scheme now covers 10% of the agricultural land in England and a reasoned case could be made to support inclusion of sustainability of water resources' as an additional threat to the environment which merits consideration within the ESA scheme.

Communication

4.3.10 A number of circumstances are now known to raise significantly the risk of groundwater contamination by Cryptosporidium. In any of the following situations, rapid communication is crucial if the water company and local public health professionals are to assess the impact on water supplies:

4.4 Operational aspects of groundwater abstraction

Flow and level measurement

4.4.1 Records of groundwater level, quantities abstracted and in the case of springs, total flow, provide useful information for assessing risk. Increased risks are likely if level or flow increase rapidly after rainfall, particularly if hydrological changes are also associated with deterioration in water quality. It is important to establish whether level or flow changes are a consequence of the influence of surface waters or a consequence of changes in pumping regimes. Where continuous fixed rate abstraction takes place, steady state conditions may be approached after a period although this can take months or even years. In some aquifers, a steady state may never be achieved because of seasonal variations in rainfall. Intermittent abstraction (eg to reflect changes in demand or take advantage of power tariffs) is being used increasingly. These factors cause short term, even diurnal, variations in water level in the vicinity of the abstraction point on top of the longer term variations in the aquifer as a whole. Where appropriate, data from observation boreholes remote from local influences should be considered.

4.4.2 Groundwater levels should be monitored regularly and compared with abstraction, rainfall and quality data. A rise in level will normally either be caused by reduced abstraction or by increased rainfall recharge. An unexpected rapid rise in level should be investigated and the possibility of ingress of water of recent surface origin should be considered, particularly if it can be correlated with recent rainfall or changes in water quality or water temperature. In the case of a spring, total flow variation will usually be the primary monitoring tool. However, this still needs to be considered in conjunction with groundwater level, quality and rainfall data.

Construction characteristics

4.4.3 Current practice for sinking new groundwater abstraction points characteristics involves drilling a borehole and casing with solid lining tubes. These are grouted in through drift and underlying strata until the lining reaches sufficient depth to create a sanitary seal. Below that point the borehole will continue with a solid or perforated casing, or as an open hole depending primarily on structural stability and hydrogeological factors. Such structures should present a low risk of contaminated surface water entering the borehole provided that linings and seals are maintained adequately, in accordance with recommendation 14.22 of the First Report of the Expert Group (Badenoch 1990).

4.4.4 At older abstraction sites, construction methods rarely reflect current standards, although many are in adequate sanitary condition. Some pose a higher risk because the fabric has deteriorated or because the nature of the recharge catchment has changed. In some cases the facilities were not originally intended for water abstraction, eg where abandoned mine workings are now used as a water resource. Many of these older abstraction points remain in use and even when abandoned can still present a pathway for contaminants to reach the aquifer. Shallow water-bearing strata present greater risks because they are more vulnerable to contamination and there is little or no opportunity of creating a secure sanitary seal. An extreme example is the use of radial collector wells to abstract from gravel in a river valley. In these situations, the abstraction points should be considered as a surface water source. However, all sources with adits or galleries would comprise an inherently high-risk grouping.

4.4.5 Newer boreholes will have only limited data available for use in risk assessment, although they should have been constructed to a higher standard. Where intrusive maintenance has been carried out on older structures, existing records may no longer be representative. It may therefore be appropriate to assign a higher level of risk in these circumstances until sufficient relevant operating information has been obtained. The risks associated with construction factors should therefore be assessed on three broad sets of criteria: whether the location and surface geology favour a sound sanitary seal; whether a sound sanitary seal has actually been achieved; and the extent and reliability of knowledge of the abstraction structure and strata contributing water to the source.

4.5 Water quality testing

4.5.1 Monitoring raw (untreated) water from groundwater sources is essential for assessing risk from contamination by Cryptosporidium. Monitoring can vary from minimal surveillance for a few determinands to detailed investigations with automatic samplers and on-line continuous measurement. Table 4.3 and the following paragraphs review the application of monitoring activities in assessment of risk from Cryptosporidium in groundwater.

Table 4.3 Application of monitoring activities in the assessment of risk

EVIDENCE FOR CRYPTOSPORIDIUM RISK TO GROUNDWATERSIGNIFICANCE
Detection of Cryptoporidium oocysts in source waterdirect evidence of contamination
Detection of Cryptosporidium oocysts in distribution system sedimentsevidence of recent or historic contamination
Regular detection of E.coli in source waterIndication of faecal contamination
Detection of Clostridium perfringens in source waterPossible surrogate for Cryptosporidium
Transient changes in turbidity of source waterPossible rapid influence of surface water
Micro-temperature or conductivity changes detected by down-well loggingMay reveal influence of major inflows at shallow depths
Concentrations of certain characteristic dissolved inorganic speciesMay indicate recent surface water inflow

4.5.2 Although there is no evidence for any consistent correlation between detection of Escherichia coli indicator bacteria and oocysts, the regular detection of E.coli does warrant investigation for Cryptosporidium. This is particularly the case where there is evidence of rapid recharge conditions or Cryptosporidium-generating activities within the catchment area. Nevertheless, E.coli does occur in the absence of oocysts and vice versa. This arises because: (i) the temporal distribution of Cryptosporidium in polluted groundwater recharge will be different to that of E.coli; (ii) differences in adsorption characteristics between oocysts and bacteria and (iii) greater persistence of oocysts in the subsurface compared with E.coli. Oocysts present a larger cross-section than the environmentally-stressed form of coliform bacteria and are likely to be subject to stronger filtration effects in all but highly-fissured strata. The detection of oocysts in groundwater indicates that contamination has occurred and that further investigation to assess the overall risk is necessary. There is considerable interest in the feasibility of using simple and reliable surrogates for Cryptosporidium (Edberg et al 1997). There is some evidence that spores of the bacterium Clostridium perfringens may serve as a surrogate for Cryptosporidium in groundwater (DOE 1997) and in surface waters (Payment et al 1997). The former reference indicates also that monitoring for faecal streptococci provides a more reliable indication of faecal contamination than can be obtained from coliform monitoring, particularly at low sampling frequencies.

4.5.3 Several physical and chemical measures can help establish whether groundwater might be under the rapid influence of surface water. Of these, turbidity is most convenient and warrants further consideration. The presence of significant turbidity events in groundwater does not necessarily indicate the presence of oocysts, nor does low turbidity indicate the absence of oocysts. Sudden, unexplained peaks in groundwater turbidity should be investigated by use of particle size analysis and microscopic investigations to distinguish between aquifer material and organisms or debris derived from surface water bodies (Boutros 1992). There is a need for research into the mechanisms causing, and the significance of, turbidity in groundwater to establish the role of rapid influence by surface water and assessing the use of turbidity as a monitoring tool. Such investigations are currently limited by the restricted deployment of continuous turbidity monitoring at groundwater sources or the inadequate calibration and maintenance of installed equipment.

4.5.4 Logging of micro-temperature and micro-electrical conductivity in boreholes is a well established technique for identifying the depth and characteristics on major individual inflows (BSI 1988). Other chemical parameters such as chloride, potassium, phosphate, strontium, bromide and boron, as well as measurements of the concentrations of the stable oxygen and hydrogen isotopes have been used to determine the extent of surface water ingress (Edmunds et al 1976; Rodke 1981). However, in upper reaches of streams there may be little difference between surface and groundwater chemistry. Hydrogeological transport studies using tracers will greatly increase confidence in the diagnosis of rapid surface water ingress.

4.5.5 Cryptosporidium oocysts have been identified in mains and service reservoir sediments during investigation of suspected source contamination events. Such identification may provide evidence for intermittent contamination over a long time scale, although it is unlikely that oocysts in sediments will constitute an infection hazard. However, failure to identify oocysts in sediments does not provide confirmation that contamination of the water source has not taken place.

4.6 Guidance for water utilities

4.6.1 Water utilities should assess systematically the vulnerability of their groundwater sources to contamination by Cryptosporidium in the light of the risk factors set out in Tables 4.2 and 4.3 and take decisions concerning the need for provision of treatment or other appropriate actions. The assessment must take account of (i) the hydrogeological factors which render an aquifer prone to rapidly transmit oocysts; (ii) whether there are activities in the catchment which might constitute a source of oocysts; and (iii) construction and design factors that might predispose the abstraction point to contamination. The risk assessment should be linked to a risk management programme as illustrated in Figure 4.1. The assessment of hazard and pathway factors should be carried out in liaison with the EA. Where there is doubt about the extent and reliability of hydrogeological information, appropriate surveys should be considered. Water utilities should also review at the same time whether groundwater assets comply with the recommendations in the First Report of the Expert Group for borehole seals and linings (Badenoch 1990).

4.6.2 Not all groundwater is consistently high quality. Utilities should be especially vigilant for the possibility of intermittent rapid transmission of water from the surface into boreholes, wells and springs. The catchment, resource and source characteristics should always be reviewed against water quality data. If it is necessary to undertake further work, sampling should include recharge periods or times when losing reaches of surface water are most contaminated.

4.6.3 Where an unacceptable risk is identified and treatment is already available, it is important to assess the effectiveness of the treatment process to remove or inactivate Cryptosporidium. Rapid gravity filtration (and granular activated carbon filtration) without chemical pre-treatment will be less effective than flocculation/sedimentation (or flotation) followed by filtration. Ion exchange and biological de-nitrification are unlikely to significantly remove Cryptosporidium. Ozone treatment for pesticide removal may reduce the viability of oocysts but effectiveness of treatment is dependent on temperature, ozone dose and contact time (Badenoch 1995). Only physical barriers will remove oocysts.

4.6.4 The isolation of oocysts in groundwater soon after rainfall recharge is a high risk circumstance which warrants immediate investigation. This should include an assessment of historical reported rates of human cryptosporidiosis. However, it is essential to ensure health authorities have used comparable reporting practices across the review period. A recommendation on making cryptosporidiosis laboratory reportable is made in paragraph 3.3.11. When assessing the significance for health of occasional exposure to low numbers of oocysts via groundwater, it is necessary to bear in mind that oocysts are not always absent from surface water derived drinking water. There is considerable evidence that occasional exposure to low numbers of oocysts does not lead to outbreaks of cryptosporidiosis (Parker et al 1993; National Cryptosporidium Survey Group 1992). For this reason identification of occasional low numbers of oocysts in water, in the absence of other risk factors, may not indicate the need for installation of treatment or other action. Local circumstances should be the defining factor.

4.6.5 After making a risk assessment, water utilities should assess the possibility of minimising risk of contamination by reviewing catchment control options or by operational improvements to the security or integrity of the groundwater source. In rare cases the risk may be so unacceptably high that treatment installation is required. In the majority of cases, however, it will be necessary to carry out surveys and further investigations to confirm an unacceptable risk of groundwater contamination with Cryptosporidium before adopting a treatment solution. All risk assessments should be regularly reviewed, especially following any significant change in the catchment, the condition of the water supply source, or the demand on the source.

4.7 Research needs

4.7.1 This section has concentrated on Cryptosporidium and its potential to contaminate groundwater. However, much of the preceding information also relates to the identification of general 'at risk' situations where there is potential for surface-derived contamination from a range of potential pathogens. There appears to be a need for development of operational tools and a general guidance manual for the operation of groundwater abstraction and treatment processes.

4.7.2 In addition, the following specific research needs have been identified:

(i) development of operational monitoring tools to improve the detection of rapid influence of surface water sources on the quality of groundwater;

(ii) transport and fate of Cryptosporidium and other pathogens in groundwater systems;

(iii) application of chemical and particulate tracers to investigate the transport and attenuation of pathogens in groundwater;

(iv) significance and nature of turbidity changes in groundwater and its role as a monitoring tool for rapid surface water ingress; and

(v) attenuation rates for Cryptosporidium in soils and unsaturated zones following application of farm wastes and sewage sludge to land.

4.8 Recommendations

4.8.1 Water utilities should systematically assess and rank the potential risk of groundwater contamination by Cryptosporidium by application of a tripartite approach which assesses source, catchment and hydrogeological factors.

4.8.2 Continued use should be made of existing national groundwater vulnerability maps and source protection zoning schemes to assess risk of contamination by Cryptosporidium. 

4.8.3 For Cryptosporidium risk assessment, a fourth classification 'extreme vulnerability' is recommended for use with vulnerability maps and zoning schemes. 

4.8.4 In order to ensure that groundwaters are properly protected from agricultural activity, the Ministry of Agriculture, Fisheries and Food should promote further the application of the the Code of Good Agricultural Practice Water within the farming industry. 

4.8.5 Careful attention should be given to the operational aspects of groundwater abstraction.

Figure 4.1 Groundwater risk assessment and management

4.9 Glossary of technical terms used in Chapter 4

Abstraction - The removal of water from surface water or groundwater, usually by pumping.

Adit - horizontal or near-horizontal tunnel extending outward from a well or shaft below the water table, designed to increase well productivity; passage from the surface by which a mine is entered and drained.

Aquifer - a permeable geological formation that is capable of both storing and transmitting water in significant amounts.

Arenaceous - a term applied to rocks that have been derived from sand or that contain sand.

BGS - British Geological Survey

CCTV - closed circuit television

Collector well - a particular design of well, usually constructed in unconsolidated alluvial or fluvio-glacial formations, comprising a central large-diameter shaft from which radial galleries or collectors have been driven to increase the production potential of the source.

Dual porosity aquifer - aquifer in which a certain proportion of the total storage capacity of the system is provided by the interstices in the rock matrix, while the fractures provide the dominant flow-path.

Fissure flow - the preferential flow of groundwater through dilated cracks, joints, bedding planes or other features of secondary porosity within an aquifer.

Flow lines - lines indicating the direction of groundwater movement.

Fluvio-glacial - pertaining to streams flowing from glaciers or to the deposits made by such streams.

Groundwater - naturally occurring sub-surface water in the saturated zone of a rock.

Groundwater vulnerability - the tendency or likelihood for contaminants to reach a specified position in the groundwater system after introduction at some location above the uppermost aquifer.

Hazard - a property or situation that in particular circumstances could lead to harm.

High transmissivity - capable of transmitting a large amount of water

Hydraulic gradient - the prevailing inclination of the water table or piezometric surface which provides the driving force to transmit groundwater through an aquifer.

Intergranular flow - flow occurring between the grains of a rock.

Intrinsic aquifer vulnerability - groundwater vulnerability determined without reference to the attributes and behaviour of particular contaminants.

Karst (Karstic) - an area of limestone or other highly soluble carbonate rock, in which the landforms are of dominantly solutional origin and in which the drainage is underground in solutionally enlarged fractures and conduits.

Losing reaches of rivers - locations in a watercourse where surface water is percolating through the bed of the watercourse into the underlying aquifer.

Porosity - the ratio of the volume of the interstices to the total volume of a rock, expressed as a fraction or a percentage. Effective porosity includes only the interconnected pore spaces available for groundwater transmission.

Risk - a combination of the probability, or frequency, of occurrence of a defined hazard and the magnitude of the consequences of the occurrence. Risk estimation is concerned with the outcome or consequences of an intention, taking account of the probability of occurrence; risk evaluation is concerned with determining the significance of the estimated risks for those affected, it therefore includes the element of risk perception; risk perception is the overall view of risk held by a person or group and includes both feeling and judgement; risk assessment consists of risk estimation and risk evaluation.

Sinkhole - the point at which a surface stream sinks underground.

Soil leaching potential - a composite measure of the ability of a soil to attenuate a diffuse source pollutant.

Solution feature - closed depressions a few metres to a few hundred metres in diameter and depth formed by solution action in soluble rocks, notably limestones.

Source protection zones - a series of concentric zones around an abstraction within which special policies apply to activities which might affect groundwater. The outermost zone covers the complete catchment area of the source, which is also called the well capture zone.

Specific capacity - the yield of a well per unit of draw down.

References

ADRIS. (1995) Groundwater Protection Strategy for Scotland. Perth: Association of Directors and River Inspectors of Scotland.

Badenoch, J. (1990) Cryptosporidium in water supplies. Report of the Group of Experts; Department of the Environment, Department of Health. London, UK. HMSO. 230pp.

Badenoch, J. (1995) Cryptosporidium in water supplies. Second Report of the Group of Experts; Department of the Environment, Department of Health. London, UK. HMSO. 108pp.

Bell, D.F., McDonald, A.M., Morris, B.L., Lilly, A. (1997) Scotlands Minor Aquifers: A Scoping Study to Assess Groundwater Source Protection. BGS Technical Report WD/97/63. Edinburgh; BGS.

Boutros, S.N. (1992) Microscopic Particulate Analysis (MPA) in Studies of Groundwater. In Proceedings of the Water Quality Technology Conference, Toronto, Ont. Denver: American Water Works Association.

British Standards Institution. (1988) British Standard Guide for Geophysical Logging of Boreholes for Hydrogeological Purposes BSI 7022. BSI: London, UK.

DOE. (1997) Cryptosporidium Incidence in Private Water Supplies and Correlatory Indicators. Department of the Environment. London, UK.

DETR. (1997). Digest of Environmental Statistics. Department of the Environment, Transport and the Regions. London: TSO.

Edberg, S.C., LeClerc, H,. Robertson, J. (1997) Natural Protection of Spring and Well Drinking Water against Surface Microbial Contamination. Critical Reviews in Microbiology 23(2), 179-206.?

Edmunds, W.M., Owen, M., Tate, T.K. (1976) Estimation of Induced Recharge of River Water into Chalk. Boreholes at Taplow using Hydraulic Analysis, Geophysical Logging and Geochemical Methods. Report 76/5. London: Inst. Geol. Sciences.

Harvey, R.W. (1992) Transport of Protozoa Through an Organically Contaminated Sandy Aquifer. In Proceedings of the First International Conference on Ground Water Ecology. Tampa Fla: American Water Works Association.

Hancock, C.M., Rose, J.B. and Callahan, M. (1998) Crypto and Giardia in US groundwater. JAWWA 90 (3) 58-61.

Lawrence A.R., Williams A.T., Brewster L.J., Bowker J.A., Bird M.J., Ward R.S. (1996) Pathogen Transport in the Chalk Aquifer: an assessment of the risk to groundwater sources. British Geological Survey Report WD/96/48.

Lewis W.J., Foster S.S.D., Drasar B.S. (1982) The Risk of Groundwater Pollution by On-Site Sanitation in Developing Countries. Geneva: IRCWD.

MAFF. (1991) Code of Good Agricultural Practice for the Protection of Water. Ministry of Agriculture, Fisheries and Food. London, UK.

MAFF. (1998) Code of Good Agricultural Practice for the Protection of Water (The Water Code). Ministry of Agriculture, Fisheries and Food. London, UK.

National Cryptosporidium Survey Group. (1992) A survey of Cryptosporidium in Surface andGroundwaters in the UK. Journal of the Institute of Water and Environmental Management. 6 697-703.

National Rivers Authority (1992) Policy and Practice for the Protection of Groundwater. Bristol: NRA.

Parker J.F.W., Smith H.V., Girdwood R.W.A. (1993) Survey of Loch Lomond to Assess the Occurrence and Prevalence of Cryptosporidium spp. Oocysts and their Likely Impact on Human Health. Report FR 0409. Marlow: Foundation for Water Research.

Payment P., Menard B., Prevost M. (1997) Enteric Viruses, Giardia and Cryptosporidium Level in the Raw Water of Water Treatment Plants along the Saint-Lawrence River. In Proceedings of the Water Quality Technology Conference, Denver, Co. Denver: American Water Works Association.

Rodke A. (1981) Spring Flood - Meltwater or Groundwater ? In Nordic Hydrology 12 21-30.

UKWIR. (1998) Management of Cryptosporidium Risk in Groundwaters. UK Water Industry Research Limited, London, UK.

WSA. (1995) Wise Ways with Waste. Water Services Association. London, UK.

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