WATER P-NOTES ISSUE 30 JANUARY 2009 47361 Protecting the Quality of Public Water-Supply Sources A Guide for Water Utilities, Municipal Authorities, and Environmental Agencies W ater-supply quality is too often taken-for- Identifying the threats to granted. Because we can see rivers and streams, they command most attention groundwater quality when talk turns to water quality--but subsurface aquifers are every bit as important as a source of "Local conditions require careful assessment"--is one public water-supply and are also under threat of of the fundamental lessons for groundwater quality pollution. Acting now to protect them makes sound protection. The groundwater below our feet exists ev- economic sense, because it is always cheaper to erywhere in a unique combination of circumstances-- maintain the quality of groundwater resources, and from the thickness and characteristics of the ground of individual water-supply sources, than to mitigate above it to the type of activity at the land surface cre- the damage once done. ating a pollution risk. And these conditions can vary spatially over relatively short distances. As a result, it But timely action depends on awareness of the is necessary to fine-tune assessments of the the level urgent need to protect groundwater--and to do of actual and potential risk of groundwater pollution. this we must be able to identify clearly the threats they face. Because it is unrealistic to prohibit all Groundwater in aquifers exist in a range of potentially-polluting activities and the economically confinement (or degree of hydraulic isolation from sound approach is to identify what are the most the land surface)--and, in effect, this is inversely significant pollution threats, which parts of the land proportional to the degree of vulnerability to pollu- surface are most vulnerable to pollution of underly- tion. Highly-confined aquifers are typically the least ing groundwater and whether any such pollution vulnerable, because they have an unbroken and will impact existing public water-supply sources. relatively impermeable layer above them. At the Such a procedure, which is described in this book, other end of the spectrum are unconfined aquifers, provides the direct focus required on the protection whose groundwater is regularly replenished from measures necessary to conserve the quality of any rainfall and/or streamflow on the land surface, and given groundwater supply source. which are consequently more vulnerable to contam- ination--although the precise level of vulnerability will depend on the characteristics and thickness of the ground above the water-table. The following note highlights important information from Groundwater Quality Protection: A Guide for Water Utilities, Municipal Authorities, and Environmental Agencies, by Stephen Foster, Ricardo Hirata, Daniel Gomes, Monica D'Elia, and Marta Paris (Washington, DC: World Bank, 2002, ISBN 0-8213-4951-1) ­ a GW-MATE publication. Visit www.worldbank.org/gwmate for more information. Readers may download the complete paper from www.worldbank.org/water. WATER P-NOTES Figure 1. Common Processes of Groundwater Pollution solid waste tip industrially-polluted industrial site leaking in-situ farmyard leaking wastewater agricultural or landfill `losing' river drainage storage tanks sanitation drainage sewers lagoons intensification The type and intensity of human activity on the tifying the vulnerability of groundwater to pollution. land surface exerts a major influence on the quality The acronym refers to the three primary variables of groundwater, especially where it is more vulner- that determine groundwater vulnerability: Ground- able to pollution. Some activities, such as those water confinement, Overlying strata characteristics in Figure 1, introduce percolates that can include and Depth to groundwater body (and in some cases highly toxic compounds which do not break down a fourth variable relating to the Soil cover properties readily once deposited or leached into the subsur- is added) with the `final score' being an indicator of face environment. groundwater vulnerability to pollution. Groundwater pollution risk arises as a result of Vulnerability maps for depicting the interaction of the natural intrinsic vulnerability to pollution and the contaminant load (or pres- potential pollution hazards sure) resulting from human activity. An aquifer may be highly vulnerable to pollution but not be at risk Groundwater accumulates in the subsurface after because of the absence of a significant contami- percolating through the soil--and thus contamina- nant load. Conversely, it may have a high contami- tion reaching the soil zone has the potential to leach nant pressure but little risk of pollution due to low into aquifers. The greater the depth of the soil pro- groundwater vulnerability. Groundwater pollution viding a buffer between the land surface and the vulnerability mapping thus examines the range of groundwater table, the less vulnerable will be the subsoil conditions in the area concerned and as- aquifer to pollution, since soil processes naturally sesses their potential to attenuate pollution and thus attenuate many types of contaminants. However, the vulnerability of groundwater to pollution. not all subsoils are capable of completely absorb- ing a given contaminant and thus preventing it from Whilst very valuable tools for raising awareness harming groundwater. This is the case, for example, and evaluation groundwater pollution, the vulner- where the subsoil is particularly rocky. The process of ability concept and mapping have their limitations. groundwater pollution also has to be distinguished First, considerable simplification must be accepted from contamination by the solution of naturally-oc- in order to express conditions numerically (as is the curring elements that can sometimes also occur. case with any type of modeling of natural systems) Second, mapping is based on best available data The so-called `GOD Index' is one relatively and should be updated when new information simple but scientifically consistent method for quan- becomes available. Third, any characterization of 2 ISSUE 30 · JANUARY 2009 Figure 2. Components of groundwater pollution hazard assessment used for groundwater protection land surface zoning B Predominant Lithology of Confining Beds of C Unsaturated Zone (APV) A ollutionP protection esourcesR providing Depth of aquifier or 50 40 water table Aquifier unerabilityV 60 Elements groundwater Source Protection Area (SPA) Sources Vertical flow Horizontal flow underground conditions and their ability to contain Armed with this information, water resource attenuate pollution incidents will always be subject regulators and land-use planners can make better- to significant uncertainty and thus a `conservative informed choices on the measures necessary (in approach' is usually recommended. In addition cer- terms of controlling contaminant load) to protect tain types of contaminant (such as nitrate) are very the quality of groundwater. stable in the majority of subsurface conditions and others (such as some synthetic solvent compounds) In addition, the delineation of flow directions enter groundwater in the immiscible phase--and and specific flow capture zones around individual if discharged or leached to the subsurface will public-supply sources will provide a direct focus on sooner-or-later impact most groundwater systems ir- the quality protection of existing water-supplies--and respective of their vulnerability. the smaller zones of the land surface that require `special protection measures' in the immediate inter- est of conserving drinking water quality. Putting pollution risk assessments Since it is usually impractical to prohibit eco- to work in land-use planning nomic activity over the entire recharge area of an aquifer, the most cost-effective way to balance the goals of groundwater quality protection and eco- Groundwater pollution generally takes a long time nomic development is to take advantage of areas to become evident, and this time- lag leads to the of low groundwater pollution vulnerability--in effect possibility that irreversible environmental damage to follow a strategy of `groundwater zoning' that may occur before it is detected. Thus protecting facilitates objective tradeoffs between environmental groundwater resource quality requires a `planned and economic priorities. Using the aquifer pollution protective approach' through: vulnerability map as a guide, it is possible to ac- · mapping the vulnerability of groundwater to commodate both goals by allowing economic de- pollution velopment to take place where it will have the least · surveying the current and potential contaminant impact on groundwater quality. load (or pressure) on the subsurface, and, Larger industrial developments utilizing more · from their interaction assessing the groundwater harmful contaminants can be located farthest from pollution risk. groundwater vulnerable areas. But interestingly, the 3 WATER P-NOTES impact of the scale of economic activity on ground- groundwater resources are an importance source water quality is not always straightforward--the of public drinking water-supply. They are neither intuitive assumption would be that larger facilities not costly nor difficult to perform. The mapping of would have a greater negative impact than smaller groundwater pollution vulnerability and delinea- ones, but this is not necessarily so. Larger industrial tion of groundwater flow capture zones around facilities often invest considerable financial resourc- public water-supply sources provides a method of es and design effort into reducing contaminant load land-surface zoning to directly reflect the interest of on the subsurface, and often have an economic groundwater quality conservation--and surveying and social interest in avoiding regulatory costs and existing (and potential future) generators of con- conflicts, whereas smaller ones may not have suf- taminant load allows an assessment of groundwa- ficient financial resources to engage in pollution ter pollution risk and required control measures. control and may try to escape the notice of regula- tory authorities. Such mapping and assessment work in effect makes `groundwater resources more visible' and provides a solid foundation for land-use planning A future that protects decisions by suggesting preferred locations for po- tentially-polluting developments. It is also the start- groundwater quality ing point for raising the awareness of land users, regulatory authorities, and potential polluters to the A systematic approach to groundwater qual- economic and environmental imperative of protect- ity protection measures is always required where ing groundwater quality. Figure 3. Significance of contrasting aquifer pollution vulnerability high aquifer vulnerability low aquifer vulnerability times cale of urban area rural area urban area rural area downward water flow weeks M N S C F N S P M N S C F N S P years shallow unconfined aquifer deep semi-confined aquifer decades M heavy metals N nitrate S salinity Corganic carbon F faecal pathogens P pesticides The Water Sector Board Practitioner Notes (P-Notes) series is published by the Water Sector Board of the Sustainable Development Network of the World Bank Group. P-Notes are available online at www.worldbank.org/water. P-Notes are a synopsis of larger World Bank documents in the water sector. 4 THE WORLD BANK | 1818 H Street, NW | Washington, DC 20433 www.worldbank.org/water | whelpdesk@worldbank.org