Water for people.pdf - WHO Thailand Digital Repository

T H E N A T U R A L W A T E R C Y C L E / 8 1Table 4.5: Groundwater exploitation and associated problemsSocio-economic benefits• economical provision of good-quality urban water supply• low-cost development of drought-reliable rural water supplies• accessible and reliable water supply for irrigated crop cultivation• improved drainage and salinity alleviation in some areasSustainability problems• inefficient resource utilization on a very widespread basis• growing social inequity in the access to groundwater in some regions• physically unsustainable abstraction rates in more arid regions• reduction in dry weather baseflow in some downstream watercourses• irreversible aquifer damage locally due to saline intrusion/upconing• localized land subsidence due to aquitard compaction• damage to some groundwater-dependent ecosystemsSources: Alley, 1999; Foster et al., 2000.seriously) natural aquifer discharge to the aquatic environment, andresource development involving consumptive use of groundwater (orexport from the sub-basin concerned) has the greatest impact. Thisshould be an important consideration in resource planning andenvironmental management, but it is one that has been all toowidely overlooked in the past.The integrity of the soil layer overlying aquifers plays a key role inallowing groundwater recharge to take place. Anthropogenicinfluences can be highly significant in this context. For example, thereis mounting evidence from across the African Shield that clearance ofnatural vegetation has led to soil erosion and compaction. As aconsequence, infiltration and aquifer recharge and discharge havebeen reduced, leading to the fall of dry-weather flow in many smallerrivers which are vital to human survival and livelihood.Natural groundwater quality problemsWhile the quality of unpolluted groundwater is generally good,some groundwater naturally contains trace elements, dissolved fromthe aquifer matrix, which limit its fitness for use (Edmunds andSmedley, 1996). These elements can be troublesome for domesticuse (iron) or pose a public health hazard (fluoride, arsenic). Withthe introduction of more systematic and comprehensive analysis ofgroundwater supplies, supported by hydrogeochemical research,detailed knowledge of their origin and distribution is steadilyincreasing with the hope that associated problems can either beavoided or treated on a sound footing in the future.There are significant areas of the globe where serious soil andgroundwater salinization are present or have developed as a result of:■ rising groundwater table, associated with the introduction ofinefficient irrigation with imported surface water in areas ofinadequate natural drainage;■ natural salinity having been mobilized from the landscape,consequent upon vegetation clearing for farming developmentwith, in these cases, increased rates of groundwater recharge; and■ excessive disturbance of natural groundwater salinitystratification in the ground through uncontrolled wellconstruction and pumping.Such situations always prove costly to remedy (Foster et al., 2000).Vulnerability of aquifers to pollutionAquifers are much less vulnerable to anthropogenic pollution thansurface water bodies, being naturally protected by the soil andunderlying vadose (unsaturated) zone or confining strata. But, as aresult of large storage and long residence times when aquifersbecome polluted (see figure 4.4), contamination is persistent anddifficult to reverse (Clarke et al., 1996).Some aquifers are more vulnerable than others, and can beaffected by a wide range of pollutants discharged or leached at theland surface. Moreover, most aquifers will (sooner or later) beaffected by relatively persistent contaminants (such as nitrate,salinity and certain synthetic organics), if widely leached intogroundwater in aquifer recharge areas.The more spectacular groundwater pollution incidents, withlarge plumes of high concentration, are associated with industrialpoint sources from accidental spillage or casual discharge invulnerable areas. However, more insidious and persistent problemsare associated with diffuse pollution sources generated throughintensification of agricultural cultivation or from unsewered urbanand industrial development. The compilation of maps of aquifervulnerability provides land use managers with a valuable tool for theestablishment of preventive and protective measures (Vrba andZaporozec, 1994).Certain clear tendencies, including widespread qualitydeterioration of shallow aquifers in areas of rapid urbanization andagricultural intensification, have been identified (Foster andLawrence, 1995). However, it is not possible to make reliableestimates of the proportion of active replenishment or ofgroundwater storage affected by pollution, because few nationshave adequate groundwater quality monitoring networks set up for