Assessing groundwater vulnerability

Abstract: Groundwater is protected to varying degrees by the overlyinggeological strata. Groundwater vulnerability assessment is anevaluation of how effective the geological material is atreducing pollutants. The response of groundwater to aciddeposition may provide us with an example of how theinteractions of a pollutant with the soil and subsoil reduceits potentially negative impacts. The geological materialcounter-acts the effects of pollutant input. In the short-term,ion-exchange reactions are most effective. However the exchangecapacity of the soil and subsoil is limited. Groundwaterresources are vital for drinking-water supplies, which meansthat groundwater vulnerability assessments need to be reliablein the long-term. The vulnerability assessments are thereforedirected toward determining the rate of weathering. Weatheringof most common minerals in Sweden is slow with the exception ofcarbonate minerals. Three different methods used to assess thegroundwater vulnerability have been examined. Two of themethods are qualitative and represent different methods ofweighting together the parameters considered to be important.The assessments were compared with alkalinity measurements ofwell water within the region. The results show that the areasleast vulnerable to acidification could be distinguished. Thisis mostly attributable to the strong influence of carbonateminerals in some areas. In other areas, local conditions nearthe wells were more important, making it impossible todistinguish between wells representing intermediate or lowvulnerability conditions. A method providing a quantitativemeasure of the vulnerability through modelling the mineralweathering rate was used for comparisons. As the stronginfluence of carbonate minerals was not reflected in the inputdata set, the regional description of weathering wasincomplete. However, the method did give an acceptable resultfor the level of weathering in non-carbonate areas. In theacidification case the occurrence of carbonate minerals was ofparamount importance. Importantly, the difficulties experiencedin distinguishing between medium and low vulnerability may actas a warning for assessments of other types of pollutants. Ahigh dependence on the local flow systems and difficulties indetermining these may seriously flaw the assessment ofvulnerability.