Geophysical methods turns up to be very useful, when vulnerability of ground waters is mapped in areas with a heterogeneous geological setting. In Denmark groundwater accounts for almost 100 % of the drinking water resource. Only groundwater that satisfy the quality standards of drinking water is used. Thus nitrate becomes a mayor threat in rural areas where nitrate loads generally exceed the quality limit of 50 mg/l. Previous work in Denmark concentrated on nitrate reduction under homogeneous geological condition/1/. In areas with thick day layers above the aquifer nitrate was found to penetrate about 3 - 8 m into the til! and never reached the groundwater /2/. In sandy areas nitrate was reduced in the aquifer by pyrite at the redox boarder where oxygen and nitrate were depleted at the same depth and below reduced conditions with iron and mangese was found. /3/ These homogeneous conditions accounts for about 50 % of the Danish ground waters, but in the rest of the country the geological settings are often very heterogeneous due to disturbance form ice movements during the last glaciation. Therefore both the top layers above the aquifer and the aquifer are very irregular with Iayers of altemating gram size from impermeable sediments with up to 15 % day minerals to coarse gravel deposits. The aim of this work is to study the effect of geological heterogeneity on groundwater quality and redox conditions in the aquifer. This work is part of the Danish Environmental Research programme (1992-1996).


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