AQUIFER-VULNERABILITY ASSESSMENT USING IN-PLACE TRACERS IN DRINKING WATER

The degree to which an aquifer is innately vulnerable to chemical contamination from the surface is becoming much more important information in hydrogeology. Economical but strong evidence was needed statewide here. Conceptual evaluations based on geologic conditions typical of large areas were then tested and confirmed or modified using extensive in-place tracer evidence from public-supply wells. The latter includes specially analyzed isotopic evidence (¹⁴C, ³H, ¹³C, ²H, ¹⁸O) that gives direct (ranging down to inferential) indications of ground-water "age" and thus distance from the recharge area where contamination is easiest. The most economical (i.e., already produced for other purposes) and among the strongest tracer evidence comes from routinely analyzed surface-derived contaminants found in drinking water: e.g., traces of nitrate, refined petroleum, solvents or their degradation products, agricultural chemicals. Where encountered in well water, these tracers clearly document a local vulnerability and strongly suggest a high vulnerability in similar aquifer settings in the wider area. Linked computerized databases and GIS systems greatly facilitate and economize this method. Collectively, the several methods show (1) the piedmont fractured-rock aquifer is highly vulnerable, despite the often thick saprolite residuum layer atop it and the opposite interpretation that is made sometimes, (2) the sandhills, in the innermost coastal plain, are highly vulnerable but decline so coastward, (3) much of the middle and lower coastal plain has low vulnerability in all but surficial aquifers, although (4) there are important local areas of demonstrated exception and risk.