NUTRIENT MIGRATION IN CARBONATE AQUIFERS IN RESPONSE TO STORMS: A COMPARISON OF TWO GEOLOGICALLY CONTRASTING AQUIFERS

Karst aquifers occur in a wide variety of geologic and geomorphologic settings, which can affect their geochemical behavior. We investigated the migration of two nutrients—nitrate and potassium—in response to storms in two karst aquifers in highly contrasting settings: the Chalk aquifer (Normandy, France) and the Edwards aquifer (Austin, Texas, USA). In these settings, the Chalk aquifer is typified by high matrix porosity, thick overlying deposits (as much as 30 m), and agricultural land use, and the Edwards aquifer is typified by low matrix porosity, outcropping limestone, and urban land use. Following one to three storms, from 5 to 16 samples from springs and wells were analyzed for major ions, and physico-chemical properties were monitored continuously. Comparison of the chemographs indicated some generalized responses, including an increase in turbidity and potassium concentrations and a decrease in major ion and nitrate concentrations with infiltrating storm runoff, but further interpretation was complicated by the number of constituents, the number of sites and events, the sparseness of data at some sites, and hydrologic differences between storms. Factor analysis of major ions and turbidity, however, revealed strikingly similar loadings of the chemical variables for the two aquifers. The first two factors, which explained more than 75% of the variability, illustrate three geochemical groups with distinct responses: nitrate and other major ions except potassium; turbidity; and potassium. A third factor explained an additional 10% of the variability for the Chalk aquifer and indicated an additional component of nitrate dynamics for this aquifer. The results demonstrate that potassium and nitrate are effective tracers of infiltrating storm runoff and resident ground water, respectively, and the similar results for these two highly contrasting aquifers suggest that the dynamics identified might be applicable to karst systems in general.