DELIVERY OF CHLORIDE AND NITRATE BY GROUND-WATER TO THE GREAT-LAKES - CASE-STUDY FOR THE DOOR PENINSULA, WISCONSIN

A calibrated, two-dimensional, finite-element transport model of the dolomite aquifer of the Door Peninsula of Wisconsin is used to ascertain the mass loadings of chloride and nitrate delivered to Lake Michigan and Green Bay by ground water. Inclusion of unsteady transport effects and verification of the model have not been possible due to data limitations. With that caveat, the model shows that only 43, 33, and 38% of the water, chloride, and nitrate, respectively, entering the aquifer's upper surface as recharge actually discharges directly to the lake as ground water. The remainder is discharged to wells and other sinks beforehand. A concentration of agricultural and urban land use sources along the Green Bay coast results in 58 and 50% of the total chloride and nitrate, respectively, delivered by ground water going to the west shore, despite that side's receiving only 38% of the water volume. On the Lake Michigan side, agricultural land use is the largest source of both chemicals, with the second largest source from background contributions. Point sources are relatively minor contributors of both chemicals. In an earlier paper, Cherkauer and McKereghan (1991) showed that ground-water discharge was focussed into embayments along the shore. That phenomenon repeats itself in solute loads as the greatest mass discharges occur in the bays. The simulated results show that discharge to both bays and nonbays on a given coast have similar solute concentrations, so the load differences are strictly the result of discharge focussing. Ground water's delivery of the solutes to these bays generally greatly exceeds that from precipitation. Comparison of loading to other sources or to mass budgets for the entire lake is not possible at this time.