Understanding long-term baseflow water quality trends using a synoptic survey of the ground water-surface water interface, Central Wisconsin

The relationship between stream water quality and landscape activities is difficult to evaluate where the principal source of stream flow is ground water seepage because the average travel time from ground water recharge areas to stream discharge positions can be on the order of decades. We tested the idea that past and future baseflow water quality can be predicted based on a synoptic survey of ground water recharge age-dates (based on chlorofluorocarbon [CFC] measurements) and water quality measurements obtained at the ground water-surface water interface. In this study we (i) characterize the discharge-weighted age distribution and water quality of ground water seepage into the Little Plover River (LPR); (ii) use this information to backcast and forecast basellow NO3- concentrations; and (iii) evaluate NO3- backcasts against historical baseflow data (1960 to 2000). The discharge-weighted apparent CFC age of ground water seepage into the LPR was 23.7 (+/- 7) yr. Basellow backcasts matched the four decade rise of baseflow NO3- from 2 to 8 mg L-1. Baseflow forecasts included three scenarios. Scenario A projects the historical rise of NO3- in the LPR basin's ground water recharge through 2050. Scenario B projects a leveling (off of NO3- in ground water recharge in the year 2000. Scenario C projects a leveling off in the year 1985. Under Scenario A, LPR baseflow NO3- will increase steadily from 8 to 19 mg L-1 between 2000 and 2050. Under scenarios B and C basellow NO3- will plateau at 13 mg L-1 in 2030 and at 10 mg L-1 in 2010, respectively. The approach developed in this study can be used to (i) reconstruct historical basellow water quality patterns in the absence of long-term monitoring data and (ii) project the effects of potential management decision on future water quality.