Is Phosphorus-Enriched Groundwater Entering Wisconsin Streams?

Groundwater phosphorus (P) is generally hypothesized to be a negligible surface water input. But direct evidence for this supposition is sparse. Our recent work suggests that higher concentrations of dissolved P have been slowly developing since the 1960s beneath agricultural landscapes and that P enriched groundwater has begun to discharge to agricultural drainages in Wisconsin. The goal of this study will be to evaluate whether the groundwater transport of P to agricultural drainages is a potential threat to the trophic status of surface water. We will seek direct evidence by focusing on groundwater seepage as it enters agricultural drainages in southern Wisconsin. The work will include four tasks: (a) A survey of the breadth and frequency distribution of apparent chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6) recharge age-dates of groundwater seepage entering representative agricultural drainages in southern Wisconsin. (b) A partitioning of the population of seepage sites into recharge age-date or residence time classes (e.g., decadal from 1950 to present), a selection of sampling locations to obtain a representative sub-population of each recharge age-date class, and three synoptic surveys (late spring, mid-summer, fall) of water quality and recharge age-dates at these locations. (c) An analysis of trends in dissolved P concentration in groundwater seepage within and between recharge age-date classes to determine whether there is robust evidence of groundwater P enrichment of surface water in southern Wisconsin agricultural drainages. (d) A forecast of the impact of the dissolved P trends on the surface water (baseflow) trophic classification using the theoretical approach developed by Browne and Guldan (2005). This study will provide a set of recommendations/questions for research into groundwater P enrichment. The data will be ofinterest to water quality managers and researchers addressing agricultural P management.