Elevated Dissolved Phosphorus in Riparian Groundwater along Gaining Urban Streams

Findings of low concentrations of dissolved phosphorus in groundwater in large surveys [e.g., United States Geological Survey's National Water-Quality Assessment (NAWQA) Program (Dubrovsky, N. M.; et al. The Quality of Our Nation's Water: Nutrients in the Nation's Streams and Groundwater, 1992-2004. U.S. Geological Survey Circular 1350; USGS: Reston, VA, 2010.); >5000 wells] support the common perception that groundwater is generally of little importance for transporting phosphorus. Here, we address whether this applies to urban riparian settings, where discharging groundwater may potentially contribute to urban stream syndrome and downstream eutrophication problems. This survey study. includes 665 samples of groundwater collected along gaining stream reaches at six urban sites. Considering the combined sample set, 27% had soluble reactive phosphorus (SRP) concentrations >0.1 mg L-1, which is more than double that determined in the NAWQA Program (12%), while for individual sites the range was 12-52%, excluding one site with consistently low SRP (0%). None of the sites showed significant correlation between SRP and the artificial sweetener acesulfame, a promising wastewater indicator, including two with known wastewater contamination (but the lowest SRP). Rather, high SRP concentrations were associated with geochemically reducing conditions. This could mean that natural aquifer or stream sediment materials were a primary contributor of the elevated SRP observed in this study.