THE ORIGIN OF ELEVATED GROUNDWATER PHOSPHATE IN THE ALLUVIAL AQUIFER AT EMPORIA STATE UNIVERSITY

An understanding of the origin of the phosphate entering Midwestern rivers and streams is important to stream ecology and protection of drinking water resources. High groundwater phosphate concentrations (2.5-9.2 mg/L) have been observed in an alluvial aquifer near Emporia State University during the 10 years of monitoring. The sand and gravel aquifer is confined by 9 m. of silty clay that grades laterally into interbedded sand and clay at the nearby Neosho River. The highest concentrations occur in the deepest part of the aquifer. River water phosphate concentrations (0.01-0.32 mg/L) are near the EPA’s recommended limit for surface waters entering lakes. The accumulation of fertilizer phosphorous in the soil was examined as a potential phosphate source. A fluctuating water table and reversal of stream-aquifer exchange during drought were thought to contribute dissolved soil phosphate to the river. However, soil phosphate concentrations of 17.5-37/5 mg/kg at 3 locations on the floodplain are below natural levels observed in similar soils and thus, an unlikely source. Above normal phosphate concentrations (1.4-1.6 mg/L) were measured in a small campus lake that recently experienced an algal bloom. Additional investigation of the lake as a potential source of the groundwater phosphate, and its hydrologic connection to the aquifer, is part of this ongoing research.