STREAMBANK SEDIMENTS: POTENTIAL SINKS AND SOURCES OF PHOSPHORUS IN A EUTROPHIC HEADWATER STREAM

Excessive phosphorus (P) in surface water can cause problems with excessive algal growth leading to drinking water issues. Understanding internal stream P transfers is thus important in controlling eutrophication. To determine the direction of groundwater and surface water interactions and evaluate P retention within streambank sediments, groundwater well pairs, about 5 m deep, were installed at three locations (A, B and C) along a second-order, eutrophic stream in north-central Texas. Well cores were analyzed for P, and groundwater levels were monitored for about two years. Water levels in wells furthest upstream (A) always indicated a losing stream, while wells furthest downstream (C) showed a gaining stream except during storm events when flow reversals occurred. Total-P from well cores ranged from 54-254 mg kg^-1 and was typically high near the surface, decreased downward until redoximorphic features were encountered and then increased notably with depth to near or above surface concentrations. Very little extractable P occurred in sediments from the two upstream well sets; however, the set furthest downstream showed extractable P throughout with a high of 21 mg kg^-1 near the bottom. Repeated wetting-drying at sites A and B as noted by redoximorphic features may have shifted P into more stable sediment-bound forms. The decrease in extractable P at sites A and B compared to site C may be explained by conditions at C that were wetter and potentially anaerobic. Because the overall stream reach was more often losing than gaining, there appears to be a mass flow of P into streambank sediments. Interaction between surface water and groundwater in losing streams may lead to long-term phosphorus retention in streambank sediments with the long-term stability dependant on phosphorus forms and erosion potential.