PHOSPHORUS FATE IN A SEASONALLY ENDORHEIC LAKE RECEIVING WASTEWATER

Frank Lake is a constructed lake in a semi-arid climate that has received increasing amounts of wastewater since the 1980s, with a management goal of minimizing outflow to mitigate phosphorus export. The lake volume increases significantly during spring run off, before experiencing progressively lower water levels that are accompanied by seasonally increasing δ¹⁸O and δD values and increasing ion concentrations (to the point of carbonate and gypsum mineral saturation).

Although Frank Lake experiences significant evaporation, a water balance suggests that groundwater likely provides a significant, previously unaccounted for, fraction of water entering the lake. Mass balance indicates the lake is storing chloride every year. The fate of a significant fraction of the annual chloride entering the lake is not accounted for by evaporation, mineral precipitation, or discharge. Since the net groundwater flow direction is into the lake, chloride loss is not via discharge to groundwater. Elevated chloride concentrations in the shallow groundwater in peripheral lake areas suggest chloride (and other ions) are accumulating in the sediments.

Since the discharge of nutrients into the freshwater river downstream of Frank lake is a significant management concern, a phosphorus balance is of particular interest. The mass flux of P discharged from the lake is small (less than 8% of the estimated mass entering the lake each year). In addition to accumulation in the near-shore sediments, phosphorus minerals in the lake-bottom sediment were identified by microprobe analyses.