Phosphorus in legacy sediments of shallow, eutrophic Utah Lake

Phosphorus (P) accumulation in shallow lakes leads to eutrophication and harmful algal blooms. Decades of nutrient loading from agricultural runoff and wastewater treatment plants (WWTP) on the east side of Utah Lake has resulted in elevated P concentrations in the lake sediments and water column. In an effort to mitigate further eutrophication, the Utah Division of Water Quality proposed new limitations of P loading in the lake from WWTPs. Limiting external loading, however, may not lead to improvements due to a legacy of nutrients in the lake sediments. To investigate P cycling in Utah Lake, we collected water column, pore water, and sediment samples during 2015-2016. Water samples were analyzed for total dissolved P (TDP) and sediment samples were analyzed for total P (TP). A subset of sediment samples was subjected to a sequential extraction procedure to evaluate P speciation. TP concentrations in sediment ranged from 306-1710 mg/kg, with the highest concentrations on the east side of the lake near WWTP inputs. Sediment TP was reflected in water samples, with TDP concentrations as high as 1.7 mg/L in the water column and 10.8 mg/L in pore water on the east side of the lake. Sequential extractions indicated that 25-47% of sediment TP is associated with carbonate minerals and 41-61% is associated with Fe oxide/hydroxide minerals. The carbonate-associated P fraction is likely immobile, but the Fe oxide-bound P is potentially bioavailable under changing redox conditions. Currently we are using sediment flux chambers to monitor dissolved oxygen concentrations in relation to P release at the sediment-water interface. Ultimately, we plan to quantify internal P loading to determine the internal versus external P on nutrient budget for Utah Lake. This information is much needed as the state considers nutrient regulations for local WWTPs.