INTERNAL PHOSPHORUS CYCLING IN UTAH LAKE: DETERMINING POTENTIAL FOR FUTURE EUTROPHICATION USING A CONCEPTUAL MODEL

Internal nutrient cycling in Utah Lake, Utah, USA, was examined in order to assess future eutrophication potential. Previous studies of Utah Lake have not addressed internal nutrient cycling, acknowledging that these processes are poorly understood. Therefore, we are attempting a first step in this direction by parameterizing a basic model of P cycling in Utah Lake.

Since the lake is a net sink for P, we investigated likely pathways for P sequestration. We characterized sediment samples from the lake by X-Ray Diffraction (XRD), selective dissolution, and P-adsorption experiments. Six sediment samples were analyzed using XRD and RockJock (USGS) analysis software. Surface areas of two samples were determined. A selective dissolution sequence quantified fractions of P bound by the following sorption modes: dissolved, ion-exchange, Fe and Al sorbed, apatite-bound, Ca-bound and organic-bound P. Ca-bound P was determined to be the dominant fraction.

We constructed a STELLA model based on inferences from experimental results and assumptions about the system. The model included modules for P-coprecipitation with calcite, aqueous P flux, P-sorption onto sediments, and sediment sequestration. Water fluxes and P concentrations were assumed to be at steady state under present conditions, consistent with records over the past few decades. We assumed that P precipitated with calcite would be permanently sequestered, due to the lake being supersaturated with respect to CaCO3. We also assumed a sedimentation rate of 1 mm/yr, and the top 1 ft. of sediment to be available for adsorption reactions.

Responses of P concentration to changing input levels from wastewater treatment plants were then modeled. The model reestablished steady-state conditions within 2.5 to 4 years.

Future modifications of the model will include optimization of predicted P coprecipitation rates based on electron microprobe data, and sequestration of P in refractory organic matter.