Technical Note: Pineview Reservoir Nutrient Loading, Unloading, and the Role of Groundwater in the Estimates

Abstract

The total maximum daily load (TMDL) study for Pineview Reservoir was completed in 2002. As is often the case, the data used in the TMDL analysis were sparse. Concerns over the accuracy and implementation of the TMDL led the Utah Water Research Laboratory and the Weber Basin Water Conservancy District to collaborate on a study of the reservoir starting in October 2007. An objective of the research work was to expand the information available about the sources of nitrogen and phosphorus to Pineview Reservoir and to improve the description accuracy of nutrient transport through the watershed. It was determined that: (1) thermal stratification occurred in most of the reservoir and that the hypolimnion became anoxic by late July; (2) phosphorus and nitrogen accumulated in the anoxic hypolimnion and were exported with reservoir withdrawals; (3) nutrient concentrations and phytoplankton production responded to water column turnover; and (4) turnover and nutrient mixing into the water column appeared to be related to both draw-off of the hypolimnion and wind-mixing under near-isothermal conditions. These findings led to a detailed study of nutrient transport from the watershed to Pineview Reservoir. Grab sampling and high-frequency monitoring of both groundwater and surface water were conducted. Nitrogen and phosphorus transport through surface water and groundwater have been determined. The study has shown that groundwater loadings of dissolved nitrogen and soluble reactive phosphorus are 35% to 40% lower than those estimated in the TMDL study. Nitrate-N loading from surface water was 1.8-fold higher and total phosphorus loading was 2.4-fold higher than was predicted in the TMDL study.