Phosphorus distribution in reservoir sediments: implications for groundwater transport.

Phosphorus (P) impairs reservoir water quality and managers impose limits on external P loading to address this problem. Even with P reductions, restoration has taken longer than expected at many reservoirs. Researchers attribute this to internal P sources, such as the interactions between the reservoir water column and sediment pore water. Subsurface nutrients and groundwater/surface water interactions seem to dominate many of these reservoirs. Data sets to characterize sediment P distribution are rare, but critical, as studies show that P types and amounts vary spatially. Deer Creek Reservoir (DCR), Utah exhibits ongoing degradation even after significant reduction in external P loading. A recent DCR construction drawdown provided a unique opportunity to study delta sediments and determine the potential amounts, geochemical sources, and spatial distribution of P. Traditional reservoir studies are limited by the ability to take large numbers of core samples. We were able to obtain samples and data in higher resolution than in previous studies. We took 91 sediment samples in six transects to define the spatial distribution and geochemical form of P over an approximate 750,000 m (185 acre) area. In addition to surface samples, at selected sites we took samples to characterize vertical P distribution. We analyzed all samples for water soluble P, and 19 samples for four other P forms. Water soluble P ranged from 2.28E-03 to 9.81E-03, KCl-P from 2.53E-03 to 1.10E-02, NaOH-P from 5.30E-02 to 4.60E-01, HCl-P from 1.28E-01 to 1.34E+00, and organic (residual) P from 8.23E-01 to 3.23E+00 mg g-1. Initial geostatistical analysis indicated spatial trends in the P distribution that follow expected sediment distribution patterns.