Potential for release of sediment phosphorus to Lake Powell (Utah and Arizona) due to sediment resuspension during low water level
The water level in Lake Powell, an important water-supply reservoir on the Colorado River, United States, decreased in most years from 1999 to 2010, exposing the sediment delta in the inflow region of this reservoir. This study assesses the potential for sediment-associated phosphorus (P) to enter the water column during sediment resuspension. We collected sediment samples from the reservoir inflow region and performed a sequential extraction and a sorption experiment that focused on P. We also collected water samples at locations upstream and downstream of the exposed sediment delta and measured soluble reactive P during base flow and during the yearly flood associated with spring runoff. Results indicate that extractable P is associated with various operationally defined fractions as follows: 50% with calcite and biogenic apatite, 29% with detrital apatite, 10% with Fe(III)-oxide minerals, 6% with organic matter and 4% with easily exchangeable solid-phase association sites. Sorption experiments indicate that the equilibrium P concentration is 0.19 ± 0.07 μM. Particle size is correlated with sediment P; coarse sediment contains and sorbs much less extractable P than fine sediment does. Measurements of water samples indicate that sediment resuspension probably releases P from sediment to overlying water during base flow, but the effect during floods is less clear. These results indicate that lowering reservoir levels introduces sediment-associated P to the reservoir when sediment resuspension occurs.