Publication Summary

During the summer of 2003, the Department of Ecology′s Environmental Assessment Program undertook a variety of Total Maximum Daily Load (TMDL) field studies affecting the Deschutes River and Percival Creek watersheds. These studies were to assess current stream temperatures, water quality, streamflows, and other environmental factors. This part of that TMDL effort was undertaken to gain a better understanding of how groundwater discharge to gaining stream reaches affects the temperature, water quality conditions, and flows of these streams.

This report describes the results of a hydrogeologic investigation that was undertaken to support a Total Maximum Daily Load (TMDL) evaluation of the Deschutes River and Percival Creek watersheds in Thurston County, Washington. The Deschutes River was included on Washington State′s 1996 and 1998 303(d) lists for temperature, pH, fecal coliform, and fine sediment violations of surface water quality standards. Percival Creek, although not formally listed, was also included in the evaluation due to temperature, fecal coliform, and pH impairments.

Multiple field techniques were employed to (1) evaluate the direction, volume, and timing of surface water and groundwater interactions, and (2) estimate the potential loading of phosphorus and nitrogen-based nutrients that groundwater contributes to gaining reaches of the Deschutes River and Percival Creek. The field techniques included stream seepage runs, installation and monitoring of instream piezometers, analysis of groundwater quality samples, and monitoring of streambed thermal profiles.

During baseflow seepage runs conducted in the summers of 2003 and 2004, the Deschutes River gained 41.4 ft3/s, between river miles 42.3 and 0.5, while Percival Creek gained 1.7 to 2.6 ft3/s between river miles 3.3 and 0.1. The reach-based streamflow gains and losses observed during seepage runs were generally supported by point-based vertical hydraulic gradients and streambed thermal profiles measured in instream piezometers.

Measurable concentrations of dissolved orthophosphate (0.008 to 0.086 mg/L) and dissolved total phosphorus (0.046 to 0.152 mg/L) were found at all sampled piezometer sites. Measurable concentrations of dissolved nitrate+nitrite-N and ammonia were found in roughly half of the piezometers sampled at concentrations ranging from 0.011 to 4.76 mg/L and 0.032 to 0.206 mg/L, respectively. The average estimated unit-area-mass loading to the Deschutes River by discharging groundwater varied by location and ranged from 2.8 to 66.4 mg/d/m2 for dissolved total phosphorus to 68.6 to 3913 mg/d/m2 for dissolved nitrate+nitrite-N.

Link to EIM data for User Study ID MROB0001