Scientific Investigations Report 2005–5215
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005–5215
A total maximum daily load (TMDL) management plan was developed for the lower Boise River after it was listed as water-quality limited in 1992, in accordance with Section 303(d) of the Clean Water Act. The management plan includes TMDLs for nutrients, suspended sediment, bacteria, elevated water temperature, and low dissolved-oxygen concentrations. A 2001 synoptic study estimated that as much as 150 pounds per day of the nutrient, dissolved phosphorus, from ground-water seepage was entering a 3 river-mile reach of the lower Boise River. Better estimates of ground-water seepage into the lower Boise River are required if further nutrient transport studies are to be done. This study was designed to determine ground-water seepage estimates for the lower Boise River using the USGS model VS2DI and the parameter estimation code PEST. These seepage estimates potentially can be used to help determine the approximate amount of nutrients being transported to or from the stream.
To obtain seepage rates, a two-dimensional heat and water transport simulation model was created for each of four transects distributed over 3 river miles on the lower Boise River and representing conditions from April to August 2003. As many as seven piezometers were installed at each transect to obtain monthly stage and ground-water head values, continuous temperature, and saturated hydraulic conductivity from slug tests.
Analysis of model results indicated that three of the four transects gained water, with seepage rate estimates ranging from 6 to 73 cubic feet per second per mile [(ft3/s)/mi]. The fourth transect showed seepage from the Boise River to ground water, with rates ranging from 2 to 18 (ft3/s)/mi. One benefit of this seepage estimation method is the ability to model how seepage changes over time. The trend of seepage to the river increased for most of the 5-month study period in the three transects that gained water. Likewise, the transect that lost water showed a decreasing trend of seepage from the river. Seepage estimates for this study were higher than those of previous studies using different methodologies. Seepage values for this study are representative of a unit meter stream length as opposed to long stream reaches. Comparison of seepage estimates for all transects also aids in identifying the seepage variability within a river reach.
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U.S. Geological Survey
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