Scientific Investigations Report 2005–5215
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005–5215
Water quality in the lower Boise River has changed over time due to anthropogenic activities such as land-use changes, increased urbanization, and an altered flow regime in the Boise River. The lower Boise River was listed as water-quality limited in 1992 in accordance with Section 303(d) of the Clean Water Act. This listing required the development of a total maximum daily load (TMDL) management plan for the lower Boise River. 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-mile reach of the Boise River.
Results of the synoptic study indicated (1) the potential of significant seepage of nutrients to the lower Boise River from ground water, and (2) the need for a more focused study of ground-water and surface-water interaction. In 2003, this study was designed and implemented to identify and estimate seepage between ground water and the lower Boise River. Seepage estimates can potentially be used to help determine the approximate amount of nutrients being transported to or from the stream.
Water exchange between surface water and ground water affects temperatures not only in the two water resources, but also in the sediments near them. Therefore, analyses of subsurface temperature patterns provide information about surface-water and ground-water interaction. The USGS model VS2DI is a two-dimensional, variably-saturated, ground-water flow model that has been modified to simulate heat transport by advection and conduction. It was used to estimate seepage between ground water and surface water within a 3 river-mile reach of the lower Boise River. Two-dimensional models of four transects along this reach were modeled representing temperature and flow conditions from April to August 2003.
As many as seven piezometers were installed to the same depth at each of four transects along the lower Boise River near Parma. Hydraulic conductivity of the subsurface sediments was estimated using slug tests in the piezometers, monthly ground-water head and stream stage values, and continuous temperature was measured at various depths and locations in piezometers throughout the transect. The parameter estimation code, PEST, was used to refine two VS2DI model parameters: (1) saturated hydraulic conductivity, and (2) the horizontal to vertical saturated conductivity anisotropy ratio. Comparisons between simulated and measured temperatures and ground-water head measurements were used to evaluate the VS2DI models.
The VS2DI modeling resulted in a large variability of seepage estimates among the four transects. Models of three of the four transects resulted in seepage gains to the Boise River, with rates ranging from 6 to 73 ft3/s. The model for the other transect resulted in losing stream conditions, or seepage from the river at rates of 2–18 ft3/s. One of the benefits of VS2DI modeling is the ability to see how the seepage rates change over time. All models that resulted in gaining stream conditions showed an increasing trend of seepage to the river from April to August 2003. In the case of the losing stream transect, the seepage from the river decreased during the modeling period.
Previous estimates of seepage in the lower Boise River were conducted with a different technique using seepage runs. A seepage estimate during the non-irrigation season in November 1971 that included the upper one-half of the study area for this study estimated an average seepage of ground water to the Boise River of 3 (ft3/s)/mi. A 1996 seepage study in canals near this project’s study area during the pre- and post-irrigation season yielded seepage ranges of -10 to 4 (ft3/s)/mi (negative values indicate seepage from the river), with the late summer seepage values being less than those for early summer. Although these previous studies gave lower estimates of seepage to the Boise River than this study did, it should be noted that they were conducted over long stream or canal reaches. The seepage estimates from this study are focused at transects with a unit meter width, and they indicate the variability of seepage throughout the 3-mile river reach.
The VS2DH modeling could be improved by using continuous head recorders for both the piezometers and stream stage. At least one additional piezometer installed at depth would help to identify the vertical hydraulic gradient of the subsurface. Although VS2DH is not currently available in a three-dimensional format, three-dimensional modeling would improve seepage estimates for river reaches, thereby providing additional information beyond that available from two-dimensional transect models. SUTRA (Voss and Provost, 2002), a heat and ground-water transport model with three-dimensional capabilities, has been successfully applied using three-dimensional temperature and head data to examine surface-water/ground-water exchanges (Burow and others, 2005). However, three-dimensional modeling requires considerably more field data than two-dimensional modeling, which could affect the economic feasibility for this type of seepage modeling over a large river reach.
For more information about USGS activities in Idaho, visit the USGS Idaho Water Science Center home page .
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U.S. Geological Survey
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