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Scientific Investigations Report 2008–5162

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5162
Version 1.1, December 2008

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Introduction

Lake Tahoe, an approximately 191 mi2 lake along the state line between western Nevada and eastern California, is a natural resource known for its deep, clear water. Protection of its renowned clarity has become important in the past half century, as clarity has been decreasing by about 1 ft each year (Goldman, 2000). Decreased clarity has been attributed to human activities that increase nutrient and sediment inputs to the lake (Goldman, 1988). In spite of numerous projects implemented to mitigate decreasing clarity, including exportation of all waste-water effluent and erosion-control regulations, lake clarity has continued to decline (Goldman, 2000). Nutrients enter the lake from streams, atmospheric deposition, erosion of shorelines and intervening areas, and by ground-water inflow (Reuter and others, 1998; Reuter and Miller, 2000). Estimates of nutrient inputs to Lake Tahoe by ground water have been made using regionalized values of hydraulic properties coupled with averaged nutrient concentrations (Thodal, 1997; U.S. Army Corps of Engineers, 2003). Several nearshore areas of Lake Tahoe have been identified as having increased turbidity and algal production that consistently are elevated compared to the midlake. Although atmospheric deposition of nitrogen is estimated to contribute most of the nutrient loading to the lake and may be responsible for overall decline of lake clarity, nearshore clarity losses may be caused by local influences, including nutrient-enriched ground-water discharge (Taylor, 2002).

Collection, detention, and infiltration of stormwater runoff in constructed basins is considered an effective best management practice (BMP) for achieving water-quality criteria related to total maximum daily load regulations (Schuster and Grismer, 2004). However, while this type of BMP may mitigate surface-water loads of suspended sediment to the lake, infiltrated stormwater in the detention basin may contaminate shallow ground water and increase ground-water gradients and flow to the lake. In addition, contaminants associated with stormwater runoff often include organic compounds and metals that potentially are toxic when consumed. Processes that affect ground-water contamination from stormwater have only just begun to be considered (Thomas and others, 2004; Prudic and others, 2005; Green, 2006), but understanding these processes is important because growing numbers of environmental improvement program projects are planned to encourage infiltration of stormwater runoff.

Background

Past detention-basin studies have focused more on suspended-sediment reductions than nutrient reductions. Martin (1988) experimented with routing flow through a detention basin and wetlands in series and found that while suspended-sediment loads were reduced, nutrient-load reductions were variable. The efficiency rates were between 0 and 72 percent (Martin, 1988), depending on the type of nutrient, the detention basin, and wetlands. Similarly, Reuter and others (1992) routed flow through a wet meadow and found a reduction in suspended solids but little change in nutrient concentrations. Ground water underlying detention basins near Lake Tahoe may have increased water-quality degradation because much of the stormwater runoff occurs during snow­melt, when vegetation is dormant and not assimilating nutrients (Prudic and others, 2005).

Purpose and Scope

This report documents 2005–07 hydrogeologic conditions in a shallow ground-water flow system adjacent to Lake Tahoe and the effects of an engineered stormwater-control system. Descriptions of the basin-fill aquifer and a stormwater collection and control system; quantification of components of the ground-water budget; and characteristics of the quality of stormwater, bottom sediment from a stormwater detention basin, ground water, and nearshore lake and interstitial water are included. Results of a three-dimensional, finite-difference, ground-water flow model also are presented as a tool to evaluate responses of ground-water flow to stormwater runoff accumulation in the stormwater-control system.

The city of South Lake Tahoe hired 2NDNATURE, LLC, in June 2005, to “design and implement a two year data-collection program to promote a quantitative understanding of the impacts of urban surface water infiltration via stormwater-treatment systems on the quality of shallow groundwater resources” (Maggie Mathias, 2NDNATURE, LLC, written commun., September 2006), which provided the opportunity for collaboration and data sharing. Ground-water flow and seepage across the lake interface was simulated by the U.S. Geological Survey (USGS).

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