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

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Location and General Features

The study area is in the Lake Tahoe basin, along the California–Nevada state line. It coincides with a 2.91 mi² intervening area, delineated topographically as an area that is tributary to Lake Tahoe without a perennial stream draining into the lake (site 69; Jorgensen and others, 1978). The area is bounded by the drainage basins of Bijou and Heavenly Valley creeks in California, by Edgewood Creek Basin in Nevada, and by about 8,000 ft of Lake Tahoe shoreline (fig. 1). The area’s land surface slopes steeply (0.46 ft/ft) from the ridge northwest of Heavenly Valley Creek to the consolidated rock-alluvium contact at 6,350 ft. Land surfaces slope more gradually (about 0.03 ft/ft) from that contact to the legally defined minimum lake stage altitude of 6,223 ft above sea level. The nearshore lakebed slopes more gradually (0.015 ft/ft) to about 0.75 mi offshore, before plunging steeply (0.11 ft/ft) to the 600 ft depth contour about 1.5 mi offshore.

Consolidated rocks in the study area are light-gray, fine- to medium-grained, well foliated granitic type rocks of Cretaceous age (about 90 million years ago; Armin and John 1983; Bonham and Burnett, 1976; Saucedo, 2005). These consolidated rocks have little porosity and generally are impermeable to ground-water flow, except where fractured or weathered. Unconsolidated basin-fill deposits are weathered mostly from consolidated granitic rocks and sorted to varying degrees by fluvial processes. During the Pleistocene Epoch, glaciers dammed outflow from Lake Tahoe resulting in formation of lacustrine deposits of moderately to poorly sorted silt, sand, and gravel that accumulated as broad terraces 15–30 ft above current lake levels. Pleistocene beach deposits are gravelly, coarse arkosic sand that have been moderately sorted by wave action during higher lake levels. Younger (Recent Holocene) beach deposits are moderately sorted fine- to very coarse-grained sand to gravelly arkosic sand coincident with modern lake levels. A small, manmade deposit of varying composition was imported for construction of the casinos and associated development along the state line. Two narrow bands of alluvium have been poorly to moderately sorted by intermittent streamflow from the area’s upland areas. The distribution of these deposits is shown in figure 2.

The area between the lake and the consolidated-rock alluvium is urbanized (fig. 2). Development prior to about 1940 was limited to logging, agricultural, and fishing communities, and several roadhouses catering to trans-Sierra travelers and those attracted to the health and pleasure attributes of Lake Tahoe. Tourism and recreation became increasingly important to the economy of the area, especially following World War II, and most of the area’s manmade impervious surfaces were originally constructed between 1940 and 1969 (C.G. Raumann, U.S. Geological Survey, written commun., 2007). Along the Highway 50 corridor within the study area are several casinos in Nevada and numerous side streets with motels, shopping complexes, and residential developments that result in increased impervious land cover and stormwater runoff. The mountainous uplands of the area are forested with residential developments, motels, and shops catering primarily to Heavenly Valley Ski Resort. The city of South Lake Tahoe incorporated in November 1965, with a population of about 14,000 (Crippen and Pavelka, 1970, p. 26). According to the 2000 U.S. Census, the population of the city of South Lake Tahoe, California, was 23,609 and Stateline, Nevada, was 1,215.

Prior to a 1970 mandate to treat and export all domestic wastewater from the Lake Tahoe Basin, disposal practices had evolved from privies and wastewater cesspools and lagoons to septic tank-leachfield systems to land application of treated municipal effluent. These practices resulted in nitrogen contamination of the regional ground-water flow system that was reported in low-flow stream samples (Perkins and others, 1975) and in ground-water monitoring results (Thodal, 1997).

Park Avenue Stormwater Control System

Stormwater-drainage networks in South Lake Tahoe have developed in a piecemeal fashion since rain and snowmelt were diverted from unpaved wagon trails in the early 1900s. By May 2008, an estimated 1,500 drainage inlets collect stormwater from street and parking lot gutters in the city with 15 stormwater treatment vaults and 100 outfalls to surface waters that are tributary to Lake Tahoe (City of South Lake Tahoe, 2008, p. 6-29). The Federal Water Pollution Control Act (also referred to as the Clean Water Act) was amended in 1987 to include controlling pollutants in stormwater runoff under the National Pollutant Discharge Elimination System. The U.S. Environmental Protection Agency promulgated regulation in 1990 to require separate municipal stormsewer systems serving a population of 100,000 or more to obtain stormwater permits. These regulations were amended in 1999 to require permits for stormwater discharges from small conveyance systems and from construction sites disturbing between 1 and 5 acres of land. The Lahontan Regional Water Quality Control Board adopted Order 6-92-02 in 1992 that requires stormwater permits for all municipal separate stormsewer systems (MS4s) on the California side of the Lake Tahoe basin. This permitting system provides a mechanism to work with the local municipalities to improve stormwater-management and maintenance practices, and requires permittees to develop comprehensive stormwater-management programs in the Lake Tahoe area (California Regional Water Quality Control Board, Lahontan Region, 2005).

Application for a stormwater permit typically requires one or more BMPs (methods that have been determined to be the most effective, practical means of preventing or reducing pollution from stormwater runoff). These practices may include behavioral BMPs such as education, structural BMPs such as source and treatment controls to treat runoff before it discharges to the storm drain or local waterways, and other practices that prevent or reduce pollutants from reaching the stormdrain or other waters (California Regional Water Quality Control Board, Lahontan Region, 2005, attachment A).

The Park Avenue stormwater control system is a BMP selected for investigation of ground water responses to infiltration of stormwater runoff. It is a structural treatment control that includes two detention basins near Park Avenue, in South Lake Tahoe, north of Highway 50 (fig. 3). The system is designed to collect runoff from a pre-existing urban stormwater-drainage network into detention basins that allow suspended sediment and associated nutrients to settle or be assimilated by vegetation, or infiltrate through soil to the shallow ground water. One detention basin (site PA1; fig. 4A) was constructed in 2000 and a second detention basin (site PA2; fig. 4B) was constructed in 2002. PA1 normally is a perennial wetland/shallow detention basin, with cattails, rushes, sedges, and duckweed (genera Typha, Juncus, Carex, and Scirpus, respectively) and a benthic periphyton community capable of assimilating biologically available nutrients. PA2 is a dry basin that receives stormwater only when inflow to PA1 exceeds the basin’s capacity. Runoff from upgradient areas serviced by the stormdrain network flows into PA1 via two culverts (sites PA1_inletA and PA1_inletB; fig. 4A) and when PA1 reaches its capacity, overflows into a culvert (PA1_out) that conveys water to PA2 and into an open unlined ditch. A dam in the ditch can be operated manually to divert flow to a wet meadow in an adjacent neighborhood during excessive stormwater runoff.

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