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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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Methods

Data-collection sites used for this investigation are listed and described in table 1 and locations are shown in figure 3. 2NDNATURE, LLC, purchased, installed, and maintained instrumentation used to monitor water-level changes in the detention basins and selected wells, and to monitor stormwater inflow. These data are presented courtesy of 2NDNATURE, LLC.

Surface Water and Park Avenue Basins

2NDNATURE, LLC, installed and maintained two recording pressure transducers and staff plates to monitor water stage in basins PA1 and PA2 and two flow-velocity meters and automated samplers (Sigma®) in two culverts (sites PA1_inletA and PA1_inletB) that convey stormwater into PA1. A third pressure transducer was installed to correct nonvented transducers for changes in barometric pressure. The transducers recorded average pressure every 30 minutes that was converted to depth of water below the measuring point; the staff plates provided a visual check of the altitude of the water surface in each detention basin. USGS used a real-time kinematic Global Positioning System to survey locations and altitudes of each transducer to convert all data to a common datum and also surveyed the bathymetry of each detention basin (fig. 4). Elevation of the surface of Lake Tahoe was obtained from the water-stage recording gage that is operated by USGS (station number 10337000) on the U.S. Coast Guard pier on the north shore of Lake Tahoe. The datum of the gage is 6,220.00 ft above Bureau of Reclamation datum, which is 6,218.86 ft above the National Geodetic Vertical Datum of 1929. Lake elevations referred to Bureau of Reclamation datum because that datum is used as the official reference point by all Federal, State, and local agencies (available at http://waterdata.usgs.gov/nwisweb/local/state/ca/text/10337000-manu.html).

Ground Water

Well Construction

Seven observation wells were installed during October and November 2005 to characterize local aquifer hydraulic properties, monitor ground-water levels, and to sample ground water (fig. 4). The wells also were used to evaluate responses of local ground water to infiltration of stormwater that accumulated in detention basins (PA1 and PA2). Boreholes used for well installation were drilled using a trailer-mounted hollow-stem auger that produced a borehole diameter of about 6.6 in. Wells were installed in boreholes drilled to depths of 14–30 ft below land surface and constructed of flush-thread, 2-in. nominal diameter ASTM 480-88A, schedule 40 polyvinyl chloride well casing, screens, and end points. Well screens (5 ft long with 0.020 in. factory slots) were positioned below the water table and surrounded with clean, coarse (#6) aquarium gravel. Clean, fine (#12) sand was used to fill the borehole to about 1 ft above the water table and high-swelling 100 percent pure sodium bentonite (certified by the National Sanitation Foundation, International, to meet ANSI/NSF Standard 60; Drinking Water Treatment Chemicals–Health Effects) was emplaced from the top of the fine sand to 2 ft beneath land surface to provide a sanitary seal. The top of well casings extend about 3 ft above land surface to avoid inflow of surface water and a 0.3 ft by 0.3 ft locking well protector was cemented into the remaining 2 ft of annulus and the finish hand-troweled so that surface water drains away.

In addition to the seven wells constructed in 2005, 2NDNATURE had contracted construction of four observation wells in August 2003 for an earlier project. These wells were used to monitor water levels and ground-water quality for this investigation. Domestic wells and wells not included in the real-time-kinematic survey were located with a hand-held Global Positioning System and altitudes were determined with a laser level (table 1).

Aquifer Characteristics

Hydraulic conductivity near the Park Avenue detention basins ranged from 0.3 to 20 ft/d for sandy clay and medium sand (table 2). Hydraulic conductivity was estimated from the results of slug tests in eight wells that were analyzed with the method described by Bouwer and Rice (1976). Results from these slug tests constrained ground-water velocity estimates utilized in the ground-water flow model, which control transport and travel times of nutrients.

Sub-Littoral Ground-Water Discharge to Lake Tahoe

Water exchange between Lake Tahoe and the adjacent aquifer system is controlled by the hydraulic gradient and permeability of the lakebed and aquifer material. Lakebed permeability is variable due to fluvial processing of deltaic sediments and texture of basin-fill deposits, wave sorting of beach deposits, and the mineralogy of parent rock (which controls the size and uniformity of sediment grains resulting from weathering). The hydraulic gradient controls the direction of ground-water exchange between lake and aquifer, and the energy available to move water through variably permeable deposits. Lake Tahoe receives most of its inflow as snowmelt runoff and is managed as a storage reservoir, such that the hydraulic gradient may be subject to artificial variability and possible gradient reversals (Winter and others, 1998, p. 18).

Estimates of ground-water discharge to Lake Tahoe have been made using regionalized values of onshore aquifer hydraulic properties. The mass of nutrients transported to the lake with ground water was estimated by coupling ground-water volume with averaged nutrient concentrations (U.S. Army Corps of Engineers, 2003; Thodal, 1997). Growing acceptance that ground water is a significant variable to consider in water and nutrient budgets of surface water (Winter and others, 1998; Brock and others, 1982) has led to the development of a variety of novel hydrological tools used to quantify and corroborate the connection between lakes and ground-water systems (Schuster and others, 2003).

Water-Quality Sampling Procedures and Analytical Methods

A total of 95 ground-water and 37 stormwater samples were collected following procedures described in USGS National Field Manual for the Collection of Water-Quality Data (variously dated). The collection of samples were made prior to and during snowmelt runoff (November 2005–May 2006 and December 2006–April 2007) and sent to High Sierra Water Laboratory in Truckee, California, for determination of nutrients (Kjeldahl nitrogen; ammonium; nitrate plus nitrite, phosphorus and soluble reactive phosphate; American Public Health Association, 1998; Solorzano, 1969; Liddicoat and others, 1975; Woodworth and Conner, 2003). Ground-water samples were collected after well purging by using a bailer dedicated to each observation well or a portable submersible pump. Subsamples were filtered through a 0.45-µm mixed cellulose ester membrane for laboratory analyses. Unfiltered stormwater samples were collected by the open-mouth bottle method with subsamples filtered through a 0.45-µm mixed cellulose ester membrane for determination of ammonium, nitrate plus nitrite, and orthophosphate-phosphorus. Concentrations of organic nitrogen and hydrolysable phosphorus were estimated by subtracting laboratory values of ammonium-nitrogen from Kjeldahl-nitrogen, and orthophosphate-phosphorus from phosphorus, respectively. In addition, ground-water samples were collected in February 2006 for field determination of filtered concentrations of bicarbonate and alkalinity (Rounds, 2006) and laboratory determination of stable isotope ratios of oxygen and hydrogen (Epstein and Mayeda, 1953; Coplen and others, 1991; Coplen, 1994) and filtered concentrations of organic carbon (Brenton and Arnett, 1993), major ions, silica, chromium, copper, iron, lead, nickel, and zinc (Fishman and Friedman, 1989; Fishman, 1993). Isotope samples were shipped to the USGS Reston Stable Isotope Laboratory in Reston, Virginia and other samples were shipped to the USGS National Water Quality Laboratory in Lakewood, Colorado.

Ten samples of lake and interstitial water also were collected from five locations along the shoreline of Lake Tahoe (fig. 3). A multiparameter meter was used to measure temperature, pH, specific conductance, and dissolved oxygen of the lakewater prior to sample collection. Lakewater samples were collected from 1 ft beneath the water surface and from the water-lakebed interface using 0.25-in. diameter polyethylene tubing and a 60 mL syringe at a rate of 30 mL/min. Interstitial water was collected by pushing a 0.5-in. diameter minipeizometer 0.8 ft beneath the lakebed and withdrawing water through 0.25-in. diameter tubing and a 60 mL syringe at 30 mL/min. Unfiltered water was collected for stable isotope analysis (Epstein and Mayeda, 1953; Coplen and others, 1991; Coplen, 1994). Subsamples were passed through 0.45-µm Supor® syringe filters into 125-mL opaque polyethylene bottles and chilled on ice for overnight shipment for nutrient analyses by the USGS National Water Quality Laboratory (Fishman, 1993; Patton and Kryskalla, 2003). Laboratory reporting levels of each analyte are listed in table 3. All water-quality data are listed in appendix A (at back of report).

Collection of Bottom-Sediment Samples

Two bottom-sediment samples were collected from site PA1 (fig. 4A) on August 31, 2005, using a 2-ft split-spoon sampler with polyethylene core liner and soft-fingered core catcher. The sampler was driven 2 ft into the sediment about 20 ft from the PA1_inletA. The resulting core was extruded into two glass bowls for processing. One bowl included the uppermost 0.2 ft (0–0.2 ft) of dark grey sediment and the other bowl contained 0.2 ft of brown-orange sediment from 1.5–1.7 ft beneath the sediment surface. Each sample was homogenized thoroughly with a Teflon policeman and about 100 g was transferred into 1-L widemouth glass jars that had been baked to 450˚C. These samples were placed in an ice chest and shipped overnight to the USGS National Water Quality Laboratory for determination of selected polycyclic aromatic hydrocarbon compounds (table 4) following analytical procedures described by Arbogast (1996), Hageman (2007), Taggart (2002), and Briggs and Meier (1999). The sediment remaining in each bowl was sieved through precleaned 62.5-µm nylon mesh using a Teflon policeman and native water. About 10 g of the resulting fine-grained fraction was placed in 500-mL polypropylene widemouth jars and shipped to the USGS Branch of Geochemistry Laboratory in Lakewood, Colorado, for determination of selected metals, and inorganic and organic carbon (table 4), following analytical methods describe by Olson and others (2004). Laboratory reporting levels of each analyte are listed in tables 3 and 4. All bottom-sediment quality data are listed in appendix B.

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