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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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Summary and Conclusions

Clarity of Lake Tahoe, California and Nevada, has been decreasing, in part due to inflows of sediment and nutrients associated with stormwater runoff. Constructed stormwater detention basins are considered effective best management practices for mitigation of suspended sediment and nutrients associated with runoff, but consequences of infiltrated stormwater to shallow ground water are not known. This report documents 2005–07 hydrogeologic conditions in a small part of a shallow aquifer and how it interacts with a stormwater-control system and with nearby Lake Tahoe. Descriptions of the basin-fill aquifer and a stormwater-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, numerical model also are presented, coupled with chemical data to evaluate responses of ground-water flow to stormwater runoff accumulation in the stormwater-control system.

Ground-water flow in the basin-fill aquifer was modeled using five layers of constant thickness (5, 10, 15, 10 and 10 ft thick, respectively), from land surface to 50 ft and a sixth layer that varied in thickness from the bottom of layer 5 to bedrock. The large and varying thickness of this deepest layer is assumed to have no effect on the simulation of shallow ground-water movement. Information indicates lacustrine layers may be interspersed within sand and gravel deposits, but enough evidence was not available to support a continuous confining layer in this model. Bedrock underlying layer 6 represents a basal no-flow boundary and boundaries to the northeast and southwest were aligned along hydrographic basin boundaries and were represented as no-flow boundaries. Model cells exposed to Lake Tahoe in the northwest were simulated using a constant-head boundary of 6,225 ft, which is the average stage of Lake Tahoe. The southern Park Avenue detention basin also was simulated using a constant-head boundary because it was never observed to completely drain.

Model layers 1–5 were assigned hydraulic conductivity values of 2 ft/d for the simulations. Recharge was applied to the modeled area as precipitation (areal recharge) and mountain-front recharge. Areal recharge was estimated during model calibration and applied to about 62 percent of layer 1 model cells that were determined not to be impervious. Mountain-front recharge was applied to all cells along the southeast boundary, as well as two zones that extend downgradient from the mountain front. Annual precipitation averaged 4,400 acre-ft in the watersheds upgradient of the model area between 1971 and 2000, of which 2 percent was estimated to be in-place recharge, 54 percent is consumed by evapotranspiration, and the remaining 44 percent is attributed to potential runoff. The percentage of the potential runoff that actually becomes recharge was estimated during model calibration.

A network of stormwater drainage ditches and sewers scattered across South Lake Tahoe were simulated as drains. These drains were placed 5 ft below land surface. In this way, if the simulated ground-water surface came within 5 ft of land surface, ground water would be able to discharge to the stormwater ditches, and then directly to Lake Tahoe.

The steady-state model was calibrated to water-level measurements in 18 wells and a mean ground-water discharge from the model domain was estimated to be 256 acre-ft/yr to Lake Tahoe. About 0.61 acre-ft/yr infiltrates from detention basin PA1 to ground water and particle tracking indicated 25 percent (0.15 acre-ft/yr) of this infiltration ultimately discharges to Lake Tahoe within 60 ft of the shoreline. The remaining 0.46 acre-ft/yr discharged to local stormdrains that convey water to nearby wet meadows and directly to Lake Tahoe, depending on diversion-dam configuration.

Settling of suspended nutrients and sediment, biological assimilation of dissolved nutrients, and accumulation of chemicals of potential concern are the primary stormwater treatments achieved by the detention basins. Comparison of mean concentrations of unfiltered nitrogen and phosphorus in stormwater samples indicate that 55 percent of nitrogen and 47 percent of phosphorus is trapped in the detention basin. Cadmium, copper, lead, mercury, nickel, organic carbon, phosphorus, and zinc in the uppermost 0.2 ft of bottom sediment from the detention basin were all at least twice as concentrated compared to sediment collected from 1.5 ft deeper. Similarly, concentrations of 28 polycyclic aromatic hydrocarbon compounds were all less than laboratory reporting limits in the deeper sample, but 15 compounds were measured in the uppermost 0.2 ft of bottom sediment. Published concentrations determined to affect benthic aquatic life were exceeded for benz[a]anthracene, copper, phenanthrene, pyrene, and zinc in the uppermost 0.2 ft of bottom sediment.

Concentrations of filtered major ions indicate that upgradient ground water is a mixed cation-bicarbonate type with 152 mg/L total solutes that evolves to a mixed cation/mixed anion, sodium-chloride type and sodium-bicarbonate type waters with up to 390 mg/L total solutes, possibly due to residual leachate from abandoned septic-tank systems and recharged stormwater runoff. Concentrations of filtered nitrogen ranged from 82 to 4,600 µg/L and phosphorus ranged from 4 to 420 µg/L. Coupling mean concentrations of phosphorus (39 µg/L) and nitrogen (1,100 µg/L) with the steady-state ground-water flow model yields annual estimates of 26 lb of phosphorus and 770 lb of nitrogen that may be transported with ground water to Lake Tahoe from the modeled area (1.5 percent of the total area that is tributary to the lake).

The isotopic composition of water, expressed as ratios of oxygen‑18 relative to oxygen‑16 (delta oxygen‑18) and deuterium relative to hydrogen-1 (delta deuterium), of local ground water is different from that of Lake Tahoe due to evaporative fractionation of lake water that has an estimated residence time of 700 years. Comparison of delta oxygen‑18 and delta deuterium ratios for samples of shallow ground water, lake water, and interstitial water from Lake Tahoe indicates the lake water was well mixed with a slight ground-water signature in two of five lake-water samples collected near the lakebed. One of two interstitial water samples from 0.8 ft beneath the lakebed was nearly all ground water and concentrations of nitrogen and phosphorus were comparable to concentrations in shallow ground-water samples. The other interstitial sample fell along a mixing line between ground water and lake water, and nutrient concentrations appeared diluted with lake water. Nitrate was less than laboratory reporting levels in both interstitial samples, indicating a dissimilative nitrate reduction to ammonium by micro-organisms. Based on average nitrogen and phosphorus concentrations of interstitial-water samples and ground-water discharge to Lake Tahoe directly from detention basin PA1, it is estimated that PA1 contributes loads of less than 0.3 lb of nitrogen and less than 0.1 lb of phosphorus per year.

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