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

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Ground-Water Discharge and Nutrient Loading to Lake Tahoe

In order to estimate the load of nutrients discharged to Lake Tahoe directly from detention basin PA1, an estimate of the amount of stormwater infiltrated from the detention basin was needed. Detention basin PA1 was not instrumented to record basin outflow; therefore, a calibrated-infiltration rate from the ground-water flow model of 73 ft³/d was used. Thus, an estimated 18 ft³/d of stormwater that infiltrated through the bed of detention basin PA1 discharges to Lake Tahoe based on particle tracking results of 25 percent direct discharge. The remaining 55 ft³/d (0.46 acre-ft/yr) of infiltrated stormwater discharged to stormdrains that discharge into Lake Tahoe by way of a wet meadow or was evapotranspired by riparian vegetation. The volume of infiltrated stormwater that discharges to Lake Tahoe as ground water represents about 0.1 percent (0.15 acre-ft/yr) of the total volume of ground-water discharge from the study area (256 acre-ft/yr). Assuming a mean filtered nitrogen concentration in ground water of 1,100 µg/L and filtered phosphorus concentration of 39 µg/L, it is estimated that ground water within the study area contributes 765 lb of nitrogen and 27 lb of phosphorus each year to Lake Tahoe and infiltrated stormwater from detention basin PA1 contributes less than 0.45 lb of the nitrogen load and less than 0.1 lb of the phosphorus load to the lake.

The distribution of ground-water discharge across the lake sediment is important because heterogeneity of aquifer permeability can “focus” ground-water discharge, such as ground-water discharge to springs. The calibrated ground-water flow model was used to calculate zones of discharge to the lake (fig. 9). About 41 acre-ft/yr (16 percent) of total ground-water discharge to Lake Tahoe occurs within about 60 ft of the shoreline. Approximately 55 acre-ft/yr also discharges in a zone between 60 ft and 600 ft offshore.

Particle tracking was used to observe where infiltrated stormwater into the Park Avenue detention basins would discharge. Nearly 9,000 particles were placed in model cells representing detention basin PA1. The number of particles placed in each model cell was proportional to the ground-water flux through that particular cell. Results indicate that 75 percent of infiltrated stormwater discharges to nearby stormdrains northwest of detention basin PA1 while the remaining 25 percent discharges to Lake Tahoe within 60 ft of the shoreline. Ground water discharged to stormdrains is conveyed to nearby wet meadows and directly to Lake Tahoe depending on diversion-dam configuration.

Other methods were attempted to determine locations of ground-water discharge to Lake Tahoe. Differences in the temperature and electrical conductivity of ground water compared to the receiving lake water have been used to locate submerged ground-water discharge for subsequent measurement of limnologic responses (Lee, 1985). Paired thermocouples attached to a data logger were dragged behind a boat along the Lake Tahoe shoreline, but wave action and air-temperature variations affected the data logger performance, preventing useful results.

An attempt was made to apply Raman Spectra fiber optic distributed temperature sensing technology in collaboration with researchers from the University of Nevada, Reno; Oregon State University; and others in June 2007. The system precisely measures temperature (±0.05°C) along a 3,000 ft length of standard optical communication cable with 3-ft spatial resolution (Hausner and others, 2007). Unfortunately, the equipment was available only during early June 2007 when a cold front moved in with wind and snow that again obscured any differences in lakebed temperatures.

On July 26 and August 2, 2007, a multiparameter water-quality probe was again dragged along the nearshore lakebed. However, no variations in temperature or electrical conductivity were observed. Thick mats of attached algae were observed in a linear pattern parallel to the lakeshore that indicated focused ground-water discharge. A seepage meter and minipiezometer were inserted into the lakebed sediment, and lake level and intercepted ground-water levels were compared using transparent tubing. Two samples of interstitial water and eight samples of lake water were collected for laboratory determination of stable isotopes of hydrogen and oxygen and for filtered concentrations of nitrogen and phosphorus.

Three lake samples were collected 1 ft beneath the lake surface and five were collected at the lakebed-water contact. Relations between the two isotopic ratios (fig. 14) indicate that the lake water was well mixed with a slight ground-water signature in the samples collected at site L4B, near the lakebed (table A7). One interstitial water sample was nearly all ground water (site L3C) and the other (site L1C) falls along a linear mixing line between ground water and lake water. Filtered concentrations of nitrogen and phosphorus were less than laboratory reporting limits for all lake water samples except for one sample collected near the lakebed (site L3B) that had 71 µg/L of nitrogen. The sample of interstitial water indicated a mixture of lake and ground water (site L1C). This sample had 143 µg/L of nitrogen and 36 µg/L phosphorus comprised mostly of organic nitrogen and orthophosphate, respectively. The other sample (site L3C) had 720 µg/L of filtered nitrogen comprised of 65 percent ammonium, 35 percent organic nitrogen and detectable nitrite (3 µg/L), and 40 µg/L filtered phosphorus comprised of 72 percent orthophosphate and 28 percent hydrolyzable phosphorus. Filtered nitrogen concentrations in ground-water samples collected from wells averaged 1,000 µg/L with nitrate representing nearly 70 percent of the concentration and ammonium only 10 percent. Nitrate was less than reporting levels in the interstitial water (estimated as 10 µg/L from site L1C) indicating a dissimilative nitrate reduction to ammonium by sediment micro-organisms (Sørensen, 1978).

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