Scientific Investigations Report 2008–5162
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5162
Version 1.1, December 2008
Median values of pH, specific conductance, dissolved oxygen, and temperature in ground water were 6.4 standard pH units, 450 μS/cm, 3.2 mg/L (34 percent saturation), and 9°C, respectively. Concentrations of dissolved solids in water samples collected from seven observation wells ranged from 150 mg/L for site MW194 to 390 mg/L for site MW197 and the average concentration was 300 mg/L (figs. 3 and 4A). Concentrations of major ions indicate that upgradient ground water tapped by MW194 is mixed cation (dominated by calcium) bicarbonate type water. Downgradient samples from wells near detention basin PA1 (wells MW196–MW200) have evolved to mixed cation (sodium and calcium)/mixed anion (mostly dominated by chloride) type water (Piper and others, 1953). This evolution is most likely due to the chemistry of recharged stormwater runoff that may be contaminated by deicing road and sidewalk treatments and residual leachate from abandoned septic-tank systems mixing with regional ground water. Decreased concentrations of dissolved oxygen and sulfate in ground-water samples collected downgradient of PA1 may be the result of activity of sulfate-reducing bacteria.
Concentrations of filtered nitrogen were generally largest in ground-water samples collected upgradient of the stormwater control system compared to unfiltered samples of stormwater and filtered ground-water samples collected downgradient of the stormwater control system (fig. 12A). In contrast, concentrations of filtered phosphorus were generally smallest in ground-water samples collected upgradient of the stormwater control system and largest in stormwater (fig. 12B). The maximum concentration of filtered phosphorus was measured in ground water collected downgradient of the stormwater control system (site MW199A; 420 µg/L).
Maximum concentrations of unfiltered nitrogen and phosphorus (table 6, fig. 13) are highest among the 22 samples of stormwater inflow to PA1. Maximum values for nitrogen (7,400 µg/L) and phosphorus (1,500 µg/L) were measured in a stormwater sample collected on March 2, 2007, after several inches of snow had accumulated. Air temperature was less than 0°C during the week prior to March 2, 2007, and only intermittent snowmelt runoff entered PA1 during this time (fig. 13A). Minimum values of unfiltered nitrogen (390 µg/L) and phosphorus (79 µg/L) were measured in a stormwater sample collected on April 18, 2006, after several inches of snow had melted for 6 days prior to sample collection (fig. 13B) and meltwater had flushed the stormwater-collection system. Although the study area typically receives less precipitation than Fallen Leaf Lake (fig. 1), data for the snow pillow at Fallen Leaf Lake, operated by the U.S. Department of Agriculture Natural Resources Conservation Service offered the closest representation of daily snowmelt near the shore of Lake Tahoe (Natural Resources Conservation Service, 2008 [http://www.wcc.nrcs.usda.gov/snow]). Median values of unfiltered nitrogen (2007: 1,300 µg/L; 2006: 870 µg/L) and phosphorus (2007: 470 µg/L; 2006: 240 µg/L) in stormwater samples were 1.5 and 2.0 times as large in 2007 as median values for 2006. Precipitation in 2006 was about 2.4 times the precipitation in 2007 (fig. 13C). The correlation between lower precipitation and higher nutrient concentrations indicates that increased runoff may dilute nutrient concentrations entering the detention basins. However, the total mass of nutrients delivered by stormwater to the Park Avenue detention basins may be similar from year to year.
Unfiltered concentrations in eight samples of stormwater outflow (seven from site PA1-out and one from PA2; table 6) had the smallest statistical-distribution variables (mean, median, maximum, and minimum) of nitrogen and phosphorus. Comparison of inflow and outflow concentrations indicates that about 55 percent of total nitrogen and 47 percent of total phosphorus may be retained in detention basins due to settling of suspended nutrients. However, filtered concentrations of nitrogen and phosphorus are larger in outflow samples than in detention-basin samples because outflow was sampled during stormwater-runoff events during water years 2006 and 2007 and samples of standing detention basin water were collected only during water year 2007 when precipitation was much less (fig. 13C). The term “water year” means a 12-month period beginning on October 1 and ending on September 30.
The largest mean concentrations of unfiltered nitrogen (table 6; 1,500 µg/L) and phosphorus (570 µg/L) in stormwater samples are for seven samples of standing detention basin water from PA1, but concentrations in filtered samples collected at the same time had the smallest mean concentrations of nitrate plus nitrite nitrogen (110 µg/L), ammonium nitrogen (17 µg/L), and phosphorus (49 µg/L). Relatively small concentrations of filtered nitrate plus nitrite, ammonium, and phosphorus indicate that these more biologically available nutrients are assimilated during photosynthesis by aquatic plants in PA1.
Ninety-five samples of ground water from 11 observation wells had the largest statistical-distribution variables (mean, median, maximum, and minimum) of filtered nitrogen concentrations and, except for the anomalous maximum concentration (420 µg/L), had the smallest distribution of phosphorus concentrations for all filtered water samples. Concentrations of filtered nitrogen and phosphorus in ground water averaged 1,100 and 39 µg/L, respectively. Nitrogen ranged from 82 to 7,700 µg/L and phosphorus ranged from 3 to 420 µg/L. Nitrogen concentrations measured in individual wells varied 380 µg/L (410–790 µg/L) for samples from site MW198A to 4,200 µg/L (200–4,400 µg/L) for samples from site MW201A (figs. 3 and 14A). The maximum nitrogen concentration from site MW201A was measured in a sample collected November 7, 2005. Phosphorus concentrations measured in individual wells varied 9 µg/L (13–22 µg/L) for samples from site MW198B to 410 µg/L (7–420 µg/L) from site MW199A (fig. 14B). The maximum phosphorus concentration was measured in a sample collected on May 9, 2006, from site MW199A. During well purging of almost 16 gal, the discharge water frothed as if it contained detergent or other surfactants, such as naturally occurring dissolved organic carbon. One other filtered sample from site MW199A had phosphorus concentration greater than 100 µg/L and six of the nine samples were 20 µg/L or less.
The isotopic composition of water, expressed as oxygen‑18 relative to oxygen‑16 (18O/16O) and deuterium relative to hydrogen‑1 (2H /1H), of local ground water was shown to be different from that of Lake Tahoe due to evaporative fractionation of lakewater that has an estimated residence time of 700 years (Thodal, 1997). By convention, each ratio is related mathematically to the comparable ratio for an international reference standard known as the Vienna Standard Mean Ocean Water (VSMOW) and expressed as “delta oxygen‑18” (δ18O) and “delta deuterium” (δ 2H); the units of measure are parts per thousand (abbreviated permil). A negative delta value indicates that the sample water is lighter isotopically than the standard (depleted). Evaporation preferentially removes the lighter isotopes (16O and 1H) as water vapor and the heavier isotopes (18O and 2H) remain in the liquid water. Figure 15 shows the relation of stable-isotope values for samples from Lake Tahoe, shallow ground water near the Park Avenue stormwater collection system, and two samples of interstitial water that fall along a linear mixing line (δ2H = 26.19+(5.59 (δ18O)) between lakewater and ground water. The meteoric water line (δ2H = 26.19+(5.59 (δ18O)); Craig, 1961) also is shown.
Eight water samples were collected from five nearshore locations in Lake Tahoe, July 26 and August 2, 2007, that averaged 5.2 permil δ18O (5.6 to 5.1 permil δ18O) and 55.9 permil δ2H (59.0 to 54.4 permil δ2H). One water sample collected from Lake Tahoe in 1980 was 5 permil δ18O and 56 permil δ2H. The five samples collected from the lakebed/lakewater interface averaged 5.2 permil δ18O and 55.9 permil δ2H compared to three samples collected 1 ft beneath lakewater surface that averaged 5.1 permil δ18O and 54.8 permil δ2H. The isotopic composition of 11 shallow ground-water samples averaged 13.94 permil δ18O (15.05 to 12.70 permil δ18O) and 104.0 permil δ2H (110.4 to 97.6 permil δ2H) and are comparable to 32 samples of ground water collected from wells and a spring in the Lake Tahoe Basin in 1990 that averaged 14 permil δ18O and 104 permil δ2H (Thodal, 1997). Isotopic compositions of two samples of interstitial water, collected 0.8 ft beneath the lakebed using a 0.5-in. diameter minipeizometer, were 13.85 permil δ18O; 102.5 permil δ2H and 9.76 permil δ18O; ‑80.8 permil δ2H. Both interstitial-water samples fall on a linear mixing line between lakewater and ground water, with the isotopic signature for the interstitial water sample collected from site L3 (fig. 3) falling in the middle of values measured in well-water samples (fig. 15).
One 2 ft core of bottom sediment was collected from PA1 (fig. 4A; site PA1) and divided into two samples for laboratory analyses to assess pollutant retention by the infiltration basin (tables B1 and B2). Laboratory determinations of selected chemicals of potential concern associated with urban stormwater runoff included chromium, copper, lead, mercury, nickel, organic carbon, phosphorus, and zinc. Sediment samples also were analyzed for selected polycyclic aromatic hydrocarbons (PAHs). These compounds are found in petroleum products and tar, and are produced by combustion of petroleum as well as by forest fires and wood-burning stoves (Smith and others, 1988, p. 64–67). Comparison of concentrations in surface sediment to those in sediment collected from depth provides a qualitative evaluation of the ability of the detention-basin sediment to retain contaminants, the potential for adverse environmental effects to wet basin ecology, and economic and regulatory considerations for contaminated-sediment disposal. Selected published sediment toxicity screening values also are provided for comparison with data from PA1 (table 7).
Concentrations of organic carbon, cadmium, copper, lead, mercury, nickel, phosphorus, sulfur, and zinc in the surface sample are all at least twice as large as concentrations in the deeper sample (6.4, 2.8, 3.2, 2.8, 3, 2.2, 3.2, more than 7.2 and 6.1 times, respectively), but chromium in the surface sample was only 1.1 times more than the deeper sample. Cadmium, copper, lead, nickel, and zinc are metals used in the fabrication of tires and brake linings (Hjortenkrans and others, 2007, p. 5224–5225). Other studies in the Lake Tahoe basin also demonstrate increased concentrations of metals in shallower sediments compared with deeper samples. Concentrations of lead and mercury in sediment core-samples collected from Lake Tahoe (Heyvaert and others, 2000) were 6 and 5 times larger, respectively, in samples estimated to have been deposited in the mid-20th century compared to sediment deposited prior to 1850, indicating regional atmospheric sources of these contaminants. Concentrations of 28 PAHs were all less than laboratory reporting limits in the deeper sample, but 15 compounds were quantified and the concentration of an additional compound, acenaphthalene, was estimated in the surficial sample.
No constituents measured in the deeper (1.5–1.7 ft) bottom-sediment sample exceeded concentrations for chemicals of potential concern for protection of benthic aquatic life, but concentrations in the surface sample (0–0.2 ft) exceeded the severe effect level for copper and the median effect, probable effect, and consensus-based probable effect levels for zinc. Probable effect levels also were exceeded for benz[a]anthracene (0.4 µg/g), phenanthrene (0.8 µg/g), and pyrene (1.0 µg/g; table 7) in the surficial bottom sediment sample.
Settling of suspended particles, accumulation of chemicals of potential concern, and biological assimilation of dissolved nutrients are the primary stormwater treatments achieved by the Park Avenue detention basins. Suspended particulates may include suspended micro-organisms (algae, bacteria, and fungi), organic detritus, and suspended inorganic sediment particles to which ammonium, phosphate, metals, and hydrophobic organic compounds have sorbed. Dissolved nutrients are available for biological assimilation by algae, bacteria, fungi, and aquatic vascular plants. Additionally, bacteria and fungi can decompose particulates to derive energy from carbon and assimilate nutrients.
Contributions of the species of phosphorus (suspended phosphorus and filtered orthophosphate and hydrolyzable phosphorus) relative to total concentrations indicate that most of the phosphorus is associated with particles that are larger than 0.45 µm (nominal pore size of cartridge filter; fig. 16). Suspended phosphorus, estimated as the difference between unfiltered and filtered phosphorus values, had mean concentrations that averaged 67 percent of the total phosphorus (range: 26–94 percent). Suspended phosphorus in samples from detention basin PA1 averaged 86 percent of total phosphorus (range: 71–96 percent) and outflow samples averaged 55 percent of total phosphorus (range: 16–74 percent). The phosphorus concentration (1,400 µg/g) in the surface sample of bottom sediment supports the observation that about half of unfiltered phosphorus in stormwater inflow to detention basin PA1 settles out and is retained by the stormwater-control system. However, assuming that the phosphorus concentration in one sample of bottom sediment is representative of sediment throughout detention basin PA1, about 50 lb of phosphorus has accumulated in the top 0.2 ft of sediment in PA1 while almost 200 lb of phosphorus is estimated to be associated with inflow to PA1.
Suspended plus organic nitrogen in six samples from detention basin PA1 were 96 percent of total nitrogen (range: 91–99 percent) but suspended plus organic nitrogen in seven outflow samples averaged 73 percent of total nitrogen (range: 54–90 percent). However, organic nitrogen was not determined in filtered stormwater samples. Suspended plus organic nitrogen, estimated by subtracting mean filtered concentrations of ammonium and nitrate plus nitrite from total nitrogen, measured in 22 samples of inflowing stormwater averaged almost 78 percent of the total nitrogen (range: 8–97 percent). Concentrations of filtered nitrogen and phosphorus probably are decreased due to photosynthesis and assimilation by suspended biomass (algae, bacteria, and fungi) as well as by attached aquatic vegetation. However, because concentrations of filtered organic nitrogen in stormwater samples were not determined, it is not known how much of the total nitrogen was suspended nitrogen and how much was filterable organic nitrogen.
Concentrations of filtered nitrogen (440 µg/L) and phosphorus (20 µg/L) in ground-water samples from site MW194 (upgradient of most development) predominately are organic nitrogen and hydrolyzable phosphorus (fig. 15). The mean concentration of nitrogen is larger than values for samples from three other wells and phosphorus concentration is larger than values for two other wells. Nutrient-enriched surface runoff in the Lake Tahoe basin has been attributed to accumulation of forest litter due to fire suppression (Miller and others, 2005). Recharge from an intermittent stream near this site may be the source of nitrogen and phosphorus in samples from this well.
Sites MW196 and MW197 are upgradient of detention basin PA1 and have the two largest mean concentrations of filtered nitrogen (2,000 µg/L and 2,700 µg/L, respectively) that are more than 90 percent nitrate plus nitrite. This indicates that nitrate contamination of the regional shallow aquifer, possibly by past wastewater-disposal practices, continues to persist since its early recognition in low-flow stream samples (Perkins and others, 1975) and ground-water monitoring results (Thodal, 1997). Filtered concentrations of phosphorus (19 µg/L and 23 µg/L) are comparable to concentrations for site MW194, with hydrolyzable phosphorus slightly more dominant.
Filtered nitrogen concentrations in samples from sites immediately downgradient of detention basin PA1 average much less than concentrations in the two upgradient sites, with ammonium and organic nitrogen accounting for 84–99 percent of the average filtered nitrogen concentrations. This indicates that a recharge mound beneath detention basin PA1 has displaced the regional ground water with infiltrated stormwater with a lower mean concentration of nitrate (44 µg/L) that also has diluted the high nitrate concentrations and introduced elevated concentrations of ammonium and organic nitrogen. Mean concentrations of filtered phosphorus for the four downgradient wells (17–24 µg/L) are comparable to the concentrations in upgradient wells (19–23 µg/L) with hydrolyzable phosphorus contributing more than 70 percent of the filtered phosphorus. However, the mean phosphorus concentration for site MW199A (fig. 3) was 72 µg/L and only one sample of the nine collected from site MW199A had orthophosphate accounting for more than 40 percent of filtered phosphorus. The anomalous sample collected on May 9, 2006, had 420 µg/L of filtered phosphorus with 90 percent as orthophosphate. The mean phosphorus concentration for site MW200 was 39 µg/L, but seven of nine samples were less than 30 µg/L. The largest concentrations of filtered phosphorus (260 µg/L) and filtered orthophosphate (240 µg/L) in the outflow from detention basin PA1 was in a sample collected on May 10, 2006. The second largest concentration of filtered phosphorus (84 µg/L) and filtered orthophosphate (68 µg/L) was measured in samples from site MW200 and also was collected on May 10, 2006. MW199A has the largest measurement of hydraulic conductivity (20 ft/d; table 2). Attempts to slug test at site MW200 were unsuccessful because the stressed water level recovered too quickly for quantification. The rapid recovery indicates that the hydraulic conductivity of the aquifer material tapped at site MW200 is greater than 20 ft/d, and may represent localized deposits that permit preferential flow of stormwater to ground water.
Concentrations of filtered nitrogen (560–1,300 µg/L) in samples from sites downgradient of detention basin PA2 (sites MW201A, MW201B, and MW202) are dominated by nitrate (63–92 percent) because detention basin PA2 only received stormwater inflow during flooding due to a warm rain-on-snow events that spanned December 17, 2005, through January 7, 2006. Therefore, nitrate in the shallow regional ground water is only diluted and displaced intermittently. Sites MW201A and MW202 are completed at 13 ft below land surface and site MW201B was completed at 23 ft. Samples from the deeper well had a mean filtered nitrogen concentration of 1,300 µg/L compared to 890 µg/L for site MW201A and 560 µg/L for site MW202. Mean concentrations of filtered phosphorus (29–120 µg/L) for these three wells are all at least 69 percent orthophosphate and are among the five largest mean concentrations.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubs.usgs.gov/sir/2008/5162
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