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| Scientific Investigations Report 2006-5259 |
Prepared in cooperation with the El Dorado County Department of Transportation, Tahoe Engineering Unit and California Tahoe Conservancy
Edited by Jena M. Green
Lake Tahoe (Nevada-California) has been designated as an “outstanding national water resource” by the U.S. Environmental Protection Agency, in part, for its exceptional clarity. Water clarity in Lake Tahoe, however, has been declining at a rate of about one foot per year for more than 35 years. To decrease the amount of sediment and nutrients delivered to the lake by way of alpine streams, wetlands and stormwater detention basins have been installed at several locations around the lake. Although an improvement in stormwater and snowmelt runoff quality has been measured, the effectiveness of the detention basins for increasing the clarity of Lake Tahoe needs further study. It is possible that poor ground-water quality conditions exist beneath the detention basins and adjacent wetlands and that the presence of the basins has altered ground-water flow paths to nearby streams. A hydrogeochemical and ground-water flow modeling study was done at Cattlemans detention basin, situated adjacent to Cold Creek, a tributary to Lake Tahoe, to determine whether the focusing of storm and snowmelt runoff into a confined area has (1) modified the ground-water flow system beneath the detention basin and affected transport of sediment and nutrients to nearby streams and (2) provided an increased source of solutes which has changed the distribution of nutrients and affected nutrient transport rates beneath the basin.
Results of slug tests and ground-water flow modeling suggest that ground water flows unrestricted northwest across the detention basin through the meadow. The modeling also indicates that seasonal flow patterns and flow direction remain similar from year to year under transient conditions. Model results imply that about 34 percent (0.004 ft3/s) of the total ground water within the model area originates from the detention basin. Of the 0.004 ft3/s, about 45 percent discharges to Cold Creek within the modeled area downstream of the detention basin. The remaining 55 percent of ground water is either consumed by evapotranspiration, is discharged to Cold Creek outside the modeled area downstream of the detention basin, or is discharged directly to Lake Tahoe. Of the 45 percent discharging to Cold Creek, about 9 percent enters directly downstream of the detention basin and 36 percent enters further downstream.
Geochemical and microbial data suggest that a seasonal variation of chemical constituents and microbe population size is present at most wells. The geochemical data also indicate that construction of Cattlemans detention basin has not substantially changed the composition of the ground water in the area. High concentrations of ammonia, iron, and dissolved organic carbon, low concentrations of sulfate and nitrate, and large populations of sulfate-reducing microbes imply that the major geochemical process controlling nutrient concentrations beneath the detention basin is sulfate reduction. High concentrations of total nitrogen indicate that oxidation of organic carbon is a second important geochemical process occurring beneath the basin. The influx of surface runoff during spring 2002 apparently provided sufficient oxidized organic carbon to produce iron-reducing conditions and an increase in reduced iron, sulfate, and iron-reducing microorganisms. The increase in recharge of oxygenated water to the ground water system beneath the basin in future intervals of increased recharge may eventually redistribute nutrients and speed up transport of dissolved nutrients from the ground water system to Cold Creek.
Abstract
Introduction
Purpose and Scope
Acknowledgments
Cattlemans Detention Basin Site Description
Well Construction
Aquifer Characteristics
Methods
Ground Water
Cold Creek
Water-Quality Sampling Procedures
Analyses of Water Samples
Core Collection and Ground-Water Sampling
Microbial Sampling
Chemical Analyses
Simulation of Ground-Water Flow by Jena M. Green and Keith J. Halford
Flow Model Description
Boundary Conditions
Initial Conditions
Model Calibration
Model Results
Model Sensitivity
Model and Data Limitations
Water Quality in and near Cattlemans Detention Basin by Michael S. Lico
Chemical Composition of Water
Cold Creek
Ground Water
Stormwater
Precipitation
Effect of Detention Basin on Water Quality
General Chemical Composition
Spatial Changes in Water Quality
Temporal Changes in Water Quality
Processes Affecting Water Quality near the Detention Basin
Changes in Microbial Activity by Ean Warren, David E. Prudic, and E. Michael Godsy
Concentrations of Microbial Populations
Microbial Populations and Redox Conditions
Effects of Stormwater Infiltration on Microbial Populations
Summary and Conclusions
References Cited
Appendix A. Description of deposits from boreholes drilled in the vicinity of Cattlemans
detention basin, South Lake Tahoe, California, November-December 2000 (in PDF)
Appendix B. Measured water levels for monitoring wells in the vicinity of Cattlemans detention basin, South Lake Tahoe, California, water year 2001–05, in feet above mean sea level (in PDF)
Appendix C. Water-Quality Data -- Appendix C is provided as an Excel spreadsheet (.xls)
1. Map showing Cattlemans detention basin in the Cold Creek watershed and Lake Tahoe, California and Nevada
2. Aerial photograph (August 10, 2000) showing historic Cold Creek channel changes
3. Map showing Cattlemans detention basin study area, South Lake Tahoe, California
4. Schematic diagram showing general construction of observation wells used in study of Cattlemans detention basin, South Lake Tahoe, California
5. Vertical cross section through Cattlemans detention basin showing distribution of alluvial deposits, South Lake Tahoe, California
6. Distribution of horizontal hydraulic conductivity near Cattlemans detention basin, South Lake Tahoe, California
7. Map showing locations of temporary microbial sampling sites at Cattlemans detention basin, South Lake Tahoe, California
8. Cross section showing sampling intervals for temporary microbial sampling sites in relation to Cattlemans detention basin and nearby wells
9. Schematic depicting A, ground-water model layer one with boundary conditions; B, ground-water model vertical discretization
10. Graph depicting A, slope analysis of cc03S hydrograph method; B, cumulative stage rises from precipitation events method; and C, feet of calculated monthly recharge based on cumulative stage rises method
11. Hydrographs showing examples of simulated head values that A, underestimate; B, fit well with; and C, underestimate peaks with measured head values
12. Graph of cumulative percent of absolute difference versus absolute difference between simulated and measured water-level measurements
13. Plots of simulated ground-water levels in the vicinity of Cattlemans detention basin, South Lake Tahoe, California
14. Graphs showing simulated ground-water level contour comparison of 2002 (dry) and 2003 (wet). A, March 2002, stress period 75; B, August 2002, stress period 80; C, November 2002, stress period 83; D, March 2003, stress period 87; E, August 2003, stress period 92; and F, November 2003, stress period 95
15. Model sensitivity to independent changes in selected calibration parameters
16. Equivalents diagram showing the ionic composition of ground water, stormwater, water in Cold Creek, and precipitation in samples collected from Cattlemans detention basin study area, South Lake Tahoe, California
17. Boxplot showing the distribution of specific conductance values in Cold Creek, stormwater and ground-water samples from Cattlemans detention basin study area, South Lake Tahoe, California
18. Boxplot showing chloride concentrations in runoff to Cattlemans detention basin, South Lake Tahoe, California.
19. Boxplot showing dissolved iron concentrations in water samples collected from Cattlemans detention basin, South Lake Tahoe, California
20. Concentrations of major ions, iron, and dissolved organic carbon in ground-water samples collected from January 2001 to June 2005 from wells in Cattlemans detention basin study area, South Lake Tahoe, California.
21. Concentrations of nutrients in ground-water samples collected from January 2001 to June 2005 from wells in Cattlemans detention basin study area, South Lake Tahoe California.
22. Boxplot of dissolved sulfate concentrations in water samples from the Cattlemans detention basin, South Lake Tahoe, California
23. Graphs showing concentrations of attached and suspended aerobic microorganisms on log scale
24. Graphs showing concentrations of attached and suspended denitrifying microorganisms on log scale
25. Graphs showing concentrations of attached and suspended fermenting microorganisms on log scale
26. Graphs showing concentrations of attached and suspended iron-reducing microorganisms on log scale
27. Graphs showing concentrations of attached and suspended sulfate-reducing microorganisms on log scale
28. Graphs showing concentrations of attached and suspended methanogenic microorganisms on log scale
29. Concentrations on log scale of attached microorganisms on shallow sediment during the course of the study
30. Graphs showing trends in sulfate concentrations in water sampled from wells along transect A-A´
31. Graphs showing trends in iron concentrations in water sampled from wells along transect A-A´
32. Graphs showing trends in ammonia concentrations in water sampled from wells along transect A-A´
1. Well name, land-surface altitude and construction data from wells at Cattlemans detention basin
2. Sample dates, land-surface altitudes, depths of cores and water samples below land surface, and lithologic descriptions from temporary microbial sampling sites, Cattlemans detention basin, South Lake Tahoe, California
3. Percentage of the total thickness assigned to each ground-water model layer
4. Grouping of recharge parameters as observed by SNOTEL data
5. Average daily evapotranspiration rates in the vicinity of Cattlemans detention basin, South Lake Tahoe, California
6. Simulated ground-water flow model parameters and corresponding estimates
7. MODPATH results showing the percentage of ground water from Cattlemans detention basin that discharges to Cold Creek
8. Alternative ground-water flow models
9. Ground-water model results from selected simulations as compared to the preferred run
10. Median concentrations, range of concentrations, and number of observations of nutrients in the study area, 2001–05
11. Concentrations of attached and suspended aerobic microorganisms from temporary sampling sites at Cattlemans detention basin, South Lake Tahoe, California, 2001–04
12. Concentrations of attached and suspended denitrifying microorganisms from temporary sampling sites at Cattlemans detention basin, South Lake Tahoe, California, 2001–04
13. Concentrations of attached and suspended fermenting microorganisms from temporary sampling sites at Cattlemans detention basin, South Lake Tahoe, California, 2001–04
14. Concentrations of attached and suspended iron-reducing microorganisms from temporary sampling sites at Cattlemans detention basin, South Lake Tahoe, California, 2001–04
15. Concentrations of attached and suspended sulfate-reducing microorganisms from temporary sampling sites at Cattlemans detention basin, South Lake Tahoe, California, 2001–04
16. Concentrations of attached and suspended methanogenic microorganisms from temporary sampling sites at Cattlemans detention basin, South Lake Tahoe, California, 2001–04
17. Concentrations of dissolved inorganic constituents in water samples collected from temporary sampling sites at Cattlemans detention basin, South Lake Tahoe, California, 2001–04
18. Concentrations of dissolved nitrogen, phosphorus, and carbon compounds in water samples collected from temporary sampling sites at Cattlemans detention basin, South Lake Tahoe, California, 2001–04
19. Concentrations of ammonium, nitrate, and Olsen phosphorus on deposits collected from temporary sampling sites at Cattlemans detention basin, South Lake Tahoe, California, 2001–04
Send questions or comments about this report to the author, Jena M. Green
For more information about USGS activities in Nevada, visit the USGS Nevada Water Science Center home page.
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Page Last Modified: Thursday, 01-Dec-2016 19:22:38 EST