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Scientific Investigations Report 2005-5260

Changes in Ground-Water Flow and Chemistry after Completion of Cattlemans Detention Basin, South Lake Tahoe, California--November 2001 to November 2003

By David E. Prudic, Jena M. Green, James L. Wood, and Katherine K. Henkelman

Version 1.0

ABSTRACT

A study of the Cattlemans detention basin in South Lake Tahoe, California began in November 2000 to evaluate the effectiveness of the detention basin in reducing sediment and nutrient loads from urban runoff to nearby Cold Creek. Detention basins commonly are used to minimize sediment and nutrient loads from urban runoff in the Lake Tahoe Basin, but the effects of these detention basins on changes in ground-water flow and chemistry are largely unknown. This report summarizes changes in ground-water flow and chemistry for two years after completion of Cattlemans detention basin in October 2001. The report includes a comparison of dissolved nutrient loads from ground-water discharge to Cold Creek below the detention basin with dissolved loads in Cold Creek.

Ground-water flow is through sand and gravel lenses within unconsolidated deposits in the vicinity of the detention basin. Ground water generally is less than 10 feet below land surface and is within meadow deposits consisting of gray silt and sand with lenses of sand and gravel. Ground-water levels rose during late winter and early spring and declined during the summer and fall. The general direction of ground-water flow remained persistent from east to west across the detention basin even when runoff into and precipitation on the detention basin caused an increase in ground-water levels during late winter and spring. A consistent downward gradient in water levels between paired wells on the west edge of the detention basin compared with generally upward gradients from the middle to the eastern edge of the detention basin indicates an abrupt thickening of the unconsolidated deposits beneath the western part of the detention basin. This downward flow into the unconsolidated deposits allows some of the ground water beneath the western part of the detention basin to flow underneath Cold Creek; however, a consistent gradient along the west side of the detention basin to a man-made meander on Cold Creek has focused ground-water discharge along the meander. The annual mean ground-water discharge along this meander was two to three orders of magnitude less than the annual mean discharge of Cold Creek during water years 2002 and 2003.

The chemical composition of ground water for a two-year period after completion of the detention basin did not change substantially from the chemical composition of ground water before completion of the detention basin. The principal dissolved cations remained sodium, iron, and calcium, and the principal dissolved anions were bicarbonate and chloride. Nitrogen concentrations in ground water from the meadow deposits also did not change substantially after completion of the detention basin. Ammonia remained the most common form of nitrogen and ranged from 0.001 to 15 mg/L. Nitrate plus nitrite concentrations were always less than 0.33 mg/L. The mean and median concentrations of dissolved phosphorus, orthophosphate, and organic carbon increased after completion, although the range in the concentrations did not substantially change.

Dissolved nitrogen, phosphorus, iron, and organic carbon loads from ground-water discharge along the meander below the detention basin were estimated by multiplying the range in annual ground-water discharge with annual mean concentrations in ground water immediately next to Cold Creek. Although dissolved nitrogen and phosphorus concentrations were 10 times greater in the ground water than dissolved concentrations in Cold Creek, the much greater discharge of Cold Creek compared with the discharge of ground water resulted in estimated loads from ground water of two orders magnitude less than the estimated annual loads in Cold Creek. Because dissolved iron concentrations in ground water were 250 times higher than in Cold Creek, the dissolved iron load from ground-water discharge is a larger fraction of the dissolved iron load in Cold Creek, which is consistent with the increase in measured dissolved iron concentrations in Cold Creek from above to below the detention basin.

Contents

Abstract

Introduction

Purpose and Scope

Acknowledgments

Abstract

Introduction

Purpose and Scope

Acknowledgments

Description of Cold Creek and Cattlemans Detention Basin

Methods

Ground Water

Cold Creek

Water-Quality Sampling Procedures

Analyses of Water Samples

Ground Water

Principal Aquifer

Seasonal Trends in Water Levels

Changes in Direction of Flow

Changes in Flow to Cold Creek

Changes in Ground-Water Quality

Chemical Composition

Nutrient Concentrations

Cold Creek

Chemical Composition

Nutrient Concentrations

Nutrient Loads

Summary and Conclusions

References Cited

Figures

Figure 1. Location of Lake Tahoe and study area adjacent to Cold Creek, South Lake Tahoe,
California

Figure 2. Location of wells, storm drains, and Cold Creek in relation to Cattlemans detention
basin at the end of Cattlemans Court, South Lake Tahoe, California

Figure 3. Distribution of unconsolidated deposits beneath Cattlemans detention basin.

Figure 4. Trends in ground-water levels from January 2001 through November 2003 (A) along eastern side, (B) along northern side, (C) along southern side, and (D) along western side of Cattlemans detention basin, and (E) in meadow distant from detention basin

Figure 5. Daily mean water levels in wells with recording pressure transducers near Cattlemans detention basin from January 2001 through November 2003

Figure 6. Contours of ground-water levels at Cattlemans detention basin for selected dates.

Figure 7. Ground-water gradients and ground-water discharge near Cattlemans detention basin

Figure 8. Comparison of daily mean ground-water discharge to Cold Creek below Cattlemans detention basin near well cc14 with daily mean discharge of Cold Creek at Pioneer Trail, South Lake Tahoe, California

Figure 9. Changes in concentrations of selected dissolved constituents in ground water from
shallow wells before completion (January 2001 to November 2001) and for two years
after completion of Cattlemans detention basin (December 2001 to November 2003)

Figure 10. Trends in concentrations of selected dissolved constituents in ground water from shallow wells (A) cc3S, (B) cc8S, (C) cc13S, and (D) cc14 at Cattlemans detention basin from January 2001 to November 2003

Figure 11. Changes in concentrations of dissolved nitrogen, phosphorus, and iron in ground water from shallow wells next to Cattlemans detention basin, next to Cold Creek, and in meadow before completion (January 2001 to November 2001) and for two years after completion of detention basin (December 2001 to November 2003)

Figure 12. Trends in concentrations of dissolved nutrients in ground water from shallow wells (A) cc3S, (B) cc8S, (C) cc13S, (D) cc14,(E) cc16, and (F) cc21 at Cattlemans detention basin from January 2001 to November 2003

Figure 13. Trends in concentrations of dissolved constituents in Cold Creek above and below Cattlemans detention basin from September 2001 to September 2003.

Figure 14. Trends in concentrations of dissolved nutrients in Cold Creek above and below Cattlemans detention basin from September 2001 to September 2003.

Tables

Table 1. rmy Well #1Table 1. Comparison of dissolved inorganic constituents and properties in ground water from shallow wells sampled before completion of Cattlemans detention basin (January to November 2001) with ground water sampled for two years after completion of detention
basin

Table 2. Comparison of dissolved inorganic constituents in ground water from deeper wells sampled before completion of Cattlemans detention basin (January to November 2001) with ground water sampled for two years after completion of detention basin (December 2001 to November 2003)

Table 3. Concentrations of dissolved inorganic trace elements in ground water sampled from shallow wells before completion of Cattlemans detention basin (January to November 2001) and ground water sampled for one year after completion of detention basin
(December 2001 to November 2002)

Table 4. Comparison of dissolved nutrient concentrations in ground water from shallow wells sampled before completion of Cattlemans detention basin (January to November 2001) with ground water sampled for two years after completion of detention basin (December 2001 to November 2003)

Table 5. Comparison of dissolved nutrient concentrations in ground water from deeper wells sampled before completion of Cattlemans detention basin (January to November 2001) with ground water sampled for two years after completion of detention basin (December 2001 to November 2003)

Table 6. Comparison of dissolved constituents and properties in water from Cold Creek above and below detention basin

Table 7. Estimates of dissolved nitrogen, phosphorus, iron, and organic carbon loads from ground-water discharge to Cold Creek below detention basin for water years 2001– 2003

Table 8. Comparison of nitrogen, phosphorus, iron, and organic carbon loads from ground- water discharge to Cold Creek below detention basin with dissolved loads in Cold Creek above detention basin for water years 2002 and 2003

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