Bijou Ground Water

Lake Tahoe is an outstanding natural resource known for its deep, clear water. Protection of its renowned clarity has become important in the past half century, as clarity has been decreasing by about 1.0 ft each year. Decreased clarity has been attributed to human activities that increase nutrient and sediment inputs to the lake, but in spite of numerous projects implemented to reduce quantities of sediment and nutrients, including exportation of all waste-water effluent, lake clarity has continued to decline. Nutrients are believed to enter the lake from streams, atmospheric deposition, intervening areas, shoreline erosion, and by ground-water inflow. Estimates of nutrient inputs to Lake Tahoe by ground water have been made using regionalized values of hydraulic properties coupled with averaged nutrient concentrations and several near-shore areas of Lake Tahoe have been identified as having elevated turbidity and algal production that are consistently elevated compared to the mid-lake. Although deposition of atmospheric nitrogen is estimated to contribute most of this nutrient and may be responsible for decline of lake-wide clarity, near-shore clarity losses may be caused by local influences, including nutrient-enriched ground-water discharge.

Basins constructed to collect stormwater runoff are considered an effective BMP (best management practice) for meeting water-quality criteria related to TMDL (total maximum daily load) regulations. However, while this type of BMP may abate surface-water loads, infiltrated stormwater may contaminate shallow ground water and increase ground-water gradients and flow to the lake. In addition, contaminants associated with urban runoff often include organic compounds and metals that are potentially toxic when consumed with drinking water. Processes that affect ground-water contamination from stormwater have only just begun to be considered, but understanding these details is important because growing numbers of environmental improvement program (EIP) projects are planned to encourage infiltration of urban runoff to comply with TMDL regulations.

Objectives

Document the interaction of a small local ground-water system with a selected stormwater-control/infiltration basin and the quality of accumulated stormwater and associated ground water in terms of nutrients and selected contaminants associated with stormwater runoff, examine how one cycle of seasonal variability affects nutrient concentrations and ground-water flow, characterize processes that influence nutrient transport from detention basins to shallow aquifers, and estimate mass of nutrients transported by shallow ground water and identify locations where nutrient-enriched ground water seeps into Lake Tahoe.

Strategy and Approach

South Lake Tahoe, California, is one of the more urbanized communities in the Lake Tahoe basin with several stormwater-control projects constructed to mitigate runoff. An infiltration basin near the lakeshore will be instrumented to record surface-water stage, inflow, and outflow. Several shallow observation wells will be constructed in pairs to measure hydraulic properties and quality of ground water that is near the water table and about 10 feet deeper near the selected basin, downgradient of the basin, and along the lakeshore. Hydraulic conductivity of aquifers will be measured in selected wells by slug test and depth to water, especially during runoff, and will be monitored to evaluate ground-water response to surface water in the basin. Temperature-surveys will be made along the lakeshore to identify zones of ground-water seepage. Ground-water flow away from the basin and into Lake Tahoe will be estimated by numerical modeling and seepage meter. Water and bottom sediment from the infiltration basin will be sampled for chemical analyses. Samples from surface-water inflow, selected wells, and ground-water discharging to Lake Tahoe also will be collected for chemical analyses.

Relevance and Benefits

The Lake Tahoe Restoration Act was signed into Federal law on November 13, 2000, partly to ensure that Federal entities continue to work to improve water quality. Information gathered for this study may be used to evaluate effectiveness of an existing stormwater-control project and the role ground water may play in the lake’s declining clarity. Human and environmental health are water-related issues identified in the Science Plan for the USGS water-resources discipline in Nevada and science related to environment and natural resources is a critical part of the USGS mission.

Publications

A draft final report is complete and in the peer-review process.