Lake Tahoe Basin Ground-Water Resources Inventory

Ground water in the Lake Tahoe Basin is a valuable source for domestic and municipal water supplies.  In most places throughout the basin, ground water is withdrawn from alluvial deposits.  These deposits may be in single or multiple layers and may be hydraulically connected to each other and to the underlying bedrock. However, in areas where alluvial deposits are thin or absent, ground water is often obtained from fractured-rock aquifers. The amount of ground water that can be withdrawn from fractured rock depends on the degree of fracturing and jointing of the rock. Although widely used as a source of water in the basin, the spatial extent, thickness, and distribution of these aquifers is either not well known or not available in a format to conduct a basin wide ground-water assessment.  Therefore, there is a need to consolidate and standardize all pertinent data into one geospatial database. 

In 2007, the U.S. Forest Service (USFS) requested that the USGS inventory the known alluvial and fractured-rock aquifers in the Lake Tahoe Basin. The inventory consists of compiling existing geospatial data and other information needed to determine the extent and characteristics of the aquifers in the Tahoe Basin. Geospatial data includes geologic maps and soil surveys of the entire basin and for specific watersheds within the basin at the best available scales; land-use maps from several remote-sensing datasets; well information from various local, state, and federal agencies; geophysical surveys; and results of available ground-water studies. The compilation and development of a ground-water inventory geospatial database will assist the USFS in better assessing the present and future impacts on ground-water resources within the Lake Tahoe Basin. 

Objectives

The overall objective is to provide a geospatial compilation of existing hydrogeologic data for the Lake Tahoe Basin within a GIS database to determine extent and characteristics of the aquifers in the basin. Specific objectives are to improve the understanding of the hydrologic framework of the Lake Tahoe Basin and develop a geospatial database that will include consolidating and attributing existing pertinent well data throughout the Lake Tahoe Basin, compiling and standardizing data from existing published reports and non-USGS databases, and compiling existing basin-wide ancillary GIS data.

Strategy and Approach

The intent of this study is to provide the USFS with a standardized geospatial database using existing data to support science-based ground-water resources decisions. The approach will be to use existing geologic data and studies to compile a map of the Tahoe Basin that shows the extent of the hydrogeologic units and if available, thickness of potential aquifers, and develop a standardized geodatabase with compiled information on point and areal data, including information from published ground-water reports. A USGS Scientific-Investigations Report describing the study results will be published in the spring of 2008.

Relevance and Benefits

The proposed study is relevant to issues one and five of the Nevada Water Science Center Science Plan. These two issues address the need for information to better quantify water budgets, and to assist other federal agencies in the use and regulation of water resources in areas of potential conflict. This study is also consistent with the Environment and Natural Resources goal of the USGS Strategic Plan, which is to provide water-resources managers with accurate, reliable, and impartial scientific information. The work will benefit the USFS in their management of Lake Tahoe Basin’s public land by providing information necessary for sound resources development and management decisions.