Groundwater Quality in California's Sierra Nevada

The Sierra Nevada Regional study unit, by aquifer rock type. USGS.

About the assessment

Groundwater supplies more than 40% of California's drinking water. The state's GAMA Priority Basin Project, run with the USGS, assesses untreated groundwater in the part of the aquifer used for public supply — not the water delivered to taps — comparing it with drinking-water benchmarks. Data here come from sampling in 2008 and the state database for 2006–2008.

The study unit

• About 25,000 square miles of the Sierra Nevada's mountains and foothills, divided by rock type: granitic (51% of the primary aquifer), metamorphic (30%), volcanic (12%) and sedimentary (7%).
• Unlike most study units, it is mostly fractured bedrock: water sits in cracks that may or may not connect, so water levels, yields and quality vary widely. Sedimentary basins hold glacial, alluvial-fan, river and lake deposits.
• About 25% of public-supply sources are springs. Most wells are 150–600 feet deep, cased or sealed to 50–200 feet.
• Precipitation ranges from 11 inches at low elevations to more than 80 up high, mostly as snow; the range's rivers supply much of the water used elsewhere in California.
• Land is about 90% natural (about 60% national forests, parks and other federal land), 5% urban and growing fast, and 5% agricultural.
• Recharge comes mainly from streams and from rain and snowmelt soaking in; sedimentary basins also get water from the surrounding mountain fronts.

How "high" is defined

• High: above a drinking-water benchmark (a health-based limit where one exists, otherwise an aesthetic or advisory level).
• Moderate: above half the benchmark for inorganic constituents, or a tenth for organic and special-interest ones.
• Low: below that, or not detected.

Results are the share of the primary aquifer's area in each class.

Share of the primary aquifer system with high, moderate and low concentrations. USGS.

Inorganic constituents

Inorganic constituents. USGS.

Organic constituents

The project's labs detect VOCs and pesticides far below health benchmarks, which helps trace water from the land surface into aquifers.

• Solvents: high in under 0.1% (the solvents PCE and 1,2-dichloroethane), moderate in about 3%.
• Trihalomethanes (from chlorinating water supplies) and gasoline oxygenates: none high; moderate in under 0.1%.
• Other VOCs, such as organic synthesis reagents and refrigerants: not found at high or moderate levels.
• Pesticides: not found at high or moderate levels.

Organic constituents. USGS.

Why quality differs by rock type

Water quality differs among the four rock types because of their minerals, nearby land use, the kinds of wells in each, and other factors.

• Nitrate: highest in sedimentary aquifers, where wells have more urban and farm land nearby. These aquifers are only 7% of the primary aquifer but hold more than 20% of the public-supply wells.
• Uranium: more often high or moderate in granitic and sedimentary aquifers, because uranium- and thorium-bearing minerals are commoner in granite — the main source of the region's sediments.
• Arsenic: the trace element most often high or moderate in all four aquifer types, with the most high values in volcanic aquifers, where it went with oxygen-rich, high-pH water. Elsewhere, high arsenic went with low-oxygen water and with the southwestern part of the study unit, where granite is richer in arsenic and other trace elements.

Arsenic, uranium and nitrate by aquifer rock type. USGS.

Sources

Based on Miranda S. Fram and Kenneth Belitz, "Groundwater quality in the Sierra Nevada, California," U.S. Geological Survey Fact Sheet 2014–3096 (2014), with the California State Water Resources Control Board; a work of the United States government in the public domain. The map, charts and the study-unit description, which the import did not carry, are taken from the fact sheet's PDF.