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The program
Groundwater supplies more than 40 percent of California's drinking water. The state's Groundwater Ambient Monitoring and Assessment (GAMA) Program, through its Priority Basin Project, assesses groundwater quality statewide with the USGS and the State Water Resources Control Board. The Cascade Range and Modoc Plateau (CAMP) study unit, in northeastern California, is one of its areas.
The CAMP study unit and its six study areas. USGS.
The study unit
- Aquifers: the volcanic rocks can be highly productive, holding water in porous zones between lava flows, cooling fractures and lava tubes — connected or isolated, so water levels, yields and quality vary locally. Sedimentary basins hold young alluvial-fan, river and lake sediments.
- The primary aquifer system: the parts tapped by public-supply wells and springs. About 10 percent of sources are springs; most wells go 140 to 400 feet deep, sealed from the surface down 50 to 220 feet.
- Climate: 10 to 20 inches of precipitation a year over much of the area, and over 80 inches at high elevations in Lassen Volcanic National Park and on Mount Shasta.
How quality is judged
The project tests untreated groundwater — not the water delivered to taps — against drinking-water benchmarks, federal and California maximum contaminant levels where they exist.
| Rating | Inorganic constituents | Organic and special-interest constituents |
|---|---|---|
| High | above the benchmark | above the benchmark |
| Moderate | above half the benchmark | above one-tenth of the benchmark |
| Low | below that, or not detected | below that, or not detected |
Results (share of the primary aquifer system)
Overall: inorganic constituents with health benchmarks were high in about 9 percent and moderate in about 15 percent. Organic constituents — volatile organic compounds and pesticides — were never high or moderate; lab methods detect them far below health benchmarks, useful for tracing water from the land into aquifers.
| Constituents | High | Moderate | Notes |
|---|---|---|---|
| Trace elements | 6% | 14% | mostly arsenic, boron, molybdenum, vanadium — natural, from rocks and sediments |
| Uranium and radioactivity | 3% | under 1% | mostly uranium, from natural uranium and thorium decay |
| Nutrients | 1% | 2% | only nitrate was high; sources include fertilizer, septic systems and waste |
| Total dissolved solids (salinity; non-health benchmark) | 2% | 9% | |
| Manganese or iron (non-health benchmark) | 15% | 2% | released from minerals in low-oxygen groundwater |
| Perchlorate (regulated in California since 2007) | 0% | 2% | used in rocket fuel, fireworks and flares; also natural at low levels |
Why quality varies
Inorganic constituents were high or moderate in 24 percent of the aquifer system overall — from 9 percent in the Eastside Sacramento Valley to 64 percent in Honey Lake Valley. Rocks, land use and well depths differ among the six areas.
In Honey Lake Valley, arsenic, boron, molybdenum, vanadium, uranium and nitrate were all elevated, because groundwater there mixes with hydrothermal fluids, reacts with granitic rocks of the Sierra Nevada and ancient lakebed sediments, and includes old, low-oxygen groundwater. High nitrate goes with shallow wells, young groundwater and farming.
High and moderate concentrations of health-related inorganic constituents by study area. Pie charts show the high (yellow), moderate (green) and low (blue) shares. USGS.
About the project: data came from the project's own sampling in 2010 and state data for 2008–2010. The project covers more than 120 basins statewide, uses both a statistical well network and a study of the factors behind water quality, and measures constituents far below benchmarks. A separate Domestic Well Project covers the shallower aquifers private wells use.
Sources
Based on Miranda S. Fram and Jennifer L. Shelton, "Groundwater Quality in the Cascade Range and Modoc Plateau, California," U.S. Geological Survey fact sheet, with the California State Water Resources Control Board; a work of the United States government in the public domain. The map and chart are taken from the fact sheet's PDF, which also supplies the study-unit description missing from the imported text.
Lizenz: CC0 1.0 (gemeinfrei) · Bearbeitet nach pubs.usgs.gov
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