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Groundwater supplies more than 40% of California's drinking water. The state's Groundwater Ambient Monitoring and Assessment (GAMA) Program, through its Priority Basin Project run by the USGS and the State Water Resources Control Board, measures the quality of untreated groundwater. This is the summary for the Klamath Mountains of northwestern California.
The study unit
The Klamath Mountains study unit. USGS.
| Area | more than 8,800 square miles |
| Rock | mostly metamorphic, ultramafic and granitic — high peaks, deep river gorges, few groundwater basins |
| Aquifer | unusually for GAMA, fractured bedrock; fractures may connect or not, so water levels, yields and quality vary locally |
| Water sources sampled | springs (about 18% of sites) and wells (about 82%), most 25 to 400 feet deep |
| Land use | about 97% natural (forest, grassland, bare rock), 2% urban, 1% agricultural |
| Rain | 69 inches a year in the west, 40 in the center and east |
| Drainage | Klamath-Trinity, Sacramento and Smith rivers |
Recharge is rain and snowmelt seeping into fractures at higher elevations; water leaves through wells, evaporation and streams.
The overall picture
Share of the primary aquifer system with high, moderate or low concentrations. USGS.
Concentrations are compared with drinking-water benchmarks for context, though this is untreated water. High means above a benchmark. Moderate means above half of it for inorganic constituents, or above a tenth for organic ones, which are usually rarer.
Inorganic constituents
| Group | High | Moderate |
|---|---|---|
| Trace elements (arsenic, antimony and boron were the ones found high) | about 3% | about 5% |
| Radioactive constituents (from natural uranium and thorium) | none | about 5% |
| Nutrients such as nitrogen | none | about 1% |
| Total dissolved solids (taste, scaling) | about 1% | about 3% |
| Iron and/or manganese (taste, odor, staining) | about 13% | about 10% |
Organic constituents
- Volatile organic compounds: none at high levels. The gasoline additive MTBE was moderate in about 2%. Solvents, trihalomethanes (which can form when water is disinfected) and refrigerants were found only at low levels, and infrequently.
- Pesticides: none at high or moderate levels; herbicides were detected infrequently at low levels.
Iron and manganese
No health-based constituent was high in more than 3% of the aquifer. But iron exceeded its aesthetic benchmark of 300 µg/L in about 8%, and manganese its benchmark of 50 µg/L in about 11% — within the 7 to 31% range seen in most GAMA study units. Harmless at natural levels, they make water taste metallic, smell foul, and stain surfaces black, brown or red.
Iron or manganese by groundwater age and oxygen conditions. Older water and water low in oxygen carry more. USGS.
High iron and manganese went with low dissolved oxygen and older groundwater. Their minerals dissolve more easily in oxygen-poor (reducing) water, given enough of them in the rock and enough time. In the Klamath Mountains those conditions were rare: most water was young and well oxygenated.
About the assessment
GAMA's Priority Basin Project looks at the primary aquifer system used for public supply (its Domestic Well Project covers shallower water) in more than 120 basins. It does not evaluate the water delivered to consumers, which may be treated. Here, samples were collected in 2010, with state data from 2007–2010.
Sources
Based on George L. Bennett V and Miranda S. Fram, "Groundwater quality in the Klamath Mountains, California," U.S. Geological Survey Fact Sheet 2014–3031, prepared with the California State Water Resources Control Board; a work of the United States government in the public domain. Figures are taken from the fact sheet's PDF. Full results: Bennett, Fram and Belitz, USGS Scientific Investigations Report 2014–5065.
Лицензия: CC0 1.0 (общественное достояние) · По материалам pubs.usgs.gov
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