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The Northern Coast Ranges study unit: interior (NOCO-IN) and coastal (NOCO-CO) study areas. USGS.
About the assessment
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 with the USGS, assesses the quality of untreated groundwater in the part of the aquifer used for public supply — not the water delivered to taps. Data here come from GAMA sampling in 2009 and the state health department's database for 2006–2009.
The study unit
- 633 square miles in 35 groundwater basins and subbasins, grouped as interior basins (NOCO-IN) and coastal basins (NOCO-CO) on the Pacific.
- Interior: alluvial basins of sand, silt, gravel, clay and thin volcanic ash, with groundwater following the land and streams; a Mediterranean climate of hot, dry summers and cold, wet winters.
- Coastal: alluvial fan, floodplain and terrace deposits, with groundwater moving west to the ocean; cool summers and cold, wet winters.
- Wells: 36–400 ft deep inland and 15–400 ft on the coast.
- Rain: 22–79 inches a year. Rivers: the Eel, Russian, Mad, Navarro, Smith, Klamath, Noyo and Big.
- Land use: about 60% natural (grassland and forest), 29% agricultural (pasture, row crops, hay, vineyards, timber) and 11% urban — the largest towns are Crescent City, Arcata, Eureka, Fort Bragg, Willits, Ukiah and Lakeport.
- Recharge comes from rain and irrigation water, runoff from surrounding hills, seepage from rivers and creeks, and inflow from surrounding rock.
How "high" is defined
Concentrations are compared with drinking-water benchmarks — health-based limits where they exist, otherwise aesthetic or advisory levels.
- High: above the benchmark.
- Moderate: above half the benchmark for inorganic constituents, or above 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.
Overview
Share of the primary aquifer system with high, moderate and low concentrations. USGS.
Inorganic constituents
| Constituent | High | Moderate | Notes |
|---|---|---|---|
| Trace elements | about 10% | — | mostly boron and arsenic; barium high in under 2% |
| Nutrients (nitrate) | none | about 4% | from fertilizer, septic systems and waste |
| Uranium and other radioactive constituents | none | none | from natural decay of uranium and thorium |
| Iron or manganese (aesthetic) | about 40% | about 9% | released from minerals where groundwater lacks oxygen; cause staining and taste |
| Total dissolved solids (aesthetic) | none | about 7% | from weathering of minerals |
| Perchlorate | none | about 2% | regulated in California since 2007; used in rocket fuel and fireworks, found in some fertilizers, and natural at low levels |
Inorganic constituents. USGS.
Organic constituents
The GAMA labs detect volatile organic compounds (VOCs) and pesticides at levels far below health benchmarks — useful for tracing water from the land surface into the aquifer.
- VOCs: high in under 1%, moderate in about 2%.
- Gasoline components: the oxygenate MTBE was high in under 1%.
- Trihalomethanes, which form when water is chlorinated and can reach groundwater through landscape irrigation: moderate in about 2%.
- Vinyl chloride, an organic synthesis reagent used to make PVC pipe: moderate in about 2%.
- Solvents: not found at high or moderate levels.
- Pesticides: none at high or moderate levels; herbicides occasionally at low levels.
Organic constituents. USGS.
Why boron is high
Boron was the constituent most often high: about 7% of the aquifer exceeded California's notification level of 1,000 micrograms per liter. It is natural and very soluble, coming from igneous rocks, evaporite minerals, seawater and geothermal waters.
All the high boron, and most of the moderate, was in the interior basins. High boron went with warmer groundwater, chemistry like geothermal water, and nearness to known geothermal areas — pointing to groundwater mixing with hydrothermal systems. Earlier work in the Geysers–Clear Lake area found the same signatures: warm water, high boron, arsenic and lithium, and elevated helium isotope ratios.
Boron in the interior basins, with hot springs and geothermal wells, and groundwater temperature plotted against boron. USGS.
Sources
Based on Timothy M. Mathany and Kenneth Belitz, "Groundwater quality in the Northern Coast Ranges Basins, California," U.S. Geological Survey Fact Sheet 2014–3114 (2015), with the California State Water Resources Control Board; a work of the United States government in the public domain. The maps, charts and the study-unit description, which the import did not carry, are taken from the fact sheet's PDF.
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