Groundwater Quality in Domestic Wells Around Redding and Red Bluff, California

Share of the groundwater used for domestic drinking water at high, moderate and low concentrations. USGS.

The study

Groundwater supplies more than 40 percent of California's drinking water. The state's Groundwater Ambient Monitoring and Assessment (GAMA) Program protects it; its Priority Basin Project, run with the U.S. Geological Survey, assesses groundwater quality across the state and makes the results public.

This study covers the Redding–Red Bluff study unit: about 1,200 square miles in Shasta and Tehama counties at the north end of the Sacramento Valley, with the Sacramento River running through it.

• Aquifers: marine, continental and volcanic sediments washed down from the Cascade Range (east), the Klamath Mountains (north) and the Coast Ranges (west).
• Land: mostly natural (60 percent); urban use is commoner around Redding (36 percent), farming around Red Bluff (20 percent).
• Wells: private domestic and small-system wells here draw mainly from shallow aquifers. They are typically 80–338 feet deep — shallower than public-supply wells (typically 115–450 feet) — with water 15–163 feet below the surface (10th–90th percentiles).
• Sampling: 50 wells, December 2018 to April 2019, spread to represent the whole resource statistically. The water was tested untreated; what comes out of a tap can differ, after plumbing, air or treatment. Private domestic wells aren't regulated in California.

Earlier findings: public-supply wells here were low in inorganic and volatile organic compounds, except arsenic (4.6 percent). A state survey of 223 domestic wells in Tehama County found arsenic above the benchmark in 13 percent, mostly in the southeast — though those wells weren't spread to represent the aquifer.

How results are rated

Results are compared with drinking-water benchmarks: federal (EPA) and state regulatory limits for health (MCLs) where they exist, otherwise non-regulatory aesthetic limits (SMCLs) and health levels (screening levels, response and notification levels, lifetime advisories). High means above a benchmark; moderate, above half of one (for inorganic constituents); low, below that or not detected.

ppb, parts per billion; ppm, parts per million; pCi/L, picocuries per liter; TT, treatment technique.

What they found

Trace elements (health benchmarks) come naturally from the minerals in aquifer rock and sediment.

• About 2 percent of the groundwater had one or more at high levels — strontium and barium — and about 10 percent at moderate levels — arsenic, boron and chromium.
• Radioactivity: gross alpha activity was above the benchmark in 2 percent of wells. Most radioactivity in groundwater comes from the decay of uranium and thorium in aquifer materials.
• Nitrate: moderate in about 4 percent. Nutrients are naturally low in groundwater; high levels usually come from fertilizer, septic systems, or human and animal waste.

Hexavalent chromium, Cr(VI), a form of chromium that may be toxic, can come naturally from sediments of ultramafic rocks (serpentinites), or from industry such as stainless-steel making. It was high in 4 percent and moderate in 8 percent. (The benchmark is the USGS screening level of 20 ppb, because California's MCL of 10 ppb was withdrawn in 2017.)

Aesthetic constituents (taste, color, odor, staining, scaling):

• Total dissolved solids, a measure of salinity from weathering minerals, was above California's upper limit in about 2 percent.
• Manganese or iron was above aesthetic limits in about 4 percent. Anoxic water — dissolved oxygen below 0.5 ppm — can release them, and other trace elements, from minerals.

Organic constituents: no volatile organic compounds (VOCs) or pesticides were at high or moderate levels. The labs detect them far below health benchmarks, as tracers of water moving from the land into the aquifer: VOCs turned up at low levels in 16 percent of wells — mostly disinfection by-products — and pesticides in 4 percent, mostly atrazine and simazine.

Microbes: total coliforms were in 8 percent of wells and Enterococci in 4 percent (not the same wells); E. coli wasn't found. Both are natural in soil and animal guts, and signal possible fecal contamination — though confirming a detection needs repeat sampling, which this study didn't do.

North versus south

Moderate and high concentrations in domestic wells, northern (Redding–Red Bluff) and southern Sacramento Valley study units. USGS.

Compared with domestic wells in the southern Sacramento Valley, the northern wells were cleaner for several constituents:

Hexavalent chromium, which dissolves best in oxygen-rich water, can come from natural water–rock reactions or, in places, industrial releases.

About these assessments

The Priority Basin Project is assessing more than 120 basins and areas across California. This study focused on domestic wells, which tap shallower water than public-supply wells — and water quality can change with depth. The project uses two approaches: a statistical network of wells to describe groundwater quality, and water-quality, hydrologic and geographic data to understand what controls it. Its lab work goes beyond routine compliance testing, including very low concentrations and tracers of where groundwater comes from and how it moves.

More: USGS GAMA · State Water Resources Control Board GAMA.

Sources

Based on Groundwater Quality in the Redding–Red Bluff Shallow Aquifer Study Unit of the Northern Sacramento Valley, California, USGS Fact Sheet 2020–3025 (April 2020), by Jennifer S. Harkness and Jennifer L. Shelton, prepared with the California State Water Resources Control Board, U.S. Geological Survey; a work of the United States government in the public domain. The charts and benchmark table are taken from the fact sheet's PDF. Its study-area map, on a base that includes commercial data and whose credit line is an unfilled template, is not reproduced; the table there labels total dissolved solids "CA-MCL," though its text calls those benchmarks secondary, so this page gives only "CA."