Groundwater Quality in the North San Francisco Bay's Shallow Aquifers

The North San Francisco Bay Shallow Aquifer study unit and its two study areas. Base from USGS and other federal and state data. 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 statewide and makes the results public.

The North San Francisco Bay Shallow Aquifer study unit covers more than 1,800 square miles near the coast.

• Geology: 6 groundwater basins, some with subbasins, plus the surrounding hills, split into two areas — the alluvial Valleys and Plains, and the volcanic, metamorphic and ultramafic Highlands.
• Climate: warm, dry summers and cool, wet winters, with about 30 inches of rain a year.
• Land: about 73 percent natural (forest, grassland, bare rock), 16 percent urban and 11 percent farmland — pasture, vineyards, flower nurseries and orchards. The biggest cities are Santa Rosa, Petaluma, Rohnert Park and Napa.
• Water in and out: recharge comes mainly from rivers and streams soaking in and from rain; water leaves through municipal pumping, evaporation and flow to streams.
• The shallow aquifer is the part above the depths tapped by public-supply wells (assessed separately in an earlier study). Wells sampled ran 22 to 755 feet deep, open to the aquifer between 8 and 550 feet. Data were collected in 2012.

Its quality can differ from the deeper public-supply aquifer. Water was tested untreated; tap water can differ after plumbing, air exposure or treatment.

How results are rated

Results are compared with drinking-water benchmarks: federal and California regulatory health limits (MCLs) where they exist, otherwise non-regulatory health advisories and notification levels, and secondary limits for taste and odor (SMCLs).

• High: above a benchmark.
• Moderate: above half of one for inorganic constituents; above one-tenth for organic and special-interest constituents.
• Low: less than moderate, or not detected.

ppb, parts per billion; ppm, parts per million; pCi/L, picocuries per liter; HAL, lifetime health advisory; RSD5, risk-specific dose at a risk factor of 10⁻⁵.

What they found

Share of the shallow aquifer at high, moderate and low concentrations. USGS.

Inorganic constituents (many occur naturally, though people can affect them too) were high in about 27 percent of the shallow aquifer and moderate in about 21 percent.

• Trace elements: high in about 25 percent, moderate in about 21 percent. Arsenic, boron, fluoride and manganese were the ones found high.
• Radioactivity: only radon-222 was high, in 2 samples. Its danger is as a gas in indoor air, so it isn't in the radioactivity chart.
• Nutrients such as nitrogen, usually from fertilizer, septic systems and waste: high in about 2 percent, moderate in about 3 percent.
• Perchlorate — used in rocket fuel, fireworks and flares, found in some fertilizers, and regulated in California drinking water since 2007: moderate in about 3 percent.

Aesthetic constituents: total dissolved solids, or the ions chloride and sulfate, were high (above the upper limit) in about 6 percent and moderate in about 16 percent. Iron, released from minerals in low-oxygen water, was high in about 20 percent and moderate in about 5 percent.

Organic constituents — from household, business, industrial and farm products — were high in about 1 percent and never moderate.

• VOCs (solvents, trihalomethanes, refrigerants and more) were found only at low levels.
• Pesticides were high in about 1 percent — the insecticide dieldrin, the only one found high — and none moderate. (The labs detect such compounds far below health benchmarks, which helps trace water from the land into the aquifer.)

Where the arsenic comes from

Arsenic concentrations against dissolved oxygen, by study area and groundwater age. USGS.

Groundwater age and chemistry often go with trace-element levels. Here the highest arsenic was in water low in dissolved oxygen and premodern or mixed in age:

• in oxygen-poor water, arsenic can be released from iron or manganese oxyhydroxides;
• the longer water sits against arsenic-bearing rock, the more time it has to react;
• some areas have hydrothermal water — heated by the Earth — rising along faults and fractures to mix with groundwater. Minerals dissolve more at higher temperatures, so arsenic, boron and fluoride can rise. The wells with the most arsenic had slightly warm water, hinting at that mixing.

About these assessments

The Priority Basin Project studies both the public-supply aquifer and the shallow aquifer (where domestic wells often draw) in more than 120 basins across California. It uses a statistical well network to describe groundwater quality, plus water, hydrologic and geographic data to understand what controls it — measuring well below health benchmarks and tracing where groundwater comes from. It does not assess the drinking water delivered to consumers.

Sources

Based on Groundwater Quality in the North San Francisco Bay Shallow Aquifer, California, USGS Fact Sheet 2018–3007 (February 2018), by George L. Bennett V and Miranda Fram, 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 map, charts and benchmark table are taken from the fact sheet's PDF.