Groundwater Quality in the Western San Joaquin Valley, California

Groundwater supplies more than 40 percent of California's drinking water. To protect it, the state set up the Groundwater Ambient Monitoring and Assessment (GAMA) Program. Its Priority Basin Project, carried out by the U.S. Geological Survey with the California State Water Resources Control Board, assesses groundwater quality across the state and makes the information easier for the public to find. The Western San Joaquin Valley is one of the areas it studies.

The study area

The Western San Joaquin Valley study unit covers about 5,600 square miles, taking in the Delta–Mendota and Westside subbasins of the San Joaquin Valley groundwater basin. Its freshwater aquifer is made mostly of river-laid sediments of late Tertiary to Quaternary age, washed down from the Coast Ranges to the west and the Sierra Nevada to the east. A clay-rich lake deposit, the Corcoran Clay, divides it into upper and lower zones.

The study unit, its two subbasins and the extent of the Corcoran Clay. USGS map.

People have changed the area's water system profoundly. Naturally, groundwater was recharged mainly by the sparse streams from the Coast Ranges soaking in through alluvial fans, and by the San Joaquin and Kings Rivers, mostly in the winter rains. Farming began more than a century ago, and about 70 percent of the land is now farmed. Irrigation, with groundwater and imported surface water, is now the main source of recharge, and pumping for farms the main way water leaves the aquifer. Drainage water from the farmland can carry high concentrations of trace elements and salts, and has to be managed.

What was studied

The study looked at the parts of the aquifer tapped by the public supply wells in the state's database. In the Delta–Mendota subbasin these wells are usually 150 to 600 feet deep, with solid casing from the surface down to 130 to 290 feet and screens below that. In the Westside subbasin they are usually 575 to 1,550 feet deep, cased to 375 to 650 feet. Both subbasins draw public water from above and below the Corcoran Clay. Water in shallower or deeper parts of the aquifer can differ.

The project measures untreated groundwater, not the water delivered to homes, which may be disinfected, filtered, blended and exposed to air first. Results are set against drinking-water benchmarks for context: a concentration above a benchmark is high; for inorganic constituents, above half a benchmark is moderate; for organic constituents, which are generally rarer and lower, above a tenth of a benchmark is moderate. Everything else, including no detection, is low. Data were collected in 2010, alongside the state's regulatory monitoring data for 2007–10.

Share of the aquifer area with high, moderate and low concentrations. Inorganic constituents: 53, 30 and 17 percent. Organic constituents: 3, 3 and 94 percent. USGS.

Inorganic constituents

Many occur naturally, and both natural processes and human activity affect their levels. One or more inorganic constituents were high in about 53 percent of the groundwater used for public drinking water.

• Trace elements, which come from the minerals in rocks and sediments, were high in about 52 percent. Six exceeded benchmarks: boron (51 percent), hexavalent chromium (25), arsenic (10), strontium (5), molybdenum (4) and selenium (3). Which ones are high varies across the area, because the rocks the water passes through, and the chemistry that controls how minerals dissolve, vary.
• Radioactive constituents, mostly from the natural decay of uranium and thorium in aquifer minerals, were high in about 3 percent: uranium, radium and gross alpha-particle activity.
• Nitrate occurs naturally at low levels; high levels usually come from human activity — fertiliser on crops and landscaping, septic systems, and human and animal waste. It was high in about 4 percent.

Constituents with aesthetic rather than health benchmarks — affecting taste, colour or odour, or causing staining and scaling — are measured against non-regulatory secondary limits and are left out of the overview charts:

• Total dissolved solids, a measure of salinity, were above California's upper limit in about 50 percent.
• Manganese or iron, released from minerals where the water is low in oxygen, were high in about 46 percent.

Perchlorate, regulated in California drinking water since 2007, is used in rocket fuel, fireworks, safety flares and some fertilisers, and also occurs naturally at low levels. It was not found at high levels, and was moderate in about 15 percent.

Organic constituents

Organic constituents come from products used at home, in business, industry and farming, and reach the environment through normal use, spills or careless disposal; some, like petroleum, also have natural sources. They were high in about 3 percent. The project's lab methods detect them far below health benchmarks, which helps trace water moving from the land surface into the aquifer.

• Volatile organic compounds, which evaporate readily, were high in about 3 percent: benzene, probably from nearby natural petroleum deposits, and tetrachloroethene (PCE), a common solvent.
• Pesticides were not found at high levels. The one at moderate levels was the fumigant DBCP (1,2-dibromo-3-chloropropane), whose use as a soil fumigant in California ended in 1977.

Where the water came from

The source of recharge is one of the things that shape water quality. The ratios of stable oxygen and hydrogen isotopes in the water tell recharge from Sierra Nevada runoff (delta oxygen-18 below –9.5 per mil) apart from Coast Ranges runoff (above –8.5 per mil).

• Sierra Nevada water is generally lower in dissolved solids and turns up in some wells on the east side of the area, near the San Joaquin River and Fresno Slough.
• Coast Ranges water varies with the geology it drains. Runoff from the Franciscan Complex, mostly metamorphic rock, is generally low in dissolved solids; runoff from marine sedimentary rocks is generally high in dissolved solids and sulfate, the sulfate coming from the oxidation of biogenic pyrite in organic-rich marine shales.

Noble gases dissolved in groundwater give the temperature of the water when it entered the aquifer. Most of the area's groundwater entered at below 20 °C, matching natural recharge from winter runoff. Irrigation water enters warmer, because most irrigation happens in the warm growing season. The warm-recharge water here (above 22 °C) also has delta oxygen-18 above –8.5 per mil, so it is probably Coast Ranges groundwater that was pumped up and used for irrigation. It is found both above and below the Corcoran Clay, which supports other studies' conclusion that the clay is no longer an intact confining layer.

Dissolved solids in wells grouped by source of recharge (top), and recharge temperature against delta oxygen-18 (bottom), by depth relative to the Corcoran Clay and by side of the valley. USGS.

About the project

The Priority Basin Project assesses groundwater used for drinking-water supply, with ongoing work in more than 120 basins and areas outside basins across California. This study focused on water used for public supply; domestic wells usually tap shallower parts of aquifers, and quality can vary with depth. The project uses a network of wells to assess groundwater quality statistically, and combines water-quality, hydrological and geographical data to understand what affects it. Its analyses go beyond regulatory monitoring, measuring concentrations far below health benchmarks and tracers of where groundwater comes from and how it moves.

Sources

• Miranda S. Fram, Groundwater Quality in the Western San Joaquin Valley, California, USGS Fact Sheet 2017–3028, April 2017. https://doi.org/10.3133/fs20173028
• Fram, M.S., 2017, Groundwater quality in the Western San Joaquin Valley study unit, 2010—California GAMA Priority Basin Project, USGS Scientific Investigations Report. http://pubs.usgs.gov/sir/2017/5032/
• GAMA: http://ca.water.usgs.gov/gama