Groundwater Quality in the Shallow Aquifers of the Tulare, Kaweah and Tule Basins

Groundwater supplies more than 40 percent of California's drinking water. To protect it, the state created the Groundwater Ambient Monitoring and Assessment (GAMA) Program, whose 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. One of the areas it has studied is the shallow groundwater of the Tulare, Kaweah and Tule basins and the neighbouring highlands of the southern San Joaquin Valley.

The study area

The Tulare Shallow Aquifer study unit covers about 2,320 square miles: the Tulare, Kaweah and Tule groundwater basins and the adjacent highlands of the Sierra Nevada. In the basins, the freshwater aquifer is made of river-laid sediments of Quaternary age, mostly washed down from the Sierra Nevada, and is divided into upper and lower zones by a clay-rich lake deposit, the Corcoran Clay. The highlands have fractured-rock aquifers, mostly in Mesozoic granite. People — above all, farming — have greatly changed the southern San Joaquin Valley's water system.

This study looked at the groundwater used by domestic wells, which here are usually 150 to 500 feet deep — shallower than public-supply wells, which are usually 300 to 1,400 feet deep; water quality varies with depth. To give a statistically representative picture, 96 wells across the area were sampled between November 2014 and April 2015. Earlier studies had found nitrate, fumigants, microbial indicators and several trace elements at elevated levels in some domestic wells.

The four study areas — Tulare, Kaweah, Tule and the Highlands — and the sampled wells. USGS.

How results are measured

The project measures untreated groundwater, which can differ from the water people drink because of household plumbing, contact with air or treatment. Results are set against drinking-water benchmarks for context: above a benchmark is high; for inorganic constituents, above half a benchmark is moderate; for organic constituents, above a tenth; everything else, including no detection, is low.

Share of the groundwater used for domestic supply with high, moderate and low concentrations. Inorganic constituents: 54, 35 and 11 percent. Organic constituents: 1, 4 and 94 percent. USGS.

Inorganic constituents

One or more inorganic constituents were high in about 54 percent of the groundwater used for domestic drinking water.

• Trace elements, which come naturally from rocks and sediments, were high in about 29 percent: arsenic (19 percent), molybdenum (8), vanadium (4) and manganese (3).
• Radioactive constituents, mostly from the decay of uranium and thorium in aquifer minerals, were high in about 17 percent: uranium, gross beta-particle activity and adjusted gross alpha-particle activity.
• Nitrate, naturally low but raised by fertiliser, septic systems and human and animal waste, was high in about 27 percent.

Measured against guidelines for taste, colour, odour, staining and scaling rather than health:

• Total dissolved solids, a measure of salinity, were above California's upper limit in about 2 percent.
• Manganese or iron, released from minerals where oxygen is low, exceeded the aesthetic guidelines in about 14 percent; for manganese, the aesthetic limit is lower than the health-based one.
• Perchlorate — regulated in California since 2007, used in rocket fuel, fireworks, flares and some fertilisers, and also naturally present at low levels — was high in about 2 percent.

Organic constituents and microbes

The project's lab methods detect pesticides and volatile organic compounds (VOCs) far below health benchmarks, which helps trace water moving from the land into the aquifer.

• Fumigants were high in about 1 percent: DBCP (1,2-dibromo-3-chloropropane) at high and moderate levels and EDB (ethylene dibromide) at moderate levels. Their use as soil fumigants in California ended in 1977 and 1983. No other pesticides, and no other VOCs, were found at high or moderate levels.
• Total coliform bacteria, which have both natural and human sources and point to possible fecal contamination, were found in 21 percent of the wells sampled. E. coli was not detected.

Why it varies

Nitrate, arsenic, uranium and fumigants were the constituents at high or moderate levels in the largest shares of the domestic-supply groundwater. Nitrate and uranium in San Joaquin Valley groundwater have risen as land and water use have changed, so groundwater with moderate levels now could reach high levels in future.

• The Tulare area had the largest share with high or moderate arsenic.
• The Kaweah and Tule areas had the largest shares with high or moderate nitrate.
• Uranium was similar across all four areas.
• Only the Tule area had high or moderate fumigants.

The differences reflect the rocks and sediments that make up each area's aquifers, the chemistry that governs how easily constituents move, farming practices on the land above, and other factors.

Share of domestic-supply groundwater with high or moderate nitrate, arsenic, uranium and fumigants, by study area. USGS.

Sources

• Miranda S. Fram, Groundwater Quality in the Shallow Aquifers of the Tulare, Kaweah, and Tule Groundwater Basins and Adjacent Highlands Areas, Southern San Joaquin Valley, California, USGS Fact Sheet 2017–3001, January 2017. https://doi.org/10.3133/fs20173001
• Data: Bennett, G.L., Fram, M.S., and Johnson, T.D., 2017, USGS data release. http://dx.doi.org/10.5066/F7BP00W8