Private Well Water in California's Northern San Joaquin Valley

By Zeno F. Levy, Mariia Balkan and Jennifer L. Shelton. USGS Fact Sheet 2021–3040, July 2021, with the California State Water Resources Control Board.

Groundwater supplies more than 40 percent of California's drinking water. The state's Groundwater Ambient Monitoring and Assessment (GAMA) Program, through its Priority Basin Project (PBP), assesses groundwater quality and makes the data public. This study looked at private domestic and small-system wells in the Northern San Joaquin Valley (NSJV), which draw from shallower parts of the aquifers than the region's deep public-supply wells. Private well water is not regulated in California.

The study area

The study unit covers about 3,095 square kilometers of the northeastern San Joaquin Valley near Stockton, from Dry Creek in the north to the Stanislaus River in the south. Land is mostly natural in the east, by the Sierra Nevada foothills, and agricultural and urban toward the San Joaquin River. Beneath lie several hundred meters of sediment, the deepest marine and salty; shallower aquifers are separated from them by volcanic deposits of the Mehrten Formation, about 300 m down near Stockton.

The study unit, its 50 grid cells and sampled wells. Map: USGS.

Sampling: the area was split into 50 equal cells of 62 km², and one domestic or small-system well was sampled in each of 46 cells from July through September 2019. Wells were 18 to 145 m deep (median about 80 m). One extra "delta well," on an island in the San Joaquin Delta, was sampled to see water from delta sediments.

Overall results

Share of domestic groundwater resources with high, moderate and low concentrations. Chart: USGS.

"High" means above a drinking-water benchmark such as a maximum contaminant level (MCL). Untreated groundwater can differ from tap water after plumbing, air or treatment.

• Fumigants — soil pesticides — were applied to crops together or in rotation. California banned DBCP in 1979 and 1,2-DCP in 1984.
• Coliforms and Enterococci occur naturally in soil, animal guts and wastewater; they signal possible fecal contamination.

What drives it

In all, arsenic, nitrate or fumigants were high in 29% of domestic groundwater.

Nitrate versus arsenic in each sampled well, by oxygen conditions. Graph: USGS.

• Nitrate and fumigants: legacy of agriculture, and they persist in oxygen-rich (oxic) water. Fumigants appeared only in oxic samples, mostly with high or moderate nitrate.
• Arsenic: released from sediments in low-oxygen (reducing) water and, less so, at high pH (7.5 or above). It is highest where groundwater touches delta sediments, as the delta well showed.
• Shallow versus deep: decades of heavy pumping have pulled oxic farm recharge from shallow zones into deeper, reducing public-supply aquifers. An earlier GAMA study found public-supply aquifers had more high-arsenic water (17%) and little high-nitrate water (3%). Here, high nitrate was more common in the shallow domestic aquifer — always oxic, often with fumigants — and some high-pH oxic samples had both high nitrate and high arsenic.

About the program

GAMA's PBP has studied more than 120 basins and areas across California. It uses a statistical well network to describe water quality and combines chemistry with hydrologic and geographic data to find causes, measuring many constituents far below health benchmarks, plus tracers of groundwater's source, movement and age. It assesses untreated groundwater, not treated drinking water.

Sources

Based on Quality of Groundwater Used for Domestic Supply in the Northern San Joaquin Valley, California, by Zeno F. Levy, Mariia Balkan and Jennifer L. Shelton, USGS Fact Sheet 2021–3040, U.S. Geological Survey; a work of the United States government in the public domain. The study-area description, map, charts and scatter plot are taken from the fact sheet's PDF; the import had lost the study-area section.