Groundwater Quality in the Shallow Aquifers of Monterey Bay and the Salinas Valley

U.S. Geological Survey Fact Sheet 2018–3026 (version 1.1, June 2018), with the California State Water Resources Control Board. By Carmen A. Burton.

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, assesses groundwater quality statewide and makes the information public. One study unit covers the shallow aquifers around Monterey Bay, the Salinas Valley and the neighbouring highlands, which supply household wells.

The study area

• About 7,820 square kilometres in Santa Cruz, Monterey and San Luis Obispo Counties, in four areas: Santa Cruz, Pajaro Valley, Salinas Valley and Highlands.
• More than 75 percent of drinking water in this Central Coast region is groundwater, and there are more than 8,000 drillers' logs for household wells.
• Water comes from sediments — the Purisima Formation, Aromas Sand, Monterey Formation and young alluvium — except in the Highlands, where it comes from granitic, metamorphic or hard sedimentary rocks.
• 100 wells and 70 household taps were sampled between October 2012 and May 2013. Household wells here are typically 7–275 metres deep, shallower than public-supply wells (20–600 metres).
• Earlier studies had found seawater intrusion and high nitrate, arsenic, boron, molybdenum and gross alpha radioactivity.

The four study areas and the sampled wells and taps. USGS.

How results are judged

Untreated groundwater is compared with drinking-water benchmarks — federal and California maximum contaminant levels, and non-regulatory secondary levels, health-based screening levels and lifetime health advisories. High means above a benchmark; moderate means above half of it for inorganic constituents, or a tenth for organic ones. Tap water can differ from groundwater because of plumbing, air or treatment — and household wells are not regulated in California.

Results

One or more inorganic constituents were at high levels in about 51 percent of the groundwater used by household wells; organic constituents in less than 1 percent.

Health-related inorganic constituents

• Trace elements: high in about 37 percent — arsenic 4 percent, boron 2, molybdenum 14, selenium 3, strontium 10 and manganese 6.
• Radioactivity, mostly from natural uranium and thorium: high in about 19 percent, as uranium, gross beta and adjusted gross alpha activity.
• Nitrate, generally from human activity such as fertiliser, septic systems and animal waste: high in about 14 percent.

Taste, colour and scaling

• Total dissolved solids (salinity): above California's upper limit in about 27 percent.
• Manganese or iron, released where water has little oxygen: high in about 31 percent. (Manganese's aesthetic limit is lower than its health-based level.)

Perchlorate — regulated in California since 2007, found in rocket fuel, fireworks, flares and some fertilisers, and naturally at low levels: high in less than 1 percent, moderate in about 35 percent.

Organic constituents

• Volatile organic compounds (solvents, gasoline components, refrigerants): none high or moderate; found at low levels in 17 percent.
• Pesticides: none high; the fumigant 1,2-dichloropropane was moderate in 0.7 percent.

What drives the differences

Nitrate, dissolved solids, molybdenum and adjusted gross alpha radioactivity were high or moderate in the largest shares of the groundwater.

• The Highlands had the most molybdenum, alpha radioactivity and dissolved solids; the Pajaro and Salinas Valleys the most nitrate; Santa Cruz the least molybdenum and nitrate.
• The differences reflect the rocks and sediments of each aquifer, chemical conditions underground, farming above, and other factors.
• Nitrate in the Salinas Valley rose with irrigated agriculture and the use of nitrate fertilisers and animal waste, and places now at moderate levels could reach high levels in future. Molybdenum, alpha radioactivity and dissolved solids are natural, from the rocks and sediments.

Share of household-well groundwater with high or moderate levels of four constituents, by study area. USGS.

About the programme

The Priority Basin Project assesses groundwater used for public and household supply in more than 120 basins and areas across California — untreated groundwater, not drinking water — combining a statistical network of wells with studies of what affects water quality, and analyses more sensitive than regulatory monitoring.

Sources

• Burton, C.A., 2018, Groundwater quality in the shallow aquifers of the Monterey Bay, Salinas Valley, and adjacent highland areas, California (ver. 1.1): U.S. Geological Survey Fact Sheet 2018–3026. https://doi.org/10.3133/fs20183026 · https://pubs.usgs.gov/publication/fs20183026
• Burton, C.A., and Wright, M.T., 2018, Status and understanding of groundwater quality in the Monterey-Salinas shallow aquifer study unit, 2012–13: U.S. Geological Survey Scientific Investigations Report 2018–5057.
• The study area, sampling, overall results, map, chart and closing section come from the fact sheet's PDF.
• Rewritten in hubnx's own words.