North San Francisco Bay Groundwater: Public-Supply vs. Shallow Aquifers

By George L. Bennett V, U.S. Geological Survey, with the California State Water Resources Control Board. Fact Sheet 2018–3064, September 2018.

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 (GAMA-PBP) — assesses the quality of that water statewide and makes the results public.

Two studies, one place, two depths

The North San Francisco Bay study units and their sampled wells, between the Pacific and San Pablo Bay. USGS.

The two cover nearly the same ground but different depths. Each was split into two study areas:

• Valleys and Plains: alluvium-filled basins;
• Highlands: the surrounding volcanic, metamorphic and ultramafic hard rock.

The shallow aquifer (domestic wells) lies above the public-supply aquifer (deeper, high-yield wells). Not to scale. USGS.

Definitions:

• Public-supply aquifer: the depth zone tapped by public-supply wells — deeper, screened over long stretches, built for high yields.
• Shallow aquifer: above the productive zones; used by domestic and small community wells serving one to a few households, with low yields.

Method: most data came from USGS sampling (plus a state drinking-water database for the NSF-PA). Wells were chosen on a grid, giving unbiased estimates of how much of the aquifer has each constituent at low, moderate or high levels compared with EPA and California drinking-water benchmarks.

How the wells differ

• Depth: in the Valleys and Plains, median public-supply wells were 303 feet deep against 172 feet for shallow wells; in the Highlands, the gap was smaller.
• Age of the water: measured by tritium, a short-lived hydrogen isotope (half-life 12.32 years) spread through the air by nuclear weapons tests from the mid-1950s. Water is pre-modern (under 0.2 tritium units), mixed (0.2 to under 1) or modern (over 1). The shallow aquifer held significantly more modern water in both study areas — as expected of shallower wells. Young water can carry surface contaminants to a well quickly; old water has had more time to react with the rocks.
• Land use within 1,640 feet of each well was similar; more urban land around public-supply wells, especially in the Valleys and Plains, fits their being near towns.

Measuring quality

GAMA tests untreated groundwater, not tap water, but compares it with drinking-water benchmarks: legal maximum contaminant levels (MCLs) where they exist, otherwise health advisory levels, or secondary levels for taste and odor.

Differences were tested for statistical significance (two-by-two contingency tables, α = 0.1).

What was found

Inorganic constituents — few differences overall. But in the Valleys and Plains, the shallow aquifer had significantly more area with high levels of:

• any inorganic constituent with a health benchmark;
• trace elements as a group;
• nitrate;
• manganese.

Arsenic and manganese were the trace elements most often high. In the Highlands, high levels were more common in the public-supply aquifer; in the Valleys and Plains, in the shallow one. Geothermal systems feeding mineral-rich water into parts of the Valleys and Plains may explain the high trace elements.

Share of aquifer area with high (dark), moderate and low levels: A, any inorganic with a health benchmark; B, trace elements; C, nitrate; D, manganese — public-supply (purple) and shallow (green) aquifers, Highlands and Valleys and Plains. USGS.

Organic constituents:

• Detected in about 36 percent of the aquifer in the Valleys and Plains and 21 percent in the Highlands, in both studies; no significant difference between areas.
• High or moderate in only 3 percent of the shallow Valleys and Plains and public-supply Highlands.
• Only trihalomethanes and herbicides were found in more than 10 percent of any study area, plus carbon disulfide — mostly natural here. Trihalomethanes and herbicides were found more often in the Valleys and Plains; the only significant difference was herbicides in the public-supply Valleys and Plains.

Summary

The differences are real but subtle:

• the public-supply aquifer holds more pre-modern water; the shallow aquifer, more modern water;
• in the Valleys and Plains, the shallow aquifer has more area with high inorganic constituents, trace elements, nitrate and manganese;
• organic constituents were rarely high (under 2 percent in each study) and less common in the Highlands, where there is less urban land.

Sources

Based on Comparing Public-Supply and Shallow Aquifer Groundwater Quality in the North San Francisco Bay Aquifers, California, by George L. Bennett V, USGS Fact Sheet 2018–3064, U.S. Geological Survey; a work of the United States government in the public domain. It draws on Kulongoski and others (2010) for the 2004 study and Bennett (2018) for the 2012 study. The opening, the map, the aquifer diagram and the pie charts are taken from the fact sheet's PDF, which the import had left out.