How Geology Shapes Well Water in California's Northern Sierra Nevada Foothills

The study region's generalized geology, watersheds and sampling sites. U.S. Geological Survey.

The question

About 2 million Californians drink groundwater from domestic wells. The state and the U.S. Geological Survey (USGS) created the Groundwater Ambient Monitoring and Assessment Program Priority Basin Project (GAMA-PBP) to assess that water and find where drinking-water resources are vulnerable.

Many rural homes in the northern Sierra Nevada foothills pump from fractured-bedrock aquifers, and the region's varied geology can change groundwater chemistry and quality over short distances. In 2015–17, GAMA-PBP sampled 142 domestic wells and springs in the Yuba, Bear, North American, South American, Cosumnes and Mokelumne River watersheds, chosen for their many well-dependent households.

A patchwork of rock

The foothills' bedrock began forming more than 100 million years ago, as ocean floor collided with western North America. As heavy ocean crust slid beneath the continent, magma rose and cooled into huge bodies called plutons, the backbone of the range. The foothills sit in the Western Metamorphic Belt, where slices of ocean crust were scraped onto the growing mountains, metamorphosed by heat and pressure, and intruded by plutons. Folds and steep faults are common, especially in the Melones fault zone in the northwest.

The study grouped the rocks into five aquifer types:

Wells here tap the old fractured bedrock, not the younger glacial and river sediments; most sampled wells were in granitic or metavolcanic rock.

What was found

Domestic well water isn't regulated, but the study compared it with public drinking-water benchmarks — health-based, or aesthetic (color, taste, odor). For hardness (dissolved calcium and magnesium), it used the "very hard" level of 180 milligrams per liter as calcium carbonate.

• Health: only 5 percent of wells exceeded a health benchmark for a naturally occurring trace element or radioactivity indicator (arsenic, barium, boron, molybdenum, strontium, alpha radioactivity).
• Aesthetics: 43 percent exceeded an aesthetic benchmark — iron (12 percent), manganese (23 percent) or hardness (27 percent). Iron and manganese give water a metallic taste, stain it, and form oxides that clog pipes; hard water weakens soap — though calculations suggest mineral scale is unlikely to clog pipes here.

Wells exceeding benchmarks for trace elements and radioactivity (A), iron (B), hardness (C) and manganese (D), by rock type. U.S. Geological Survey.

Why the rock matters

• Health exceedances occurred only in mafic-ultramafic, granitic and metavolcanic aquifers, which also had the hardest water — over 40 percent of mafic-ultramafic and metavolcanic sites exceeded the hardness benchmark. Their serpentine minerals weather into alkaline water rich in trace elements and magnesium.
• Iron and manganese exceeded benchmarks at about one third of sites, highest in metasedimentary aquifers, whose marine sediments carry iron and manganese oxide coatings that dissolve in low-oxygen water. Manganese also exceeded the benchmark at about 20 percent of granitic and metavolcanic sites; the northwest's ophiolites hold concentrated manganese deposits.
• Volcanic aquifers had high oxygen (about 8 mg/L) and low pH (about 5.5) — like rain or snowmelt that has barely reacted with rock — and the best water: ridgetop wells produced particularly high-quality groundwater.
• Higher pH with more trace elements and hardness in mafic-ultramafic and metavolcanic aquifers points to weathering serpentine in the Melones fault zone, where faults may also let deeper, more reacted groundwater rise toward shallow wells.

Dissolved oxygen and pH by aquifer rock type. U.S. Geological Survey.

What it means

Very few foothill wells exceeded health benchmarks. The main problems are aesthetic — iron, manganese and hardness — but they can be costly to treat, so rock type and water chemistry are worth weighing when more homes turn to these aquifers.

Sources

Based on Geologic Influences on the Quality of Groundwater Used for Domestic Supply in the Northern Sierra Nevada Foothills, California, U.S. Geological Survey Fact Sheet 2021–3013, by Zeno F. Levy and Miranda S. Fram, USGS Publications Warehouse; a work of the United States government in the public domain. Figures recovered from the fact sheet's PDF.