Groundwater Quality in the Santa Barbara Coastal Plain, California

Groundwater supplies more than 40 percent of California's drinking water. The State's Groundwater Ambient Monitoring and Assessment (GAMA) Program, through a Priority Basin Project run with the U.S. Geological Survey, assesses groundwater quality statewide and makes the results public. The Santa Barbara Coastal Plain is one of its study units.

The study unit

More than 48 square miles of coastal plain in Santa Barbara and Ventura Counties, between the Santa Ynez Mountains and the Pacific Ocean: five groundwater basins — Goleta, Foothill, Santa Barbara, Montecito and Carpinteria, west to east. The climate is Mediterranean, with hot, dry summers and cool, moist winters and about 17 inches of rain a year. Mission Creek and other intermittent streams run south from the mountains to the sea.

The study unit, from Goleta to Carpinteria: urban (pink), agricultural (green) and natural (gray) land between the Santa Ynez Mountains and the Pacific. USGS.

• Land: about 64 percent urban, 24 percent natural and 12 percent farmland, mostly in the east and on the western outskirts, on coastal hills sloping to the sea.
• Aquifers: mainly Quaternary alluvium, over the Santa Barbara and Casitas Formations. Public-supply wells are 150 to 1,230 feet deep, cased solid for the top 110 to 350 feet and open below.
• Recharge: rain and irrigation water soaking in, imported water leaking from canals and aqueducts, and seepage from streams. Discharge: pumping for municipal supply, underground flow to other basins, and evaporation.

The project samples the part of the aquifer used for public drinking water — untreated water, not what reaches the tap — and compares it with drinking-water benchmarks. Above a benchmark counts as high; above half a benchmark (a tenth, for organic and special-interest constituents) as moderate. Data were collected in 2011 and supplemented with state records from 2008–11.

Share of the aquifer area at each level. USGS.

Constituents with health benchmarks

• Trace elements and minor ions — high in about 5 percent of the aquifer system (only boron and fluoride) and moderate in about 11 percent.
• Radioactive constituents — from natural uranium and thorium in the aquifer sediments — never high, moderate in about 17 percent.
• Nitrate — usually from fertilizer, septic systems and human and animal waste — high in about 1 percent and moderate in about 16 percent.
• Volatile organic compounds — none high. The gasoline additive MTBE was moderate in about 11 percent; the solvents 1,2-dichloroethane and perchloroethene in 6 and 1 percent.
• Pesticides — none high or moderate. The herbicide simazine was the only one detected.
• Perchlorate — used in explosives, rocket fuel, fireworks and flares, found in some fertilizers and at low levels in rain, and regulated in California drinking water since 2007 — never high, but moderate in 50 percent.

Taste, staining and scaling

• Total dissolved solids (TDS) — high (above the upper limit) in about 32 percent of the aquifer system and moderate in about 63 percent.
• Iron, manganese or both — released from minerals where groundwater has little oxygen — high in about 37 percent and moderate in about 21 percent.

Why so much dissolved solids

High TDS (above 1,000 parts per million) is the unit's notable problem. Across seven coastal basin study units in Central and Southern California, it ranged from 2 to 35 percent of the aquifer, with Santa Barbara and the Santa Clara River Valley the highest. Water high in TDS usually also had high or moderate sulfate or chloride (limit 500 ppm each); sulfate was the more common in five of the seven units, Santa Barbara among them. Possible sources include seawater intrusion, upwelling of connate saline water, reactions with rock, evaporation from shallow groundwater, and recharge from irrigation return flow, wastewater and runoff.

TDS, sulfate and chloride at moderate and high levels in seven coastal study units, from San Francisco Bay to San Diego. USGS.

The seven coastal study units compared. USGS.

Sources

• Tracy A. Davis and Kenneth Belitz, Groundwater Quality in the Santa Barbara Coastal Plain, California, USGS Fact Sheet 2016–3058, August 2016, with the California State Water Resources Control Board. https://doi.org/10.3133/fs20163058
• GAMA Program: http://ca.water.usgs.gov/gama · http://www.waterboards.ca.gov/gama