Get to know your groundwater source in Sonoma County

The Sustainable Groundwater Management Act, the first law in state history to regulate pumping subsurface water, applies to about 9,000 wells in three groundwater basins in Sonoma County.

Reports by the county Water Agency in 2015 described conditions in the three basins, which essentially cover the county’s flatlands, also its biggest population and farming centers.

Santa Rosa Plain: Covers 78,720 acres from Rohnert Park and Cotati north to Windsor, including Santa Rosa and the east edge of Sebastopol. It has an estimated 6,000 wells.

Groundwater levels in the southern part of the plain declined in the late 1970s through the early 1990s, followed by recovery in the early 2000s. Water quality is generally high, with naturally occurring elements such as iron, manganese, boron and arsenic problematic in some areas and increasing chloride in southern parts of the plain.

Total groundwater use from 2004-2010 was estimated at 42,000 acre-feet, nearly half of total water consumption in the basin.

An acre-foot is 326,000 gallons - the amount of water that would cover 1 acre to a depth of 1 foot.

Members of the basin’s groundwater sustainability agency: Santa Rosa, Windsor Sebastopol, Rohnert Park and Cotati. Sonoma County, the Sonoma County Water Agency and Sonoma Resource Conservation District are members of all three local agencies.

Petaluma Valley: Covers 46,000 acres from Penngrove down to San Pablo Bay. It has an estimated 1,000 wells.

Groundwater levels in many areas are stable, with long-term declines in some parts of Petaluma Valley. Elevated levels of nitrate found in western portions of the valley were attributed to past land-use practices. Saltwater intrusion from the Petaluma River prior to 1962 was detected in shallow aquifers with no further intrusion as of 1982, but the risk of intrusion since then was deemed unknown.

Petaluma Valley groundwater use estimates were not available, according to the Water Agency. Annual recharge to the basin was estimated in 1982 at 40,000 acre-feet per year.

Members of the basin’s groundwater sustainability agency: Petaluma, North Bay Water District.

Sonoma Valley: Covers 44,700 acres from Kenwood to San Pablo Bay. It has an estimated 2,000 wells.

Groundwater levels in deep aquifers, primarily in southeastern and southwestern Sonoma Valley, have been declining for a decade or more. Water levels in many wells in these areas are dropping several feet a year and have fallen below sea level. Groundwater quality is generally good, except for brackish water affecting wells in southernmost Sonoma Valley and representing a threat if groundwater levels continue to drop.

Groundwater use in Sonoma Valley in 2012 was estimated at 10,500 acre-feet, representing nearly 60 percent of total water use. Studies indicate an average annual groundwater storage loss of about 1,400 acre-feet.

Members of the basin’s groundwater sustainability agency: Sonoma, Valley of the Moon Water District, North Bay Water District.