Hydrologic and geochemical characterization of the Petaluma River watershed, Sonoma County, California

Principal water agencies in the Petaluma Valley groundwater basin seek to understand the availability of groundwater to meet increasing regional water supply demand caused by increasing agricultural and urban water demands. Surface water from the Russian River is the primary water supply in the basin, but the future availability of this surface-water supply is uncertain. Groundwater is an important supplemental source of water for the City of Petaluma and the primary supply for agriculture and domestic use by rural property owners. Furthermore, supplies of recycled water are becoming more available. Water managers face the challenge of meeting increasing water demand with a combination of surface water, local groundwater resources, and use of recycled wastewater.

In 2014, California adopted legislation to manage groundwater: the Sustainable Groundwater Management Act (SGMA). The SGMA requires the development and implementation of Groundwater Sustainability Plans (GSPs) in 127 medium- or high-priority groundwater basins, and the Petaluma Valley groundwater basin was designated as a high-priority basin. Sustainability is defined within the SGMA in terms of avoiding undesirable results: significant and unreasonable groundwater-level declines, reduction in groundwater storage, seawater intrusion, water-quality degradation, land subsidence, and surface-water depletion. The U.S. Geological Survey (USGS), in cooperation with the Sonoma County Water Agency (SCWA) and the City of Petaluma, characterized the hydrology of the Petaluma valley watershed (PVW) and developed an integrated hydrologic model that can be used to improve understanding and management of the groundwater system and develop a GSP.

The objectives of the study are to (1) develop an updated assessment of the hydrogeology and geochemistry of the PVW and (2) develop an integrated hydrologic model for the PVW. The purpose of this report is to describe the conceptual model of the hydrologic, hydrogeologic, and water-quality characteristics of the PVW and a numerical groundwater-flow model of PVW.