Groundwater Monitoring San Mateo Plain Basin

The County of San Mateo Sustainability Department administers groundwater monitoring in the San Mateo Plain Subbasin (Basin) of San Mateo County. Following completion of the San Mateo Plain Groundwater Basin Assessment in 2018, the Basin has been under continuous groundwater monitoring and is sampled biannually with data uploaded to the California Statewide Groundwater Elevation Monitoring (CASGEM) online system. The California Department of Water Resources (DWR) has approved the County's CASGEM Monitoring Report, and the Basin has maintained a Very Low Priority status through continued sampling and reporting.

The CASGEM Program, created by the DWR under SBx7-6, establishes a permanent, locally managed system for tracking groundwater elevations. Local agencies can volunteer to be “Monitoring Entities,” responsible for overseeing groundwater monitoring in their basins. If local agencies do not assume this role, DWR will default to managing it, potentially affecting the eligibility for certain state funding.

The goal of this project is to maintain the Very Low Priority status through continued biannual sampling and reporting, which is crucial for regulatory compliance and securing funding.

The San Mateo Plain Subbasin (Basin) is one of four subbasins within the Santa Clara Valley Groundwater Basin, as defined by the Department of Water Resources (DWR). Spanning approximately 37,708 acres, this Basin is situated along the eastern edge of the San Francisco Peninsula, nestled between San Francisco Bay and the Santa Cruz Mountains. The Basin occupies a structural trough filled with unconsolidated alluvial sediments, with bedrock cropping up at shallow depths in the northern end near Hillsborough and San Mateo, and a notable bedrock hill at the San Francisco Bay shoreline. To the south, the Basin is bounded by the San Mateo-Santa Clara County Line, following the course of San Francisquito Creek, which runs through its alluvial fan.

The Basin serves a population of about 292,000, encompassing 13 cities and unincorporated areas across Santa Clara and San Mateo Counties. Historically, large portions of the Basin were tidal marshes, which have since been diked, filled, and urbanized starting as early as 1873. Despite this urban development, some areas still retain marshes or salt evaporation ponds. The land use within the Basin is predominantly urban, extending westward from the coastal plain into the upland regions.

Groundwater production in the Basin has been relatively modest over recent decades, with most water supplied by the Hetch Hetchy system from the San Francisco Public Utilities Commission (SFPUC). Although there is some groundwater use for domestic and landscaping purposes, especially in the southern part of the Basin, it remains limited. Historical groundwater pumping, which peaked before 1960, caused significant drops in groundwater levels, leading to issues like saltwater intrusion and land subsidence. However, groundwater levels have stabilized since then. A 2018 San Mateo Plain Groundwater Basin Assessment confirmed that the Basin could maintain its 'low priority' status, underscoring the need for ongoing groundwater monitoring amidst renewed interest due to recent drought conditions.