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Groundwater Level Monitoring

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An estimated 95 percent of water used in Monterey County is extracted by wells from groundwater aquifers. With nearly 200,000 acres of land under cultivation in the Salinas Valley, total water pumped in the valley is approximately 500,000 acre-feet per year, of which about 450,000 acre-feet are used for agriculture. Assuring sufficient quantities of good quality groundwater is central to managing water resources in Monterey County today.

Map of the Salinas River Groundwater Basin subareas


Changes in groundwater storage occurring within unconfined or “water table” aquifers throughout the County (in the East Side, Forebay and Upper Valley Subareas) are tracked by monitoring water levels. In the confined aquifers of the Pressure Subarea of the northern Salinas Valley, however, seasonal fluctuations of subsurface water levels (or more accurately “pressure head” levels) result from changes in hydrostatic pressure distribution within the aquifer rather than changes in storage volume.

When groundwater from coastal aquifers is extracted (pumped) at a faster rate than it can be replenished, water pressure in these “confined” aquifers drops, resulting in lower groundwater levels. Chronically low coastal groundwater levels have enabled seawater to move inland into freshwater aquifers.

photo of staff measuring well
Staff taking a groundwater
level measurement

Groundwater levels, as measured in water wells, are indicators of both the quantity of groundwater in storage and the susceptibility of coastal aquifers to the intrusion of seawater. Well level measurement programs in the Salinas Valley began in the 1940s with state and county investigations into the causes and nature of seawater intrusion, which was first observed in the 1930s. By monitoring groundwater levels throughout the Salinas Valley over time, the Agency has been able to track storage changes and provide an ongoing assessment of the ability of groundwater supplies to meet pumping demands. Long-term monitoring is fundamental to understanding and addressing seawater intrusion.

In the Salinas Valley Groundwater Basin, seawater-intruded coastal aquifers are overlain with low-permeability clays and thus receive little direct recharge from either the overlying land surface or Salinas River percolation. Replenishment to these confined aquifers, is derived primarily from adjacent inland aquifers.

The Agency’s first approach to slowing seawater intrusion was to construct the Nacimiento and San Antonio dams in 1957 and 1967, respectively, which increased replenishment of over-pumped aquifers through percolation of water released from the reservoirs. Groundwater level monitoring programs are key to enabling the Agency to assess the effectiveness of basin replenishment and account for water rights associated with the dams and subsequent water projects.

The Agency conducts monthly and annual surveys to monitor seasonal fluctuations and long-term changes in groundwater levels throughout the Salinas Valley Groundwater Basin. A survey is also conducted each August to monitor changes in coastal groundwater zones that most directly affect the inland movement of seawater. Data are collected from over 450 wells in these investigations. Because of the prohibitive cost of constructing dedicated monitoring wells, privately owned agricultural production wells are the primary source of groundwater level data collected by Agency staff. When practical, these wells are used for the collection of both water level and water quality data and thereby provide an understanding of current aquifer conditions.

Of the over 450 program wells twenty-seven are Agency-owned small diameter monitoring wells. These include a network of dedicated monitoring wells instrumented with electronic data loggers that record hourly groundwater levels. Data from these instrumented wells are used to evaluate, verify and supplement groundwater level data collected in each of the other monitoring programs. Each of the Agency’s water level surveys are discussed in more detail in the following paragraphs.

Pages Under Groundwater Level Monitoring

Monterey County Water Resources Agency
1441 Schilling Pl., North Bldg., Salinas, CA 93901

    Phone (831) 755-4860   |   FAX (831) 424-7935   |   After Hours (831) 796-1166