Each summer, Agency staff conducts an intensive groundwater level survey of the northern Salinas Valley. Groundwater levels (GWLs) are sampled at approximately 150 wells from Chualar to the coast, on a single day, to obtain a “snapshot” survey of conditions within and beyond the mapped extent of seawater intrusion. This is done during a time of the year when aquifers are most stressed from groundwater pumping. One purpose of the survey is to monitor and assess the forces driving seawater intrusion; in particular regional groundwater troughs and local inland-sloping groundwater level gradients. Both of these features tend to be most pronounced in summer, when pumping is at its seasonal peak and natural recharge does not occur.

Groundwater elevation contour maps are built from data collected for the Annual Fall and August monitoring programs and illustrate the locations and magnitude of groundwater troughs at the time of a given survey. Troughs are indicated by concentric contours with elevations decreasing toward the center. The contours are referenced to a mean sea level datum. Contours with a negative value represent water surface elevations below sea level. The more negative the value, the further below sea level the water surface lies. Aquifers within the Pressure Subarea are largely confined by overlying clay layers, resulting in pressurized conditions. Groundwater level contours in these aquifers are piezometric or “pressure head” surfaces. Aquifers in which pressure heads are chronically below sea level induce seawater to intrude and contaminate the aquifer.