Groundwater Level Monitoring
An estimated 95 percent of all water used in Monterey County is derived from groundwater wells. With nearly 200,000 acres of land under cultivation in the Salinas Valley, agricultural pumping exceeds 495,000 acre-feet per year. Combined with urban and other uses, total water pumped in the Salinas Valley is approximately 520,000 acre-feet per year. Assuring that there are sufficient quantities of good quality groundwater is the most important aspect of managing water resources in Monterey County today.
Map of the Salinas River Groundwater Basin subareas
The ability of groundwater supplies to meet pumping needs can be determined through monitoring fluctuations in groundwater levels and the corresponding changes in groundwater storage. Programs of consistent well measurement have been ongoing since the 1940s when Monterey County and the State of California began investigations to determine the cause of seawater intrusion along the coast. They found that intrusion was caused by pumping the basin beyond rates that it could recharge through the natural groundwater system. The aquifers near the coast are covered by heavy clays and receive little direct recharge from the overlying land surface or the Salinas River. Therefore, replenishment to these aquifers must travel from inland sources through the aquifer which acts as a conduit confined by the overlying clays. If water is pumped at a rate faster than it can be replenished by the conduit, then water pressure in the pressurized aquifer is lowered and seawater is allowed to move into the aquifer. The first steps in dealing with the intrusion problem was to increase water supplies through the construction of the dams. Along with the construction of the dams and other water supply projects came the need to monitor replenishment to the basin and also to account for water being added to the basin for water rights purposes. Continuation of the water level measurement program was a natural course of action to provide the needed information.
Changes in groundwater storage occur annually in the unconfined aquifers throughout the County. No significant change in storage occurs in areas where groundwater aquifers are overlain or confined by continuous beds of impermeable clay. In these strata, the seasonal fluctuation of subsurface water levels is determined by the hydrostatic pressure distribution within the aquifer rather than a change in the storage volume. These pressurized groundwater strata are termed “pressure aquifers.”
Staff taking a groundwater level measurement
Surveys to determine the fluctuations in groundwater levels are strategically conducted by the MCWRA. Existing production wells are used, with the owner’s permission, to facilitate these investigations. A number of dedicated monitoring wells have been constructed to augment this effort, but because of the additional cost of these wells, it is more cost effective to utilize existing wells. High production agriculture wells are used most often because they extract a substantial amount of water and provide better representation of current aquifer conditions. When practical, these designated study wells are used for the collection of both water level and water quality data.
The groundwater survey program consists of measurements of key wells to monitor monthly fluctuations, and annual measurements of an established network of wells to determine relative changes in storage. A survey is also conducted each August to monitor changes in coastal groundwater zones that will affect the inland movement of seawater. In addition to these three surveys, the MCWRA maintains a network of dedicated monitoring wells instrumented with electronic data loggers that record data at regular intervals. Each of the water level surveys are discussed in more detail in the following paragraphs.