Slowing Seawater Intrusion

To our knowledge, MRWPCA operates the world's largest water recycling facility designed for raw food crop irrigation

The primary source for water in Monterey County is from aquifers hundreds of feet below the ground. The reserve is diminishing as the number of farms, businesses and residences have increased. So much water has been removed, in fact, that intruding seawater has come within two miles of Salinas's wells.

In addition to threatening the drinking water supply, seawater intrusion threatens the region's multi-billion dollar agricultural economy.

In the mid 1970s, a group of community leaders began discussing the idea of recycling wastewater. This led to the extensive five-year Monterey Wastewater Reclamation for Agriculture Study that began in 1980. The final results of this research proved that recycled water is safe for the irrigation of crops that are consumed without cooking. Today, this definitive report is used as the standard in countries all over the world.

In 1992, MRWPCA and the Monterey County Water Resources Agency formed a partnership to build two projects: a water recycling facility at the Regional Treatment Plant; and a distribution system including 45 miles of pipeline and 22 supplemental wells. Its objective was to retard the advance of seawater intrusion by supplying irrigation water to nearly 12,000 acres of farmland in the northern Salinas Valley. This would significantly reduce the draw of water from the undergound aquifers. The $75 million projects were completed in 1997 after three years of construction.

The use of highly treated wastewater to irrigate landscaping has been practiced for years, yet for food crops, it is relatively new. The recycled water facility is capable of producing an average of 29.6 million gallons of recycled water per day. This is the equivalent of one foot of water over 91 acres of land.

In the future, MRWPCA plans to additionally supply recycled water to city parks, roadway landscape and golf courses.

Changing Wastewater into Safe Water

Wastewater entering the Regional Treatment Plant from homes and businesses passes through primary, secondary and tertiary treatments that clarify and extract sediment. Primary treatment consists of gravity separation; secondary treatment utilizes microscopic organisms found naturally in the environment. The tertiary, or recycled water treatment, moves secondary treated wastewater through the water recycling facility where it is further filtered and disinfected.

During the filtration process, treated water filters through a 6-foot bed of coal, sand and gravel in which minute particles are trapped. This is the same as the filtering process performed for drinking water.

The disinfection process destroys bacteria and germs by maintaining a specific chlorine level in the water for two hours. The final product is clear, odorless and safe to use for irrigation.

Technicians perform frequent water quality tests and monitor the system to ensure that safety standards are being maintained.

After treatment, the recycled water is held temporarily in an 80-acre/foot storage pond before it is distributed to farmlands via an underground pipeline system. During the rainy season, when the growers don't need the treated water, it is safely discharged two miles into the Monterey Bay.