Castroville Seawater Intrusion Project Overview

All wastewater, regardless of where it originates, enters the Monterey One Water Regional Treatment Plant for primary and secondary wastewater treatment. After secondary treatment, the effluent reaches a fork in the road. Here the water is safe and it has two potential route options.  1) ocean discharge via our outfall pipe, 2) it is pumped to one of our two water recycling projects: the Castroville Seawater Intrusion Project (CSIP) or the Pure Water Monterey Groundwater Replenishment Project.

Quick Stats

Source Water:

4 Types of Wastewater 

are diverted to Monterey One Water for treatment prior to purification: 1) municipal wastewater, 2) industrial processing water, 3) crop drainage water, and 4) urban stormwater runoff.


Salinas Valley Reclamation

After treatment, the Recycled water is held temporarily in an 80-acre foot storage pond before it is distributed to farmlands in the CSIP system.  


3 step treatment process

  • Flocculation
  • Tertiary filters
  • Chlorine disinfection


4 billion gallons

of recycled water produced a year for food crop irrigation to further reduce groundwater pumping in the vulnerable CSIP area. 

Monterey Wastewater Reclamation for Agriculture Study

In the mid-1970s, a group of community leaders began discussing the idea of recycling wastewater. The objective was to retard the advance of seawater intrusion by supplying irrigation water to farmland in the northern Salinas Valley. This would significantly reduce the draw of water from the underground aquifers.

This led to an extensive 11-year Monterey Wastewater Reclamation for Agriculture Study (PDF) which began in 1976. The final results of this research proved that recycled water is safe for the irrigation of food crops that are consumed without cooking. Today, this definitive report is used as the standard in countries all over the world.

By 1983, several phased construction projects were completed utilizing the old coastal treatment plants as pumping stations, connected to one of 3 interceptor pipelines, and a 60-inch diameter outfall pipeline extending 2 miles into the Monterey Bay. Visit the Wastewater Conveyance page for additional information.

Offering economies of scale and concentration of expertise, M1W proposed to operate one regional plant by replacing eight older (and some overloaded) wastewater facilities in Northern Monterey County.