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Multiple Benefits of Treatment Wetlands

As we develop the Multi-Benefit Land Repurposing Program, it's insightful to consider successful examples from the Salinas Valley, such as the Castroville Slough Treatment Wetland. This project showcases the potential of multi-benefit initiatives.

Aerial view of the Castroville Slough Treatment Wetland

Completed in 2016 and located off of Hwy 1 just south of Elkhorn Slough, the Castroville Slough Treatment Wetland provides 18 acres of restored freshwater wetland habitat in the Moro Cojo Slough. In addition, it removes pollutants from water coming from the surrounding farms. Water drained from approximately 800 acres of farmland is pumped into the wetland system and then fed by gravity through a 1.25 km sinuous channel before flowing into Castroville Slough, out to the Moro Cojo Slough, and eventually into the Monterey Bay.

Water entering the wetland contains high concentrations of nitrates, ranging from 30-60 mg/L, but by the time the water leaves the wetland about 3 days later it contains less than 1 mg/L of nitrates. High concentrations of nitrates are harmful to both humans and aquatic species. For humans the drinking water limit for nitrates is 10 mg/L, and in aquatic habitats high concentrations of nitrates can cause large algal blooms that reduce the available oxygen for fish and other species, leading to die-off events.

After the construction of the Castroville Slough Treatment Wetland, current water quality monitoring downstream at the mouth of the Moro Cojo Slough shows the slough to be meeting the water quality objectives set by the Central Coast Regional Water Quality Control Board, the state agency responsible for regulating water quality. This project illustrates the effectiveness of treatment wetlands for achieving real water quality improvements and is a case study we often refer to when working with partners to develop projects for the Multi-benefit Land Repurposing Program.

So how does a wetland remove excess nitrates and other nutrients from polluted water? Through a series of naturally occurring physical, chemical and biological processes. One of the key processes for reducing nitrates is carried out by bacteria that thrive in the low oxygen, water saturated environment of wetland soils. These bacteria metabolize nitrates, which are harmful in high concentrations, transforming them into inert nitrogen gas that is released into the atmosphere.

A diagram showing the physical, chemical, and biological processes that occur in wetlands to how f

Through studying the nitrate reduction capacity of the Castroville Slough Treatment Wetland, the Central Coast Wetlands Group has developed a tool to calculate the necessary size of a treatment wetland based on the starting nitrate concentration, amount of time water spends in the wetland, and objective output nitrate concentration. When developing projects for the Multi-benefit Land Repurposing Program this tool can help us estimate the expected water quality benefits from a potential treatment wetland. We can use this estimation of benefits to work with partner agencies and farmers to decide if a project is useful as designed, whether the design needs to be reassessed, and what other projects and practices are needed to meet the water quality goals for a sub-watershed.

The Castroville Slough Treatment Wetland has been a huge success. Not only has it provided substantial water quality and habitat benefits, but it is an example and guide for what can be done with repurposed lands. It gives shape to our vision for what we hope to achieve with the Multi-benefit Land Repurposing Program.


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