The Ventura County Watershed District will build five new wastewater-filtering systems in Oak Park this fall in an attempt to reduce pollution downstream.

Last month, the Ventura County Board of Supervisors approved a $146,600 contract with Santa Paula-based Aguilera Brothers Construction Inc. to install urban runoff pollution reduction systems adjacent to stormwater catch basins near Medea Creek.

“The object of the project is to identify effectiveness of these units and install more in the future,” said Ricardo Moreno, water quality planner for Ventura County Watershed Protection District officials.

“The systems will capture dry weather runoff water before it goes in and after it comes out to test and help regulate discharge,” Moreno said.

The new catch basins will be at the end of a cul-de-sac on Tamarind Street, on the northeast and northwest corners of Conifer Street and Smoke Tree Avenue, at the end of Medea Creek Lane, and near a bridge on Oak Hills Drive.

During a recent presentation in Oak Park, watershed representatives said new technology available allows effective treatment with low maintenance.

The inlets will look like typical concrete stormwater inlets but include an ornamental tree on top. The tree will absorb nutrients from the wastewater.

Residents who live near the new retention filters will have the option to choose one of three trees proposed for the sites. The trees are western redbud, desert willow and crape myrtle.

Construction will begin in September and take about two months to complete.

Surface water flowing into the new inlets will pass through a filter made up of mulch, peat, sand and crushed rock before collecting in an outfall pipe.

The filter controls bacteria, pathogens and other pollutants stemming from trash, animal waste, irrigation runoff, faulty septic tanks and leaking sewer pipes before water is discharged through existing drains.

“The driving force for this project is that Malibu Lagoon is impaired by bacterial pollution originating upstream,” Moreno said.

Federal and state laws require cities, sewer treatment plants and other public agencies to comply with strict guidelines governing water pollution.

Moreno said bacteria levels exceeded quality objectives at four out of seven sampling locations within the unincorporated areas flowing into the Malibu Creek watershed.

Oak Park is one of the four locations that exceeded bacteria levels.

The excesses could trigger penalties by the Regional Water Board against the watershed district and Ventura County, Moreno said.