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RISK ASSESSMENT OF DECENTRALIZED WASTEWATER TREATMENT SYSTEMS IN HIGH PRIORITY AREAS IN THE CITY OF MALIBU, CALIFORNIA – CONCEPTUAL HYDROGEOLOGIC MODEL

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Paper number  701P0104,  . (doi: 10.13031/2013.15757)
Authors:   D. Morrissey, M. K. Clark, B. Douglas, A. Macrellis, V. Peterson, and C. Dean
Keywords:   bacteria, California, data analysis, environmental impact, environmental protection, groundwater, hydrologic modeling, mathematical models, nitrate, risk assessment, B30wastewater

The City of Malibu, California is conducting a risk assessment to evaluate environmental impacts of current onsite wastewater management practices. Malibu is located on the north shore of Santa Monica Bay, northwest of Los Angeles. The project will use a three-dimensional numerical modeling tool to evaluate the impacts of onsite wastewater treatment systems on groundwater quality and to delineate directions of groundwater flow from major wastewater dispersal areas.

A review of existing reports, studies, and other data was used to develop a conceptual understanding of sources and sinks of groundwater, nutrients, and bacteria in the hydrogeologic system. Sources of water to the system include recharge from upland runoff, recharge from approximately 430 onsite and off-site wastewater systems, infiltration of precipitation, and infiltration from Malibu Creek. Sinks for groundwater include discharge to the Pacific Ocean and Malibu Lagoon.

Groundwater quality in the study area is variable depending on the location of the monitoring point relative to onsite wastewater dispersal systems and other sources of contamination. A total of 20 monitoring wells are being sampled for a one-year period to enhance the conceptual understanding of the system and to quantify the impact of onsite wastewater disposal systems on water quality.

The risk assessment portion of this project will evaluate the appropriate siting of groundwater monitoring wells and the impact both of current wastewater disposal systems and of newly installed advanced treatment systems on groundwater quality. Malibu will use the outcome of this risk assessment approach to refine its citywide onsite wastewater management program.

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