Sifting the sands

The safety and quality of private septic systems are currently under scrutiny by the Regional Water Quality Control Board and the subject of proposed legislation, recently defeated in the State Senate.

A long-term study completed in June indicates the possibility of pathogens from septic systems contributing to pollution in Malibu Creek and Lagoon is of “minimal concern.” However, sediment samples taken at three major storm drains contained bacteria that may have come from septic overflows to drains.

The septic study performed by environmental research firm Woodward Clyde was limited to investigating one commercial septic system in the Civic Center area, the leach field of which is closest to Malibu Creek.

A very large dose of a nonpathogenic biophage (PRD-1) that mimics an enteric virus was introduced through a toilet. Ground water was sampled immediately below the test leach field and at distances of approximately 10 feet, 80 feet and 150 feet from the edge of the leach field. No PRD-1 was detected in the ground water at any of the sampling locations throughout the testing period.

A similar dose of MS-2 was introduced directly into the groundwater below the test leach field. It was detected a distance of 10 feet away within 24 hours. There was no detection at 80 feet.

The study showed pathogens are effectively removed from effluent as long as a minimum of 1-1/2 feet of unsaturated soil exists below the leach field. Thus bacteria and viruses are not reaching groundwater and therefore not contaminating the creek/lagoon or surf zone as long as groundwater separation is maintained.

However, if groundwater levels rise to reach the bottom of the leach field, then pathogens may be transported in the groundwater over distances that depend upon the hydraulic gradient (ordinarily less than 1 foot of drop in 100 yards in the Civic Center area). Based on the low hydraulic gradient, the groundwater flow rate is estimated at approximately 1 to 4 feet per day. To increase this flow rate to mimic a worst-case condition, the tracer and microbial transport testing was done using a pump that induced a gradient of more than 6 feet of drop in 100 yards. This would be comparable to the rate during a lagoon breaching event.

Since an actual breaching event did not occur during the testing period, experts say further study is necessary to determine just how far pathogens could travel under the worst-case condition and how much groundwater levels rise during extended periods of lagoon closure when the lagoon water level is highest.

Groundwater levels were found to be within 5 to 7 feet below ground surface throughout the study period. This means there was between 1.5 feet and 4 feet of unsaturated soils between the bottom of the leach field and groundwater. The lagoon was open to the ocean throughout the test period.

Nitrate and phosphate levels in groundwater were below 10 mg per liter, suggesting the nutrients are being effectively removed from effluent as it passes through unsaturated soils beneath the septic leach fields.

Conditions at three major storm drain outfalls were another matter. Sediment sampling found levels of bacteria and sterols indicative of impact by fecal matter, probably from both human and animal sources. Highest levels were found at the Cross Creek Road drain outlet into Malibu Creek just north of the PCH bridge. Testing of sediments for linear alkyl benzene compounds (surfactants found in detergents) indicate ongoing contamination from detergents. Surface runoff from parking lot washing, restaurant washing and gray-water systems are the most likely sources and need further investigation, the report states.

As a follow up to this study, the City Council has approved undertaking an extensive water quality monitoring program this summer. Who will do this and how and where it will be done have yet to be decided.

13StarsManager
13StarsManagerhttps://malibutimes.com
The Malibu Times is the first newspaper in Malibu, serving the community since 1946.

Related Articles

Advertisement

Advertisement

Latest Articles

%d bloggers like this:
×