Todd Engineers

In December 2007, Todd Engineers, Kennedy/Jenks Consultants, and Lawrence Berkeley National Laboratory completed a three year investigation conducted for the County Sanitation Districts of Los Angeles County. The studies were designed to assess the transport and fate of n-nitrosodimethyamine (NDMA) between wastewater treatment plants (WWTPs) and groundwater. While there is currently no enforceable drinking water standard for NDMA, the California Department of Public Health has established a notification level of 10 nanograms per liter (ng/L) for NDMA in drinking water. As shown in the map, highly treated wastewater from the WWTPs is incidentally recharged along unlined portions of the San Gabriel River and Rio Hondo and actively recharged at instream facilities on the San Gabriel River and at the Montebello Forebay Spreading Grounds. NDMA is both formed and degraded in the wastewater treatment process, and levels in effluent can vary substantially with different treatment processes.

The study found that NDMA is attenuated in both surface water and groundwater systems. First, water quality data from surface water sampling demonstrated that NDMA is significantly attenuated in surface water due to ultraviolet degradation from exposure to sunlight. Based on the data, a half-life of 2.2 hours in surface water was estimated for NDMA.

Second, biodegradation of NDMA in groundwater was demonstrated based on analysis of changes in mass over time and through numerical groundwater modeling.

Key Issues
Assessment of fate and transport of NDMA Potential groundwater impacts of NDMA in recycled water Regulatory effluent limits

The estimated mass of NDMA in groundwater was estimated for both September 2006 and July 2007. The September 2006 conditions represent groundwater impacts from relatively high effluent concentrations (several thousand ng/L), while the July 2007 conditions reflect changes in wastewater treatment processes that resulted in reduction of NDMA effluent concentrations to several hundred ng/L. Over that time span, NDMA concentrations decreased from 3.1 to 1.1 kilograms (kg). This mass reduction (about 65%) indicates that NDMA is likely being removed in the groundwater system due to biodegradation. If NDMA were not removed by biodegradation in groundwater, the size of the area of detectable NDMA and the NDMA mass would increase over time as more NDMA is continually added to the system.

The following maps show NDMA groundwater concentration contours in September 2006 and July 2007. This work is the first comprehensive investigation showing direct evidence of NDMA biodegradation in groundwater under field conditions.

NDMA groundwater concentrations in September 2006
NDMA groundwater concentrations in July 2007

A FEFLOW groundwater flow and transport model was also constructed to predict attenuation and dilution of NDMA in the southern San Gabriel Groundwater Basin, Whittier Narrows, and Montebello Forebay portion of the Central Basin. The model was calibrated to observed groundwater levels and NDMA concentrations. The calibrated groundwater biodegradation half-life was 69 days over most of the model domain, and provided a very good match between simulated and monitored concentration-time profiles at several shallow monitoring wells. The calibrated NDMA biodegradation rate in groundwater also is very close to laboratory results presented in the scientific literature.

This project included data compilation and management; installation and testing of monitoring wells and soil borings; a program of synoptic stream gauging; a comprehensive multi-year water quality monitoring program of effluent, surface water, recharge water, and groundwater; development of a conceptual hydrogeologic model; and development, calibration, and application of the coupled surface water and groundwater mathematical model.