Map showing the coverage area of the model
The domain of the surface water and groundwater flow model

Key Issues

  • NDMA is both created and destroyed in wastewater treatment processes
  • NDMA has a drinking water notification level in groundwater of 10 parts per trillion
  • Understanding of the fate and transport of NDMA is needed to support determination of appropriate effluent limits

Solutions

  • Field work in combination with modeling
  • A comprehensive field data collection program to provide specific data
  • A technically-robust surface water and groundwater flow and transport model to demonstrate NDMA field attenuation in both surface water and groundwater

Field Degradation of NDMA Demonstrated

Los Angeles County, California

A field investigation and surface water and groundwater flow and transport model demonstrated significant attenuation of n-nitrosodimethylamine (NDMA) in both surface water and groundwater downstream of wastewater discharges.

Todd Engineers (now Todd Groundwater) led a team including Kennedy/Jenks Consultants and Lawrence Berkeley National Laboratory in conducting a three year investigation to characterize the fate and transport of NDMA downstream of Water Reclamation Plants (WRPs) operated by the County Sanitation Districts of Los Angeles County. Highly treated wastewater from the WRPs 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. Study area features are shown in the figure below.

Map of the area addressed by this NDMA degradation study
Map of the study area, showing Recharge Basin and Waste Water Treatment Plant (WWTP) locations

NDMA is both formed and degraded in the wastewater treatment process. 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.

Water quality data from surface water sampling along a lined stretch of river channel demonstrated UV attenuation of NDMA, with a half-life of 2.2 hours.

Attenuation of NDMA in groundwater was demonstrated based on estimates of changes in mass over time through plume mapping and through groundwater model calibration. The estimated NDMA mass reduction demonstrated by plume mapping was approximately 65 to 82% depending on the area assessed. The calibrated NDMA half-life in the subsurface was 69 days over most of the model domain; this provided a very good match between simulated and monitored concentration-time profiles at several shallow monitoring wells.

Graph of observed and simulated NDMA levels
	         at a specific monitoring well
Graph of observed and simulated NDMA levels
	         at a specific monitoring well
Observed and simulated NDMA levels at representative monitoring wells

The calibrated in-situ NDMA attenuation rate is consistent with laboratory results presented in the scientific literature. Based on the literature, the subsurface attenuation is presumed to be due to biodegradation.

Conclusions

The study findings indicate that NDMA is significantly attenuated in the surface water system due to ultraviolet degradation from exposure to sunlight and in the groundwater system due to biodegradation.