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Project: Comparative Mass Emissions to the SCB

Background and Objectives

Mass emission estimates for constituents of concern (mass released over time) are one indicator used to evaluate the risk of environmental impairment. Mass emissions data allows assessment of whether discharges are increasing or decreasing in magnitude relative to other sources. This enables managers to identify the contaminant source(s) of highest concern. It also allows comparison of the discharge sources to the levels found in various receptors of contamination (e.g., offshore sediment, mussels, or fish).

SCCWRP has conducted mass emission comparisons from a variety of sources to the Southern California Bight (SCB) at periodic intervals dating back to 1970. Estimates of mass emissions from large publicly owned treatment works (POTWs) have been made annually. Estimates from other sources (such as small POTWs, industrial dischargers, and oil platforms) have been conducted at frequencies of about every five years. Efforts to compile monitoring data on stormwater mass emissions have only been initiated recently, though earlier studies modeled stormwater mass emissions using information on coastal watershed land uses.

The goal of this project is to estimate mass emissions from all sources in order to determine: 1) combined mass emissions; 2) relative contribution of each source; and 3) trends in mass emissions from each source since the early 1970s.


This is an ongoing project.


Previously, trends in regional mass emissions and relative pollutant contributions were evaluated at several time points since 1970. Major sources included in this ongoing study are: large POTWs, small POTWs, power generating stations, oil platforms, industrial facilities, dredged material disposal, and stormwater. Data is gathered for each southern California discharger’s NPDES monitoring report, which provides effluent flow and chemistry data. Local stormwater agencies are also able to supply some monitoring data on flows and constituent levels in stormwater. Concentrations are then standardized to monthly time steps and used to calculate annual discharge volumes and constituent mass emissions.


The first effort to compare mass emissions to the SCB looked at data from 1971 to 2000. Though coastal population grew by 56% and total effluent volume increased by 31%, mass emissions of nearly all constituents decreased, most by greater than 65%. The median decrease in metals emissions was 88%, while total DDT and PCB emissions each decreased by three orders of magnitude. Nitrate was the only constituent load to increase over time.

Percent change in total discharge volume and mass emissions of selected constituents from 1971 to 2000.

Large POTWs were the dominant sources of most constituents throughout the study period. However, as point source treatment has improved over time, the relative contribution of non-point sources such as stormwater runoff has increased. For nitrate, stormwater runoff was the dominant source in each time period examined. Better data integration is needed to improve the precision of regional mass emissions estimates, particularly for non-point sources.

Relative contribution to combined metals emissions from major sources between 1971 and 2000.
For more information on Comparative Mass Emissions to the SCB, contact Eric Stein at (714) 755-3233.
This page was last updated on: 7/1/2014