ASSESSING THE IMPACT OF AIR TOXIC EMISSIONS USING HARP

Wastewater and combustion processes at wastewater treatment facilities, the combustion of fossil fuels at power plants, and operations of other industries leads to the emissions of a number of air contaminants, including air toxics. Many states have developed air toxic programs in order to control the risks from these and other types of air toxic sources, such as Arizona's Ambient Air Quality Guidelines (AAAQGs), and Utah's Toxic Screening Levels (TSLs). California's Air Toxics Hot Spots Program requires stationary sources to report the types and quantities of certain substances that their facilities routinely release into the atmosphere. The objectives of the “Hot Spots” Act (AB2588) are to collect emission data, identify facilities having localized impacts, ascertain health risks, and to notify nearby residents of any potential significant risks.To standardize the approach to this complex air toxics program, California Air Resources Board (ARB) developed a computerized program, Hotspots Analysis and Reporting Program, or “HARP”. HARP integrates all of the data management, dispersion and risk analysis functions required for statewide air quality management into a single windows-based program. This program includes an emissions inventory database, a dispersion modeling module, and a risk assessment analysis module. Released in December 2003, HARP is used by California air pollution control and air quality management districts, facility operators, and other parties as part of the permitting process to evaluate emissions inventory data and the potential risk levels and health impacts associated with the emissions of toxic air contaminants. In addition, if any specific regulatory thresholds are exceeded, reduction measures and control strategies should be evaluated and implemented.This paper will discuss the features of the HARP program and its application to a municipal wastewater treatment plant in the South Coast area of California. The sources of air toxic emissions at this wastewater treatment plant include open tanks (aeration basins), combustion sources (engines and boilers), and odor control scrubber stacks. These sources were represented as area and point sources within the dispersion module of HARP. The development of the emissions database, the set-up of the dispersion model, the selection of appropriate receptor locations, the characterization of potential risk scenarios, the results of the analysis with respect to the permitting thresholds, and potential mitigation measures will be discussed.Both short-term (1-hour) and long-term continuous exposure to carcinogenic and noncarcinogenic toxic air contaminants were evaluated as part of the health risk assessment.The contribution of various source types to the maximum predicted offsite health effect impacts are discussed along with a breakdown of the contribution from individual toxic air contaminants. These results are also discussed in relation to recommended control strategies based on the risk assessment results. Finally, the paper will discuss the implications and usefulness of using HARP on the evaluation of risk management and risk reduction control measures.