NATURAL ATTENUATION FACTOR WITH FIRST-ORDER DEGRADATION IN RISK ASSESSMENT

摘要

Many examples show that groundwater drinking pathway is the main exposure pathway threatening human health at chlorinated compounds contaminated sites. When making a Tier 2 or Tier 3 risk assessment, correctively estimation of natural attenuation factor (NAF) is critical to accurate risk calculation. Calculating natural attenuation factors should consider mechanisms of dispersion, diffusion, adsorption, biodegradation, volatilization and others. NAF of one trichloroethylene (TCE) contaminated site was calculated using RBCA Tool Kit for Chemical Releases, the results show that with degradation considered, NAF data is more than 4 orders larger than NAF data without considering degradation. The result indicates that degradation item is very significant in naturally degradable contaminant risk assessment. However, its significance depends on the magnitude of degradation rate for certain, how the NAF is calculated is also critical to perform a risk assessment. Besides Domenico model in RBCA Tool Kit, another analytical Domenico model (1987) accepted in ASTM E 1739-95 is also widely used. Calculation using this model gives a NAF data of 1/60 of RBCA Tool Kit result at the same site. In order to make a more accurate prediction, a three-dimensional groundwater flow model was setup, MODFLOW code was used to realize the groundwater flow and MT3D code was used to predict TCE transport. It is confirmed by numerical simulation result that Domenico model in RBCA Tool Kit suppose both phases of dissolved and sorbed contaminants are degradable and have same degradation rates. On the other hand, Domenico model in ASTM E 1739-95 suppose only dissolved phase contaminant is degradable. This assumption is consistent to assumptions in BIOCHLOR and RT3D, which are widely used nowadays to simulate natural or enhanced degradation of chlorinated compounds in groundwater. MT3D can be used to simulate both degradations of dissolved phase and adsorbed phase if they have different degradation rates, however, it is only limited to parent products. Another point is that, in Domenico model, the source is supposed to be a vertical constant concentration plane source, perpendicular to groundwater flow, without considering source dimension at groundwater flow direction. Three-dimensional MT3D simulation result show that both of the analytical models have tendency of giving a smaller concentration prediction at pint of exposure (POE), which leads to a lower estimation of risk.