Age Dating Groundwater Plumes Based on the Ratio of 1,1-Dichloroethylene to 1,1,1-Trichloroethane: An Uncertainty Analysis.

In principle, when 1,1-dichloroethylene (DCE) is formed from the hydrolysis of 1,1,1-trichloroethane (TCA), the ratio of DCE to TCA can be used to determine how long hydrolysis has been occurring and, consequently, the age of a TCA release to groundwater. The rate of hydrolysis appears to depend only on groundwater temperature, and sorption does not appear to have a significant effect on the hydrolysis rate. This article focuses on the uncertainties and biases in estimating the age of a TCA release from DCE/TCA ratios. Two methods are examined: (1) the use of one-time ratio measurements together with a laboratory-derived hydrolysis rate constant and (2) the use of time series data collected over a number of years. When the uncertainty in concentration measurements is small, the principal uncertainty for one-time measurements is due to the uncertainty in estimating the rate of hydrolysis at groundwater temperatures from laboratory data. For one-time measurements, we estimate 95% confidence levels for time estimates under favorable conditions of about ±25%. When time series data are available, using these data may be more or less accurate. The principal source of uncertainty then is due to uncertainty in concentration measurements. In interpreting field data, the age obtained should be recognized as an average based on when contact between water and TCA began. This may be different than a spill date for pure phase spills of TCA. Under some circumstances time estimates can be made when biodegradation of TCA as well as hydrolysis occurs. [ABSTRACT FROM AUTHOR]