Multiyear Temporal Changes in Chlorinated Solvent Concentrations at 23 Monitored Natural Attenuation Sites

Long-term (e.g., 5–15 years) groundwater concentration versus time records were compiled from 47 near-source zone monitoring wells at 23 chlorinated solvent sites (52 total records). Chlorinated volatile organic compound (CVOC) concentrations decreased significantly in most of the 52 temporal records, with a median reduction in concentration of 74%. A statistical method based on a Mann–Kendall analysis also showed that most sites had statistically significant decreasing concentration trends over time. Median point decay rate constants (kpoint) values were calculated for nine sites containing tetrachloroethene (PCE); 13 sites containing trichloroethene (TCE); two sites containing cis-1,2-dichloroethene (DCE); and six sites containing 1,1,1-trichloroethane (TCA). The TCA sites had the highest kpoint values (0.34/year) followed by PCE, DCE, and TCE (0.23/year, 0.16/year, and 0.11/year, respectively) (equal to decay half-lives of 2.0, 3.0, 4.3, and 6.1 years, respectively). If the median point decay rates from these sites are maintained over a 20 year period, the resulting reduction in concentration will be similar to the reported reduction in source zone concentrations achieved by active in situ source remediation technologies (typical project length: 1–2 years).