Long-term ͑e.g., 5-15 years͒ groundwater concentration versus time records were compiled from 47 near-source zone monitoring wells at 23 chlorinated solvent sites ͑52 total records͒. Chlorinated volatile organic compound ͑CVOC͒ 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 ͑k point ͒ values were calculated for nine sites containing tetrachloroethene ͑PCE͒; 13 sites containing trichloroethene ͑TCE͒; two sites containing cis-1,2-dichloroethene ͑DCE͒; and six sites containing 1,1,1-trichloroethane ͑TCA͒. The TCA sites had the highest k point values ͑0.34/year͒ followed by PCE, DCE, and TCE ͑0.23/year, 0.16/year, and 0.11/year, respectively͒ ͑equal to decay half-lives of 2.0, 3.0, 4.3, and 6.1 years, respectively͒. 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 years͒.