Although regulatory authorities are often willing to consider site closure or reduced monitoring at low risk sites with exceedances of groundwater standards, the time required to achieve these standards can be an important consideration.  Few tools are currently available to evaluate the likely timeframe required for natural attenuation to achieve drinking water or other low concentration groundwater standards.  For this project, we have conducted a detailed evaluation of groundwater monitoring results at three UST sites in southern California with long (14 to 15 years) groundwater monitoring records.  These monitoring data have been used to i) evaluate long-term trends in the dissolved plume size and concentration, ii) estimate source remediation rates and remediation timeframes and iii) evaluate the impact of active remediation on observed concentration trends.
At all three of the sites evaluated, the plume size and concentration decreased significantly over the course of the monitoring record.  Decreasing benzene (15 wells) and MTBE (11 wells) concentration trends were observed in all wells evaluated for concentration trends.  In some wells, constituent concentrations decreased by more than three orders of magnitude to levels consistently below analytical detection limits indicating that matrix diffusion and other “exhausted plume” factors are not important at all locations.  At the three evaluation sites, the average estimated source half-life was 0.9 to 1.5 years for benzene and 1.0 to 1.7 years for MTBE.  Based on these observed source decay rates, the best estimate of the additional remediation time required to achieve drinking water standards at all locations ranged from 3.4 to 10 years for benzene and 1.7 to 9.3 years for MTBE.  
At the three sites, the concentration trends and remediation rates were similar during periods with and without active remediation.  These findings indicate that natural attenuation was responsible for the majority of observed decreases in dissolved concentrations.