A toolkit is i) to evaluate the overall source attenuation rate while accounting for the effect of random variability in apparent attenuation rates in individual monitoring wells and ii) to determine the amount of groundwater monitoring data required to characterize the long-term source attenuation rate for sites undergoing long-term monitoring (i.e., during an MNA process, either when MNA is a stand-alone remedy or when MNA is applied as a polishing step after active remediation).