(687a) Use of Monte Carlo Simulation Techniques Assess Remedy Complete Time Frame for First Order Contaminant Decay with Concentration Fluctuations

Many sites undergoing in situ treatment for dilute groundwater plumes utilize monitored natural attenuation (MNA) as a last element of a treatment train approach after a period of active treatment, and with MNA, groundwater concentrations commonly exhibit first-order concentration decay characteristics for individual monitoring wells. In some instances, attainment of a remedy complete (no further action or no monitoring required) is hindered by slow contaminant attenuation rates exhibited by the existing data. If the existing data also exhibits concentration fluctuations in addition to the first order decay trend, it can be difficult to determine a date of future cleanup objective attainment with acceptable uncertainty, as is frequently needed by the financially responsible parties. It is common practice to extend a decay trend into the future to determine the expected date of cleanup achievement, using first order attenuation rates. This approach gives only a single future date, however it is evident that, with a case of significant fluctuating concentration data, a single date will not best describe the future occurrence. A range of dates representative of both the decay trend and the concentration fluctuations is a more suitable representation, and likely much more helpful for the responsible part in their program planning. A Monte-Carlo approach to future groundwater concentration prediction with first order attenuation is presented, which provides a range of dates at which attainment of the groundwater cleanup level is anticipated.