Sampling Frequency of Groundwater Monitoring and Remediation Delay at Contaminated Sites

This study examines the frequency of sampling at contaminated sites located in heterogeneous subsurface environments. The impact of delays in remedial response is also investigated in terms of the growth that such delays incur on contaminated areas and remediation costs. Our work utilizes high-resolution numerical Monte Carlo realizations to simulate contaminant movement in heterogeneous, two-dimensional aquifers and to calculate the probabilities of detection P (d) attained by various monitoring well arrangements. For all types of soils P (d) was seen to decrease as sampling became less frequent, with 8 wells sampled daily, or 20 wells sampled monthly required in order to maintain detection probabilities that are higher than failure probabilities. Irrespective of the density of a monitoring network at highly dispersive subsurface environments a very rigorous sampling schedule must be maintained in order to retain the detection performance of the network. Highly heterogeneous soils through the presence of low permeability zones appeared to impede the spread of the contaminants and hence, ameliorate the effects of dispersion. Analysis of the time lag, between the time that contaminants first appeared at monitoring locations and the time they were observed, as well as of the enlargement of the plume area that resulted from this time lag led to the conclusion that monthly sampling is required for a wide range of hydrogeologic environments. Finally, in highly dispersive environments the remediation response must be of the order of a few months if one does not wish the contaminated areas and remediation costs to grow significantly.