Multi-layer sampling in conventional monitoring wells for improved estimation of vertical contaminant distributions and mass

“Traditional” approaches to sampling groundwater and interpreting monitoring well data often provide misleading pictures of plume shape and location in the subsurface and the true extent of contamination. Groundwater samples acquired using pumps and bailers in conventional monitoring wells yield data which are largely dependent upon the length of the screened interval, the purging and sampling method employed, and the purge volume extracted prior to sample collection. Accurate delineation of plume boundaries and vertical concentration gradients is desirable, to accurately characterize waste sites and optimize remedial strategies. The objective of this study was to compare sampling results using four different sampling approaches and devices. Conventional monitoring wells were sampled with an electric submersible pump using low-flow sampling techniques and with a bailer using “traditional” sampling methods. The same wells were also sampled with a passive multi-layer sampling system (DMLS®, Margan Ltd.). Finally, aqueous concentrations were also determined in the formation adjacent to the monitoring wells studied using a Geoprobe® and short (30 cm) screens. Results indicated that “traditional” sampling methods can provide misleading information regarding contaminant distribution and mass and indeed can miss the presence of contamination altogether.