Bioremediation in Fractured Rock: 2. Mobilization of Chloroethene Compounds from the Rock Matrix

Abstract

A mass balance is formulated to evaluate the mobilization of chlorinated ethene compounds (CE) from the rock matrix of a fractured mudstone aquifer under pre- and postbioremediation conditions. The analysis relies on a sparse number of monitoring locations and is constrained by a detailed description of the groundwater flow regime. Groundwater flow modeling developed under the site characterization identified groundwater fluxes to formulate the CE mass balance in the rock volume exposed to the injected remediation amendments. Differences in the CE fluxes into and out of the rock volume identify the total CE mobilized from diffusion, desorption, and nonaqueous phase liquid dissolution under pre- and postinjection conditions. The initial CE mass in the rock matrix prior to remediation is estimated using analyses of CE in rock core. The CE mass mobilized per year under preinjection conditions is small relative to the total CE mass in the rock, indicating that current pump-and-treat and natural attenuation conditions are likely to require hundreds of years to achieve groundwater concentrations that meet regulatory guidelines. The postinjection CE mobilization rate increased by approximately an order of magnitude over the 5 years of monitoring after the amendment injection. This rate is likely to decrease and additional remediation applications over several decades would still be needed to reduce CE mass in the rock matrix to levels where groundwater concentrations in fractures achieve regulatory standards.

Bioremediation in Fractured Rock: 1. Modeling to Inform Design, Monitoring, and Expectations

“…Groundwater fluxes quantified by the model also were essential to evaluating contaminant mass mobilized by the bioaugmentation, as discussed in the companion paper (Shapiro et al 2017c). Results emphasize that design of effective amendmentbased groundwater remediation in settings where discrete fractures host the majority of groundwater flow requires detailed knowledge of the distribution of hydraulic conductivity as well as information about groundwater fluxes and advective pathways for solute transport.…”

“…This pumping removed some residual Br −1 mass in the aquifer that remained from the Br −1 tracer test ( Figure 3D). The actual injection at 36BR-A to initiate the bioremediation consisted of a total volume of 669 L of amendments and flush water (Shapiro et al 2017c). This initial value (∼4 mg/L) is 100 times higher than background Br −1 values measured prior to the tracer test, and reflects residual Br −1 mass in the lower-permeability zone ( Figure 3D).…”

“…North Carolina), and bioaugmentation with a consortium of bacteria (KB-1™; SiREM, Guelph, Ontario, Canada) (Revesz et al 2014;Shapiro et al 2017c). Dilute EOS™ solutions have density and viscosity similar to water (Clayton and Borden 2009), so the advection of EOS™ is similar to that of bromide.…”

“…Furthermore, the quantification of groundwater fluxes permits the calculation of chemical fluxes under pre-and postbioremediation conditions, which enables evaluation of the contaminant mass mobilized by the treatments, as discussed in the companion article (Shapiro et al 2017c). This application of modeling is a novel approach to bioremediation design in fractured rocks, in that it explicitly accounts for the spatial variability of hydraulic properties in the vicinity of the treatment area, and the effect of this variability on fluxes and transport paths.…”

“…Use of the fluxes is then extended beyond the design stage, to inform bioremediation monitoring strategies, and to set expectations about the extent of the treatment zone. Furthermore, the quantification of groundwater fluxes permits the calculation of chemical fluxes under pre-and postbioremediation conditions, which enables evaluation of the contaminant mass mobilized by the treatments, as discussed in the companion article (Shapiro et al 2017c).…”