Harte, P.T. and W.C. Brandon. | Remediation 30(2):37-53(2020)

A newly developed method assesses the effect of matrix diffusion on contaminant transport and remediation of groundwater in fractured rock. The method utilizes open-borehole monitoring wells in fractured rock to conduct backward diffusion experiments on CVOCs in groundwater. Testing was performed on relatively unfractured zones over short intervals in open boreholes at the former Pease Air Force Base in Portsmouth, New Hampshire to investigate back diffusion of cis-1,2-DCE. Post-sparging concentrations of cis-1,2-DCE showed initial rebounding followed by declines, excluding an episodic spike in concentrations from a groundwater recharge event. Three processes were theorized to control concentration responses in the test zones post-sparging: 1) the limited back diffusion of CVOCs from a halo or thin zone of rock around the borehole contributes to the initial rebounding; 2) aerobic degradation of cis-1,2-DCE occurred causing declines in concentrations in the test zone; and 3) microflow from microfractures contributed to the episodic spike in concentrations following the groundwater recharge event. In active flow zones, the latter two processes are not measurable due to equilibration from groundwater transport between the borehole and active flowing fractures.