High-Resolution Investigation of Vapor Intrusion in Fractured Sedimentary Rock
The presentation will detail methodologies and findings of investigations and testing for vapor intrusion associated with chlorinated ethenes in siltstone and sandstone located in the mid-Atlantic U.S. There is little scientific or regulatory precedence for assessment of vapor intrusion in fractured soil or rock environments, as most of the empirical experience is derived from work at granular soil sites, with little relevance to fractured rock settings. Fractured rock may be distinguished from a granular soil setting through lower gas-filled porosity to support transport and the possible influence of fracture orientation.
Our presentation will focus on how the historical site conceptual model, developed from investigations conducted with long open bedrock boreholes, was refined through higher resolution investigation and pilot testing of remedy enhancements. Application of discrete fracture network (DFN) style investigation techniques (Parker 2007) included high-resolution logging of core and open boreholes, physical and chemical testing of core samples, and multi-level monitoring of water and subsurface gas under stressed and unstressed conditions. We will present estimates of fracture porosity and effective hydraulic aperture derived from this testing compared against effective porosity estimates derived from gas and aqueous phase tracer testing, and apparent specific yield estimates derived from hydraulic testing.
The data derived from this testing support that upward vapor migration in the subsurface is limited by the presence of sparsely-fractured aquitard intervals and a predominance of near–horizontal, bedding-plane parallel fracturing. The patterns of fracturing as indicated by logging of core and boreholes area are supported by the findings of multi-level monitoring of water and gas under unstressed conditions and under applied stresses from relatively large–scale, dual-phase extraction testing. Given the predominance of near-horizontal fracturing, we found that it is possible to establish a vacuum field underlying many acres of land through vacuum dewatering of fracture networks.
