Brownfields redevelopment hasn’t gotten any easier under the new Vapor Intrusion regulations. Source that could previously be left in place must now be removed, and once source remediation is complete, subslab mitigation must operate for many years, until stringent indoor air screening levels are met in down-gradient plumes. Such is the plight of many large commercial, light industrial manufacturing buildings looking for a new ownership and a new purpose. Such buildings typically have hundred of thousands of square feet of floor space. One such building in New England comprises half a million posits. The building is so large that is covers both the source area and a portion of the down-gradient groundwater plume. To get the building sold and re-occupied, subslab soils and sill material must be made free of chlorinated solvents, and depressurized. In addition, the groundwater plume at the water table below the building must also be reduced, especially if the subslab mitigation system should have any chance of ever being turned off.

Delivering bio-amendments or oxidants, or recovering soil gas for in situ remediation under a large building slab normally requires dozens of points being vertically drilled and driven through such a large building slab, often requiring that workers are disrupted and on stand-by while each of the subslab systems is put in place: The soil vapor extraction (SVE) system, followed by a bio-amendment or oxidant injection system, and finally the subslab depressurization system. Each of these are installed at a different depth, at the zones of influence of the vertical well points. A smarter solution is a set of horizontal well screens with zones of influence that span the entire unsaturated zone and can therefore be used for SVE or bioamendment injection into the source material.

Directional Technologies, Inc. (DTI) made a horizontal environmental well design and installed such a system featuring variable slot size and spacing along the environmental well screen section. Up to six screen zones with different open areas enabled the horizontal remediation wells to operate first at high flow rates and vacuum for the initial horizontal SVE; then at low flow rates for bioamendment or oxidant injection; and finally at low flow rates and low vacuum levels for long-term subslab mitigation.

In the course of installing the horizontal wells only a few feet below the building slab, and running both below and above underground utility lines, DTI’s experienced directional drilling supervisor was able to identify underground storm sewer lines that even the ground penetrating radar (GPR) of a $30,000 underground utility survey could not identify. At the end of the environmental well design installation, the client had developed a detailed understanding of the subsurface utilities. Armed with this detailed information, the client is able to evaluate potential preferential pathways beneath the building that may interfere with the effectiveness of the subslab mitigation system. This knowledge is especially valuable in any negotiation with the responsible state and federal regulatory agency.