CONTINUOUS SAND-IRON REACTION WALL FOR GROUNDWATER REMEDIATION

CLARK, LISA M., lisa.clark@rmtinc.com, RMT, Inc., Greenville, SC 29606; and ROWAN, M. ELIZABETH, beth.rowan@rmtinc.com, RMT, Inc., Nashville, TN.

In 1992, a pre-acquisition audit of a former industrial property in South Carolina identified a number of environmental risks which led to the discovery of volatile organic compounds (VOCs) in soil and groundwater at the site. Closure activities were initiated in potential source areas, and hydrogeologic investigations were conducted to evaluate aquifer conditions and constituent concentrations in groundwater.

The site is located in the Coastal Plain physiographic province of South Carolina. Three aquifers - a water table aquifer, an intermediate aquifer, and a deep aquifer - underlie the site. The upper two aquifers are impacted by chlorinated hydrocarbons. The thin upper (water table) aquifer is comprised primarily of silty to sandy fill material. A clay unit, which grades from light to dark with depth, underlies the upper aquifer. The clay unit contains variable amounts of fine silt laminae and very fine sand layers, which comprise the intermediate aquifer. The lower part of the clay unit provides a competent confining layer between the intermediate and deep aquifers.

Constituents detected in groundwater include trichloroethene (TCE) and associated degradation products. TCE concentrations are highest in the water table aquifer, with historical concentrations in the central portion of the plume on the order of 25 mg/L in monitoring wells near the source area. VOCs detected in the intermediate aquifer are generally an order of magnitude less, and show limited areal distribution. VOCs have not been detected in the deep aquifer. A permeable iron filings reactive wall was designed and installed for the site to intercept the heart of the VOC plume closest to the property boundary, where the greatest potential exists for off-site migration of constituents. The wall extends vertically through the intermediate aquifer, to address not only migration of impacted groundwater through the water table aquifer, but also potential future migration of impacted groundwater through the intermediate aquifer. Data collection and analysis conducted to support design of the reactive barrier included:

--geotechnical borings drilled at 50-foot and 25-foot spacings along the center line of the wall, to map the base of the intermediate aquifer

--laboratory treatability testing and degradation modeling to establish design residence time for the reactive wall

--laboratory premeability testing and particle size analysis of various sand-iron mixes, to evaluate hydraulic properties of the wall relative to the aquifer materials

--groundwater flow modeling to evaluate flowpaths through the reactive wall and assess the potential for groundwater flow around the edges of the wall.

The final reactive wall design included a 50 percent iron filings/50 percent sand mixture, by volume, as a means to control material costs. Groundwater monitoring conducted since installation of the wall indicates that remediation of impacted groundwater is occurring as intended.