Horizontal extraction well and treatment system installed at a former grain storage facility in Kansas
EVS scientists have implemented the first phase of a remediation plan at Hanover, Kansas, a small community about 90 mi northwest of Topeka, Kansas. The remedy, developed by the Commodity Credit Corporation (CCC, an agency of the U.S. Department of Agriculture [USDA]) and Argonne, is targeted to reduce carbon tetrachloride contamination in groundwater beneath the site of a former grain storage facility that operated from 1950 to the early 1970s.
During its operation, the grain storage facility at Hanover employed the use of commercial grain fumigants containing carbon tetrachloride to preserve stored grain. Over time, this contaminant moved into the groundwater beneath the site. In 2009-2010, EVS scientists conducted a site investigation on behalf of the CCC/USDA to characterize the hydrogeology and extent of contamination at the site. As part of this work, four groundwater zones were identified (see animation). Zone 1, the uppermost zone, consists of a few discrete, thin, saturated horizons (1-3 ft thick) developed along bedding planes and fractures within interbedded limestone and shale, with an average flow rate of 14 ft/yr; its lateral extent is restricted to the upland on which the former facility was located. Zone 2, which underlies Zone 1, consists of relatively thick saturated intervals in shale and limestone; it has a total thickness of about 10 ft. This zone is laterally more extensive than Zone 1, and it is thicker, more permeable, and more capable of groundwater production. Zones 3 and 4 are moist-to-wet units that lie deeper than Zone 2. Zone 3 consists of gray dolomitic shale; Zone 4 occurs at the base of a red shale unit. Each of these zones has a thickness of less than 2 ft. Carbon tetrachloride has been found in the shallow groundwater of Zone 1 at concentrations exceeding 600 µg/L in places (with the highest concentrations correlated to areas of high hydraulic conductivity); it has also been identified in Zone 2, along a relatively narrow pathway extending downgradient from the former facility. Zones 3 and 4 are currently uncontaminated.
The main objectives of the remediation plan are to reduce carbon tetrachloride in the highest contaminated area in Zone 1 and to minimize contaminant migration from Zone 1 to Zone 2. The preferred remedial alternative, therefore, involved the construction of a horizontal extraction well, oriented along the plume axis, and a treatment system consisting of a diffused aeration stripper. The horizontal well offers the advantage of a longer screen interval (as compared to a vertical well) for maximum contact with contaminated groundwater. Because the plume at the site is longer and wider than it is thick, this well configuration creates a greater zone of influence and increases the efficiency of contaminant recovery. Once treated, groundwater is conveyed through an underground pipe to a discharge point east of the former facility, with seasonal diversion to a storage tank for beneficial reuse as irrigation water for the school district's football field.
The treatment system has been up and running since November 1, 2017, and is almost six months into its anticipated 20-year lifespan. Its performance is monitored and controlled by a remote access system that operates on a cellular internet connection. This system records pumping flow rates and volumes, influent and effluent carbon tetrachloride (and degradation products) concentrations, groundwater levels at the extraction pumps, system status (e.g., air stripper pressure and air flow rate) and alarms. It also allows remote online reading and control of these operating parameters.
Initial and subsequent sampling of the influent and effluent from the air stripper indicate that the system is performing well, collecting groundwater with carbon tetrachloride concentrations as high as 800 µg/L and reducing concentrations in the effluent to non-detection levels.