Copyright American Society of Civil Engineers Mar 1998| [Headnote] |
| A contaminated manufacturing site demonstrates how pumping and treating with horizontal wells and soil venting can effectively remediate groundwater. |
Based on the size and shape of a contaminated site in northern Chicago, engineers ruled out a traditional vertical well system and used a two-tier system that taps into contaminated groundwater and cleans it through aeration.
Seepage from underground storage tanks had contaminated groundwater under a portion of a former manufacturing site and threatened to spread to neighboring property. The 30-acre site was sold in 1995, but the former owners retained responsibility for cleaning it. During excavation to remove underground tanks prior to selling the property, four 5,000 gal. and two 15,000 gal. tanks were found to be leaking. One 5,000 gal. tank was part of the original diesel and oil heating system, and the other three had stored solvents used for manufacturing, which once included light machining, cleaning, painting and assembly operations. The 15,000 gal. tanks had stored fuel for the heating system.
Shepherd Miller Inc. (SMI), Fort Collins, Colo., was hired to rehabilitate the site, using pump-and-treat methods. Although much maligned because of its expense and reportedly inconsistent success, pump-andtreat was chosen. However, instead of using up to 40 typical vertical wells to treat the wide and shallow site, SMI ultimately decided to pump and treat with two horizontal wells.
In characterizing the site, sl's goal was to identify:
The extent of unsaturated soil near the former tank locations;
The extent of contaminated groundwater on site;
The extent of contaminated groundwater beneath the building;
Locations for monitoring wells.
Eight monitoring wells provided data to the Illinois Environmental Protection Agency (IEPA) that characterized the level of contamination in the soil and groundwater. Data showed contaminants in a thin saturated zone-5 to 8 ft thick-sitting approximately 5 ft below the surface. The soil there is silty sand over sand and gravel and is underlain by a highly impermeable clay/silt that acts as an aquiclude.
Two groundwater plumes flow beneath the site. Solvents and volatile organic compounds (vocs), including trichloroethene and 1,2-dichloroethene and some fuel, spread throughout a 1-acre area. A second, smaller plume, less than a quarter acre, is contaminated by diesel fuel. No free product, either as light non-aqueous-phase liquid fuel or dense non-aqueous-phase liquid solvents, has been detected in the monitoring wells.
Groundwater at the site flows to the north-northeast and is locally influenced by the storm-water recharge pond at the western edge of the site. Contaminated groundwater has migrated beneath the manufacturing building and possibly off-site to the northeast.
CONCETUALIZATION AND DESIGN
The conceptual design SMI presented to the client and IEPA in late 1993 included groundwater extraction with horizontal wells and soil venting near the source. Groundwater would be treated with a diffused bubble aeration system (DBAS) and carbon, if necessary. A separate, vertical groundwater extraction and soil venting system with vertical wells would be installed at the smaller fuel plume. For soil venting, a vacuum applied to a well draws air through contaminated soil, speeding up natural biodegradation of contaminants.
The remediation system includes two horizontal wells, 1,500 ft of trench, more than 7,500 ft of piping and conduit, four aboveground groundwater pumps, three pump houses, motorized valves, a DBAS, secondary carbon treatment, a three-phase power connection, a telephone/modem line, a freestanding treatment building and a telemetry system that allows pumping rate changes and operational parameter analysis via a modem connection. The system was installed with piping to accommodate four vertical soilvented wells to be installed in the summer of 1998 in the area of highest concentration of vocs. SMI and the client have requested that the installation of the second ventilation system in the smaller plume be postponed, because concentrations in groundwater have fallen to levels below cleanup standards as the contaminants appear to be degrading naturally. SMI will continue to monitor this area.
Several components of the design are expected to circumvent problems typically associated with pump-andtreat systems. The National Research Council in 1994 reported that pump-and-treat is neither a fast nor a cost-effective way to remediate contaminated groundwater. In a typical system, when groundwater rises and falls, contaminated water and floating fuel smear across a porous medium, becoming trapped in its pores. As the water table rises during natural seasonal fluctuations, contaminants are locked in the medium. If solvents are present, they are rarely recovered in this process.
In this project, pump-and-treat became a dewatering system, draining the saturated zone surrounding the contaminant Remediation was then carried out using soil venting. The plume outside the source area will draw groundwater to the middle of the site and treat as much water in this area as possible.
SMI installed one 500 ft stainless steel well parallel to the direction of flow along the axis of the large plume, and an 800-ftlong well perpendicular to groundwater flow in the area with the highest concentration of contaminants. Sand was allowed to cave in around the wells to stabilize them. Both wells were installed with directional drilling techniques and were located along the top of the aquiclude. All four well ends terminate in steel well boxes to allow access and future cleaning.
 | |
 | |
SMI chose soil venting because of the anticipated low flow rate (less than 10 gal./min total flow), the ease of removing the vocs by air treatment, the relative simplicity of the system and the low cost to purchase, install and operate the system. Installation cost about $700,000, with an annual operating budget of $160,000. Water is pumped into one end of the DBAS unit and through a series of perforated baffles. Air is forced through the perforated baffles, maximizing air/water contact. This air/water contact forces volatilization of the majority of the contaminants, which are discharged under a state of Illinois air permit. Treated groundwater is drained to a sump, where it is either pumped through two activated carbon vessels installed in series or routed directly to the sanitary sewer under a National Pollutant Discharge Elimination System permit. The entire system is connected to a telemetry system that can be monitored remotely via modem. Electronically actuated ball valves can be opened and closed remotely. The design was approved by IEPA; the system was built in the winter of 1995-96 and began operating in April 1996.
OPERATION
Since operations began, workers have completed routine system cleaning, filter replacement, carbon replacement and pump maintenance. In addition, the former owner is monitoring groundwater. SMI collects samples from site monitoring wells quarterly, and results are reported to the former and the new owners and to IEPA. SMI plans to experiment with temporary shutdowns to see if contaminant levels in the groundwater remain low or if a rising water table causes contaminant levels to rise.
The former owner is contractually responsible for remediating the site quickly and expects to meet objectives in less than 10 years. As remediation goals are met, the client will petition IEPA for a clean closure letter, which will fulfill the client's obligation to the current site owner. At that time, the system will be shut down and dismantled.
When the client sold the property, IEPA standards stipulated that no more than 50 employees could be on-site. Today, because groundwater contamination has been curbed, the facility operates three shifts, 24 hours per day, and employs more than 1,300 people.
| [Author Affiliation] |
| Adam H. Hoffman, P.E., is a senior engineer for Shepherd Miller Inc. in Fort Collins, Colo. |
My Research
