- Aquifer Site Investigation & Remediation
In 1985, trichloroethylene (TCE) contamination of the Arkansas River Alluvium Aquifer was discovered beneath the Cessna Aircraft Co.’s Midcontinent Facility. Cessna retained Burns & McDonnell to define the extent of contamination and to begin remediation as quickly as possible. Burns & McDonnell conducted an extensive site investigation while at the same time beginning remediation of the groundwater with a pump-and-treat system.
Upon completion of the site investigation and a limited feasibility study, the pump and treat system was enlarged to treat the entire contaminant plume. The treatment system has been active for more than 15 years, in which time the plume has been reduced to less than one-half the original size. The remediation system was enhanced by the addition of an aquifer air sparging and soil vapor extraction system (AS/SVE) to address the source area.
- Subsurface investigation
- Remedial investigation
- Feasibility study
- Groundwater modeling design
- Groundwater monitoring
- Site remediation
The initial phases of the project involved both the compilation of existing site data and development of new information. Using data gathered by Cessna and the Kansas Department of Health & Environment (KDHE) on contaminant levels in existing wells, Burns & McDonnell performed a preliminary subsurface and site history investigation to identify the general distribution of the contamination. The information gathered by Burns & McDonnell included data from employee interviews and an extensive geophysical/geological investigation. Burns & McDonnell prepared the safety, sampling, and quality assurance/quality control plans.
A feasibility study was performed for Cessna Aircraft, Wichita, Kan., to determine the appropriate remediation scenario for the TCE contamination in the groundwater. The contamination plume in the alluvial aquifer of the Arkansas River Valley extends off the site and was the subject of litigation. The objective of the feasibility study was to determine the optimum remedial scenario with the goals of:
- Minimizing the volume of water withdrawn relative to the amount of contamination removed
- Expediting the cleanup to show good-faith efforts toward the plaintiffs in the court case
- Minimizing the cost of remediation, to the extent possible, by effective placement of remedial wells and innovative engineering design
Nine remedial scenarios were modeled on the computer with different combinations of wells and flow rates for each well. The program used in modeling was the U.S. Geological Survey Modflow three-dimensional, finite difference groundwater flow model. The remedial scenario selected included the use of five existing wells and four new wells. The capital cost of the four new off-site remedial wells was estimated at approximately $100,000. The actual cost was approximately $70,000 due to the innovative design of placing two pumps in each well.
Remediation presently consists of nine withdrawal wells, five on the site and four off, pumping at a combined flow rate of approximately 1.8 million gallons per day. There are two pumps in each well that pump independently from separate deep and shallow portions of the aquifer. Effluent from the air stripper is discharged to a surface water flood control ditch.
Enhanced remedial efforts included the design and construction of an efficient AS/SVE system to address the source area. Air is injected below the water table to strip contaminants in-situ and a vacuum is applied to extract the volatized contaminants from the unsaturated zone soils.
To date, approximately 34,000 pounds of contaminants have been successfully removed by the combined remedial pumping and soil vapor extraction systems. Further groundwater monitoring costs have been significantly reduced by employing a two-year sampling schedule. This two year sampling program is detailed as follows:
- Year 1
- Round 1, Short List
- Round 2, Short List and Compliance Wells
- Year 2
- Round 1, Short List
- Round 2, Full List
This modified sampling program resulted in a cost savings of approximately $30,000 over the two-year period.
The geophysical/geological investigation included:
- Surface electrical resistivity and ground conductivity (electromagnetic) surveys
- Drilling and constructing monitoring wells
- Drilling temporary soil borings for collection of soil and groundwater samples
- Analysis of soil samples collected from borings and monitoring wells
- Sampling and chemical analysis of groundwater samples from monitoring wells
- Extensive water level measurements
- Full-scale pumping test
- Groundwater modeling
- Data evaluation and reports