Evaluation of Hydraulic Control and Remediation of Contaminated Groundwater through Use of Plant Systems
An integrated remediation system incorporating large-scale plantings and in-stream wetlands was designed and engineered to mitigate and control dissolved carbon tetrachloride being discharged from groundwater to surface water in a creek at a contaminated site in Murdock, Nebraska. The hydraulic control component of the remediation system was designed by using calibrated groundwater flow and transport models. The three-dimensional MODFLOW-2000 and MT3D codes were used for model development and simulation. Modules for evapotranspiration, well pumping, and drainage were applied in MODFLOW-2000 to simulate the functions of shallow plantings, deep plantings, and the hydraulically connected wetlands, respectively. The model was well calibrated with water level and contaminant data, plus measured flows from drain tiles in the area. The simulated results identified an optimal remedial configuration of shallow and deep planting zones and in-stream wetlands. The simulated hydraulic effects suggested (1) local drawdown in the planting area, resulting in significant hydraulic containment of contaminated groundwater; (2) a reduction of 90% in seepage of carbon tetrachloride to the creek; and (3) further mitigation in the wetlands.
Since system installation in 2005, extensive monitoring for performance evaluation has included surface water and groundwater chemistry; plant tissue analyses; and plant growth, water flow, and weather data -- plus sap flow data collected recently. Beneficial hydraulic effects have been recorded and partly quantified even at very early growth stages. The effects will be validated as the trees grow larger and increasingly exert their influence on the system.