WaterSubject

The hydraulic control of contaminated groundwater is commonly achieved through the use of pumping wells. When long-term hydraulic containment is required, the cost of treating the water pumped from the aquifer can become prohibitive. An open, vertical barrier wall may be used in conjunction with extraction wells to provide hydraulic containment at significantly lower pumping rates than can be achieved using wells alone. The cost of constructing the barrier wall may be justified by the cost savings associated with the decreased pumping rate, over the lifetime of the project.

Bayer et al. investigated the hydraulic efficiency of pump and treat systems supported by vertical barrier walls. They examined the effects of several wall configurations using numerical models of groundwater flow and transport under both homogeneous and heterogeneous aquifer conditions, and concluded that open barriers can significantly reduce the pumping rate of the extraction wells used to isolate a plume. Bayer et al. developed an economic model and presented a comparative cost analysis for pump and treat systems with and without barrier walls, for a hypothetical flow case. They found that in addition to reducing pumping rates, the use of barriers can reduce the uncertainty in the pumping rate required to isolate a plume. More recently, Bayer and Finkel investigated the performance of barrier-supported pump and treat systems in highly heterogeneous aquifers subject to uncertainty in the direction of regional flow. Again, they found that barrier-supported systems can significantly reduce the average pumping rate required to control a plume.

Anderson and Mesa presented a mathematical model and analytical solutions for the head and discharge fields for steady, two-dimensional groundwater flow to one or more pumping wells, past an arc-shaped vertical barrier wall. They investigated, by capture zone analysis, the potential reduction in discharge that could be achieved by including a vertical barrier wall in the design of a containment system, and presented dimensionless capture zone envelopes to be used for the preliminary design and cost estimation of remedial systems.

Both the analytical model of Anderson and Mesa and the numerical models used by Bayer et al. include steady, two-dimensional, groundwater flow around impermeable barrier walls; none of the models includes recharge to the aquifer. Lerner showed, however, that recharge will significantly alter capture zone geometries. Kinzelbach et al. demonstrated that the shape and extent of the capture zone envelope of a well, depend on the rate and areal pattern of groundwater recharge. The recharge rate can be an important parameter that should be included in capture zone analysis, particularly when problems of long-term hydraulic containment are considered. Addition of areal recharge leads to a reduction in the area contained by a capture zone envelope, and it is expected that larger pumping rates will be required to control a contaminated region of aquifer.

Here we extend the analytical model of Anderson and Mesa. We present a mathematical model of steady, local groundwater flow near a barrier wall and pumping wells, in an aquifer with areal recharge. Explicit analytical solutions are developed for the analysis of remedial systems that include open vertical barrier walls. We use the term local model here to indicate a groundwater flow model that does not explicitly include distant boundary conditions to represent real aquifer features such as streams, lakes, or rock outcrops. In the local model, the effects of the distant aquifer features on the local flow field are represented by a combination of uniform flow from infinity and a constant areal recharge rate. Although the flow domain extends to infinity in the local model, we are only interested in the flow field near the pumping wells and barrier wall.

As in the previous paper, the local model and dimensionless results presented here are intended to be used as screening tools in the preliminary phases of engineering design. When combined with site-specific information (e.g., the costs of installing the barrier wall and the costs of treating the extracted water), the results may be used for cost comparisons and preliminary design. The analysis provides insight into the behavior of groundwater flow near systems with combined vertical barrier walls and wells when recharge to the aquifer occurs. The benefits of including a barrier wall in a containment system are quantified, and dimensionless capture zone envelopes are presented for practical well and barrier placements over a wide range of parameters.