Improved Well Modeling Tools for Unsaturated Flow Pump-and-Treat Remediation Studies

Many Department of Defense military sites and Environmental Protection Agency superfund sites benefit from pump-and-treat systems for their remediation. One of the ways used to determine the effectiveness of a pump-and-treat system is by using a groundwater finite element method program to compute flow patterns from proposed well placement. As the governing Richards' equation for unsaturated flow is highly nonlinear, the finite element discretization often has trouble converging. This is especially true when (1) a significant part of the flow region has unsaturated flow, (2) soils such as sand have relative hydraulic conductivities that go from almost vertical to almost horizontal, (3) water in very dry soil is being applied at the top of the flow region, and (4) the wells become partially above the free surface when the pumping is turned on. This paper focuses on an improved well-modeling technique that allowed the finite element solution to go from nonconvergent to converging in 15 nonlinear iterations for the Higgins Farm Superfund site The Higgins Farm Superfund site, which is approximately 75 acres in size and currently operated as a cattle farm, is located in a rural area along Route 518 in Franklin Township, Somerset County, New Jersey. The site is primarily pastureland where a drum burial dump was discovered. Extraction wells were placed around the perimeter with an onsite treatment plant. The finite element run not only did not converge but also the modeling of wells was initially very tedious. A given well has a specified flow, and the user would manually try to distribute this flow among the given set of nodes modeling the well. During the iteration toward convergence, if a node of a well became above the free surface (pressure head less than zero), the code stopped. The user then manually redistributed the flow for that well and started the process again. However, combining all of the nodes for a well into one well supernode was determined to be the best solution. The nodes representing a partially penetrating well are combined into a single degree-of-freedom well supernode. Now, one total value of flow is used as input to the well supernode, and one single value of total head is computed as output. Further improvement was added to the well model by implementing a nonreversible elimination of well nodes above the water table. This paper will give the details of this computational experience using the high performance computers at the ERDC and illustrate the general applicability of this approach to other studies.