A numerical assessment of large screened monitoring wells on groundwater flow fields and solute distribution

Proper design and construction of monitoring wells is essential to obtain representative samples for further analysis such as fate and transport modeling, risk assessment and feasibility of remediation projects. Monitoring wells with large screens have been used for many years to monitor groundwater, and although this practice has been largely replaced by smaller screens in recent years, several legacy, large-screened monitoring wells are still in use at many sites, leading to potential for cross-connection between aquifers and less than efficient in situ remedies. This paper conducts a number of simulations using groundwater flow, particle tracking and solute transport analysis to assess potential impacts of such wells in providing short cuts for solute transport across different layers. The detailed numerical experiments use Modflow and MT3D to simulate groundwater flow and transport through porous media with large screened monitoring wells. The study compares using Modflow’s Multi Node Well (MNW) package as well as zones of high hydraulic conductivity to simulate monitoring wells. A number of scenarios are analyzed including monitoring wells located in recharge and discharge areas; in areas with horizontal flow conditions, under isotropic and anisotropic conditions and heterogeneities. The study also compares results of the numerical analysis with actual data from real sites to shed light into the potential for cross connection.