ANALYTICAL MODELING OF FLOW REGIMES WITHIN AND SURROUNDING SCREENED MONITORING WELLS UNDER AMBIENT AND PUMPED CONDITIONS

Analytical modeling of monitoring well hydrodynamics provides a rapid and flexible method of quantifying ambient and pumped flow regimes within and surrounding monitoring wells. These flow regimes control the distribution of inflowing groundwater, and thus control the composition of both passive and active groundwater samples. Combinations of analytic elements allow the modeling of finite-length, finite-diameter, screened monitoring wells and interactions with the surrounding flow regimes. The screened section of a monitoring well is modeled as a cylindrical uniform head boundary established by a line doublet. A polynomial line source-sink is used to compensate for vertical gradients, maintain the vertical orientation of the cylindrical uniform-head boundary and determine the distribution of inflow and outflow due to those vertical gradients. A line sink simulates the redistribution of inflow and outflow due to pumping. Wellbore flow regimes are modeled separately and linked to the surrounding flow regime using the boundary conditions established by the external model. For example, a vertical component of the ambient gradient surrounding the monitoring well establishes a proportional vertical component to well-bore flow. Vertical gradients also redistribute inflow along the screened section of the well resulting in well water concentrations that are weighted toward the high-head end of the screen. The degree of weighting depends on the strength of the vertical gradient relative to the horizontal gradient. During pumping the inflow and outflow will be redistributed depending on the rate of pumping relative to the ambient conditions; inflow is increased, outflow is decreased and, between these regions of inflow and outflow, outflow is reversed. This region of flow reversal will grow as the pumping rate is increased and, at a rate equivalent to the ambient through flow, will encompass the entire screened section. The distribution of inflow and the water captured by the pump are controlled by the interaction of pumping with the ambient flow regime. Groundwater sampled by pump or with a passive sampler depends on the interaction of the well hydrodynamics with the ambient flow regime, geometry of the well, strength of vertical gradients and rates of pumping.