Impact of Long Well Screens on Monitoring of the Freshwater-Saltwater Transition Zone.

Deep monitoring wells with long screens crossing the transition zone between freshwater and saltwater are often used in coastal areas to characterize fresh groundwater resources and the depth of saline groundwater. However, past studies have demonstrated that long-screen wells can lead to biased observations of the transition zone, since vertical flow within the borehole can modify the shape and elevation of the transition zone in and around the borehole compared to undisturbed conditions without a well. Here, field observations and variable-density numerical flow simulations are used to evaluate, under natural flow conditions, how the installation of long-screen wells can provide time-varying biased observations of the freshwater-saltwater transition zone, and how various aquifer and well parameters affect the magnitude of these biases. Results show that long-screen wells can lead to a more dispersed interface, an upward displacement of the transition zone of between 5 and 10 m, and a salinity decrease in the saltwater portion of the well on the order of 10 to 15 g/L. The perturbations take up to 5 years to fully develop and stabilize. The degree of displacement depends on the screen diameter, screen length, aquifer anisotropy, and hydraulic conductivity, whereas the displacement is independent of the distance of the well from the coast. This analysis provides insight into which well and aquifer characteristics increase the risk of obtaining biased observations in long-screen wells, and provides orders of magnitude for these biases.