Simulation of solute movement through wellbores to characterize public supply well contaminant vulnerability in the High Plains Aquifer, York, Nebraska

A ground-water flow and transport model simulating contaminant movement to public supply wells in the High Plains aquifer near York, Nebraska was developed as part of the U.S. Geological Survey National Water-Quality Assessment Program. Water-quality samples were collected from wells screened in unconfined and confined units. Samples collected from a public supply well and selected monitoring wells completed in the confined aquifer have chemistry and age-tracer concentrations consistent with a mix of young water from unconfined recharge areas combined with relatively older confined aquifer water. These results imply that there are preferential flow paths that permit shallow recharge water and contaminants to move through the confining layer. The transient ground-water flow and transport model was constructed using the multi-node well package of MODFLOW-2000, which simulates flow between different layers through the wellbore of a well. MODFLOW-GWT (Ground-Water Transport) was used to simulate solute movement through the system, while also assigning ages to the particles. The simulations indicate that summer pumping conditions, which can cause vertical head differences of up to 15 meters between unconfined and confined layers, and wells screened through multiple layers introduce shallow recharge waters and contaminants to lower layers, increasing the vulnerability of public-supply wells completed in the confined system to contamination from unconfined waters.