Groundwater–surfacewater interactions at the transition of an aquifer from unconfined to confined

A three-dimensional numerical model is developed to identify systematic changes in the interactions between the groundwater flow systems and streams that cross the transition zone where unconfined aquifers become confined. A generalized model is designed that represents hydrologic conditions where gently dipping aquifer-confining unit sedimentary sequences subcrop beneath a thin surficial aquifer and are transected by gaining streams flowing parallel to dip. A series of simulations are performed to investigate the effect of different hydrogeologic characteristics of the edge of the confining unit, where potential for facies changes or fluvial reworking, results in variations in the material properties of the confining unit. The results show an abrupt increase in groundwater discharge to the stream immediately upstream of the transition to confined conditions, and a corresponding, but not equal, decrease in groundwater discharge along stream reaches downstream of the edge of the confining unit. Model results were compared to measured stream discharges at a location where the Loosahatchie River of eastern Tennessee, USA, crosses an unconfined–confined transition in the upper Mississippi Embayment.