QUANTIFYING EXCHANGES ACROSS LAYERED AMERICAN AQUIFER SYSTEMS USING VERTICAL HYDRAULIC GRADIENT

Vertical hydraulic gradients between unconfined and confined aquifers influence the speed, direction and existence of exchanges between layered aquifers. Groundwater travels unseen below the land surface, complicating efforts to understand aquifer exchanges and groundwater movement, but measuring vertical hydraulic gradient can help quantify groundwater flow direction and rates. Understanding exchanges in layered aquifer systems can support the management of groundwater quality and quantity. Efforts to measure vertical hydraulic gradients between unconfined aquifers and their underlying confined aquifers rely on measurements of groundwater levels in co-located wells. However, groundwater levels in unconfined and confined aquifer units are rarely examined together and in conjunction with hydrogeological data, meaning we lack measurement-based insights into vertical flow potentials between aquifer units across a diverse array of aquifer systems. Here we examine monitoring well water level time series in five major aquifer systems that represent a substantial share of total US groundwater pumping: (1) the Mississippi Embayment, (2) the North Atlantic Coastal Plain, (3) California’s Central Valley, (4) the southern High Plains, and (5) the Floridan. We observe decadal-scale changes in vertical hydraulic gradients in all five aquifer systems. Specifically, four of the five aquifer systems indicate an increase in downward potential of water flow between unconfined and confined units. This may indicate either (a) a reduction or even reversal of upward-oriented flow potentials, or (b) increased potential for downward flow in places where flow was already downward-oriented. In addition to our regional aquifer analyses, we calculate vertical hydraulic gradients using thousands of pairs of shallow and deep monitoring wells across the US. Our pan-US analysis provides a wide-scope view of changes in flow potential over time. By examining how vertical hydraulic gradients have changed over time at expansive spatial scales, we provide some of the first estimates of flow potential in aquifer systems throughout the United States.