Up or Down? Vertical Hydraulic Gradients Across Layered American Aquifer Systems

Abstract:

Vertical hydraulic gradients between unconfined and confined aquifers influence the speed, direction and existence of exchanges between layered aquifers. Understanding groundwater movements between unconfined and confined aquifer units can help support efforts to manage groundwater quality and quantity. Groundwater travels unseen below the land surface, complicating efforts to understand aquifer exchanges, but measuring vertical hydraulic gradients can help quantify groundwater flow directions and rates. Efforts to measure vertical hydraulic gradients between unconfined aquifers and their underlying confined aquifers rely on measurements of groundwater levels in co-located shallow and deep wells. However, groundwater levels in confined and unconfined aquifer units are rarely examined together, 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.