The role of uplift and erosion in the persistence of saline groundwater in the shallow subsurface

In many regions of the world, the shallow (<300 m) subsurface is replenished with meteoric recharge within a few centuries or millennia, but in some regions saline groundwater persists despite abundant rainfall. Analyses of the flushing rate of shallow groundwater usually consider the permeability and recharge rate and a static landscape. The influence of landscape evolution can become important over millions of years, however. Here we present numerical simulations of fluid flow and transport in the top 1 km of a sedimentary foreland basin dominated by aquitards, where the rate of uplift and erosion ( 20 m Ma(-1)) balances that of meteoric flushing. Paleozoic age saline groundwater and brine persist at shallow depths that might otherwise have contained potable water. Similar hydrogeologic conditions, and uplift and erosion rates, likely exist in many other regions of the world, where a moving landscape has probably never been considered as an important contributor to groundwater quality. Plain Language Summary Most people are unfamiliar with brine, but saline water more concentrated than seawater can be encountered nearly everywhere beneath the Earth's surface. Brines five to ten times more concentrated than seawater typically occur deep below the surface, but beneath the Appalachian Plateaus in the eastern United States, brine is within 300 m of the surface. This is surprising because the brine and the rocks within which it resides are quite old (more than 250 million years). Rainfall in this humid area should have flushed the brine from the rocks over this period. This study found that erosion plays a dominant role in this landscape, because the rate of erosion is comparable to the rate of flushing, so brine will persist as the surface continues to descend. This finding has implications to other areas in the world where similar geologic processes are at work, and has particular significance to the Appalachian Basin, where hydraulic fracturing of gas wells has raised concerns that injected fluids could migrate to land surface. Shallow brine in the Appalachian Plateaus provides a geologic barrier to the upward migration of injected fluids, because the density difference between brine and the injected fluid will impede its upward flow.