The Existential, Unanticipated Threat of Groundwater Basin Closure

Every groundwater basin has its own, internal source of salinity. This is why the natural TDS of fresh groundwater is typically on the order 10² ppm while the TDS of infiltrating precipitation is on the order of 10^-1-10¹ ppm. Accordingly, salinity of the water in closed hydrologic basins (e.g., Mono Lake, Death Valley, Dead Sea) is high (10³-10⁵) because of evaporative concentration and the lack of outlets for groundwater salinity. The existence of naturally fresh groundwater is made possible by the existence of open groundwater basins containing exits for the circulating groundwater and entrained salts. Worldwide, even moderate development of groundwater is closing basins by cutting off groundwater discharge to streams or to adjacent basins, as exemplified by data and analysis of water and salt balances in aquifer systems of the southern Central Valley, California, and the North China Plain. In cases where the pumped groundwater is being used to irrigate crops, the dominant exit for the pumped groundwater becomes evaporation, virtually totally closing the basin and setting up progressive salinization on time scales of decades to centuries. Because the Green Revolution has been substantially driven by exploitation of groundwater for irrigation supply, and because long term use of groundwater in closed basins will necessitate future desalination of the degraded groundwater, basin closure poses an existential threat to not only groundwater quality sustainability, but also to global energy demand and the food supply.