Theoretical head decay in closed basin aquifers: an insight into fossil groundwater and recharge events in the Andes of northern Chile

Abstract

Many small to moderate-sized, closed aquifer basins in northern Chile show a groundwater gradient indicating flow despite an apparent lack of recharge under current climatic conditions. An examination of the mechanisms that might produce such flow suggests that the only realistic possibility is head decay from an earlier higher water level caused by ancient recharge. Model studies suggest that for small basins, such recharge must have taken place within the last 1000 years and possibly within the last few hundred years, calling into question whether such resources should be labelled fossil. Recent palaeoclimatic studies of northern Chile show several pluvial periods throughout the Late Pleistocene and Holocene, inaddition to long return period storm events, which continue to the present and which might have led to a series of pulsed flushing events in aquifers. Hence, it is incorrect to label groundwater resources as either fossil or actively recharged, as there is, in reality, a continuum between the two, and all such aquifers require site-specific evaluation and new, more flexible guidelines for development.