Chemical and Isotopic Tracers of Groundwater Sustainability: an Overview of New Science Directions

Groundwater sustainability is an emerging concept that is rapidly gaining attention from both scientists and water resource managers, particularly with regard to contamination and degradation of water quality in strategic aquifers. The sustainability of a groundwater resource is a complex function of its susceptibility to factors such as intrusion of poor-quality water from diverse sources, lack of sufficient recharge and reorganization of groundwater flowpaths in response to excessive abstraction. In theory the critical limit occurs when degradation becomes irreversible, such that remediative efforts may be fruitless on a reasonable human time scale. Chemical and isotopic tracers are proving to be especially useful tools for assessment of groundwater sustainability issues such as characterization of recharge, identification of potential sources, pathways and impacts of contaminants and prediction of how hydrology will change in response to excessive abstraction. A variety of relatively cost-efficient tracers are now available with which to assess the susceptibility of groundwater reserves to contamination from both natural and anthropogenic sources, and may provide valuable monitoring and regulatory tools for water resource managers. In this overview, the results of several ongoing groundwater studies by the U.S. Geological Survey will be discussed from the perspective of implications for new science directions for groundwater sustainability research that can benefit water policy development. A fundamental concept is that chemical and isotopic tracers used individually often provide ambiguous information, and are most effective when used in a rigorous "multi-tracer" context that considers the complex linkages between the hydrology, geology and biology of groundwater systems.