STABLE ISOTOPES OF HYDROGEN AND OXYGEN: HYDROLOGIC TRACERS OF AQUIFER RECHARGE

Groundwater levels have been declining in the Grande Ronde basalt aquifer of the Palouse Basin in southeast Washington and northern Idaho at an average rate of approximately 0.4 m (1.3 ft) per year for more than 80 years. Repeated studies have demonstrated the apparent age of this water to be >10,000 years, suggesting primary recharge during the Pleistocene with extremely limited modern recharge. Approximately 65,000 people across the Basin depend on this aquifer for drinking water, and to date, no recharge sources have been definitively identified.

Utilizing data from precipitation and surface water across the Basin, stable isotopes of hydrogen and oxygen (δ18H and δ2O) were used as hydrologic tracers to investigate water movement along a portion of the South Fork of the Palouse River (SFPR). Stable isotope data and more traditionally collected hydrologic information, including water levels and tritium (3H), are all consistent with the premise that this reach of the SFPR is a losing stream. Combined with X-ray fluorescence spectroscopy (XRF) data from previously unsampled basalt outcrops, the groundwater stable isotope data suggest that stream loss is contributing recharge to the deeper basalt aquifers.

The isotope data have also yielded unexpected clues to the structural geology along the study reach, suggesting the orientation of subsurface folding or faulting, inferred from what appears to be directional recharge. In addition, stable isotope data have proven to be a valuable tool in identifying anthropogenic input to the hydrologic system, clearly identifying input from wastewater treatment plants.

This study illustrates the increasing usefulness of stable isotopes for water resource investigations, providing high quality data at a fraction of the cost of alternatives such as 3H, 14C, and more involved traditional hydrodynamic methods which may not always be feasible.