Stable Isotopes of Hydrogen and Oxygen as Hydrologic Tracers of Aquifer Recharge
Presented on Monday, May 5, 2014
Nathan Moxley1, Kent Keller2, Rick Conrey2 and Jim Osiensky3, (1)AMEC, Seattle, WA, (2)Washington State University, Pullman, WA, (3)University of Idaho, Moscow, ID
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, and implying modern recharge is extremely limited. Approximately 60,000 people across the Basin depend on this aquifer for drinking water, and although much has been learned over the past 50 years of study, no sources of recharge have been definitively identified.
Utilizing data from precipitation and surface water across the Basin, stable isotopes of oxygen and hydrogen (δ18O and δ2H) 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 isotope data suggest that stream loss is contributing recharge to the deeper basalt aquifers.
Groundwater stable 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 groundwater 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.
Nathan Moxley
AMEC, Seattle, WA
Nathan Moxley is a hydrogeologist with AMEC in Seattle, Washington. He has an M.S. in Geology from Washington State University where his research utilized stable isotopes as hydrologic tracers of groundwater recharge.
Kent Keller
Washington State University, Pullman, WA
Kent Keller is a professor of hydrogeology and biogeochemistry in the School of the Environment, and Co-Director of the Center for Environmental Research, Outreach, and Education at Washington State University. His research interests include biogeochemical weathering and local hydrogeology.
Rick Conrey
Washington State University, Pullman, WA
Rick Conrey is a geologist by training, with a Ph.D. from Washington State University. His research interest is primarily the complex geology and petrology of the Cascade Range in Oregon. He helps operate the Peter Hooper GeoAnalytical Laboratory at WSU. That lab has provided the vast majority of the analytical data on the Columbia River Basalt Group, critical for deciphering the stratigraphy of the basalt pile, and thus has provided some of the infrastructure for progress in hydrogeologic research in the Pacific Northwest.
Jim Osiensky
University of Idaho, Moscow, ID
Jim Osiensky is a professor of hydrogeology in the Department of Geological Sciences at the University of Idaho in Moscow, Idaho. His research interests include contaminant hydrogeology, hydrogeologic site characterization, hydrogeophysical applications in hydrogeology, and groundwater resource evaluation.
