Radiogenic isotopes in Hydrospheric Studies
Isotopes in hydrologic and fluid transport studies
The isotopic composition of hydrogen and oxygen (so-called water isotopes) has been a staple of hydrologic studies for many decades. There have been fewer studies utilizing radiogenic isotopes (‘solute’ isotopes) in hydrologic investigations, but there is an increasing interest in their application (see Blum and Erel, 2003 and references therein). Radiogenic isotopes such as Sr and Pb are excellent tracers of flowpaths through different geologic media (soils, lithologically varying bedrock, etc.) with distinct isotopic compositions. For example, the strontium isotopic ratio of dissolved Sr from carbonate rocks differs markedly from that derived from dissolving silicate minerals, particularly feldspars and micaceous minerals. Even when flowpaths cannot be distinguished, radiogenic isotopes can be of tremendous value to constrain hypotheses on what geochemical processes are controlling water chemistry. There are many concrete examples of the use of radiogenic isotopes in hydrology, including our own work on remote Lake Junin watershed in Peru (Flusche et al., 2005), where we used Sr isotopes to both identify sources of water to the lake and test hypotheses on aspects of the water cycle of the lake. We showed that isotopically distinct waters entered the lake from eastern streams compared to those draining into the western part of the lake, and that virtually all of the Sr entering the lake was from surface water with groundwater playing an almost negligible role. Following that work we used Sr isotopes to trace groundwater sources and sinks in the Catskills region of New York. Variable 87Sr/86Sr ratios of the carbonate and sandstone formations in the region provided good isotopic fingerprints of the groundwater, substantiating hypotheses based on hydrogeochemistry related to both regulatory guidelines (Azzolina et al., 2007) and how long it may take for the Catskills to suffer similar effects of atmospheric acid deposition as the Adirondacks. We have used Sr isotopes to characterize mixing and hyporheic interaction in an arid stream system in western United States where, once again, we were able to characterize different sources of waters (Jin et al., 2006). Siegel recently received NSF funding to measure Sr isotopes from dissolved Sr in northern peatlands, an extension of prior work that defined regional from local flow system dynamics (Hogan et al., 2000). We are in the process of analyzing Sr isotopes from water sampled from streams entering hypereutrophic Tai Lake (China). We will distinguish, using mixing models, how much water enters the lake from various streams carrying different pollution loads. Finally, we have extended our research on using Sr isotopes in fluids beyond the surface environment. For example, Bickford et al. (2008) used Sr isotopes to determine the rate at which fluids discharge through mud volcanoes in the Mariana trench system and how quickly strontium isotopes equilibrate between slab fluids, mud, and carbonate precipitates.