Use of Sr and U isotopes to evaluate water sources and mixing in freshwater and estuarine wetlands
J.B. Paces1, J.Z. Drexler2, F. Wurster3, L.A. Neymark1
1U.S. Geological Survey, Denver, CO, USA, firstname.lastname@example.org, email@example.com
2 U.S. Geological Survey, Sacramento, CA, USA, firstname.lastname@example.org
3U.S. Fish & Wildlife Service, Suffolk, VA, email@example.com
Understanding wetland water supplies is important for managing resources and determining habitat impacts due to climate change or anthropogenic activity. However, identifying multiple sources and contributions is often difficult due to complex hydrologic, biologic, and chemical processes operating in near-surface environments. Fortunately, heavy radiogenic isotopes (in particular, 87Sr/86Sr and 234U/238U) remain largely unaffected by these processes and can be used to help identify unique signatures from different sources. Combined Sr- and U-isotope data from wetlands in Nevada and California allowed identification of 3-component mixing patterns that were not easily recognized using other chemical approaches. Isotopes of Sr and U are particularly useful because they are relatively abundant in many aqueous solutions, can be analyzed with high precision, may have large differences between sources, and are not fractionated by most near-surface processes. At both sites, mixtures of 3 dominant end members form distorted triangular nets that bound the data and allow contributions from each source to be estimated.
Sources of water in wetlands on the Pahranagat National Wildlife Refuge (PNWR) in southern Nevada consist of discharge from high-volume springs associated with the regional carbonate aquifer and from local volcanic aquifers. Dissolved ions and H-O-S isotopes are strongly affected by evapotranspiration, mineral precipitation, and sulfate reduction in heavily vegetated areas. Unlike other components, 87Sr/86Sr and 234U/238U do not show seasonal variation at individual sites and define mixtures between surface flow entering the PNWR and diffuse discharge from the shallow volcanic aquifer. Variations depend on location within the wetland and water management practices.
Variations in fluvial and tidal influence in the Sacramento-San Joaquin Delta, CA, have been evaluated over the last 6,000 years using 87Sr/86Sr and 234U/238U preserved in peat cores. Plants take up substantial amounts of dissolved Sr during growth, whereas redox reactions at the peat-water interface cause immobilization of U dissolved in the water column. Analyses of peat cores show coherent variations in space and time, reflecting substantial changes in proportions of fluvial and seawater components in the estuarine environment. Isotope data show that the largest effects on hydrologic balances in the Delta resulted from resource exploitation and land-use change caused by large-scale development starting in the mid 19th century. More recent water-management practices have started to reverse those trends.