Investigating the effects of isotopic transport and fractionation on inferred water ages in an integrated hydrologic system
Geology and Geological Engineering
The age of a water sample is an important property for determining the distribution of residence times, understanding susceptibility to contamination, and identifying changes in hydrologic systems. The techniques used to determine water ages are based on geochemical tracers that move with the water through the system, the most common of which are isotopic tracers such as Oxygen. Many assumptions are made to process and interpret the geochemical data in order to estimate an age and thus the accuracy of the inferred ages depend entirely on the validity of those assumptions. Many of the assumptions deal with the way isotopic signatures change as water moves through a hydrologic system. Physically, water falls from the atmosphere as rain or snow, moves along the surface of the earth in streams, infiltrates into the unsaturated zone, and later becomes groundwater but the effects of the different processes in each region that can affect isotopic compositions are seldom delineated in water age-dating techniques. This study will investigate how accounting for the different processes that affect isotopic signatures in each flow regime can change the inferred age of a water sample. This requires high-resolution explicit modeling of the complete hydrologic flow system which will be done using ParFlow, an integrated, parallel watershed model that simulates surface and subsurface fluid flow, coupled with the common land model (CLM) to simulate land surface processes. Isotopic transport will be simulated using a new version of the particle tracking code SLIM-FAST that is able to track water particles in subsurface, unsaturated, and surface water flows. The study site is the East Inlet watershed, high in the Rocky Mountains, where CSM students and USGS scientists have been collecting isotopic data that can be compared to the model results for model validation. The results of this study will be of broad interest to the geochemical and environmental science communities because it will identify the magnitude of the errors that are introduced into inferred water ages by not accounting for the full range of hydrologic processes when interpreting isotopic data.