Understanding human withdrawals through tap water stable isotope comparison with meteoric water isoscapes

Background/Question/Methods

Human demand for water represents an increasingly important driver of local to global-scale hydrologic patterns and is expected to continue to represent a significant axis in the interaction between ecological function (e.g., habitat for endangered species, ecological resistance to non-native species expansion) and human needs for fresh water. We report on a stable isotope survey of tap water in the US state of Texas during record drought conditions and compare the spatial distribution of water isotopes against modeled precipitation isoscapes derived from IsoMAP (http://isomap.org).

Results/Conclusions

Significant variation in tap water isotopic composition is observed (δ²H range 70‰, δ¹⁸O range 12‰), consistent with, but larger than, the expected range of precipitation inputs across this climatologically diverse state. The range of variation reflects precipitation inputs, the effects of evaporation on surface water sources, and in some cases perhaps depletion of very slowly recharged groundwater resources. Comparisons between precipitation isoscapes and tap water observations yielded regions with significant divergences that could provide information about evaporative enrichment of surface resources or groundwater resource use and dynamics, including whether that groundwater is rapidly recharged. Clear zones of significant groundwater dependence in southern Texas are evident, as are zones of significant dependence on surface water resources (e.g., around Dallas-Fort Worth). In addition, some western regions typically dependent on surface waters and therefore expected to exhibit evidence of evaporative enrichment instead show significant reliance on groundwater resources. As we come to understand the water requirements of ecological systems facing significant uncertainty in precipitation inputs and urban and rural communities face water resource shortages, it is critical to have tools to assess, at appropriate spatial and temporal scales, how human water needs affect the magnitudes and flowpaths of water. We demonstrate rich inferences that can be drawn from spatial sampling and comparison with modeled water isoscapes and suggest that tools such as IsoMAP will become increasingly important as climate and other changes put increasing pressure on these human-hydrological-ecological systems.