Isotope hydrology and baseflow geochemistry in natural and human-altered watersheds in the Inland Pacific Northwest, USA

Abstract

This study presents a stable isotope hydrology and geochemical analysis in the inland Pacific Northwest (PNW) of the USA. Isotope ratios were used to estimate mean transit times (MTTs) in natural and human-altered watersheds using the FLOWPC program. Isotope ratios in precipitation resulted in a regional meteoric water line of δ²H = 7.42·δ¹⁸O + 0.88 (n = 316; r² = 0.97). Isotope compositions exhibited a strong temperature-dependent seasonality. Despite this seasonal variation, the stream δ¹⁸O variation was small. A significant regression (τ = 0.11D^−1.09; r² = 0.83) between baseflow MTTs and the damping ratio was found. Baseflow MTTs ranged from 0.4 to 0.6 years (human-altered), 0.7 to 1.7 years (mining-altered), and 0.7 to 3.2 years (forested). Greater MTTs were represented by more homogenous aqueous chemistry whereas smaller MTTs resulted in more dynamic compositions. The isotope and geochemical data presented provide a baseline for future hydrological modelling in the inland PNW.

Keywords:

• baseflow geochemistry
• hydrogen-2
• isotope hydrology
• mean transit times
• natural and human-altered watersheds
• oxygen-18
• watershed management

Acknowledgements

The David Lamb Memorial Scholarship awarded by the Washington State Lake Protection Association (WALPA) to RSM covered a portion of the geochemical analyses.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This project was funded by the joint venture agreement [No. 10-JV-11221634–252] between USDA Forest Service Rocky Mountain Research Station and the University of Idaho.

Supplemental data

Supplemental data for this article can be accessed http://dx.doi.org/10.1080/10256016.2015.1008468http://dx.doi.org/10.1080/10256016.2015.1008468.