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

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 δ(2)H = 7.42·δ(18)O + 0.88 (n = 316; r(2) = 0.97). Isotope compositions exhibited a strong temperature-dependent seasonality. Despite this seasonal variation, the stream δ(18)O variation was small. A significant regression (τ = 0.11D(-1.09); r(2) = 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.