The hydrogen and oxygen isotopic composition of precipitation, evaporated mine water, and river water in Montana, USA

The isotopic composition of 42 samples of rain and snow collected in 2004 were used to construct a local meteoric water tine (LMWL) for Butte, Montana. The derived equation (delta D = 7.31 delta O-18 - 7.5, r(2) = 0.987), represents one of the first published LMWLs based on direct precipitation for any location in the northern Rocky Mountains. Samples of underground and surface mine waters in Butte, including the Berkeley pit-take and a nearby tailings pond, define a linear trend with a much lower slope and intercept than the LMWL (delta D = 5.00 delta O-18 - 49.5, r(2) = 0.991), consistent with non-equilibrium evaporation at an average relative humidity of roughly 65%. Detailed evaporation calculations are presented which indicate that the shallow Berkeley pit-take was approximately 25% evaporated in October, 2003, whereas the surface of the tailings pond was at least 50% evaporated. The intersection of the LMWL and mine water evaporation trend was used to calculate the average composition of recharge water to the flooded mine complex (delta D = -139 parts per thousand, delta O-18 = -18.0 parts per thousand). These values are considerably lighter than the weighted total of precipitation for the 2004 calendar year (delta D = -118 parts per thousand, delta O-18 = -15.37 parts per thousand), which is partly explained by the unusually tow snowfall that Montana experienced in 2004. Based on this study, the LMWL recently proposed by Kendall and Coplen (2001) [Kendall, C., Coplen, T.B., 2001. Distribution of oxygen-18 and deuterium in river waters across the United States, Hydrological Processes 15, 1363-1393] from regression of isotopic data from a number of Montana rivers is more accurately interpreted as an evaporation line. Isotopic trends based on river data should be treated with caution, particularly in a semi-arid region such as Montana where rivers are often influenced by dams and irrigation withdrawals. (c) 2006 Elsevier B.V. All rights reserved.