IDENTIFYING SOURCES OF GROUNDWATER RECHARGE IN SEMI-ARID, IRRIGATED LANDSCAPES IN THE NORTHERN ROCKY MOUNTAINS

The Montana Bureau of Mines and Geology recently initiated a Precipitation Isotope Network (MTPIN) monitoring program to improve qualitative and quantitative estimates of recharge from various sources. The amount, location and timing of recharge are essential inputs to groundwater flow models developed for Montana’s semi-arid, intermontane basins. Irrigated agriculture, rapid population growth, and world-class sport fisheries co-exist in these areas, generating increased water use and a desire to maintain instream flows. In addition to direct infiltration of snowmelt and precipitation, recharge occurs along losing stream reaches, leaky irrigation canals, and through applied irrigation water. Variations in the stable isotopes of oxygen (¹⁸O/¹⁶O) and hydrogen (²H/¹H) in groundwater can be diagnostic of these sources of recharge. Multiple factors affect the oxygen-18 (¹⁸O) and deuterium (²H) signature in precipitation, including elevation and temperature, and this signature is preserved upon entering the groundwater flow system. The MTPIN program supports improved recharge characterization by providing long-term records of stable isotopes in precipitation in western Montana.

MTPIN currently consists of eight precipitation collectors located across four watersheds. Monthly composite samples of rain and/or snow are analyzed for ¹⁸O and ²H. Locations of the eight collectors range in elevation from 3300 to 7100 feet above sea level. Each collector is paired with a SNOTEL or Montana Climate Office weather station. Five collectors are also paired with a groundwater or surface water monitoring site that is sampled monthly for ¹⁸O and ²H. The monitoring program uses field protocols consistent with the International Atomic Energy Agency’s Global Network of Precipitation in order to prevent evaporation at the precipitation collectors. Several methods for collecting and compositing snowfall and snow melt have been tested at the remote, high elevation locations. Groundwater samples collected within these basins show systematic variation in ¹⁸O, indicating spatially variable sources of recharge.