MONTANA’S PRECIPITATION ISOTOPE NETWORK (MTPIN): DATA AND PROTOCOL LESSONS MONTANA BUREAU OF MINES AND GEOLOGY’S (MBMG) GROUND WATER ASSESSMENT PROGRAM (GWAP)

The Montana Bureau of Mines and Geology (MBMG) developed the Montana Precipitation Network (MTPIN), a pilot program, to collect monthly precipitation samples for stable isotope (¹⁸O/¹⁶O and ²H/¹H) analysis from four basins in southwest and western Montana (the Lolo, Upper Clark Fork, Lower Blackfoot, and Upper Missouri Watersheds). Additionally, samples from two sites (Lolo Pass SNOTEL and Basin Creek SNOTEL) were analyzed for tritium as part of a Global Network of Isotopes in Precipitation (GNIP) sampling effort. At three high elevation sites (Lolo Pass SNOTEL, MacDonald Pass, and Basin Creek SNOTEL) snowpack measurements (depth, snow water equivalent) and composite samples were collected for the first winters and then transitioned to once or twice a winter. Objectives include: documenting the spatial and temporal variation in the isotopic composition of precipitation, computing local meteoric water lines (LMWLs), establishing sampling and handling protocols, and evaluating the utility and feasibility of long-term network operation.

Characterizing the seasonal and inter-annual stable isotope variations in precipitation supports the identification of groundwater recharge sources. Each sample site consists of a precipitation collector paired with a climate station; and in three of the basins the precipitation collectors are also paired with groundwater and surface-water sampling sites. Preliminary local meteoric water lines indicate little regional variation in west-southwest Montana; tritium values ranged from 4 to 14 TU, with the lowest values in the fall and the highest values in the spring. Sample results are available through the MBMG Ground Water Information Center (GWIC, www.mbmggwic.mtech.edu) and the GNIP database (www.iaea.org/services/networks/gnip).

The pilot network has been supported by collaborators from Montana Tech, the Lewis and Clark County Water Quality Protection District, the Missoula Valley Water Quality District, the University of Montana, Montana Climate Office, GNIP, the U.S Geological Survey and the United States Forest Service. New collaborators include Montana State University and the University of Montana Western.