23 Management of Hydrologic and Geologic Resources I

Thursday, 16 May 2024
8:00 AM - 12:00 PM
Maple Ballroom

USING PRECIPITATION AND GROUNDWATER ISOTOPE DATA TO DELINEATE AND MAP HYDROGEOLOGIC UNITS, RECHARGE SOURCES AND GROUNDWATER FLOW, PLAINS, MONTANA

Paper number: 23-2
Program: Theme Sessions
Time: 8:10 AM – 8:30 AM

Authors

Abstract

The Montana Bureau of Mines and Geology (MBMG) has collected groundwater, surface water and precipitation data in Lincoln and Sanders Counties as part of the Ground Water Assessment Program’s groundwater characterization effort (2018-2023). Monthly precipitation was collected in the Plains area between November 2019 and October 2023 (Montana Precipitation Isotope Network). The data include stable water isotope analyses (2H and 18O) for each month, two tritium analyses (3H) (May 2020-9.8TUs; October 2020 - 3.1TUs) and 2023 winter snowpack data. The snowpack data is a complete snowpack sample collected in March 2023 from a Cabinet Mountain SNOTEL. Four stable-water isotope samples were collected (seasonally) from the Clark Fork River at Plains. Periodic samples were collected at 3 wells and 1 spring. And another 22 wells were sampled once. Five wells were sampled and analyzed for 3H. The data are used to understand recharge sources to the aquifers, groundwater age and connection between aquifers.

Hydrogeologic units include a Precambrian fractured bedrock aquifer that surrounds and underlies a Quaternary alluvial aquifer that includes a shallow (0-35 ft), intermediate (40-180 ft), and deep layer (> 200 ft). The deep layer is exposed along the valley margins, directly above and adjacent to the fractured bedrock aquifer.

Isotope data indicate the bedrock aquifer and the deep alluvial layer are connected with respect to recharge source and age; the tritium data (<.8) indicate recharge older than 1950’s. Isotope composition for these two connected aquifers and the intermediate layer plot between relatively isotopically light winter snowpack (δ18O, -19‰; δ2H, -143‰) and the Clark Fork River’s heavier composition (δ18O, -16.5‰, δ2H, -126.5‰). The shallow layer is isotopically identical to the Clark Fork River and has a modern-day groundwater age (5.1 TU).

Groundwater-age differences between the intermediate and deep layers suggest a depositional contact between the deep and shallow-intermediate alluvium. This contact represents partial erosion of older Glacial Lake Missoula flood deposits that were subsequently covered with a younger set of flood deposits. The age of these inferred older flood deposits is unknown.

Geological Society of America Abstracts with Programs. Vol. 56, No. 4

doi: 10.1130/abs/2024CD-399733

© Copyright 2024 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.