76-4 Geologic and Hydrologic Controls on Salinity in Intermittent Prairie Streams of Central Montana-Distinguishing Natural Salinity Processes from Potential Oilfield Impacts

Session: Contaminants Near Groundwater-Surface Water Interfaces

Presenting Author:

Authors:

Abstract:

Oil production-associated brine contamination to small creeks and shallow aquifers presents an ecological health risk to aquatic life, wildlife, livestock, agriculture, and humans. North Willow Creek (NWC) in central Montana has been identified by the Montana Department of Environmental Quality (DEQ) to be impaired for aquatic life and irrigation by salinity (e.g. 29,000 µS/cm). The NWC watershed is host to multiple oil and gas fields. This study evaluated the source of salinity in NWC and its adjacent alluvial aquifer for the possibility of historic oil-production related salt contamination and salt sourced by natural processes. During Spring and Fall of 2023, field geochemical parameters were measured, and major ion and isotopic analyses were conducted on stream water from NWC, nearby Flatwillow Creek (FWC), and the Musselshell River. Associated shallow alluvial groundwater was sampled at each location along the reach of NWC and FWC. Additionally, precipitation samples were collected to develop a local meteoric waterline to assist in the analysis of the relative dating of the study waters. The results were compared to historical water quality in the deeper Tertiary through Cretaceous aquifers and in the Pennsylvanian and Mississippian petroleum-associated aquifers within the study area. Findings reveal that NWC surface waters and groundwaters are predominantly sodium-magnesium-sulfate-type waters of modern origin, while the deeper bedrock aquifers are sodium-sulfate-carbonate-chloride dominated. δ34S-SO4_(aq) and δ18OSO4 in NWC and the shallow groundwater system are much lighter than those typically observed in bedrock samples, suggesting minimal bedrock contributions to the stream.  A linear relationship between 18O and 34S in surface and groundwater suggests sulfate reduction and is consistent with low DO measurements. Finally, LMWL analysis of the NWC groundwater and surface water date the water as modern, directly related to recent climatic events.  The study demonstrates that the source of elevated salts of NWC and its aquifer is a result of natural processes of evapoconcentration and dissolution of soluble salts in the shales and sandstones. The stream salinity is exacerbated by the use of stock dams and ponds installed along an intermittent NWC, preventing the efficient flushing of salts out of the NWC watershed.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10813

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