Strontium isotope systematics of mixing groundwater and oil-field brine at Goose Lake in northeastern Montana, USA

Groundwater, surface water, and soil in the Goose Lake oil field in northeastern Montana have been affected by Cl- rich oil-field brines during long-term petroleum production. Ongoing multidisciplinary geochemical and geophysical studies have identified the degree and local extent of interaction between brine and groundwater. Fourteen samples representing groundwater, surface water, and brine were collected for Sr isotope analyses to evaluate the usefulness of 87Sr/86Sr in detecting small amounts of brine. Differences in Sr concentrations and 87Sr/86Sr are optimal at this site for the experiment. Strontium concentrations range from 0.13 to 36.9 mg/L, and corresponding 87Sr/86Sr values range from 0.71097 to 0.70828. The local brine has 168 mg/L Sr and a 87Sr/86Sr value of 0.70802. Mixing relationships are evident in the data set and illustrate the sensitivity of Sr in detecting small amounts of brine in groundwater. The location of data points on a Sr isotope-concentration plot is readily explained by an evaporation-mixing model. The model is supported by the variation in concentrations of most of the other solutes

A GIS-based vulnerability assessment of brine contamination to aquatic resources from oil and gas development in eastern Sheridan County, Montana

Water (brine) co-producedwith oil in theWilliston Basin is some of themost saline in the nation. The Prairie Pothole Region (PPR), characterized by glacial sediments and numerous wetlands, covers the northern and eastern portion of the Williston Basin. Sheridan County, Montana, lies within the PPR and has a documented history of brine contamination. Surface water and shallow groundwater in the PPR are saline and sulfate dominated while the deeper brines are much more saline and chloride dominated. A Contamination Index (CI), defined as the ratio of chloride concentration to specific conductance in a water sample, was developed by the Montana Bureau of Mines and Geology to delineate the magnitude of brine contamination in Sheridan County. Values \u3e0.035 indicate contamination. Recently, the U.S. Geological Survey completed a county level geographic information system (GIS)-based vulnerability assessment of brine contamination to aquatic resources in the PPR of the Williston Basin based on the age and density of oilwells, number ofwetlands, and streamlength per county. To validate and better define this assessment, a similar approach was applied in eastern Sheridan County at a greater level of detail (the 2.59 km2 Public Land Survey System section grid) and included surficial geology. Vulnerability assessment scores were calculated for the 780 modeled sections and these scores were divided into ten equal interval bins representing similar probabilities of contamination. Two surface water and two groundwater samples were collected fromthe sectionwith the greatest acreage of Federal land in each bin. Nineteen of the fortywater samples, and at least one water sample from seven of the ten selected sections, had CI values indicating contamination. Additionally, CI values generally increasedwith increasing vulnerability assessment score,with a stronger correlation for groundwater samples (R2 = 0.78) than surface water samples (R2 = 0.53)

Arsenic loads and source areas in and along Bear Creek, Park County, Montana, September 2000 /

"May 2002."Shipping list no.: 2002-0229-P.Includes bibliographical references (p. 24-25).Mode of access: Internet

Arsenic and metal loads and source areas in the Middle Fork Warm Springs Creek Watershed, Jefferson County, Montana, June 2001 /

"July 2003."Shipping list no.: 2003-0263-P.Includes bibliographical references (p. 21-22).Mode of access: Internet

Reconnaissance of ground-water resources of the Fort Peck Indian Reservation, northeastern Montana /

Includes text, bibliography, 3 tables, 2 cross sections, and 2 figures."September 1991."Shipping list no.: 91-0738-P.Mode of access: Internet

Saline-water contamination in Quaternary deposits and the Poplar River, East Poplar Oil Field, northeastern Montana /

"May 1997"Three maps on 3 folded leaves in pocket.Includes bibliographical references (p. 33-37).Mode of access: Internet

Hydrology of the Helena area bedrock, west-central Montana, 1993-98 /

"In cooperation with Lewis and Clark County Water Quality Protection District."Three plates of ill. and maps on 3 folded leaves in 2 pockets.Chiefly tables.Shipping list no.: 2001-0112-P."With a section on Geologic setting and a generalized bedrock geologic map by Mitchell W. Reynolds."Includes bibliographical references (p. 33-36).Mode of access: Internet

Ground-water quality for two areas in the Fort Peck Indian Reservation, northeastern Montana, 1993-2000 /

Shipping list no.: 2004-0076-P."December 2003."Includes bibliographical references (p. 15).Mode of access: Internet