A potential hydrologic effect of surface mining of coal in southeastern Montana is a change in the quality of ground water. Dissolved-solids concen- trations in water in spoils aquifers generally are larger than concentrations in water in the coal aquifers they replaced; however, laboratory experiments have indicated that concentrations can decrease if ground water flows from coal-mine spoils to coal. This study was conducted to determine if decreases in concentrations occur onsite and, if so, which geochemical processes caused the decreases. Solid-phase core samples of spoils, unmined over- burden, and coal, and ground-water samples were collected from 16 observation wells at two mine areas. In the Big Sky Mine area, changes in ground- water chemistry along a flow path from an upgradient coal aquifer to a spoils aquifer probably were a result of dedolomitization. Dissolved-solids concentrations were unchanged as water flowed from a spoils aquifer to a downgradient coal aquifer. In the West Decker Mine area, dissolved-solids concentrations apparently decreased from about 4,100 to 2,100 milligrams per liter as water moved along an inferred flow path from a spoils aquifer to a downgradient coal aquifer. Geochemical models were used to analyze changes in water chemistry on the basis of results of solid-phase and aqueous geochemical characteristics. Geochemical processes postulated to result in the apparent decrease in dissolved-solids concentrations along this inferred flow path include bacterial reduction of sulfate, reverse cation exchange within the coal, and precipitation of carbonate and iron-sulfide minerals.

The Barker-Hughesville Mining District in the Little Belt Mountains is home to a heavily mining impacted watershed called Galena Creek which has been the subject of remediation and environmental restoration due to the high levels of metals (including Cd, Cu, Pb, and Zn) which have negatively impacted aquatic life in the drainage. Galena Creek was designated a superfund site in the late 1990's/early 2000's, and since then several constructional efforts have removed mine waste from direct contact with the stream. Despite these efforts, numerous mine water discharges still enter the creek, and the water quality is still impaired. It is possible that the unique geology of the district is contributing a certain amount of background loading of metals and acidity to the stream. However, because the area was mined in the early 1900's, no pre-mining baseline water quality samples were ever collected. This thesis used several geochemical techniques to assess whether natural weathering of mineralized bedrock influences the water quality of Galena Creek. A detailed synoptic sampling of the stream and all measurable inflows was conducted in mid-summer baseflow conditions. Both filtered and non-filtered samples were taken, along with samples of mineral crusts and in-stream precipitates. Samples were collected for O- and H-isotope analysis of water, O- and S- isotope analysis of dissolved sulfate, and S-isotope analysis of fresh sulfide minerals collected from mine dumps. In addition, representative bulk samples of each of the major rock types in the watershed were collected for laboratory leachate studies. The leachate tests included samples of hydrothermally altered and pyrite-mineralized rock that is thought to comprise up to 20% of the outcrop area of the Hughesville Stock: the main host rock of the area. Results of the synoptic sampling investigation show that most of the loads of metals and dissolved sulfate in Galena Creek during baseflow conditions can be accounted for from the known mine discharges. The S- and O-isotope composition of sulfate in the stream is similar to that of sulfate in the mine discharges, and the S-isotope composition of sulfate is similar to that of sulfides on the mine dumps. The hydrothermally-altered Hughesville Stock produced leachates with very poor water quality, whereas the unaltered stock and other bedrock units in the watershed produced leachates with much lower concentrations of metals and sulfate. By mixing the leachate water chemistry from each rock type, scaled to the percentage of the total watershed underlain by each rock type, a first pass approximation of the pre-mining water quality of Galena Creek was obtained. Although this type of calculation rests on several assumptions, the results suggest that Galena Creek could have had elevated concentrations of metals and sulfate from natural weathering prior to mining disturbance. Lastly, the concentrations of several metals of interest in Galena Creek were compared to concentrations in Chicago Gulch, a small stream with natural acidity draining an unmined, but hydrothermally altered, stock in the central Judith Mountains. The range in concentrations in the two drainages overlap. Whereas Galena Creek on average has higher concentrations of Mn, Zn and Cd, Chicago Gulch has higher concentrations of Pb and Al. In summary, although the present chemistry of Galena Creek is obviously impacted by the legacy mines, some metals and acidity would have entered the creek due to natural weathering prior to mining.

Groundwater Geochemistry: Fundamentals and Applications to Contamination examines the integral role geochemistry play s in groundwater monitoring and remediation programs, and presents it at a level understandable to a wide audience. Readers of all backgrounds can gain a better understanding of geochemical processes and how they apply to groundwater systems. The text begins with an explanation of fundamental geochemical processes, followed by a description of the methods and tools used to understand and simulate them. The book then explains how geochemistry applies to contaminant mobility, discusses remediation system design, sampling program development, and the modeling of geochemical interactions. This clearly written guide concludes with specific applications of geochemistry to contaminated sites. This is an ideal choice for readers who do not have an extensive technical background in aqueous chemistry, geochemistry, or geochemical modeling. The only prerequisite is a desire to better understand natural processes through groundwater geochemistry.

The single most important factor for the successful application of a geochemical model is the knowledge and experience of the individual(s) conducting the modeling. Geochemical Modeling for Mine Site Characterization and Remediation is the fourth of six volumes in the Management Technologies for Metal Mining Influenced Water series about technologies for management of metal mine and metallurgical process drainage. This handbook describes the important components of hydrogeochemical modeling for mine environments, primarily those mines where sulfide minerals are present—metal mines and coal mines. It provides general guidelines on the strengths and limitations of geochemical modeling and an overview of its application to the hydrogeochemistry of both unmined mineralized sites and those contaminated from mineral extraction and mineral processing. The handbook includes an overview of the models behind the codes, explains vital geochemical computations, describes several modeling processes, provides a compilation of codes, and gives examples of their application, including both successes and failures. Hydrologic modeling is also included because mining contaminants most often migrate by surface water and groundwater transport, and contaminant concentrations are a function of water residence time as well as pathways. This is an indispensable resource for mine planners and engineers, environmental managers, land managers, consultants, researchers, government regulators, nongovernmental organizations, students, stakeholders, and anyone with an interest in mining influenced water. The other handbooks in the series are Basics of Metal Mining Influenced Water; Mitigation of Metal Mining Influenced Water; Mine Pit Lakes: Characteristics, Predictive Modeling, and Sustainability; Techniques for Predicting Metal Mining Influenced Water; and Sampling and Monitoring for the Mine Life Cycle.