Organic Carbon:
The Other Chemistry in the Berkeley Pit

Douglas Cameron, Ph.D.
Department of Chemistry and Geochemistry

The research for this project was funded through the Mine Waste Technology Program (MWTP). The MWTP is sponsored by the Environmental Protection Agency (EPA) through the Department of Energy (DOE), Contract Number DE-AC22-96EW96405, and implemented by MSE Technology Applications, Inc., in Butte, Montana. The underlying theme of all MWTP projects is remediation and/or control of current or future mine waste problems.

The water quality in pit lakes is highly variable and depends mainly on the geology of the surrounding area. In some areas, such as the Butte Mining District, the ore bodies are rich in sulfide minerals. The chemical interaction of the sulfide with water and the oxygen in the air produces sulfuric acid. The acid facilitates mineral dissolution, and metal and sulfide ions are released into the water, which produces more acid. The result is highly acidic water with high concentrations of dissolved metal ions, such as that found in the Berkeley Pit Lake. Water from these conditions is commonly referred to as acid-rock or acid mine drainage (AMD). A significant amount of research has been conducted to understand the chemistry that produces AMD, and work continues in the study of water chemistry in the Berkeley Pit and other pit lakes. Until recently, the chemical characterization and research have focused on the inorganic (metal ion and anion) chemistry in the Berkeley Pit Lake and other AMD waters.

A chance measurement a few years ago revealed organic carbon (ca. 5 mg carbon/liter water, or 5 ppm) in the deep water of the Berkeley Pit. About the same time, Dr. Grant Mitman, Montana Tech Biology Professor, found and began to study algae, bacteria, fungi, and protozoa in the surface water of the pit lake. Organic carbon would be expected from the decay of these organisms; however, many of the organisms identified were heterotrophic, which means they need organic carbon for growth. The food requirement for these organisms indicates the presence of a separate source of organic carbon. Organic matter in the Berkeley Pit Lake water could be the result of any or all of the following: the surface water and groundwater from the surrounding watershed; the decaying mine timbers or decaying microbiological material in the pit and adjacent mines; fuel spills, leaking fuels, or lubricants from abandoned machinery stored in the interconnecting mines.

For the past 18 months, samples have been collected from the Berkeley Pit Lake water in an attempt to identify the organic carbon, isolate its source, and to measure its effect, if any, on the water chemistry. This study is being conducted with graduate students Licette Hammer and Jim Jonas, and undergraduate student Mark Syverson. The late Dr. William Chatham provided significant assistance on this project.

Samples collected during November 1997 showed a slight decrease in the concentration of total organic carbon (TOC) with increasing depth at three lateral positions on the Berkeley Pit Lake. Surface waters had concentrations of approximately 5 ppm. The TOC concentration decreased to around 3 ppm at depths from 100 to 300 ft. The analyses of samples from May 1998 showed a slight increase in TOC with depth. Surface and near-surface water concentrations measured 4 ppm approximately, increasing to around 5 ppm near 700 ft. Differences in the TOC trends, if real, may result from changes in water temperature, amount of direct sunlight on the water surface, and a possible turnover of water layers during the spring.

Work to date has demonstrated that the majority of the organic carbon measured in the deep water and surface water offshore is not from living microorganisms or petroleum products. In-flow water (groundwater and an earlier surface-water flow) to the pit has organic carbon concentrations around 2 ppm. These determinations suggest the organic carbon is from a natural source; however, these results do not explain the difference between the carbon content in the pit lake and the current in-flow water.

When using the common standard analysis method, research results have demonstrated that the water matrix in the Berkeley Pit interferes with the TOC analysis. The interference is the result of the high sulfate concentration in the water and is also affected by the Fe²⁺/Fe³⁺ ratio. These results indicate that the measured TOC values are about 1–2 ppm too high. The interference problem goes beyond the measurement of TOC in the Berkeley Pit Lake but is inclusive of most TOC measurements in AMD. We are currently working on quantifying the interference and learning to correct the TOC measurements.

Our data indicate that the in-flow water carries most of the organic carbon into the pit lake. Because this organic carbon is from natural sources, it is reasonable to presume that the types of organic matter in the pit water are similar to organic matter in other natural waters. We believe, and are attempting to verify, that the majority of the organic carbon is from humic materials, which are heterogeneous, polymeric mixtures of decomposition products from plant materials. Humic materials are operationally classified into three categories: humin material, insoluble in water under all pH conditions; humic acids, soluble in water at basic pH conditions; and fulvic acids, soluble in water under acidic and basic conditions. From these definitions, the humic matter in the pit lake should be mainly fulvic acids.

The effects of organic carbon on the Berkeley Pit Lake chemistry or any remediation of pit-lake waters depend on the types and amounts of the carbon. If the organic carbon is mainly from humic matter, we can expect some combination of the following processes. Depending on the conditions, fulvic acids bind strongly to metal ions and can transport the metals through aqueous systems or sequester the metals by co-adsorption to solids. Fulvic acids are strong absorbers of ultraviolet and visible light.The absorbed light energy can power electron transfer reactions resulting in the oxidation or reduction of metal ions, depending on the conditions. Exploration of these new areas of the Berkeley Pit Lake chemistry will soon begin.