ARCO WETLAND DEMONSTRATION PROJECTS
by Robert F. Mueller, Ph.D.

Storm water runoff and near surface ground water transport dissolved and suspended trace metals through Butte's Metro Storm Drain (MSD) to Silver Bow Creek and eventually to the Clark Fork River. As a part of the Atlantic Richfield Company's (ARCO) remediation activities in the Clark Fork Basin, various technologies are being considered for the control of the trace metals produced by these base flows and storm water flows from the Butte area.

Graduate student, Brian Bass monitors flow rates in green house wetland simulation cells.

Conventional metal remediation systems use combinations of chemical neutralization and chemical precipitation or adsorption to remove dissolved metal species.

The majority of these systems can be characterized by high installation, high operating, and high maintenance costs. More recently increased attention has been focussed on the use of naturally occurring anaerobic processes that have been optimized for their ability to remove heavy metal contaminants from the aqueous phase. These systems are attractive because of their low operation and maintenance costs. As part of their continuing investment in the environmental restoration of the Butte area, ARCO is supporting an investigation of the use of constructed wetlands for decreasing dissolved trace metals (Cu, Zn, Cd, Mn, Fe, As, Pb) and sulfate within the MSD water prior to it entering Silver Bow Creek.

Montana Tech is conducting a three year test to determine the relative efficiencies and capacities of constructed wetlands for removing trace metals from surface and subsurface waters and will provide design parameters that can be used to estimate the cost of a full-scale treatment facility. The targeted metals in the contaminated water are zinc, copper and cadmium at concentrations of approximately 10, 1, and 0.1 ppm. Sulfate is present in the MSD water at a concentration of approximately 400 ppm. The performance of constructed wetlands is driven by microorganisms known as sulfate-reducing bacteria. Sulfate is microbially reduced to hydrogen sulfide at the expense of organic carbon. The required carbon is supplied by the addition of various fractions of compost to the cells, but plants can also contribute. The metal ions are precipitated as metal sulfide salts. All of these metal sulfides are highly insoluble and are therefore removed from the aqueous phase and are sequestered and immobilized within the test cells.

This project has evolved out of a long lasting research and educational relationship between ARCO and Montana Tech. Initial experiments conducted at Montana Tech in 1994 demonstrated the effectiveness of constructed wetland systems for metal removal, confirming the results of other research teams, Results of these meso scale testing in the greenhouse and related column experiments, showed that treated effluent waters contained zinc, copper, and cadmium consistently below the current detection limit of 0.02 ppb. These initial successes have lead to the construction of a test facility to demonstrate three of the passive treatment systems in the Butte climate on the actual surface water and subsurface groundwater present in the Metro Storm Drain channel. The field demonstration system is currently under construction on 20 acres of land between Kaw and George street in Butte and is scheduled to be completed by summer of 1996. Three different types of anaerobic wetland systems have been identified and will be tested. They are an anaerobic gravel substrate wetlands, an anaerobic organic substrate wetlands with a horizontal flow, and an anaerobic organic substrate wetlands with a upward flow. They differ in the choice of substrate materials, flow geometry, and specifics of the engineering construction. The objective of the field study is to evaluate metal removal capacities, hydraulic and microbial performance, system economics, and life expectancy of the three treatment systems. In order to obtain these essential design parameters within the three year study, an intensive set of monitoring diagnostics has been included in the field test facility. These diagnostics will allow the researcher team to chemically map the system and to describe and compare the hydrology and heat capacity of the test cells during winter and summer operation. In addition to physical and chemical monitoring, the microbial consortia developing in the test cells will be monitored over the three years to determine the impact of weather and metal loading on the biological performance of the system.

Montana Tech and ARCO have initiated a related experimental program to evaluate a innovative combined treatment system that will address both contaminated Colorado Tailings alluvium groundwater and effluent water from the Butte Silver Bow waste water treatment facility. Butte Silver Bow is expected to face more stringent discharge regulations with respect to nitrogen and phosphorous discharge in the future. Simultaneous treatment of Colorado Tailings alluvium ground water and sewage effluent water could benefit both requirements. A metal treatment system needs specific nutrients such as organic carbon, nitrogen, phosphorous, and sulfate. A large fraction of these nutrients could be supplied by the addition of the municipal waste water effluent, enabling sulfate reducing bacteria to metabolize and at the same time removing nutrients from the municipal waste water that will help improve the quality of Silver Bow creek which is the headwater for the Clark Fork River. A secondary advantage of this approach is the high heat content of the municipal effluent that could eliminate freezing problems in a constructed wetland during winter operation. Again meso scale and laboratory scale experiments are being used to estimate system performance and design parameters for a pilot scale treatment system.

Both projects are interdisciplinary and include students, engineers and scientists from chemistry, environmental engineering, biology, microbiology, hydrogeology, and soil science. The team includes Drs. Frank Diebold and Bill Chatham from geochemistry, Rod James and Bill Drury from environmental engineering, Robert Mueller from bioengineering and microbiology, John Sonderegger from hydrogeology and Rick Veeh from Plant and Soil Science. Youning Li is developing a system specific data base to enable the researchers to archive the large amount of data gathered and perform system evaluations. Student participation is very important in these projects and several graduate and undergraduate students from various disciplines are involved in laboratory experiments and monitoring of the demonstration systems. Mark Thompson designed and operated the original greenhouse experiments. Jim Jonas and Donald Pawluk are operating and evaluating vertical upflow columns to measure hydraulic performance over time and determine metal removal rates. Tom Sharp is determining the microbial composition of the various laboratory and field systems. Brian Bass and Mike Shaffer designed and are currently operating the greenhouse experiments for the combined treatment of Colorado Tailings alluvium groundwater and waste water effluent. Bill Ballinger is developing design equations for the aerobic polishing cells and Sharon Jones is investigating the hydrodynamics of the demonstration systems. Jiangming Zangh and Pengfei Zangh are chemically monitoring the demonstration systems in order to develop a chemical map of the various test cells.

The results of the field test will be used to construct performance models for each component and its relationship to the overall treatment system performance. Projections for construction, operation, and maintenance costs of an actual wetland treatment facility can be based on these models. Construction and maintenance costs can be compared to conventional treatment technologies and the viability of a constructed wetland treatment system for the Silver Bow Valley can then be evaluated based on scientific and economic considerations.