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Microencapsulation of degradative organisms enhances microorganism survivability (Stormo and Crawford 1994). The use of encapsulated cell microbeads for in situ biodegradation depends not only on microorganism survival but also on microbead transport characteristics. Two forced‐gradient, recirculating‐loop tracer experiments were conducted to evaluate the feasibility of encapsulated cell transport and bioremediation on the basis of polystyrene microsphere transport results. The tracer tests were conducted in a shallow, confined, unconsolidated, heterogeneous, sedimentary aquifer using bromide ion and 2 μm, 5 μn, and 15 μm microsphere tracers. Significant differences were observed in the transport of bromide solute and polystyrene microspheres. Microspheres reached peak concentrations in monitoring wells before bromide, which was thought to reflect the influence of aquifer heterogeneity. Greater decreases in microsphere C/Co ratios were observed with distance from the injection wells than in bromide C/Co ratios, which was attributed to particle filtration and/or settling. Several methods might be considered for introducing encapsulated cell microbeads into a subsurface environment, including direct injection into a contaminated aquifer zone, injection through a recirculating ground water flow system, or emplacement in a subsurface microbial curtain in advance of a plume. However, the in situ use of encapsulated cells in an aquifer is probably limited to aquifers containing sufficiently large pore spaces, allowing passage of at least some encapsulated cells. The use of encapsulated cells may also be limited by differences in solute and microbead transport patterns and flowpath clogging by larger encapsulated cell microbeads.

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The Bunker Hill Mine, located near Kellogg, Idaho, is situated within highly fractured metasediments comprised of quartzites and argillites. Fractures, faults, and joints are the preferentially permeable pathways for ground‐water flow. Identifying and grouting such features in an attempt to control ground‐water recharge to the mine is a potential method for acid production abatement. In order to implement an effective grouting program, preferential pathways for ground‐water recharge must be identified, delineated, and evaluated. Other researchers have identified the major flow paths and acid mine drainage producing zones within the mine. The objective of this project is to investigate the occurrence of minor structural features (fractures) that may constitute recharge pathways from unsaturated mountain slopes to underlying acid‐producing rocks in or adjacent to mined‐out areas. To accomplish this objective, a soil‐gas measuring technique was employed to measure gaseous emissions that are hypothesized to be produced by chemical reactions between the acid mine drainage and carbonate‐bearing rocks located within the mine. Carbon dioxide (CO2) was selected as a tracer for recharge pathways from mined‐out openings to the ground surface; CO2 was measured using the probe extracted static grab sample/onsite analysis method. The method was tested over an observable fault. Three surficially unobservable fractures were identified within the study area. We hypothesize that these discontinuities are capable of transmitting water to the mine during recharge events.