PRELIMINARY REVIEW

Wetland treatment concepts in EPA/A.D. LITTLE draft Engineering Evaluation/Cost Analysis (EE/CA) Report on the Elizabeth Mine

PROPOSED TREATMENT SYSTEM & DISCUSSION

The tailings at the Elizabeth Mine are discharging acidic and metal laden waters to the detriment of the local streams and the general environment. The proposed treatments are intended to remove metals to acceptable levels and to neutralize the acidity in the wastewater. The tailing piles are designated as TP-1 and TP-3 in this review (TP-2 is combined with TP-1 for the proposed treatments). It is proposed to cap TP-1 to prevent future infiltration and to divert surface runoff and shallow groundwater. There will be a decreasing amount of seepage from TP-1 after the capping is complete; it is expected to reach a low, steady-state rate after a few years.

The proposed treatment sequence for seepage from both TP-1 and TP-3 was described in the draft EE/CA report as including a holding pond followed by Successive Alkalinity Producing Systems (SAPs), followed by a constructed aerobic wetland. These treatment components were described and discussed in my previous memo of October 3, 2001. I had some serious reservations regarding the performance and the maintenance requirements of the proposed treatment process, and these were presented in my memo of October 3, 2001. This previous memo was distributed to the members of the Copperas Hill Coalition and also provided to Mr Ed Hathaway the U.S. EPA Project Officer and Mr Scot Foster of A.D. Little.

On October 10. 2001 I met with Ed Hathaway, Scot Foster, and Mr James Gusek from Knight Piesold & Company (Knight Piesold is a consulting firm, in Denver Co, specializing in treatment of acid mine drainage). The meeting was also attended by Cindy Cook, Kathy Hardy, and Fred Moody. Mr Gusek had toured the Elizabeth Mine site and had also concluded that the treatment sequence proposed in the draft EE/CA report was not the most appropriate for the type of wastes expected at the Elizabeth Mine. He is proposing elimination of the SAPs and the use of Sulfate Reducing Bacteria bioreactors (SRBs) instead. The preliminary holding pond and the final aerobic wetland would be retained, but might be different in size and configuration. This revised report is intended to describe these SRBs and their expected performance.

The originally proposed SAPs consisted of a three foot thick layer of limestone gravel under a two foot thick layer of compost, in a shallow pond. As the water flowed down through this media and out underdrain pipes, some metals would be removed and/or converted to other forms and alkalinity added from dissolution of the limestone. This media would typically require removal and replacement every 12 to 15 years. A serious maintenance issue is then caused by the presence of aluminum in the Elizabeth Mine seepage water. The chemical reactions in a SAP will convert this aluminum to Aluminum Hydroxide, the resulting sludges clog up the pores in the SAP and require flushing every six to eight months. The SAP also converts dissolved iron to another form which then oxidizes, precipitates, and settles out in the aerobic wetland. These are the intended reactions and would work effectively during the warm summer months. However, under a continuous winter ice cover there may not be sufficient oxygen for these oxidation reactions to be complete in the wetland and the system may not then meet performance expectations during these winter periods.

The SRBs are proposed to overcome these limitations. The hydraulic pathways for the SRB are similar to the SAP in that the water typically flows downward through a bed of selected material. The specific type and quantity of materials are determined in preliminary laboratory, bench, and pilot tests with the actual wastewater from the site under consideration. In general, these materials would include a variety of organic materials and limestone gravel in a mixture rather than discrete layers. Manure from cattle and horses is a major component in this mixture. Such manures contain a high organic carbon content, that is necessary to sustain the desired anaerobic (ie: no oxygen present) reactions, and they also contain the critically important sulfate reducing bacteria. Wood chips, alfalfa hay, and similar materials are also utilized as supplemental organic material. Limestone gravel is also usually included in the mixture to provide an additional source of alkalinity. The amount of these materials required for about a thirty year life is calculated based on the laboratory and pilot scale testing.

The anaerobic conditions in this SRB reactor, coupled with the activity of the sulfate reducing bacteria result in most of the metals being tied up, and retained as metal sulfides in the SRB container. The final wetland component in this case is then designed to complete the removal of manganese and to remove any excess organics that may leak from the SRB. The wetland should be capable of meeting these goals, even in the ice covered winter months. In the system proposed previously, the wetland was expected to oxidize and settle out a significant amount of dissolved iron. This function could not occur under a complete winter ice cover, so the SRB/wetland concept is a significant improvement.

The same treatment components are proposed for both TP1 and TP3; the sizes may be different depending on flow rates and metal concentrations. Two SRB units, in parallel, are proposed for each location. This is to allow maintenance when required, each unit will be designed, and sized, to receive the full design flow. The SRBs can utilize different configurations, one possibility is a lined pond, similar to the SAPs, with water standing over the media bed. Underdrain pipes collect the treated water for conveyance to the wetland. Maintenance requirements for the SRB are much simpler as compared to the previously proposed SAPs. In the case of SRBs, the major maintenance requirement will be the removal of metal residues after the 30 year design life and the placement of a new media bed. The projected useful life of 25 to 30 years is a theoretical estimate since SRBs have only been in use for five to six years. The theoretical estimate does, however, have a rational basis.

As indicated previously, the biological activity in the SRB depends on the sulfate reducing bacteria. These organisms occur in large numbers in animal manures. Their continued activity in the anaerobic SRB produces dissolved hydrogen sulfide (H2S), this H2S reacts with the metals producing the metal sulfides and releasing dissolved hydrogen gas. It is however, necessary to maintain a relatively steady flow to the SRB. If the flow is too high, the capabilities of the organisms could be overwhelmed. If the flow (and metals concentrations are too low) the organisms might produce excess H2S and there could be odors. If low flows persist, the population of organisms could be reduced and then not be fully capable of responding when higher flows again occurred. It will therefore be necessary to adopt some kind of flow control device at the outlet of the TP1 and TP3 holding ponds to regulate the flow to the SRBs. It was also indicated in the discussions with Mr Gusek that sufficient alkalinity should be available in the SRB to neutralize the acids and produce an effluent with an acceptable pH.

These changes in the proposed treatment concept render the present draft EE/CA report invalid in itÕs treatment process descriptions and cost estimates. These will have to be revised for the final EE/CA report. However, it is likely that the costs for the new proposal will be of the same order of magnitude as presented in the present draft EE/CA report. This new proposal offers a more rational basis for system design and reduces maintenance requirements to a level that should be more acceptable to the State of Vermont.

In my opinion, it is necessary to select the most positive form of seepage prevention for the TP1 cap. This would reduce the long term seepage to an absolute minimum; this should extend the useful life of the SRB/wetland system considerably and thereby further reduce the maintenance frequency. Discussion is certainly possible regarding the type and depth of protective material to be placed over the necessary (in my opinion) plastic membrane cover.

An SRB/wetland combination can also be designed to effectively treat the seepage from TP3. However, this treatment system may continue to require maintenance for generations to come if all of TP3 is retained in itÕs present configuration. I think consideration should be given to retain the major historical artifacts, and removing the most acidic portions of the waste material. This would then reduce the size, the cost, and the maintenance needs for a TP3 treatment system.

The information provided in my previous memo (October 3, 2001) regarding the proposed holding pond and the proposed wetland is still valid and will not be repeated here. If the SRBs perform as promised by Mr Gusek, then the overall system should be capable of meeting discharge limits on a year-round basis.

CONCLUSIONS

The SRB bioreactors now proposed for the Elizabeth Mine offer a more rational basis for design and achievement of performance expectations that the previous SAP concept. The theory of SRB system performance is well understood and a number of full scale systems have been in successful operation for several years.

It is necessary to remove the residues and replace the media mixture in the SRB on a projected 25 to 30 year schedule. This projection is based on valid theoretical aspects, but actual systems have only been in existence for five to six years.

On a conceptual level, the proposed SRB/wetland concept is believed capable of meeting discharge requirements on a year-round basis.

Based on current estimates, major maintenance may only be required on an infrequent 25 to 30 year basis. This should make the SRB concept much more appealing to the State of Vermont.

The discussions and cost estimates regarding the SAPs and wetlands in the draft EE/CA report are now invalid and must be revised.

I would recommend a plastic cover over all of TP! in order to reduce seepage flow to an absolute minimum and thereby extend the useful life of the system and further reduce maintenance frequency. The type and depth of protective material over this plastic membrane cover should be open for discussion.

I still believe that retention of TP3 in itÕs present form increases the risk of long term water quality problems and maintenance requirements. I would recommend removal of the more acidic materials while still retaining the major historical artifacts. This would reduce the size, the cost and the maintenance needs for the TP3 treatment system.

Other conclusions in my memo (except those related to winter operations) of October 3, 2001 are still valid and will not be repeated here.

Covering TP1 can reduce seepage flow to the absolute minimum. However, this may take a few years after construction is completed to achieve. Consideration might be given to allow a continuation of a bypass of most of these seepage flows around the new SRBÕs/wetland treatment systems until the flow rates reach their long term steady state levels. In this way the treatment systems can be smaller, and costs for construction and maintenance reduced. The time period required to reach steady state seepage conditions should be predictable. It typically takes at least two years before the wetland vegetation reaches maturity and establishes full cover, and for the wetland to become fully effective.

Sherwood C. Reed, P.E.
Principal, E.E.C.

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