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Tuesday, April 26, 2005 11:44 AM Final Conceptual Approach Bauxsol



Conceptual Approach to Demonstration Testing of In-Situ Treatment and Prevention of Acid Rock Drainage at the Skytop Area near State College, PA

Background
During the construction of road cuts in the ‘Skytop’ area, near State College, PA for construction of I-99, deposits of sulfide mineralized quartzite were removed and subsequently placed as a buttress and roadway bifurcation system, and additional sulfide mineralized rock was placed in several waste piles and in the embankment to a road bridge (Structure 317). Subsequent to aeration of the sulfide-rich rock and exposure of the deposits in the cuts, the sulfides (primarily pyrite) have begun to oxidize to form sulfuric acid and liberate metals such as iron. The result is a low pH water rich in iron and sulfate draining from the site.

Virotec has proposed the use of a ViroMine™ reagent, part of the Bauxsol™ family of products, to in-situ neutralize the existing acidity and to form a protective layer over the surface of the sulfide minerals, limiting the further oxidation of the pyrite. Since the keys to successful remediation / prevention of contamination are two-fold:
1. Select the appropriate reagent based on site geochemistry, and
2. Select the appropriate reagent delivery system(s), based on site geohydrology (Blessing and Rouse, 2002),
MWH has been selected to provide information on the appropriate delivery systems. The following is a discussion of the proposed approach for demonstration testing of treatment and prevention at three sites within the Skytop area.

Buttress / Bifurcation Area

Existing Conditions- Major sliding occurred in 2001 and 2002. The remedy to this was to place a ‘buttress’ at the toe of the slope roughly between Stations 831 to 853, and to ‘bifurcate’ the two sections of interstate highway by raising the northbound lanes onto a fill against the buttress. Much of this fill was derived from the sulfide-rich rock being excavated in the area to the east. A limestone drainage blanket was placed against the face of the cut before placement of the buttress, and a series of limestone drains or galleries was placed to daylight above the elevated northbound lanes.

Subsequent to the placement of the buttress and bifurcation fill, acidic conditions have developed. The limestone in the drains has become armored with gypsum and iron- and aluminum-hydroxide precipitates and is no longer neutralizing the discharge from the drains. Surface exposures of pyritic material have begun to oxidize, resulting in iron staining of the rock and percolation of acidic water through the unsaturated material into the ground water. The water table slopes steeply to the north and has a strong vertical downward component, leading to recharge of the ground water to the north of the highway.

Proposed In-Situ Approach- Several approaches will be utilized to in-situ neutralize existing acidity and to coat remaining sulfide minerals with ViroMine™ to inhibit future oxidation. These approaches are designed as a system, and each must be applied to achieve the optimal results. For purposes of the demonstration testing, a section of the buttress and bifurcation will be remediated between the west end of the buttress (Station 832+50) and a limestone gallery at Station 836, a length of about 350 feet.

As noted above, the limestone in the drainage blanket and galleries has become armored and no longer effectively neutralizes the acidic drainage. ViroMine™ has a high neutralization potential, and because it does not add appreciable calcium ions to the reaction, it does not result in the formation of gypsum. Rather, the minerals in the ViroMine™ (1) neutralize acid by reaction with hydroxy carbonates, (2) act as nucleation sites to remove iron and aluminum by crystal growth and (3) remove other trace metals by adsorption, authigenic mineral precipitation, co-precipitation and solid state diffusion. Experience (e.g. McConchie et al., 1999, 2003; Davies-McConchie et al., 2002; Genc-Fuhrman et al., 2004, Munro et al., 2004; see also the US EPA trench trial results for the Gilt Edge Mine, SD, attached as Appendix I) has shown that the metal removal capacity of ViroMine™ increases with time, that the bound metals are very difficult to leach from the ViroMine™, and that bound metals become more difficult to remove as spent ViroMine™ ages. Therefore, it is proposed that the effectiveness of the limestone blanket and galleries will be enhanced by percolating a ViroMine™ slurry through the drainage blanket so that crystalline iron and aluminum minerals in the ViroMine™ can act as nucleation points for crystal growth, thereby reducing the extent of armoring. The slurry will be discharged into the top of the blanket along the top end of the blanket, with the composition and strength of the slurry to be defined based on laboratory testing. Laboratory testing will also need to involve determination of the total actual acidity and the total potential acidity of sulfidic material adjacent to the limestone blanket and galleries. Monitoring will be by means of sampling the discharge from the drainage galleries.

Surface observations of the buttress material and experience with similar material in mine waste rock indicate that the oxidation of the sulfide minerals advances as an oxidation front, with the limiting factor being the influx of atmospheric oxygen and oxygen transported by infiltrating rainfall and snowmelt. Pyrite inside larger masses of rock remains unreacted until oxygen permeates into the rock mass and oxidation products of sulfate and acidity, in turn, diffuse from the rock in response to osmotic and capillary forces, resulting in a complex reaction scenario. However, in unfractured rock the reaction rates are often sufficiently low that any acid produced can be neutralized by slow reaction with other minerals in the rock mass and some sulfate may be trapped by surface adsorption and mineral salt precipitation. The moisture content on the surface of such a rock varies over time depending on infiltration and evaporation, whereas the moisture content in the core of a rock remains relatively constant. Reaction rates are largely controlled by the rates of migration of oxygen into the rock and reaction products out of the rock. Where ViroMine™ reagents are added, reaction rates can also be slowed substantially or stopped by the development of coatings on pyrite grains because the coatings impede the transfer of oxygen to the reactive surface of the pyrite, inhibit the transfer of the sulphate and acid reaction products away from the surface of the pyrite and immobilize ferric iron that could otherwise accelerate pyrite oxidation.


Figure 1: Electron microscope photograph, taken by John Drexler (University of Colorado, Boulder) on behalf of EPA Region 8, showing the development of coatings on pyrite crystals as a result of interaction with ViroMine™ reagents. The sample was taken from the ViroMine™ treated trench at the Gilt Edge Mine in South Dakota. The coatings greatly impede, and may stop, the transfer of oxygen to the surface of the pyrite grains and they immobilize ferric iron that could otherwise accelerate pyrite oxidation.

A ViroMine™ slurry will be applied to the sloping surface of the buttress fill material, at a rate to promote unsaturated percolation of moisture. By maintaining unsaturated conditions, capillary forces will be active not only in moving moisture vertically down through the buttress material, but also into the core of larger rock pieces. Application will be by means of spray irrigation using conventional irrigation sprayers as well as by means of drip emitters developed for application of leach solution to gold heap leach facilities. Monitoring of the rate of advance of the reaction front will be by means of clusters of pressure / vacuum lysimeters installed at various depths in the buttress fill. The lysimeters allow collection of actual unsaturated moisture. The samples will be tested in the field for pH and later in the analytical laboratory for iron, sulfate and selected trace metals.

Pyritic rock was used to construct the raised north-bound lanes of the bifurcation. Although the existing pavement restricts the infiltration of water and the influx of oxygen, and hence limits the rate of sulfide oxidation, evidence exists to indicate that some oxidation is occurring. In addition, there is pyritic rock in the toe of the buttress below and between the limestone drainage galleries. Addition of ViroMine™ in the lower portion of the buttress will partially be achieved by the percolation of ViroMine™ slurry from the surface; however, this may be subject to preferential flow paths. Furthermore, the suspension would not reach the pyritic material under the north-bound lanes that are currently paved. To achieve better contact, ViroMine™ slurry could be injected through a series of horizontal borings drilled from the median side of the south-bound lanes under the north-bound lanes and continued southward, to the south side of the buttress, against the native rock. Such borings would use drilling equipment commonly used to drill pressure-relief bores in road cuts. The bores would include a blank casing grouted in the first 10 feet of the bore, followed by perforated casing for the remainder of the hole. The spacing between the holes will first be determined by analysis of the particle size of the fill material as recorded by the engineers supervising the fill, and the spacing revised on the basis of observations of pressure response during injection into the first series of bores. Monitoring of the effectiveness of this treatment will utilize existing monitoring wells located within and adjacent to the test area.


Small Cut Face

Existing Conditions- During construction activities in the Bifurcation- Buttress Area, excavation of the cut exposed an area of extensive pyritic fractures bounded by two mapped fracture zones, crossing the roadway near Stations 858 and 861. The western fracture zone was mapped by Gold and Doden as a narrow zone, but the eastern fracture zone was mapped by Gold and Doden as about 10 feet wide. An acidic seep draining into the C12 drain pipe is reported to maintain a constant flow, indicating deep-seated ground water flow. Application of a PVC cover has reduced the metal concentrations in the discharge water; however, the drain pH shows an increase during precipitation events which drops again after the rainfall ends, indicating deep ground water discharge. The small cut face is located within the Buffalo Run basin, and hence is subject to stringent water-quality standards, including standards for sulfate ion concentrations.

Monitoring of the effect of the small cut face is not only accomplished by monitoring the C12 drain, but also by monitoring groundwater wells located within the test area.. Data from these wells show relatively good ground water quality at the toe of the slope, but poor water quality in the median between the north and south bound lanes.

Proposed In-Situ Approach- The small cut face will be treated in two ways. A radial fan of injection bores will be drilled in the eastern fracture zone, from one drill station at the top of the small cut face and another at the toe of the cut face, and ViroMine™ slurry and a liquid carbon source such as emulsified vegetable oil will be injected into the bores. A similar fan of injection bores will be attempted in the western fracture zone from a drill station at the toe of the cut face; however, the narrow width of the fracture zone may result in some difficulty in following the fracture zone.

In addition, the surface exposure of the fracture area will be accessed by rolling back the existing PVC cover at the top and flushing a ViroMine™ slurry and a carbon source under the cover through a series of discharge ports at the top of the cover. The cover will then be replaced to minimize flushing of the ViroMine™ by rainfall, thereby allowing more time for the full benefits of the ViroMine™ reagents to develop (including the development of coatings on pyrite crystals). The addition of a liquid carbon source will tend to reduce the sulfate concentrations in the water through bacterial action. The effectiveness of these measures will be determined by continued monitoring of wells and by the C12 Drain.

Structure 317

Existing Conditions- Structure 317 forms the southern abutment of an overpass of I-99 over Buffalo Run Road. Buffalo Run flows through a concrete culvert under the structure. Drilling indicates that isolated pockets of pyritic material were placed in the structure, especially at the northern end. Sampling of monitoring wells in and down-gradient of the structure verify the existence of acidic conditions with elevated sulfate concentrations, indicating that sulfide mineral oxidation is occurring. Ground water is below the original land surface, and Buffalo Run is reported to be a losing stream, with ground water present in karst features in limestone and dolomite bedrock.

Proposed In-Situ Approach- Two small areas on Structure 317 will be tested to verify the efficacy of ViroMine™ treatment:
• An area approximately 100 by 100 feet, on the flat surface of the structure immediately west of well MW-42, will be treated by the pressure injection of ViroMine™ slurry through a series of bores advanced from the surface to the original grade, about 86 feet below the upper surface of the structure. Injection will proceed from near the surface to the total depth in a sequential manner, with injection achieved through the drill rod, at sufficient pressure to achieve hydrostatic fracturing. Injection bores will be positioned on a grid basis, with the distance between bores determined by the texture of the rock material, as noted by engineers during fill placement. Monitoring of the effectiveness of the injections will be by means of two clusters of pressure / vacuum lysimeters installed within the bounds of the test area.
• An area approximately 50 feet wide downslope and 100 feet long along the slope, on the slope of the structure east of MW-42, will be treated by the surface application of ViroMine™ slurry through irrigation sprinklers and drip emitters. Monitoring will be achieved through two clusters of pressure / vacuum lysimeters installed in the fill and sampled for pH and sulfate, iron and manganese.

Structure 317 is sited over a region of karst features, with the regional ground water below the native soil over much of the feature. However, data indicate that there are temporary periods in which seepage at the toe of the structure occurs under perched ground-water conditions, flowing in the soil material and discharging into Buffalo Run, a high-quality stream. To minimize detrimental effects of such flow, a PRB will be constructed in local sections (to be defined in the field with detailed testing), by digging a backhoe trench and backfilling with a mixture of competent, non-pyritic rock, ViroMine™, and a carbonaceous material such as hay. The ViroMine™ will neutralize the seepage and aid in metals removal, while the carbonaceous material will tend to reduce sulfate concentrations. Treatment effectiveness will be monitored by installed lysimeters and water quality of the existing acidic seep at Route 550.

Finalization of a monitoring plan to document the performance of the three ViroMineTM Pilot tests will be coordinated with PA DEP and PennDOT and will address the locations of sample points, frequency of sampling, and parameters to analyze. If authorization to perform the pilot tests is provided by April 25, 2005 then drilling of injection ports may begin in mid May and application of ViroMine TM initiated by the end of May 2005.


References

Blessing, Todd C. and Jim V. Rouse, 2002, Keys to successful remediation of hexavalent chromium in soil and ground water, IN Proceedings, 97th Annual Meeting of the American Wood Preserver’s Association, 98: 47-51

McConchie, D., Clark, M., Hanahan, C. and Fawkes, R., 1999. The use of seawater-neutralised bauxite refinery residues (red mud) in environmental remediation programs. IN: I. Gaballah, J. Hager and R. Solozabal (eds.) Proceedings of the 1999 Global Symposium on Recycling, Waste Treatment and Clean Technology, San Sebastian, Spain. The Minerals, Metals and Materials Society, 1: 391-400.

Davies-McConchie, F., McConchie, D., Clark, M., Lin, C., Pope, S. and Ryffel, T., 2002. A new approach to the treatment and management of sulphidic mine tailings, waste rock and acid mine drainage. New Zealand Mining, 31: 7-15.

McConchie, D., Clark, M., Maddocks, G., Davies-McConchie, F., Pope, S., Caldicott, W., 2003. The use of ViroMine™ technology in mine site management and remediation. IN: Proceedings of the CIM Mining Industry Conference, Montreal, May 2003, Compact Disc Record s33a1141, 20pp.

Genç-Fuhrman, H., Tjell, J.C. and McConchie, D., 2004. Adsorption of arsenic from water using activated neutralized red mud. Environmental Science and Technology, 38: 2428-2434.

Munro, L.D., Clark, M.W. and McConchie, D., 2004. A ViroMine™-based permeable reactive barrier for the treatment of acid rock drainage. Mine Water and the Environment, 23: 183-194.

Smyth, David J.A., David W. Blowes, Carol J. Ptacek, Jeff G Bain, Shawn G. Benner, and Che W.T. McRae, May 2002. Metals removal from groundwater using permeable reactive barriers (PRBs) applications, Abstracts, Third International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA



APPENDIX I


Data for the ViroMine™ treated Waste Rock Trench
_________________________________________________________________________________________________________________
Control Result Result Result Result
Analyte – Units 2003 2001 2002 2003 2004
_________________________________________________________________________________________________________________
pH 1.93 7.9 7.96 8.35 8.62
Acidity (mg/L as CaCO3) 49,000 4 < 5 < 5 < 5
Alkalinity (mg/L as CaCO3) TDS (mg/L) 77,000 11,500 8,300 3,000 NA
Sodium (mg/L) 9,300 2,970 2,990 570 250
Sulfate (mg/L) 55,000 6,000 5,800 2,200 NA
Ag (µg/L) 150 < (1) est 1.1 < LLD (5) < LLD (5)
Al (µg/L) 1,200,000 < LLD (50) 10 66 < LLD (50)
As (µg/L) 35,000 3.1 est 3.7 < LLD (10) < LLD (10)
Ba (µg/L 99 155 est 27 35 68
Be (µg/L) 56 < 0.4 0.34 < LLD (5) < LLD (5)
Cd (µg/L) 630 0.41 est 0.4 < LLD (1) < LLD (1)
Co (µg/L) 2,200 1.5 est 11 < LLD (10) < LLD (10)
Cr (µg/L) 390 < (1) est 12 < LLD (10) < LLD (10)
Cu (µg/L) 33,000 8.2 est 7.2 < LLD (10) < LLD (10)
Fe (µg/L) 21,000,000 < LLD (25) 18 120 210
Hg (µg/L) 0.2 est < 0.1 0.2 < LLD (0.2) < LLD (0.2)
Mn (µg/L) 34,000 17 0.3 < LLD (10) < LLD (10)
Ni (µg/L) 1,600 2.1 est 1.4 < LLD (10) < LLD (10)
Pb (µg/L) 390 < 2.2 2.9 < LLD (10) < LLD (10)
Sb (µg/L) 500 < 3.7 48 < LLD (10) NA
Se (µg/L) 102 41.4 3.9 < 8.5 NA
Tl (µg/L) 200 < 5.2 3.1 < LLD (5) NA
V (µg/L) 1,700 < 0.9 1.0 < LLD (10) < LLD (10)
Zn (µg/L) 29,000 42 21 < LLD (10) < LLD (10)
_________________________________________________________________________________________________________________
Data for water leaching from sulfidic waste rock that had been treated using ViroMine™ reagent in the Trench Trial at the Gilt Edge Mine site; the data span the four years since the treatment was carried out. The control data were obtained for leachate emanating from the same type of waste rock that had not been treated with ViroMine™ reagent. < LLD indicates that the concentration is below the detection limit for the analytical procedure used (the detection limit is indicated in parentheses). NA indicates not analysed. Note: Data up to and including those for 2003 have been validated by CDM, but the data for 2004 have not yet been validated under the QC/QA procedures. ViroMine™ is a Bauxsol™ technology reagent designed for the treatment of acid rock drainage.