British Columbia Mine Reclamation Symposium

Selenium source characterization and receiving environment monitoring at the Kemess South Mine Davidson, Scott Charles; Chapman, Peter M. 2006

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SELENIUM SOURCE CHARACTERIZATION AND RECEIVING ENVIRONMENT MONITORING AT THE KEMESS SOUTH MINE   Scott Davidson, M.Sc., P.Geo.1 Peter Chapman, Ph.D., R.P.Bio.2   1 Environmental Coordinator   2 Senior Environmental Scientist, Principal   Kemess Mines Ltd.     Golder Associates Ltd.   ABSTRACT  The Non Acid Generating (NAG) Waste Rock Dump (WRD) for the minesite covers an area of approximately 147.5 ha. Selenium concentrations in Waste Rock Creek have been fluctuating above and below the BC Approved Water Quality Objectives for selenium since early 2001 with consistent exceedences of this objective since Fall 2003.  Selenium was identified as a metal of concern by the BC Ministry of Environment (MOE) during a review of the 2004 Annual Water Quality report.  This paper describes the geochemical test work conducted to characterize the Leach Cap Waste and the physical setting of this material within the WRD.  Discussion of the WRD seep chemistry and drainage management controls currently in place are presented in addition to the potential closure scenarios to mitigate long-term selenium concentrations in the WRD drainage.  The Selenium Monitoring Plan prepared for Kemess Mines by Golder Associates Ltd. to address MOE concerns associated with this metal is presented.  The Plan was submitted for review by MOE during January 2006 with implementation scheduled for Fall 2006.  INTRODUCTION  The Kemess South Mine is an open-pit gold-copper mine located approximately 15 km east of Thutade Lake, in the northern Omineca Mountains of north central British Columbia, within a claims area of approximately 15,057 ha (150 square km).  The mine is approximately 300 km northwest of Mackenzie at latitude 57 00”N, and longitude 126 45’ W.  The mine started construction in 1996 with concentrate production beginning in late 1998.  This paper presents a case study on the characterization of waste and ore material types at the Kemess Mine to determine the source and potential loadings from selenium to the receiving environment.  A discussion of the adaptive management selenium plan for the Kemess Mine is also presented.  Characterization of Selenium Sources  It is critical that proper identification and characterization of the source of selenium is conducted in order to develop a successful adaptive management plan.  The coal industry bases its revenues on the extraction of paleo-swamp deposits that are often bedded within sedimentary host rocks. These environments during their active stages would have behaved in a manner consistent with present day systems and acted as “sinks” for inorganics such as selenium.  Selenium is typically not confined to the coal that is extracted but is also found in the host rocks for the deposits, which can result in large volumes of potentially selenium-leaching waste rock (Chapman 2005a).  Metals mines, dependent on the nature and origin of the deposit, may have selenium sources that can be traced to one or more specific rock types.  Careful geochemical characterization of the different waste and ore types needs to be conducted in order to determine whether selenium releases can occur to levels of potential concern.  Whole rock geochemistry and mineralogical analyses provide information on the base level of selenium in the material type in addition to providing valuable information on the form of selenium within the rock matrix.  Methods of characterization of the potential for a material to leach selenium include field leach pads, humidity cells, meteoric leach cells and shake flask analysis.  Some of this information and test work may be obtained during the pre-development stages of a mine but, until development occurs, there often are insufficient data to provide more than a basic understanding of the potential for selenium leaching.  Pre-Development Phase Geochemical Characterization  The initial geochemical characterization for the Kemess South Mine was based on whole rock geochemistry and shake flask samples with four humidity cells being conducted on representative samples of ore and waste.  The humidity cells work in the pre-development phase was used to evaluate leachate and consisted of: blended waste materials, blended low grade ore, supergene and hypogene rock.  A break-down of the rock types used in the humidity cells and the resultant selenium concentrations is shown in Table 1, showing the level of initial detail with respect to material types.  Table 1  Distribution of Waste and Ore types for Initial Kinetic Characterization Column 1 Waste Materials Column 2 Low Grade Ore Column 3 Supergene Rock Column 4 Hypogene Rock Tertiary volcanic with sedimentary cover rocks (85%) Hypogene quartz monzonite (75%) Supergene mineralized ore and low grade rock Supergene mineralized ore and low grade rock Glacial overburden sediments (12%) Hypogene Takla Group volcanic and sedimentary rocks (15%) Hypogene Takla Group volcanic and sedimentary rocks (2%) Supergene quartz monzonite (10%) Hypogene quartz monzonite (1%) Supergene quartz monzonite (<1%)   Column Leachate Dissolved Selenium Concentrations (mg/L) Max      < 0.02 Max      < 0.02 Max         0.05 Max      < 0.02 Min       < 0.02 Min       < 0.02 Min       < 0.02 Min       < 0.02 Mean    < 0.02 Mean    < 0.02 Mean       0.02 Mean    < 0.02  The results of the kinetic test work indicated that selenium concentrations remained unchanged for Column 1, Column 2 and Column 4 while the results for Column 3 indicated a decrease followed by an increase in selenium concentrations.  The maximum concentration observed in Column 3 was 0.05 mg/L with a mean concentration of 0.02 mg/L.  All other columns returned concentrations less than the detection limit of 0.02 mg/L.  However, the humidity cells test work was initiated in 1992 and detection limits have dramatically improved since then.  Development/Operational Phase Geochemical Characterization  A detailed geochemical characterization program was initiated with the start of the development phase of the Kemess South Mine and continues through the ongoing operations of the mine. Kinetic and static test work was conducted on material types as they were encountered during pit development.  The materials characterization program resulted in the material types listed in Table 2.  Table 2   Operational Classification of Rock Types at the Kemess Mine Material Nature Concerns Test Method Overburden Waste None  Shake Flask Toodoggone Sediments Waste None Shake Flask, Humidity Cell, Field Test Asitka Group NAG Waste Metal Leaching Shake Flask, Field Test Asitka Graphitics Waste Acid Generation, Metal Leaching Shake Flask, ABA, Humidity Cell, Field Test Asitka Group PAG Waste Acid Generation, Metal Leaching Shake Flask, ABA, Humidity Cell, Field Test Hypogene PAG Ore/Waste Acid Generation, Metal Leaching Shake Flask, ABA, Humidity Cell, Field Test Leach Cap NAG Ore/Waste Metal Leaching  Column Test, Shake Flask, Field Test Takla Volcanic PAG Ore/Waste Metal Leaching Column Test, Shake Flask, Field Test Supergene Ore/Waste Metals Leaching   Column Test, Shake Flask, Humidity Cell Transition Ore Ore Metals Leaching   Column Test, Shake Flask   The increased complexity between Table 1 and Table 2 is a reflection of an increased ability to conduct detailed characterization during the development of the open pit.  This level of detail is not feasible for the pre-development phase of an operation.  Characterization of Selenium Sourcing  The results of the geochemical characterization of materials has been useful in determining the source of selenium for the Kemess Mine.  Table 3 shows a summary of the results of the characterization of selenium associated with the various material types.  Table 3  Results of Selenium Characterization Material Whole Rock Se (Max/Mean (n)) [ppm] Results of Kinetic Testwork Overburden 2.40/0.66 (43)  No long-term concern for Se leaching Toodoggone Sediments 3.20/0.51 (99)  No long-term concern for Se leaching Asitka Group NAG Cherty Siltstone/Mudstone Asitka Sediments Hematitic Chert/Siltstone Asitka Siltstone/Mudstone  6.80/0.87 (216) 10.50/1.20 (64) 8.10/1.17 (191) 3.00/0.85 (63)  No long-term concern for Se leaching Leach Cap NAG 16.80/2.60 (409)  Field pads showed initial spike followed by slowly decreasing Se-D concentrations.  Whole rock shows ~50% Se depletion within 4 years. Asitka Group PAG Cherty Siltstone/Mudstone Asitka Sediments Hematitic Chert/Siltstone Asitka Siltstone/Mudstone  11.30/1.50 (110) 13.60/2.69 (85) 8.60/4.55 (4) 5.00/1.89 (10)  No long-term concern for Se leaching Asitka Graphitics 34.30/6.34 (184)  Field pad showed initial spike followed by slowly decreasing  Se-D concentrations  Whole rock shows slow rate of Se depletion Hypogene PAG 4.90/2.09 (134)  No long-term concern for Se leaching Takla Volcanic PAG 4.00/3.53 (4)  No long-term concern for Se leaching Supergene 2.80/1.40 (7)  Some minor concern for Se leaching Transition Ore 1.00/0.90 (4)  Low concern for Se leaching  Figures 1 and 2 show the results of the analysis of leachate from sixteen field pads that have been constructed since 1999 as part of the ongoing materials characterization program.  The results from these tables show that, with the exception of the Leach Cap Waste and Graphitic Asitka materials types, the rocks at the Kemess Mine do not leach significant amounts of selenium.  The analysis of leachate for these two rock types indicates that there is a decreasing trend for the Leach Cap material while the Asitka graphitic material shows a much slower rate of decrease in dissolved selenium concentrations. Figure 1 Field Leach Pad Dissolved Selenium: Leach Cap and Asitka Graphitic Material Types 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 5/20/1999 8/28/1999 12/6/1999 3/15/2000 6/23/2000 10/1/2000 1/9/2001 4/19/2001 7/28/2001 11/5/2001 2/13/2002 5/24/2002 9/1/2002 12/10/2002 3/20/2003 6/28/2003 10/6/2003 1/14/2004 4/23/2004 8/1/2004 11/9/2004 2/17/2005 5/28/2005 9/5/2005 12/14/2005 Time D is so lv ed  S e [m g/ L] Leach Cap Leach Cap Leach Cap Asitka Graphitic   Figure 2 Field Leach Pad Dissolved Selenium: All Other Rock Types 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 6/4/1999 12/1/1999 5/29/2000 11/25/2000 5/24/2001 11/20/2001 5/19/2002 11/15/2002 5/14/2003 11/10/2003 5/8/2004 11/4/2004 5/3/2005 10/30/2005 Time D is so lv ed  S e [m g/ L] Hypogene Asitka Chert Hypogene Hypogene Hypogene Asitka Chert Asitka Chert Asitka Chert 10% GAG Cyclone Sands Toodoggone Sediments Takla Volcanics Takla Volcanics Initial Se-D concentrations  from some pads plot above scale of graph and are not shown.   Figure 3 shows the results of whole rock selenium analysis conducted on grab samples from the field pads.  The Leach Cap material type shows almost a 50% decrease in selenium concentrations within a 4 to 5 year period.  The Asitka graphitic material type shows a much slower decrease in selenium concentrations than the Leach Cap.  It is important to note that the selenium within the coarse (+2.36 mm) fraction of the graphitic material is higher than in the fine fraction by a factor of almost 2.  Figure 3 Selenium Depletion in Field Pad Rock Samples 0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 1998 1999 2000 2001 2002 2003 2004 2005 Time W ho le  R oc k Se le ni um  [p pm ] Leach Cap Leach Cap Leach Cap Asitka Graphitic   Selenium in Waste Rock Creek  The Non Acid Generating (NAG) Waste Rock Dump (WRD) for the minesite covers an area of approximately 147.5 ha.  Drainage from the WRD is collected by perimeter ditching and a sediment pond prior to entering Waste Rock Creek.  The WRD at the end of 2005 contained approximately 119,661,000 tonnes of NAG material.  This volume includes approximately 12,089,000 tonnes of the Leach Cap material type.  The disposition of this material within the WRD is shown in Figure 4.   Figure 4  Location of Waste Rock Dump and Leach Cap Waste Dump The drainage from the WRD is collected by ditching along the southern perimeter of the facility. Routine sampling has been conducted within Waste Rock Creek since prior to the construction of the WRD with seepage site sampling conducted since construction of the WRD.  The results of water quality analyses are shown in Figure 5.  From this figure it can be seen that the drainage chemistry of Waste Rock Creek has increased in dissolved selenium from below detection limits to where the concentration remains consistently above the 0.02 mg/L Water Quality Objective. The maximum observed total selenium concentration in Waste Rock Creek was 0.0637 ml/L during December 2005.  Figure 5 Waste Rock Dump Seepage/Receiving Environment Selenium 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 7/6/1996 1/2/1997 7/1/1997 12/28/1997 6/26/1998 12/23/1998 6/21/1999 12/18/1999 6/15/2000 12/12/2000 6/10/2001 12/7/2001 6/5/2002 12/2/2002 5/31/2003 11/27/2003 5/25/2004 11/21/2004 5/20/2005 Date D is so lv ed  S el en iu m  [m g/ L] WR-S1 WR-S2 WR-S3 OB1-S3 LCW-S1 WQ-14F WQ-14 MOE BCWQO Se-T  0.002 mg/L   Seepage Site WR-S3 is located directly to the south of the Leach Cap Waste Dump portion of the WRD and is located in the original Waste Rock Creek drainage channel.  The high selenium concentrations from this seepage site are attributed to the mobilization of selenium leached from the Leach Cap material as water passes through the original drainage pathway.  One other possible source of selenium that may also be contributing to loadings at WR-S3 is from the wetlands that used to occupy the original Waste Rock Creek drainage channel.  Boiling of organic deposits involving vertical displacements of greater than 5 m were observed at the toe of the dump during the advance across this drainage area.  Selenium Monitoring at the Kemess South Mine  The continued exceedance of the Water Quality Objective (WQO) for selenium has resulted in concerns with the Ministry of Environment (MOE).  To alleviate these concerns Kemess retained Dr. Peter Chapman of Golder & Associates to prepare a Selenium Monitoring Plan (Se Plan) for the mine.  The framework for the Se Plan comprises a tiered assessment framework that is discussed in detail by others (Chapman 2005b; McDonald and Chapman 2006).  A schematic showing the Se Plan for Kemess is shown in Figure 6.  To focus the Se Plan a decision was made to evaluate Waste Rock Creek to determine whether a significant impact could be determined in the “near field” effects area.  A series of beaver ponds immediately downstream of the outflow from the WRD collection ditching was selected for this first phase of the Se Plan.  Sampling of fish, benthics, and vegetation will be conducted in this area to determine whether an impact exists.  Some “far field” sampling for selenium is being conducted in conjunction with other sampling programs in order to maximize the collection of data.  This is the case with benthic invertebrates collected as part of the provincial Environmental Effects Monitoring (EEM) program.  Figure 6.  Selenium Monitoring Plan for the Kemess South Mine  The field program for Tier I of the Se Plan is scheduled for implementation during September 2006.  This coincides with sampling efforts for both the provincial and federal EEM programs and allows for maximum utilization of consultants.  The focus on “near field” effects during Tier I assessment is a reflection of the localized effects of selenium leaching at the mine as opposed to larger-scale, more regional effects that are found elsewhere.  The results of the Tier I assessment work will be used to determine whether more detailed Tier II assessment work is required.  The Tier II assessment work will focus on downstream effects within Waste Rock Creek extending to the confluence of this system with Attichika Creek. Expanded sampling of fish, benthics and vegetation would be conducted during the Tier II assessment in the extensive back channel areas of Waste Rock Creek on the floodplain of Attichika Creek.  Tier III sampling consists of detailed wildlife and fisheries studies to determine the nature of any effects of increased selenium on fish and waterfowl that utilize the area. Speciation of selenium will also be conducted during the Tier III assessment.  Post Closure Waste Rock Dump Selenium Loadings  The Tier I results will assist in determining the need for mitigation projects (identified in Figure 6) to reduce selenium loadings to the receiving environment.  These mitigative efforts would be considered in addition to closure plans for the WRD to reduce long term selenium loadings into Waste Rock Creek.  One possible mitigative measure includes the possible re-direction of Waste Rock Interception Ditch flows back into Waste Rock Creek.  This water source intercepts natural drainage and conveys it around the perimeter of the WRD.  It is anticipated that approximately 40% additional freshwater dilution could be achieved from this project although further hydrologic and geochemical evaluation is required for verification.  The clean water from this source would offset flow reductions that will occur when the pit highwall interception ditching is diverted back into the open pit at the end of mining.  The closure of the WRD will include the encapsulation of the LCWD material type to remove exposure to oxygen and water.  The design details of the encapsulation method are being evaluated as part of the next update of the Reclamation and Closure Plan for the site but will include a compacted subsurface overlain by drainage material followed by a reclamation growth media.  The overall surface of the WRD in this area will be sloped to facilitate drainage of surface water.  It is anticipated that the encapsulation of the LCWD will reduce selenium levels in Waste Rock Creek to below the provincial WQO of 0.02 mg/L.  CONCLUSION  The Kemess South Mine has receiving environment selenium concentrations that routinely exceed the BC Water Quality Objectives for this element.  Concerns over the concentrations of selenium in Waste Rock Creek were raised by MOE and as a result Kemess retained Golder & Associates to prepare a Se Plan for the site.  Geochemical characterization of waste rock at the mine has been used to determine which rock types are contributing to the selenium loadings to the receiving environment.  The geochemical materials characterization program at the site allows for the focusing of possible mitigative measures to reduce selenium loadings by identifying the sources of selenium.  The implementation of the Se Plan is scheduled for fall 2006 and is intended to coincide with provincial and federal EEM programs in order to maximize the utilization of consultants and to allow for synthesis of programs where possible.  REFERENCES CITED  Chapman, P.M. 2005a. Selenium status – Elk River Valley, BC. In: W. Price, B. Hart, B. Dixon, P. Jarman, B. Riordan, M. Freberg, and C. Howell, Proceedings of the Twenty-Ninth Annual British Columbia Mining Reclamation Symposium – “The Many Facets of Mine Reclamation”, British Columbia Technical and Research Committee on Reclamation, Abbottsford, BC, September 19-22, 2005.  Chapman, P.M. 2005b. Selenium monitoring and management – new mines In: W. Price, B. Hart, B. Dixon, P. Jarman, B. Riordan, M. Freberg, and C. Howell, Proceedings of the Twenty-Ninth Annual British Columbia Mining Reclamation Symposium – “The Many Facets of Mine Reclamation”, British Columbia Technical and Research Committee on Reclamation, Abbottsford, BC, September 19-22, 2005.  McDonald, B. G. and P. M. Chapman. 2006. Determining whether selenium is a pollutant or just another contaminant. Integr. Environ. Assess. Manag. (in press).

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