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Sediment yield from waste dumps and the potential ability of dump slide material to filter suspended.. 2009

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Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation SEDIMENT YIELD FROM WASTE DUMPS AND THE POTENTIAL ABILITY OF DUMP SLIDE MATERIAL TO FILTER SUSPENDED SEDIMENT by Kim K. Bittman, Environment Supervisor Quintette Coal Limited, Box 1500, Tumbler Ridge, British Columbia VOC 2WO ABSTRACT This paper presents a case history of the Mesa 1660 m dump failure (slide) at Quintette Coal Limited. The potential ability of dump slide material to filter suspended sediment from influent water is discussed and questions are raised regarding the amount of sediment generated by waste dumps. This paper presents data and theoretical arguments supporting the following claims: 1. That dump slide material filters suspended sediment - supported by: - Experience at an east Kootenay mine showing a dramatic decrease in sediment loads subsequent to a failure. - Experience at  Quintette Coal  Limited  indicating  that  solids concentrations below the Mesa dump slide are less than upstream concentrations in 49% of samples taken and of the same concentration in 30%. Slight increases downstream are seen in 21% of the data set. - Dye observations indicating that an approximate retention time in the slide mass is 69 hours. 2. That waste dumps are not likely to produce high sediment loads - supported by: - During the dumping process, fines remain in the upper 10% of a waste dump and in this zone, based on specific tests in the Mesa north area, comprise clay particles (0.002 mm) at 2% and fine sand or silt (0.075 mm) at 9% of the samples taken. - Natural segregation of waste dumps will preclude downward migration of fines from the upper portion to lower regions of a waste dump. - An extensive history of waste dump drainage at QCL, including a five year sampling record indicating that negligible suspended sediment loads are generated. INTRODUCTION In September of 1985, a dump failure (slide) occurred in the Mesa development at Quintette Coal Limited. The slide travelled north down a tributary of Mesa Creek and then down the Mesa Creek valley itself. This event was of great concern to Quintette Coal Limited. Rapid staff response ensured monitoring was conducted immediately to identify impacts and Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation 2 facilitate early mitigation. Water impacts were found to be minimal and the water quality not expected to deteriorate beyond an initial stage of flushing. It was subsequently realized that this slide incident had provided an opportunity to investigate the potential for sediment treatment by dump slide material. Study Area Quintette Coal Limited1s mining operations are located in northeastern British Columbia near the new town of Tumbler Ridge (Figure 1).  Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation  3 The company, managed by Denison Mines Limited, operates open pit coal workings in two areas. The Mesa/Wolverine operations are located along a ridge between the Murray and Wolverine river valleys with peak elevations ranging from 1850 metres (m) to 1720 m. To the east, in the Murray valley, are the Shikano pit and Plantsite. A 13.3 kilometre overland coal conveyor spans the distance between the Plantsite and Mesa development. Figure 2 illustrates the relative positions of the working pits, proposed pits and infrastructure. The area of study is located northeast of the Mesa pit in the 1200 to 1300 m elevation range. The climate of the area is influenced by complex mountainous terrain and the interactions of the warm, moist Pacific air mass with the colder, dryer Arctic air mass. Mean annual precipitation is 524 mm at the Plantsite and 640 mm at the Mesa/Wolverine pit area. The flow regime of small streams in the area is characterized by low winter flows from November through March, sustained high flows generated by snowmelt occurring from April through June, and short term high flows caused by rain or rain-on-snow from June through November. The 1660 m waste dump slide occurred in the headwaters of a tributary of Mesa Creek (Boulder Creek) and Mesa Creek itself, which discharges to Mast Creek and thence to the Wolverine River. Mesa Creek is relatively small at this point comprising an upper drainage area of 155 hectares and a related 10-year, 24-hour peak flow of 1.4 cubic metres per second. The dump slide study area is illustrated in Figure 3. The existing sedimentation structure, Mesa A pond, is located to the east of the slide material as shown. An interceptor ditch (Ditch 1) was constructed on the mass in 1986 to increase the inflow of water to the pond in order to provide enhanced water supply for dust suppression requirements. This interceptor ditch currently directs a portion of the original Mesa dump drainage to the pond. Figure 3 also illustrates Ditch IA which was proposed as an additional contingency scenario for dump expansion. Mesa Creek is shown on this figure along with the two Waste Management Branch receiving water sites located upstream and downstream of the waste material. The location of the permit sampling station at Mesa A pond is also provided. Background Subsequent to the 1985 dump slide event, Quintette prepared a report summarizing potential environmental impacts resulting from the failure along with our recommendations for action. A hydrological consultant (Ker, Priestman and Associates) and geotechnical consultants were brought in to review the situation. The final report described potential impacts on water quality and detailed the concept of a dump slide filter which would likely treat water from the original Mesa A pond drainage area (Ker, Priestman and Associates, 1985). In addition, several contingency scenarios were presented and were to be enacted should problems arise. Through discussion and negotiation with the Prince George, B.C. Waste Management Branch, the requirement for additional sediment treatment was waived and a multi-scenario contingency management plan approved in March of 1986. This plan is still in place and since that time, receiving water monitoring has indicated no deterioration in water quality (total Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation 4 suspended solids). Therefore, implementation of contingencies has not been necessitated. In March of 1988, an application was presented to the Ministry for expansion of the Mesa dumping area into the upper headwaters of Mesa Creek. As drainage from the dumps would discharge to the 1660 m slide mass, the proposal comprised amendment of the existing contingency plan approved in 1986. The contingency required the construction of diversion Ditch IA (Figure 3) which would be cut into the hillside above Mesa Creek, run along the failure surface and would connect with the existing Ditch 1. Two pieces of evidence were presented to support this plan, one regarding sediment treatment by the 1660 failure and the other, the source of water to be treated (Quintette Coal Limited, 1989). In February of 1990, the plan was approved by the Ministry of Environment. STUDY CONCLUSIONS The methodology utilized to investigate the potential for slide material to filter suspended sediment comprised water monitoring and analyses, a dye testing experiment, review of industry experience and direct observation. The portion of study examining the premise that waste dumps generate negligible sediment loads is supported by water quality data, literature and dump material gradation analyses. The following will describe support for the claim that, (a) treatment of suspended sediment by the 1660 m dump slide material will occur, and (b) the premise that waste dumps generate negligible sediment loads. Dump Slide Sediment Treatment East Kootenay Experience The phenomenon of filtering by dump slide material was recognized in the southeast coal block in the 1970s. The data obtained by our consultant in 1985, although not strictly comparable, indicated that there was a dramatic decrease in the creek's suspended sediment load subsequent to the slide. This was in accordance with reports by staff of the mine in question and observations by our consultant. This data and experience supported our opinion that there was a high probability for the lower portion of the Quintette 1660 m dump slide to act as a good sediment filter. We indicated that this would likely be the case during high freshet flows as well as low flows and that it would continue to provide treatment for many years (Ker Priestman and Associates, 1985). Quintette Experience Water sampling has been ongoing at receiving water stations upstream (u/s) of the slide zone and at a downstream (d/s) site from September 1985 to date. Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation 5 A record of total suspended solids (TSS) concentration over time is provided in graphic form in Figure 4. The graph shows the difference in suspended solids results between upstream and downstream stations (u/s less d/s). Points above the zero axis illustrate samples with TSS lower at the downstream site, when compared to upstream, and those below the zero axis describe samples with TSS higher at the downstream site again in comparison to upstream concentrations. As is shown, samples obtained immediately after the event (first three points) indicate the flushing process. This flushing action produced a maximum concentration of only 32 mg/L. The data set, not including the three points described above, indicate that 49% of the samples (n = 47) have a lower concentration at the downstream station as compared to the upstream site. Results in this zone range to a maximum difference of 59 mg/L during a storm event in June, 1989. Those points below the zero axis (results are higher at d/s site) comprise 21% of the total samples obtained. The concentrations in this zone range to a difference of 16.3 mg/L. This point appears to be anomalous as most describe a difference of 1 or 2 mg/L. The remainder of samples (30%) were of the same concentration. Although time lag (estimated at 69 hours) is not taken into account, these results should reflect the existing situation relatively well. This premise is based on the fact that water flow and quality conditions upstream did not vary a great deal during the majority of the sampling period. In the case of a one year storm event in 1989 (point above the zero axis at 59 mg/L) there is a likelihood that this downstream sample illustrates, the slide material filtering potential at higher flows (discharge of 0.45 m3/s). Flow-Through Characteristics In the fall of 1988 a study was undertaken to examine the flow-through characteristics of the dump slide material. Fluoroscein sodium dye was introduced to Mesa Creek at the upstream sampling site during low flow (0.001 m3/s). The downstream site was monitored on a continuous basis for 24 hours and thereafter, spot checks were undertaken daily. Evidence of the dye was never observed at the lower station, likely due to the creek flow rate at the time. However, an estimate of transit time through the slide mass was made possible through visual observations of the dye as it came to the surface of the slide and moved slowly across a surficial pond. Based on time and distance, an approximate retention period can be calculated at 69 hours. For comparison purposes, movement of dye the same distance within an open stream channel would take approximately 3 hours, assuming the same velocity as that at the upstream site. The approximate retention time within the slide mass supports the premise that the slide material provides poor flow-through characteristics. Water is held up within the mass, thereby reducing sediment loads via filtering through materials and/or settling action in surface ponds and in basins within. The runout of the 1660 m waste dump involved the high velocity and turbulent movement of coarse rock, fines (sand, gravel, till), trees and vegetation. Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation  Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation 6 The assumed slide cross section would likely reflect a heterogenous mixture of fine filter media, coarse drain sections and till layers (which could block flow). In places along the slide, interior blockages have forced water to the surface where it is observed in pools. Natural Settling Structure In addition to the surficial ponds and interior basins described above, a naturally formed presettlement pond is anticipated at the entrance to the slide mass during high stream discharge. Based on studies of dump rock drains, there is a reduction in speed or greater resistance in flow through a barrier relative to resistance to flow within an open channel upstream (Campbell, 1986). During high flow, a pool will form acting as a sedimentation trap wherein both bedload and suspended sediments are deposited. Formation of such a pond has been observed at the 1660 m slide mass during a storm event. Waste Dump Drainage Characteristics This section will present evidence supporting the premise that waste dumps do not generate high sediment loads. In regard to the application submitted by Quintette, it was explained that due to topography and subsequent mining, only dump drainage would report to the slide material. Pit runoff would not affect this treatment scenario. Waste Dump Characteristics The presence of blasted rock and fine material in the proposed Quintette dumps, or any waste dump structure, may be considered a potential source of sediment to watercourses. However, it is known that natural conditions are not conducive to this occurrence. Fines are generated by the mining operation and are end-dumped, along with coarse rock, to a waste dump where natural processes ensure that the material will be segregated within the dump structure. Conclusions from lab experiments (Nichols, 1986) and many waste dump observations indicate that upon end dumping, three distinct zones will be formed within a dump. As follows: 1. a concentration of fines in the upper 10% of the dump, 2. material which is well-graded and evenly distributed to the toe, and 3. a dispersion of predominantly coarse particles at and beyond the slope toe. This zonal segregation of material is confirmed by visual inspection of waste dumps at Quintette Coal Limited. In light of the knowledge that fines will concentrate within the upper 10% of a waste dump, we conducted material gradation analyses on samples from this portion of an active Mesa north dump. Three random samples were obtained from the dump surface and grain size analyses done on each. Combined coarse and fine material tests and hydrometer analyses were conducted. Test results (Figures 5 and 6) indicate that for the total sample, that is, combined coarse and fine material, an average of 10% of the particles are Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation  Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation 7 finer than 0.1 mm (fine sand size) and an average of 2% of the material is finer than 0.01 mm (silt or clay size). In addition, curves indicate that an average of 9% of the total sample consists of particles at 0.075 mm (200 mesh) which is the boundary between the sand and silt division. The top curve on each test sheet provides information on the finer fraction of each sample as measured by hydrometer analysis. This analyses indicates that clay fraction particles, at 0.002 mm, comprise approximately 12% of the fines fraction of the sample. Clay size fraction in the bulk samples range from only 1 to 2% of the total. In sum, of the samples obtained from the upper zone of the waste dump, only 2% of the material is of clay size and 9% consists of fine sand. It has been demonstrated, through dump simulation experiments in the laboratory and through field observations, that movement of fine particles from the upper zone to the lower regions of a waste dump will not occur. Lab models simulating dump construction (Campbell, 1986) indicate that the migration of particles within a dump column could not occur. The processes which preclude this action are the distribution of particle sizes in the vertical direction of a dump column. This arrangement constitutes a well graded filter (Campbell, 1989). The theory is also confirmed through studies conducted by Nichols (1986) and by examination of waste rock dumps by Campbell (1986 and 1989). In conclusion, the potential for high sediment loads to be generated by the proposed Mesa north dump and thus reporting to the 1660 m dump slide location is negligible due to: (a) the fact that fines remain in the upper 10% of a waste dump (b) these fines comprise a very small proportion of silt and clay size particles (c) the unlikelihood that fines would migrate to the toe of the waste dump. Suspended Solids Concentrations in Dump Drainage at QCL Concrete support of this premise is illustrated by the extensive monitoring record at Mesa A pond located in the study area. Figure 7 provides a frequency distribution graph of total suspended solids concentrations based on monthly samples from 1984 to 1989 (n = 72). As shown, the majority of samples (39%) are in the O to 5 mg/L range, 33% in the 5 to 10 mg/L range and the remainder (28%) between 10 and 50 mg/L. During this period, 98% of the monthly samples taken did not exceed 30 mg/L and none surpassed the permit 1imit of 50 mg/L. The complete data set (n = 122) which includes results obtained during weekly freshet and additional storm events, indicate a mean concentration of 10.6 mg/L. The permit limit of 50 mg/L was not exceeded during this entire period. As these results are pertinent to treated dump drainage, a question is raised regarding applicability to the reduced sediment thesis. However, evidence to support their relevance is provided in the fact that sediment buildup in the pond has not occurred nor has the pond ever required cleanout. This would indicate that typical inflow to the pond does not carry high suspended Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation  Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation 6 The assumed slide cross section would likely reflect a heterogenous mixture of fine filter media, coarse drain sections and till layers (which could block flow). In places along the slide, interior blockages have forced water to the surface where it is observed in pools. Natural Settling Structure In addition to the surficial ponds and interior basins described above, a naturally formed presettlement pond is anticipated at the entrance to the slide mass during high stream discharge. Based on studies of dump rock drains, there is a reduction in speed or greater resistance in flow through a barrier relative to resistance to flow within an open channel upstream (Campbell, 1986). During high flow, a pool will form acting as a sedimentation trap wherein both bedload and suspended sediments are deposited. Formation of such a pond has been observed at the 1660 m slide mass during a storm event. Waste Dump Drainage Characteristics This section will present evidence supporting the premise that waste dumps do not generate high sediment loads. In regard to the application submitted by Quintette, it was explained that due to topography and subsequent mining, only dump drainage would report to the slide material. Pit runoff would not affect this treatment scenario. Waste Dump Characteristics The presence of blasted rock and fine material in the proposed Quintette dumps, or any waste dump structure, may be considered a potential source of sediment to watercourses. However, it is known that natural conditions are not conducive to this occurrence. Fines are generated by the mining operation and are end-dumped, along with coarse rock to a waste dump. At this time, natural processes ensure that the material will be segregated within the dump structure. Conclusions from lab experiments (Nichols, 1986) and many waste dump observations indicate that upon end dumping, three distinct zones will be formed within a dump. As follows: 1. a concentration of fines in the upper 10% of the dump, 2. material which is well-graded and evenly distributed to the toe, and 3. a dispersion of predominantly coarse particles at and beyond the slope toe. This zonal segregation of material is confirmed by visual inspection of waste dumps at Quintette Coal Limited. In light of the knowledge that fines will concentrate within the upper 10% of a waste dump, we conducted material gradation analyses on samples from this portion of an active Mesa north clump. Three random samples were obtained from the dump surface and grain size analyses done on each. Combined coarse and fine material tests and hydrometer analyses were conducted. Test results (Figures 5 and 6) indicate that for the total sample, that is, combined coarse and fine material, an average of 10% of the particles are Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation 7 finer than 0.1 mm (fine sand size) and an average of 2% of the material is finer than 0.01 mm (silt or clay size). In addition, curves indicate that an average of 9% of the total sample consists of particles at 0.075 mm (200 mesh) which is the boundary between the sand and silt division. The top curve on each test sheet provides information on the finer fraction of each sample as measured by hydrometer analysis. This analyses indicates that clay fraction particles, at 0.002 mm, comprise approximately 12% of the fines fraction of the sample. Clay size fraction in the bulk samples range from only 1 to 2% of the total. In sum, of the samples obtained from the upper zone of the waste dump, only 2% of the material is of clay size and 9% consists of fine sand. It has been demonstrated, through dump simulation experiments in the laboratory and through field observations, that movement of fine particles from the upper zone to the lower regions of a waste dump will not occur. Lab models simulating dump construction (Campbell, 1986) indicate that the migration of particles within a dump column could not occur. The processes which preclude this action are the distribution of particle sizes in the vertical direction of a dump column. This arrangement constitutes a well graded filter (Campbell, 1989). The theory is also confirmed through studies conducted by Nichols (1986) and by examination of waste rock dumps by Campbell (1986 and 1989). In conclusion, the potential for high sediment loads to be generated by the proposed Mesa north dump and thus reporting to the 1660 m dump slide location is negligible due to: (a) the fact that fines remain in the upper 10% of a waste dump (b) these fines comprise a very small proportion of silt and clay size particles (c) the unlikelihood that fines would migrate to the toe of the waste dump. Suspended Solids Concentrations in Dump Drainage at QCL Concrete support of this premise is illustrated by the extensive monitoring record at Mesa A pond located in the study area. Figure 7 provides a frequency distribution graph of total suspended solids concentrations based on monthly samples from 1984 to 1989 (n = 72). As shown, the majority of samples (39%) are in the O to 5 mg/L range, 33% in the 5 to 10 mg/L range and the remainder (28%) between 10 and 50 mg/L. During this period, 98% of the monthly samples taken did not exceed 30 mg/L and none surpassed the permit 1imit of 50 mg/L. The complete data set (n = 122) which includes results obtained during weekly freshet and additional storm events, indicate a mean concentration of 10.6 mg/L. The permit limit of 50 mg/L was not exceeded during this entire period. As these results are pertinent to treated dump drainage, a question is raised regarding applicability to the reduced sediment thesis. However, evidence to support their relevance is provided in the fact that sediment buildup in the pond has not occurred nor has the pond ever required cleanout. This would indicate that typical inflow to the pond does not carry high suspended Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation 8 sediment loads and, that on average, the inflow from the dump area contains levels similar to that of pond outflow. Further monitoring was conducted during the 1989 freshet to directly examine dump drainage. Table 1 provides a summary of total suspended solids results from three untreated sources; the Mesa A pond interceptor ditch, the Marmot 1690 dump (reporting to Mesa A pond) and the toe of a dump to the south -Wolverine 1650 m. TABLE 1 - Quintette Coal Limited Waste Dump Drainage TSS Concentrations  As shown, results ranged from O to 63 mg/L with mean concentrations of 14 mg/L, 7 mg/L and 5 mg/L respectively. The overall mean is calculated at 9 mg/L which is very similar to outflow concentrations at Mesa A pond. This data further supports the reduced sediment thesis. CONCLUSIONS On the basis of arguments presented herein, Quintette Coal Limited, upon application to the Ministry, concluded that average suspended solids concentrations from the Mesa north dumps would not likely exceed 50 mg/L, and if it did, the 1660 m dump slide material would provide treatment via filtering and settling. This thesis/argument was accepted by the Regional Waste Management Branch in February of this year. Dumping has not as yet been initiated further to the plan. Proceedings of the 14th Annual British Columbia Mine Reclamation Symposium in Cranbrook, BC, 1990. The Technical and Research Committee on Reclamation 9 REFERENCES Campbell, D.B. (1986). Discussions of Concerns Regarding the Long-Term Performance of Rock Drains. In: Proceedings of the International Symposium on Flow-Through Drains, Cranbrook, B.C., September 8-11, 1986. Campbell, D.B. (1989). Some Observations Relative to the Performance of Flow- Through Rock Drains. In: Proceedings of the Thirteenth Annual B.C. Mine Reclamation Symposium. Water Management at Minesites, June 7-9, 1989. Ker, Priestman and Associates Ltd. (1985). Letter Report to Quintette Coal Limited, "Water Management Contingency Plans Relating to McConkey Dump Failures in the Mast Creek Drainage Basin". (Unpublished). Nichols, R.S. (1986). Rock and Segregation in Waste Dumps, Proceedings of the International Symposium on Flow-Through Rock Drains, Cranbrook, B.C., September 8-11, 1986. Quintette Coal Limited (1989). Mesa North Dump Proposal - Drainage Characteristics and Dumpslide Water Treatment. (Unpublished).

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