British Columbia Mine Reclamation Symposium

Towards closure of the Fire Road Ard site in New Brunswick Phinney, K. D.; Coleman, M. 2014

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TOWARDS CLOSURE OF THE FIRE ROAD ARD SITE IN NEW BRUNSWICK  K.D. Phinney, P.Eng.1 M. Coleman, P.Geo, P.Eng.2  1Consulting Chemist/Chemical Engineer, Halifax, NS 2Manager of Environmental Services, Mine Reclamation Inc.  (subsidiary of NB Power), Fredericton, NB  ABSTRACT  The approximately 120 ha backfilled Fire Road coal mine cut, located near Fredericton New Brunswick, has been a source of acid rock drainage since the mid 1980’s.  Lime neutralization treatment of drainage has been continuously ongoing. The cut, with depths to approximately 20 m, is located in sandstones containing iron sulphides, principally pyrite, in the range of 1 to 2 wt%.   Site water chemistry has been intensively monitored over the years using a series of groundwater wells within and bordering the disturbed areas. This has provided a considerable inventory of water chemistry data which has enabled interpretation of the characteristics of acid generation at the site; the effectiveness of “in situ” neutralization in reducing the ultimate acidity of the drainage, and trends in acidity over the years.  During the past number of years a definite trend of decreasing acidity has been observed leading to the conclusion that the site should exhibit “zero lime demand” within the next 10 years.  At present, plans are being developed for final closure of the site.  KEY WORDS  acid mine drainage, reclamation, mine water chemistry, groundwater monitoring  LOCATION AND BRIEF HISTORY  The former NB Coal Limited Fire Road coal mine, located near the village of Minto, approximately 40 km north east of Fredericton, New Brunswick, produced approximately 50000 tonnes per year of thermal coal until shutdown in 1987.  The general extent of the approximately 120 ha site is shown in Figure 1.  Photos 1, 2 and 3 provide an overview of the site at present.  The mine had been in operation since 1983 and was shutdown primarily due to an acid mine generation problem first observed in 1985.  Cut and fill mining methods were used to recover a thin seam of coal overlain by as much as 20 m of sandstone bedrock.  Both the coal and sandstones contain low concentrations of iron sulphides (< 2 wt% pyrite).  The remaining mine cut was backfilled with waste rock to the limit formed by the west highwall after cessation of mining.  Surface and ground water which flow into the site are collected in a sump at the southern limit of the site and have been treated by lime neutralization since early 1987.  The sump is maintained at a water level to ensure that all water contacting the site drains to the sump.  Treatment sludge is returned from the sedimentation ponds to the backfilled waste rock area (Coleman, Whalen, and Landva, 1997).        Figure 1. Site Layout   Photo 1. Treatment Plant    Photo 2. Natural Re-vegetation of Backfilled Area    Photo 3 – Sump   WATER CHEMISTRY OVER THE YEARS  Water chemistry data are available for a series of groundwater monitoring wells and the minewater collection sump at the south end of the site.  Annual sampling data are available from approximately 1990 for a number of the sampling locations and for a number of years for the remaining locations.    Characteristic water chemistry, as measured at the sump in June 2013, is given in Table 1.  Aluminum, iron, and manganese are the main species of concern.  An assessment of the data clearly indicates a trend toward decreasing acidity at the sump and at a number of the sampling wells within the mined (disturbed) area.  A summary of calculated acidity values is given in Table 2.  The trend towards decreasing acidity is indicated by data for a number of the sampling wells: 4A, 19S, 27S, 26S, and 26D.  Other sampling wells within the disturbed area do not indicate such a trend, 17S and 17D, and are more indicative of continuing strong acid generation.    The decrease in lime demand, as shown in Table 3, is consistent with the trend towards decreasing acidity of the acid rock drainage. The rock matrix continues to neutralize a substantial portion of generated acidity, resulting in the “residual” acidity which is measured as part of the water chemistry analyses or which can be calculated based on pH of the sample and the metals content, particularly aluminum, iron, and manganese.  Other trace metals such as copper and zinc also contribute to the acidity of the water.  An assessment of “initially generated” to “residual acidity” indicates that in the order of 30 to 70% of the generated acidity is neutralized within the rock matrix.  Water from the sump indicates a slightly higher degree of in situ neutralization, 50 to 70%, which may be due to the effect of surface runoff collected in the sump and entry of alkalinity from the adjacent sludge ponds.  The degrees of “in situ” neutralization have not changed over the years.  In situ neutralization of acid rock drainage by reaction with calcium and magnesium silicates in the host rock has and continues to be the major factor in the residual acidity of the drainage.  The significance of such neutralization reactions is indicated by the relatively high concentrations of calcium and magnesium in the drainage prior and subsequent to disposal of treatment sludge onto the mine site.  Recent data, given in Table 4, are consistent with prior-to-1992 data, given in Table 5.  These “weathering” reactions can be expected to continue until the acidity decreases to an “acceptable” level for direct discharge into the environment, due to the high proportion of silicate mineralization relative to the content of reactive sulphides, such as pyrite.  The decrease in acidity over the years is expected and will continue into the future.  The decrease can be attributed to a decrease in exposed sulphide minerals as oxidation proceeds; the effectiveness of the sludge placement within and on the waste rock surface  in reducing the rate of transfer of atmospheric oxygen to the sulphide minerals, and possible leaching of any excess alkalinity in the sludge into the groundwater zone, thereby neutralizing acidity.  Passage of local groundwater through the disturbed zone would be expected to have little neutralizing effect due to the limited alkalinity of groundwater from the undisturbed areas.  The water chemistry data for sampling locations from undisturbed areas (1D, 14S, 8A, 23, 24, 9D, 9S, 10D, and 10S) indicate limited alkalinity (order of 8 to 50 mg/L with a median of 20 mg/L as CaCO3).  As previously noted, the key contributors to acidity include iron, aluminum, and manganese.  To achieve regulatory requirements, sufficient alkalinity must be added to essentially remove iron and aluminum, which are highly insoluble at the regulatory lower pH limit of 6.5.               Table 1. Minewater Chemistry at Sump   Groundwater sampling period June 2012 July 2013 Turbidity (NTU) 3.8 4.1 pH 4.1 4.10 Total acidity (mg/L as CaCO3) 200 205 Calcium (mg/L) Magnesium (mg/L) Sodium (mg/L) Potassium (mg/L) 178 28.1 4.02 1.26 197 29.4 4.10 1.34 Aluminum (mg/L) Iron (mg/L) Manganese (mg/L) Copper (mg/L) Zinc (mg/L) 28.2 6.10 12.4 0.017 0.471 28.0 6.05 12.1 0.016 0.465 Alkalinity (mg/L)  Chloride (mg/L) Sulphate (mg/L) 0.0 1.5 680 <2 1.4 740            Table 2. Fire Road water chemistry data; summary of acidity values (calculated)                YEAR SUMP  4A 19S 17S 17D 27S 26S 26D  1988    391 1500 4700  3800 800  1990    217 2500 6400  2900 2900  1992       2800 4100 1700  1993   1300 750 1500 1400 2800 3500 1800  1994   1200 731 2800 4100 2200 2200 1700  1995   1300 615 2600 2700 1700 470 1300  1996   900 881 3700 1300 446 1600 1300  1998 680  800 1170 3800 1100  1100   1999 580          2000   600 900 5700 1200 1500  1000  2004   500 530 3200 2200 431 634 700  2006 390  480 594 2900 2600 700 440 584  2007 340  450  260 600 210 224 530  2008 380  450 516 2800 1700 145  600  2010 150  360 370 2700 1000 231 100 470  2012 200  260 260 2000 1100 250  370  2013 200  192 211 561 540 240 67 345                                                                                                                                                                                                                                                                                                     Data are for Sump and selected monitoring wells, 4A, etc.      Calculated acidity values are mg/L as calcium carbonate.      Data are for annual groundwater sampling program (summer period).                                           Table 3. Annual Hydrated Lime Consumption   Year Water pumped Purchased lime Average lime demand  m3/a t/a mg/L  1992 Not recorded 2417  1993  2113  1994  1785  1995  1492  1996  1662  1997  1009  1998  1197  1999  700  2000  623  2001  534  2002  736  2003  685  2004  591  2005  620  2006  671  2007 1 502 001 406 270 2008 1 863 564 628 337 2009 1 822 626 557 306 2010 1 745 537 370 212 2011 2 022 194 435 215 2012  1 600 650 275 172 2013 2 243 248 347 155   Average lime demand0501001502002503003504002005 2007 2009 2011 2013 2015YEARmg/L  Table 4. (Ca + Mg) for Mined Area Sample Locations (2012 data)  Sample pH SO4 (mg/L) Acidity (mg/L as CaCO3) In situ Neutralization (%) Ca + Mg (mg/L as Ca) Sump 4.1 680 200 72 224 28S 4.1 1230 74 94 482 4A 4.1 930 260 73 279 7S 5.9 92 70 28 29 6S 5.8 122 100 23 37 19S 5.7 300 260 18 60 17D 4.3 1630 1100 38 333 17S 4.0 2800 2400 19 551 16D 7.2 800 31 96 336 16S 4.1 560 140 76 184 22S 4.8 210 60 73 64 27S 4.1 370 250 35 52 26D 3.6 1030 370 66 221  Table 5. (Ca + Mg) for Mined Area Sample Locations (Pre-1990 data)  Sample pH SO4 (mg/L) Acidity (mg/L as CaCO3) In situ Neutralization (%) Ca + Mg (mg/L as Ca) Sump (1986) 3.05 790 380 54 143 19S (1988) 3.00 460 280 41 79 17D (1988) 2.85 6190 4140 36 923 17S (1988) 2.85 2480 1343 48 500 16S (1988) 3.75 516 90 83 179 26D (1988) 2.75 1390 793 45 262 26S (1988) 3.00 5940 3750 39 975    3.0 STABILIZATION OPTIONS  A number of reclamation options were considered, during the years immediately following the onset of acid generation, to stabilize the site so as to allow for closure without the continuing need for treatment of acid mine drainage.  The objective of stabilization methods is to minimize the rate of transfer of atmospheric oxygen into those areas of waste rock at which oxidation of mineral sulphides is occurring.  This objective has been achieved to a certain degree by maintaining the highest practical ground water level within the waste rock area.    The following stabilization methods were considered:   excavation and transfer of waste rock to ensure permanent disposal under water;  construction of ponds over the waste rock to form a “wet” seal;  construction of an engineered high capillary head earthen cover over the waste rock;  construction of a simple earthen cover over the waste rock, and   vegetation of the waste rock area.  Studies concluded that the cost for stabilization would be in the range of $3 M to $6 M, with the likely risk that treatment operations would still be required.  Thus, treatment operations have continued over the years since the onset of acid generation.  TREATMENT OPERATIONS  The lime addition treatment system is effective in meeting the requirements of the Province’s Approval to Operate.  Hydrated lime slurry is added to water, pumped from the sump, in neutralization tanks which overflow to sedimentation ponds for separation of the aluminum/iron sludge from the water phase.  Overflow from the ponds is discharged into the South Branch of East Brook which drains towards the Little River.  The ponds are periodically dredged, with the sludge discharged onto the disturbed mining area north of the sump.  The inert sludge assists in sealing the area and may provide some alkalinity due to leaching from the sludge.  The decreasing acidity of the mine drainage may enable consideration of passive treatment methods at some time in the future.  Passive methods are attractive as they offer the potential for minimal chemical and maintenance costs into the future.  At present, the substantial flow of minewater, in the order of 1.8 Mm3/a (206 m3/h), and the degree of total acidity (order of 200 to 400 mg/L as CaCO3) would be challenging for the application of passive methods, especially where regulatory requirements must be consistently achieved.    Passive methods can be expected to become of interest as the acidity of the drainage continues to decline and, if eventually, pumping can be stopped to allow natural water levels to re-establish, resulting in one or more low-flow discharges at minimal acidity.    CLOSURE PLANS  The site is predicted to stabilize with respect to water chemistry, as monitored at the sump, within approximately the next 10 years.  That is, water chemistry as monitored at the sump can be expected to approach similarity to the baseline water chemistry of undisturbed groundwater in the area.  If the trend towards “zero” lime demand continues, it may be possible to discharge water from the sump without treatment.      The more reactive areas along the west highwall may remain as sources of acid generation for some period beyond the next 10 years.  At present, mixing of drainage from this area with other water in the cut and contact with the backfill materials, as it flows towards the sump, results in substantial in situ neutralization.  Acidity generated along areas of the west high wall will also gradually decrease towards a “zero” lime demand but this process will likely extend beyond the approximately 10 year period.  Continuing acid generation in this area will be of concern if the level of water in the cut is allowed to re-establish at natural levels.  Thus, to meet regulatory requirements, it may be necessary to continue maintaining the lowered water level in the sump, resulting in continued pumping beyond the 10 year period (without lime treatment).  Alternatively, it may be feasible to discharge overflow(s) from the highwall area, especially if these are limited in quantity and mixed with local surface and ground waters.  Major close-out reclamation items will include restoration of the stream across the cut and filling-in/grading of the sludge ponds to the north and south of the Fire Road.  The former mine area has generally re-vegetated in a very satisfactory manner and requires no further reclamation efforts.  The ongoing surface and ground water monitoring program is very valuable in following the progress at the site.  This program will be continued.  In addition, a periodic test program is recommended to assess the quantity and water chemistry of overflows from the west highwall area.  Such tests, possibly carried out on an annual basis, would involve allowing the sump water level to rise with careful observation at the East Brook area, downstream from the highwall area.  Such observations will enable the effects on water chemistry of the proposed stream restoration to be estimated.  CONCLUDING COMMENTS  The monitoring of water chemistry over the years has proven to be very valuable in understanding acid generation at the site and in planning for eventual closure with a high degree of confidence.  REFERENCES  Coleman, M.M., P.J. Whalen and A. Landva. May 30 - June 6, 1997. Investigation on the placement of lime neutralization sludge on acid generating waste rock.  Proceedings of the 4th International Conference on Acid Rock Drainage: Application Technology. Vancouver.     


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