The Effect of Tailings Characteristics on Cover System Success. Jason Keller, Michael Milczarek, Tzung-mow Yao, Meg Buchanan

Transcription

1 The Effect of Tailings Characteristics on Cover System Success Jason Keller, Michael Milczarek, Tzung-mow Yao, Meg Buchanan

2 Presentation Overview General findings from copper tailings reclamation research and monitoring in semiarid climate: Vegetation behavior Hydraulic behavior Salinity and ph migration Facilities located in Sonoran and Chihuahuan deserts Average precipitation: mm/year Reference evaporation: >1,700 mm/year

3 Physical, Geochemical and Spatial Characteristics

4 Physical Characteristics Tailings are poorly graded Mostly silt size Highly erosive (high intensity precipitation/wind) No soil structure Impoundment construction results in additional sorting and layering beach sands slimes mixed areas Moisture retention and permeability varies by material types Variable saturation and drainage

5 Tailings Segregation and Structure Sands Slimes Mixed Zone

6 Geochemical Characteristics Can be saline to hyper-saline Ore body mineralogy can result in: High acid generation potential (and acidity) High plant available metals (i.e. arsenic) Typically low plant fertility Lack of organic matter and microbiota

7 To Cap or Not To Cap?

8 Matrix Approach Cover Depth

9 Vegetation Behavior

10 Biosolid/green waste amended circumneutral tailings

11 30 cm cover, no biosolids, native species, acid tailings

12 30 cm cover, 21 ton/acre biosolids, native species, acid tailings

13 30 30 cm cm Cover cover, - No no Biosolids biosolids, vsnative Biosolids 1.0 species, acid tailings 0.9 No Biosolids Total Vegetation Groundcover A C B A B 21 dry tons/acre Biosolids 63 dry tons/acre Biosolids B B B A A B B Different letters indicate significant differences Year

14 30 30 cm cm cover, Cover no vsbiosolids, 60 cm Cover native Total Vegetation Groundcover species, acid tailings 30 cm Cover 60 cm Cover A A A A A A A A Different letters indicate significant differences Year

15 15 cm cover on moderately acidic tailings

16 15 cm cover on circumneutral tailings

17 Beach area with 8 cm cover on circumneutral tailings

18 Decant pond with 8 cm cover on circumneutral tailings

19 Rooting characteristics

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21 Vegetation Considerations High salinity and/or acid tailings can restrict vegetation success in shallow covers Rooting characteristics: Actively root into circumneutral tailings Minor rooting into moderately acid tailings, primarily limited to cover and upper one foot of tailings Form dense root mat above cover/acid tailings contact Affected by tailings permeability Vegetative success generally greater in mixed zone than in beach areas May be affected by plant available water Vegetation characteristics varies with location (e.g. mesic (slimes) vs. xeric (sands))

22 Vegetation Considerations (cont d) Effect of cover depth Generally no difference in vegetative cover between covers > 30 cm thick (due to endemic presence of South African grasses in SW USA) BUT, greater native species success Effect of organic amendments: Can successfully reclaim raw tailings with a biosolids/green waste (compost) mix Results in significantly greater mean vegetation ground cover, grass, and forb and shrub groundcover, however, less species diversity In some cases observed to be sustained over 10 years

23 Infiltration/Net Percolation

24 Calculated 1D Flux Sensor Nest/Plot Location Total Downward Flux (cm) Annual Flux (cm/yr) Annual Flux Rate (cm/s) Estimated Flux as Percent of Precipitation Lower permeability tailings 30 cm cover, low vegetation Average (3 to 2 nests) E % Standard Deviation E % 30 cm cover, high vegetation Average (3 to 2 nests) E % Standard Deviation E % 60 cm cover, low vegetation Average (3 to 1 nest) E % Standard Deviation E % Average 60 cm cover, high vegetation Average (3 to 2 nests) E % Greater surface water runoff, less infiltration depth Standard Deviation E % Bare Tailings Average (3 nests) E % Standard Deviation E %

25 Predicted Effect of Increasing Cover Thickness Average Annual Net Percolation (mm/yr) Tailing beach Cover Depth (cm) Tailings Ksat = 3.2E-4 cm/s Tailings Ksat = 2.2E-5 cm/s Tailings Ksat = 2.2E-6 cm/s Tailings Ksat = 2.2E-7 cm/s Tailing sideslope Tailing slimes

26 Net Percolation Considerations Lower permeability tailings reduced net percolation Increasing cover thickness can have less influence on net percolation than tailings characteristics Shallow covers or direct reclaimed tailings can have less net percolation than deeper covers Tailings are an integral part of store and release and influence should be considered during cover design

27 Low-pH and Saline Solution Migration into Monolayer Covers

28 ph and EC Profiles Sample Midpoint Relative to Contact (cm) cm Cover, No Biosolids 30 cm Cover, 21 dry tons/ac Biosolids 60 cm Cover, No Biosolids 60 cm Cover, 21 dry tons/ac Biosolids Cover-Tailings Contact Sample Midpoint Relative to Contact (cm) cm Cover, No Biosolids 30 cm Cover, 21 dry tons/ac Biosolids 60 cm Cover, No Biosolids 60 cm Cover, 21 dry tons/ac Biosolids Cover-Tailings Contact Mean 1:1 Paste ph Mean 1:1 Paste Electrical Conductivity (ds/m)

29 Tailing/Cover Contact

30 Tailing/Cover Contact

31 Acid and Salinity Migration Considerations In a semi-arid environment salinity and acid migration observed to be limited to 15 cm above contact Phytotoxic levels of ph and salinity in cover material generally absent 5 cm above contact Increased migration above contact with decreased cover thickness (30 vs 60 cm) Acidity and salinity migration may be limited due to: Unsaturated hydraulic conductivities and upward flux rates greatly diminish with distance above the contact High calcium carbonate contents in the cover material neutralize low-ph solution

32 Conclusions Circumneutral tailings can be revegetated with: organic amendments (if available); net percolation may actually be lower shallow covers (< 30 cm) Low permeability tailings serve to slow down infiltration and retain water in cover; can have greater effect on net percolation than cover depth Revegetation seed mixes should consider differences between sand and slimes area; deeper covers are better for native seed mixes Cover system modeling should acknowledge ET depth into tailings Upward acidity and salinity migration into monolayer covers may be limited

33 THANK YOU!