Demonstration of Subaqueous Disposal of Mill Waste

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Demonstration of Subaqueous Disposal of Mill
Waste

• Dave Hinrichs, NewFields
• Mark Doolan, U.S. EPA
• Chris Wienecke, ATT
• Sunoco and Jasper County Group
• R. Fischer and K. Tegtmeyer, NewFields

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Mill Waste Characteristics

• Chat from Barnsdall No. 3 mine, Kansas
• Tailings from Barnsdall No. 2 mine, Missouri
• Tailings - net-alkaline, permeability 2 x 10-7
  minimal ARD
• Tailings lab 0.03 ft/ft vertical settlement
• Concentrations in tailings backfill
• Zinc 18,730 mg/kg
• Lead 570 mg/kg
• Cadmium 130 mg/kg

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Subsidence pit at Remedial Soil Repository West
of Prosperity
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Jasper Co. Site Subsidence Pit at North Edge of
Remedial repository
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Jasper Co. Site Subsidence Pits Subsidence Pit
South of Carterville off of Lewis St.
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Jasper Co. Site Subsidence PitWest of
Carterville Off of Wilson St.
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Existing Site Conditions
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Subaqueous Disposal Theory

• Place waste in the saturated zone, cap and reduce
  oxygen by 10,000x
• Establish reducing/anaerobic conditions to reduce
  or eliminate ARD and metals release
• Lower disposal and O M costs
• Eliminate pit as trash dumping site
• Eliminate as storm runoff metal load source

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Expected Results of Subaqueous Disposal

• Short-term zinc loading to aquifer
• Shift from strongly oxidizing to reducing
  conditions, i.e., lower eH
• Increase in pH to mildly alkaline
• Greatly reduced rate of sulfide oxidation, ARD,
  and metals release compared to above-ground
  repository

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Subaqueous Disposal Setting
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Mechanisms for Metals Release in Subaqueous
Setting

• Short term
• Dissolution of reactive secondary minerals
• Long term
• Stable sulfide minerals, dependent on final eH
  and pH conditions

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Galena Leach Study
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Subaqueous Field Demonstration Jasper County,
Missouri

• Selected P4 Pond at Freehold mine in Kansas
• Installed two monitoring wells in Boone aquifer
• MDNR conducted dye tracing study
• Collected 2 rounds of baseline samples,
  pre-backfill
• Backfilled 200 ft x 200 ft x 35 ft pit
• Installed Well P4 Central in July 2002
• Collected 4 rounds of post-backfill samples
• NewFields prepared summary report, Dec. 2003

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Demonstration Site Map
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Pond P-4
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Site Conditions

• Shallow aquifer confined gradient est. 0.0008
• Local shale hydraulic gradient est. 0.01
• Pit depth 35 feet Penn shale 75 feet thick
• Freehold mine depth of 120 - 180 ft in Miss.
  limestone
• Pond water level 2 13 feet higher than shallow
  aquifer, poor hydraulic inter-connection
• Water levels in pits showed dramatic rise and
  fall with storm events

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Pit Filling

• Filled pit with 58,500 yds3 of chat and tailings
  March - July 2002
• Surcharged to plus 4 feet
• Capped with 1.5 ft of topsoil
• Volunteer revegetation

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Pond 4 - filling
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Pond 4 - Post Filling
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Filled Pit
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Filled Pit Monitoring

• Installed wells P4NW and P15E Dec 2001
• Collected 2 rounds of baseline samples,
  pre-backfill
• Installed Well P4 Central in July 2002
• Collected 4 rounds of post-fill samples

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Dissolved Zinc Concentrations
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P4 Pond Redox
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ZINC LOAD ESTIMATES
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Conclusions

• Subaqueous disposal did not increase metal
  loading to shallow aquifer due to low
  permeability/poor connection
• Subaqueous disposal should substantially reduce
  metals loading to surface streams
• Post-remedy footprint reduced lower O M cost
• Jasper County FS includes subaqueous disposal as
  a permanent, moderate cost alternative to
  conventional cap and cover
• Placement of wastes in mine pits reduces pit
  associated hazards and illegal dumping

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Pit Selection

• Prioritize pits that are or can be hydraulically
  isolated from shallow aquifer. Indicators
• low or negative eH,
• low O2, poor vegetation and aquatic community
• Rapid increase in water level if recharged
• Slow to recover when test-pumped
• Prioritize pits that pose physical hazards, are
  used as trash dumps, or offer marginal aquatic
  habitat.

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Engineering Aspects

• With scrapers and dozers, cost 4.50/cy
• Tailings can be used in low-permeability caps and
  liners.
• Using chat only higher predicted zinc loads
  mix tails with chat or fly ash
• Predict consolidation before backfill
• Surcharge pits and/or allow time for settlement
  before placing the soil cap.