Presentation on theme: "Demonstration of Subaqueous Disposal of Mill Waste Dave Hinrichs, NewFields Mark Doolan, U.S. EPA Chris Wienecke, ATT Sunoco and Jasper County Group R."— Presentation transcript:
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
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
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
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
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
Pit Filling Filled pit with 58,500 yds 3 of chat and tailings March - July 2002 Surcharged to plus 4 feet Capped with 1.5 ft of topsoil Volunteer revegetation
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
Pit Selection Prioritize pits that are or can be hydraulically isolated from shallow aquifer. Indicators: low or negative eH, low O 2, 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.
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.