Ecology, Environment and Conservation Paper

AN INTEGRATED PROGRAM FOR INFILTRATION CONTROL OF GABBRO AND WASTE ROCK STOCKRIPES AT THE DUNKA MINE SITE

Francis D. Udoh

Abstract

This paper presents a methodological evaluation of techniques used to limit water infiltration into mining stockpiles near Dunka mine, northeastern Minnesota.The methodology employed involved initially screening a large number of materials for their availability, permeability characteristics, and costs as potential cover systems.Analysis of various cover designs was conducted to determine the effectiveness of the selected cover systems to stem infiltration using HELP (Hydrologic Evaluation of Landfill Performance) model. Based on model results and the most cost effective design alternatives, four different barriers were built:Twelve inches of compacted, minus 2V2 inch glacial till (Plot 1); Twelve inches of compacted, minus IA inch glacial till (Plot 2);A 20 mil PVC membrane with 6 inches of pit run sand placed above and below the liner (Plot 3); and Glacial till screened to minus 2V2 inch mixed with 5% bentonite with the barrier constructed in two six inch lifts, providing a barrier of 12 inches (Plot 4).Although all the plots significantly reduced the amount of contaminated bottom flow from the stockpiles, the barriers in Plots 2, 3, and 4 produced much less bottom flow than Plot I .The bottom flow ranged from 0.5 inches for the 20 mil PVC liner (Plot 3) to 7.5 inches for the minus 21/2 inch till barrier (Plot 1).The minus V2 inch till barrier (Plot 2) and the minus 2V2 inch till mixed 5% bentonite barrier (Plot 4) had about the same amount of bottom flow, 2.3 and 2.1 inches respectively.The residual drainage from the stockpiles was channeled to wetland treatment systems for metal removal. Nickel removal from the wetland treatment systems exceeded 90% for the first few years of operation. By the time the stockpiles which contributed to the major input to the wetland were capped, both flow and metal concentrations in the drainage were drastically reduced. Presently, there has been no evidence of nickel release from the wetland.