Research Article Open Access

Evaluation of the Hydraulic Conductivity of Cover Systems for Mineral Waste Capping Projects

Abstract

Laboratory tests and simulations were performed to evaluate the effectiveness of various capping systems in reducing drainage from mineral stockpiles. ASTM soil testing standards were used to evaluate the physical properties of the materials in order to establish a cover system that produced the highest soil density and lowest soil permeability when compacted at optimum moisture content. After laboratory tests were completed, four barrier materials were selected for further evaluation. The laboratory hydraulic conductivity of the selected materials were all less than or equal to 2 x 10-6 cm/sec., the maximum value allowed by Minnesota Pollution Control Agency (MPCA). The effective hydraulic conductivities of the barriers were estimated using the EPA HELP (Hydrologic Evaluation of Landfill Performance) model. Model results indicated that the major water loss occurred through evapotranspiration and the single most important design parameter affecting infiltration was, not surprisingly, the hydraulic conductivity of the barrier. An analysis of key input parameters in the HELP model showed that design parameters such as soil type, hydraulic conductivity, cover thickness and slope of the drainage layer affected infiltration rate to a certain extent. However, for a given barrier, the hydraulic conductivity of the cover system was the most critical design parameter affecting water infiltration. Based on the results obtained, a cap design consisting of a three-layer soil barrier was recommended for final capping of any mineral stockpile capping project.

Francis D. Udoh

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