Cover system performance — using numerical modelling to optimise monitoring systems

Abstract

Lysimeters are widely used in the mining and waste industries to directly measure the performance of cover systems installed for rehabilitation of containment facilities (the facility) to restrict net percolation (deep infiltration) and oxygen flux. As lysimeters are conceptually simple, stakeholders place significant emphasis on performance measured by these systems, which adds to the importance of obtaining representative net percolation values. However, the design of lysimeters for cover system monitoring programs in the mining industry often does not consider fundamental aspects of lysimeter design, leading to inaccurate measurement of percolation. Lysimeters also require complementary soil monitoring equipment in order to measure multiple parameters of a cover system’s water balance, and thus its overall performance. Newmont Boddington Gold Mine, located in Western Australia, implemented rehabilitation field trials for assessing the performance of an updated closure strategy for its Waste Rock Dumps (WRD). The extensive monitoring required for assessment of the cover system performance included the design and installation of a lysimeter to measure percolation. Sizing, that is the depth and area dimensions of the lysimeter, was completed through numerical modelling. Numerical modelling is used to ensure that the lysimeter performs to expectations under a wide range of conditions encountered during the life of the facility. O’Kane Consultants completed an integrated numerical assessment to develop the monitoring system design for performance monitoring of the WRD cover system. Completion of a performance analysis was required for selection of the optimal lysimeter design whilst considering installation and cost. Numerical modelling was undertaken to simulate and compare the relative performance of various lysimeter configurations. Geostudio’s VADOSE/W and SEEP/W, two commercially available software packages, were coupled to simulate long-term percolation through the cover system and seepage rates through the lysimeter’s base. This paper presents the methodology utilised, results of numerical simulations of lysimeter designs and final design of the performance monitoring system.