Model study of some factors influencing the resaturation of spoil following mining and reclamation

Abstract

This study demonstrates how groundwater flow models can be used to study the long-term response of groundwater flow in and around strip mines, particularly once mining and reclamation are complete. The two-dimensional, finite-element model used in this study is able to simulate the actual steps in mining and reclamation, and to model groundwater flow using a transient, free surface approach. Simulations presented here are carried out on two significantly different scales. In the mine-scale simulations, the mine occupies a major portion of the system. These simulations show that spoil resaturation depends on the magnitude and distribution of recharge and the hydraulic conductivity of spoil and geological units. During the early phase of spoil resaturation, high gradients along the water table can develop due to, first a lack of downward drainage through units below the mine or, second due to spoil with a low hydraulic conductivity. Conversely when conditions opposite to these develop, the gradients along the water table are relatively low. The time over which these changes in water level occur depends upon recharge rates and the storativity of the spoil. At an even smaller scale, individual hills formed during reclamation are the features of interest. Once reclamation is complete, the flow system will return to a new steady state with the position of the water table again dependent upon recharge rates and hydraulic conductivity but modified by hill size. A high water table is favoured by low hydraulic conductivities, high recharge rates, or hills with a wide base. While the addition of widely spaced surface drains is ineffective in controlling water levels, discrete open drains which are closely spaced within the spoil appear much more useful. Generally, the effect of a drain is independent of the size of the hill.