5.1 Overview

When mine workings extend below the water table, groundwater inflows to surface and underground mine workings must be managed. Methods range from a basic system of sumps from which water is pumped to surface to the extensive deployment of groundwater extraction wells. Approaches used to estimate inflows and design control systems for both underground and open pit mines are described in this section. In arid climates the regional water table may be many hundreds of meters below ground surface. Apart from minor seepage zones where perched water zones are intercepted, an open pit located above the regional water table will not need groundwater inflow controls.

Open mine workings are effectively a hydraulic sink, drawing in groundwater from the surrounding area and lowering hydraulic head in adjacent areas. Depending upon the hydrogeologic setting, this impact might extend many kilometers beyond the boundaries of the mine site. Mine dewatering carries with it a potential for substantive impacts on groundwater resources in neighboring areas. There could be, for example, a nearby well field supplying potable water for community use. Of similar concern is the potential for a dewatering system to reduce spring flows and base flows in creeks upon which either there is a community use, or this seepage provides critical support to the local ecosystem. In some jurisdictions this impact is regulated by existing water rights to extract groundwater; in other jurisdictions there may be no legal framework in place. There are instances where a proposed mining project did not proceed because impacts on neighboring groundwater users could not be mitigated or the potential impact on a valued ecosystem, even with mitigation, was judged to be unacceptable. Assessments of potential impacts in neighboring areas are evaluated using established field methods and computational tools, such as predictive, three-dimensional transient groundwater models or in some cases coupled surface water/groundwater flow models.