Open cut mining adjacent to ephemeral creek and spring systems - Pilbara, Western Australia

Abstract

The Pilbara region, located in the north-west of Western Australia, is significant as the second largest source of iron ore on the planet. Iron ore mines owned and operated by Pilbara Iron arc located within the Hamersley Basin. This paper refers to the Greater Paraburdoo mines, the 4WEST and 4EAST pits, located on the southern margin of the Harnersley basin, The groundwater model refers to ephemeral watercourses: Seven Mile and Pirraburdoo creeks, which are adjacent to the two pits and Ratty Springs, located upstream from 4WEST and within Pirraburdoo creek. The spring and creeks hold cultural heritage significance for local, Aboriginal communities. Two of these, the Gobawarrah Minduarra Yinhawanga (OMY) and Innawonga native title claim groups, initiated hydrological work on the two creeks and Ratty Springs, with the aim of determining the effects of dewatering on the natural water flows of these systems. The numerical model used to simulate drawdown in the Pirraburdoo and Seven Mile Creeks was derived from MODFLOW96 with the MT3D96 interface code, using the PIMWIN Pro software interface and the PCG2 solver package. The model covered an area of 77 km2, the calibration period was from May 1997 to December 2004 and the prediction period was from January 2005 until December 2015. The model has four layers and incorporated effects caused by mining voids, 4WEST and 4EAST ore bodies, creek bed material, weathered bedrock below creeks and surrounding banded iron formations (BIF). The model has been calibrated using the trail-and-error method set down by (Middlernis et al, 2001). The current dewatering schedule (12 ML/day) in the 4WEST and 4EAST mining areas does not affect the water table levels in the adjacent creeks significantly or for any considerable periods of time. However, changes to mine plans may occur - for example, in response to changed commodity prices - and may lead to significant increase in rates of vertical advancement. This, in turn, affects dewatering schedules, as greater the vertical advancement requires increased dewatering effort. When an accelerated mine plan is applied to the model and dewatering rates are increased to 18ML/day, draw-downs in the creeks are more noticeable. The model has therefore helped to indicate the extent to which increased dewatering rates may be acceptable, especially in terms of immediate and longer-term damage to riparian species i.e. Melaleuca and Eucalypt. Predictions from the model may also allow mine planners to schedule decreased rates of vertical advancement, where this is required to meet environmental constraints.