Practical Mitigation of the Environmental Effect of Offshore Mining
Authors: Ellis D.V. , Garnett R.T. 
Published: October 2008

There arc two main environmental mitigation targets, One is to deposit tailings within as small an area as possible surrounding a mining block by appropriately engineered outfalls, The second target is to facilitate biodiversity recovery (expected to occur substantially within 1-5 years depending on deposit grade) by contiguous placement of the extraction device (dredge. drills. etc.). i.e. not leaving gaps subject to later mining and impact. A protocol for an environmental monitoring programme to provide information that the environmental design criteria arc being met is developed based on the principles of Test and Reference. Before and After. monitoring stations, A set of Screening Criteria for appraising the environmental risks at proposed mining sites is provided.

Introduction

There arc several sets of information from which workable procedure to mitigate the environmental effect offshore mining can be derived, They come primarily from (I) West- Gold's placer mining at Nome. Alaska, 1986-1990. under strict US EPA environmental controls. and (2) a series of coastal and island mines which have discharged their tailings to the sea under Canadian environmental controls since 1971. or under similar controls elsewhere.

The information from these two sources is not put into the context of current marine mining of diamondiferous placers. The conclusions drawn shout the potential for environmental mitigation arc targeted at existing offshore operations, Precisely positioned large drill systems and underwater ‘crawlers’. together with more conventional airlifts. excavate diamondiferous gravel from water depths down to 200 meters. The slurried sediments arc raised by compressed air from the seabed and treated at sea to recover the contained rough diamonds. Then the sediments are returned as a slum. directly to the sea at its surface.

There is a simple long-known mitigation target for the tailings disposal component of mining (Figure I and Ref. 1). It is to discharge the tailings below the surface euphotic(photosynthetic) zone as a coherent slurry (turbidity current) to a defined deposition area on the seabed with minimal resuspension.

Data Sets

For the WestGold operation. the dredging procedures. environmental controls. assessments. monitoring and mitigation achieved are documented in a series of annual reports with a summary review and references in Ref. 2, There is documentation on seabed ecosystem recovery time from dredging. on outfall modelling and engineering to achieve the depositiontarget. on permit terms. and on monitoring scope and results (Table 1) for feedback to mine operators that environmental targets were being met. At WestGold a crab fishery was rnaintained during the mining on and around the dredge sites.

There is one coastal mine site that has used a Submarine Tailings Disposal (STD) system for 25 years. This is IslandCopper Mine. Canada. from which there is a 25-year series of annual environmental reports from prior to operations (1970) to the controlled Closure now underway (Ref. 3). As at the Nome dredging site. topics covered seabed ecosystem recovery time (from tailings placement and stabilization). outfall engineering to achieve the deposition target. permit terms and monitoring scope and results (Table 1).