Evaluation of Effluent Mixing Zone Size With Permit Performance Standards for an Offshore Mining Vessel

Abstract

ABSTRACT A new National Pollutant Discharge Elimination System (NPDES) draft permit was proposed to establish operational guidelines for the BIMA mining vessel operating offshore Nome, Alaska. The draft permit imposed limitations on BIMA operations which would render the project economically infeasible. The DIFCD numerical model was used to predict the turbidity rise downcurrent from the BIMA using an extensive base of environmental data collected during the previous two years for input to the model. The model results showed that the production rate of the vessel had a very minor effect on the extent of the discharge plume when compared to the effects of the silt content of dredged material, current speed and position in the proven ore reserve. Operational limitations in the draft NPDES permit were modified following a detailed numerical analysis of the BIMA tailings disposal system. The product of the numerical analysis was a series of curves, or a nomograph, which define an operational window within which compliance with water quality standards can be met with confidence. INTRODUCTION A National Pollutant Discharge Elimination System (NPDES) draft permit was issued by the Environmental Protection Agency (EPA) to Western Gold Exploration and Mining Company, Limited Partnership (WestGold) for operation of the BIMA mining vessel offshore Nome, Alaska. Compliance with water quality standards required that turbidity levels at the edge of a State of Alaska defined mixing zone of 500 m be less than 25 NTU above the ambient background turbidity. In an effort to ensure compliance with water quality standards, the NPDES draft permit proposed to limit SIMA operation to water depths greater than 8 m at lower low tide, and production rates lower than 500 m3/hr. WestGold determined that these limitations would result in a project that was not economically feasible. Previous evaluations of the BIMA tailings disposal system have been conducted by Engineering Hydraulics, Inc. (EHI, 1988, 1989). The 1988 studies combined field data and numerical model analyses to compare the effectiveness of different discharge configurations to determine the configuration producing the lowest turbidity levels and residual concentration of trace metals in the discharge plume. From these studies, It was found that all subsurface discharge configurations resulted in' rower turbidity levels than a surface discharge, but of all the subsurface configurations, no one resulted in a substantially lower turbidity level than any of the others. The 1989 study combined field data and a numerical model analysis to determine the long-term turbidity levels and trace metal dilution and to define the required mixing zone for turbidity levels to be less than 25 NTU above background on a worst-case-scenario basis when using a 1.5 m deep discharge pipe configuration. Calculations were made for production rates of 400, 650, and 900 m3/hr and depths of 5, 10, and 17 m and presented as a family of curves of allowable Initial mixing zone vs water depth for these production rates.