Discharge and surface plume measurements during manganese nodule mining tests in the north equatorial pacific

Abstract

Rates, concentrations, and composition of mining discharge and the size and structure of the ensuing surface plumes were examined during North Pacific tests of scaled manganese nodule mining systems. Discharge was composed principally of bottom water and pelagic silts and clays, although nodule fragments with diameters less than 1 mm were also discharged at widely varying rates. Average flow rates of the discharge varied from 95 to 160 litres/s, with the solid fraction varying from 550 to approximately 2000g/s. The plume, as determined by particulate concentrations in excess of ambient oceanic conditions, extended approximately 5 km from the mining ship and had a width of about 1 km. Fe and Mn signatures allowed detection of the plume nearly 35 km from the source. The plume provided evidence of settling more rapidly than expected of silt and clay-size particles: a mean settling velocity of 6 × 10 −2 cm/s for the particulates in the plume and a mixed layer vertical turbulent eddy diffusivity of 1 × 10 −2m 2/s have been inferred from the data. Field and laboratory data together suggest that the rapid settling was due to flocculation of the discharge particulates.