Monitoring environmental effects of a deep-sea mining test in shallow water

Concerns about future access to strategic raw materials for the high-tech industry have led to a renewed interest in mining of mineral resources from the deep-sea as a potential alternative for land-based mining. Polymetallic nodules, especially abundant in the eastern equatorial Pacific Ocean in water depths of 4000-6000 m, are a likely target of future deep-sea mining. However, many questions exist about the environmental sustainability of deep-sea mining, as it would involve the removal of hard substrate, disturbance of the surface sediment layer and dispersion of mobilised sediment over large areas of seabed adjacent to the mining sites. Anticipating on full-scale industrial mining tests, which are likely to start in the near future in the deep Pacific Ocean, we tried approaches for environmental monitoring of mining activities during two industry field tests in relatively shallow water offshore southern Spain, carried out in the framework of the European Blue Nodules project. The aim of these field tests was to assess technical and environmental performance of a scaled polymetallic nodule mining vehicle developed by the Dutch shipbuilder and maritime technology provider Royal IHC. Although the tests were performed in only 300 m water depth, much less than the depth where future deep-sea mining will take place, the weakly stratified bottom water, tide-dominated near-bed currents with mean magnitude around 5-10 cm s^-1, and gently sloping seabed covered with fine muddy sediment are fairly comparable to operational conditions in the deep-sea. The plume of suspended sediment mobilised by the mining vehicle, considered to represent a major environmental pressure which may extend far beyond the actual mining area, was monitored with turbidity sensors deployed with ship-operated ROV and CTD, as well as in a static array of moored sensors. It was found that the generated sediment plume extended not more than 2 m above the seabed close to the disturbance (< 100 m), but increased in height with distance away from the disturbance site. Turbidity decreased rapidly with increasing distance from the source, but a distinct signal could still be distinguished above background turbidity at 350 m away from the source. In this near-coast setting, plume monitoring suffered significant interference by bottom trawling activities in neighbouring areas. The monitoring setup proved to be well designed and the findings on the plume size and dispersion can be significantly extrapolated to account for a more realistic mining situation. Seabed surveys with ROV-based video and scanning sonar showed that the tracks of the test vehicle, exerting an average pressure of 3 kPa on the seabed, left impressions of 4±0.8 cm deep in the surface sediment. In sediment cores collected from the path of the vehicle, geotechnical testing showed an increase in undrained shear strength and bearing capacity, as compared to undisturbed sites, indicating compaction of the surface sediment. Surveys revealed ubiquitous signs of bottom trawling, including furrows of approximately 10 cm deep produced by trawl doors.