3. THE EFFECTS OF MINING ON THE MARINE ENVIRONMENT

3.1 Introduction

A primary distinction between marine mining and other elements of marine commerce is that mining involves a deliberate contact with the seafloor with a consequent redistribution of material, some of which is retained and the remainder of which is returned to the marine environment. The perturbation in the marine environment that results from mining activity will have a direct effect on three elements of the environment (figure 59). There will be a direct effect on the seafloor and on that portion, of the benthic biota which is directly contacted by the collector and there will be a direct effect on the physical/chemical characteristics of the water column resulting from the redistribution of materials (sediment, water, nodules, and biota). It should be noted that these direct effects are present regardless of the specific mining system used, but that they differ in detail (magnitude, manner of redistribution, etc.) depending on the engineering characteristics of the individual system.

Mining will have indirect effects on both the benthic and pelagic elements of the biota depending upon the manner and degree to which the physical/chemical properties of the water column and seafloor have been altered. Again, the specifics of the indirect effects will be a function of the engineering characteristics of the individual mining systems.

The direct effects of mining on the physical/chemical marine environment will be discussed in this section. Subsequent discussion in the next chapter will consider the impact on the marine biota; this includes both the direct

Figure 59. Schematic diagram showing pathways of direct and indirect

effects.

effect on the benthic biota and indirect effects resulting from modifications of the physical/chemical environment.

3.1.1 Hydraulic Systems

The direct effects of hydraulic systems are the result of the design of the mining systems (figure 60). The collector, moving along the seafloor has a direct effect on benthic biota with which it comes in contact. These, along with nodules, water, and sediment, are collected by the system. The two basic discharges (benthic and surface) are designed to provide maximum return of usable nodules, and these discharges are the source of the direct effects on the water column.

At this time, there is sufficient information to permit evaluation of the hydraulic mining systems in a generic sense.

Direct Effect on Seafloor. Obtaining a unit production of 5000 mt of dry nodules daily will require that the collector come in contact with 1.1 km2 each day, will be contacted by the collector runners or tracks but will remain unmined. An additional 0.8 km2 will remain untouched owing to the inability of the mining system to sweep the seafloor in perfectly overlapping swaths.

Figure 60.

Schematic diagram showing input and output of a hydraulic mining

system.

The total then, 1.9 km2, is the minimum area required to be mined on a daily basis to yield the unit production. The actual area required will be larger--perhaps 25% larger--due to the probable topographic limitations and low nodule coverage which will render certain areas unminable. Specifically, each mining system might work an entire year in a subarea of perhaps 750 km2 within an overall mine site.

The geometry of the area mined over a short period of time (e.g., 1 day) will vary and will depend upon many factors. On one hand, the towed system could mine at 5.5 km/h in straight line segments, roughly following lines of equal water depth, creating a swath 132 km long and 16 m wide. On the other hand, the self-propelled system could mine small areas, such as circles, and cover an equivalent area.

Direct Effect on Water Column. Benthic discharge - The hydraulic systems are designed to separate most of the bottom sediment from the nodules after they enter the collector in order to minimize the volume of sediment ascending to the surface with the nodules. Nearly all, perhaps 90% to 99%, of the sediment will be rejected in the benthic discharge within about 20 m of the seafloor. This benthic discharge will have a direct effect on the physical/chemical properties of the lower water column.

Surface discharge For the hydraulic systems, the nodules will be accompanied up the pipeline by some sediment and abraded nodule material along with bottom water sufficient to maintain 10% to 30% solids by volume in the pipeline. The nodules will be recovered on the mining vessel. The bottom water, entrained sediment, and abraded nodule material will be discharged at the sea surface and will have a direct effect on the physical/chemical properties of the upper water column.

3.1.2 Mechanical Systems (Continuous Line Bucket)

Specific information on the engineering characteristics for CLB systems is forthcoming from the CLB Syndicate but is not available at the time of writing this report. In general, the characteristics of a generic CLB system are shown in figure 61. The bucket, moving across the seafloor, has a direct effect on the benthic biota with which it comes in contact. Benthic organisms, along with sediment, water, and nodules, are collected by the bucket. In contrast with the hydraulic systems, there are no specific discharges, but rather, the undesired materials that enter the bucket are subject to washing enroute to the surface.

Direct Effect on Seafloor. The CLB system will have a direct effect on the seafloor along a narrow swath parallel to the ship's track. It will accomplish this by means of buckets mining in relatively short parallel paths at approximately right angles to the ship's track. All other characteristics, such as areal dimensions, are unknown at this time.

Direct Effect on Water Column. Although the CLB system's buckets will commence their ascent containing sediment and nodules, the fine material is expected to wash out through the relatively large open mesh of the buckets

Figure 61. Schematic diagram showing inputs and outputs of materials collected by mechanical (continuous line bucket) mining system.

designed to retain only the nodules. This action will cause a rain of fines, primarily in the lower water column, but probably continuing at a diminished rate all the way to the surface. This washing action will result in a direct effect on the physical/chemical properties of the water column.

3.2 Direct Effects on Water Column

3.2.1 Surface Discharge

Nature of Surface Discharge. The nature of the surface discharge will differ between hydraulic and mechanical mining systems. Reference to figures 60 and 61 will show these general differences. Both types of systems will discharge bottom sediment, nodule debris, and remnants of the benthic biota, but the amounts will vary. In addition, water from the bottom will be discharged from the hydraulic systems, but not from the CLB systems.

Adequate information is available (Appendix 2) on the general engineering characteristics of hydraulic systems to permit an order-of-magnitude estimate of the nature of the surface discharge. These estimates are presented in the following paragraphs, but are preliminary, and more realistic values must await actual monitoring of prototype tests.

At the time of writing, comparable information is not available for the mechanical systems, and no estimates on the nature of surface discharge are attempted here.

Resuspended bottom sediment - The amount of solids discharged at the surface each day will average about 1000 mt of which the bulk will be bottom sediment. This value is a preliminary consensus and is subject to verification during prototype monitoring although additional information from the mining industry indicates that 1000 mt/day is probably on the high side. However, as a basis for estimating the magnitude of the mining system's perturbation of the marine enviornment (Appendix 3), it will be used in this report as a representative figure.

The settling rates of bottom sediments is not adequately known at this time. Size analyses of bottom sediment from the area have been run and these indicate a mean fundamental particle size of 4-5 um diameter. On the other hand, the mining process is not expected to reduce more than 10% to 20% of the bottom sediment to fundamental size during the trip from seafloor to surface. At present, it appears that standard laboratory size analyses lead to a serious skewing of the estimated settling rates of discharged sediment toward the slower settling rates. Three independent studies are being conducted to provide a more realistic estimate of the distribution of settling rates in the discharged sediment. However, these results are not available at this time and verification will not be available until test monitoring can be conducted. In the meantime, the present report is based on the assumption that size analyses available provide a limit, albeit skewed toward the fine size, on which preliminary estimates of settling rates can be based.

The ability to estimate the nature of the sediment component of the surface discharge (and all other components as well) is limited by the precision of present knowledge of the engineering characteristics of the mining systems.

Abraded and nonabraded nodules - While most of the solids discharged at the surface are likely to be bottom sediments as described above, about 50 mt/day of nodules, nodule fragments, and abraded nodule material also will be discharged.

This

Redistributed bottom and interstitial water - About 20,000 m3 of water will be required to lift the 5000 mt daily production unit of nodules. water will comprise both interstitial water and bottom water from near the seafloor. At present there is no accurate way to determine the relative proportions of bottom and interstitial water and no attempt will be made to treat water as other than the total water required. Composition of the interstitial water from the upper several centimeters of the seabed is generally quite comparable to the overlying seawater (Table 27). In many cases, the upper few centimeters of sediment has very high water content and there appears to be little discontinuity in properties across the water-seafloor interface. Since this interstitial water (and associated sediment) will be input to the collector, rather than intersitial water originating much deeper below the seafloor, properties of the water involved in surface discharge are estimated on the basis of existing information on the bottom water.