Finding an environmentally sound, yet cost effective, mode for disposal of sulphide-containing mine waste has been a challenge facing both the mining industry and government for many decades. Given the critical role played by oxygen in the process of acid generation, thoughts towards abatement of this problem have focused around elimination of oxygen as a reactant. Consequently, arguments for subaqueous disposal arose naturally from the premise that acid generation from sulphides could be suppressed when submerged underwater where oxygen concentrations are greatly diminished relative to the atmosphere. In other words, lowering the concentration of one of the principal reaction ingredients (oxygen) would lower the oxidation reaction rate, hence the rate of generation of acid and dissolved metals. This premise was based on the well understood chemical characteristics of natural water bodies and sediments.

While founded on sound theoretical principals, the efficacy of subaqueous disposal prior to the 1980's was largely unproven and supported by only a few, limited scientific studies. In order to address the paucity of relevant data, a suite of projects created through the BCARD Task Force and MEND were designed, and involved fieldwork in a series of lakes where mine tailings had been deposited (Anderson and Mandy Lakes, Manitoba; Buttle and Benson Lakes, British Columbia). The program utilized a variety of state-of-the-art sampling, analytical and interpretive techniques designed to measure directly the reactivity and short and long-term chemical stability of subaqueous mine tailings deposits. Further, the questions to be answered by the project temporarily avoided the many and complex biological components, and focused on the geochemical environment. It was determined that once subaqueous tailings reactivity had been adequately assessed, and the geochemical processes delineated, the biological issues could be approached in a better-defined context and on a project-specific basis.

The results of the work supported the hypothesis: sulphide-rich mine tailings, when stored in the subaqueous environment, were largely chemically unreactive. In the few instances where release of dissolved metals were observed, natural secondary chemical processes within the sediments inhibited their release to the water column.

The following overview is intended to provide a summary of the MEND Project reports and a general description of the geochemical systems which contribute to the effectiveness of subaqueous disposal.

The instability or reactivity of metal sulphides arise from their mode of formation. Sulphides are formed in reducing environments (in the absence of oxygen). Consequently, they are unstable and susceptible to chemical reaction in the oxygen-rich environment of the earth's surface. Accordingly, the most stable environment in which to store sulphide-rich mine tailings is one devoid of oxygen - one that mimics their environment of genesis.

Subaqueous systems are an effective first approximation of a stable environment for sulphides not because they are devoid of oxygen (indeed, subaqueous environments most often have measurable concentrations of dissolved oxygen), but rather because they contain low oxygen levels even in their most saturated state. The maximum concentration of dissolved oxygen found in natural waters is approximately 25,000 times lower than that found in the atmosphere. Because the rate of sulphide oxidation is in part dependent on the concentration of oxygen, it is readily apparent that the generation of acid and dissolved metals will be dramatically minimized underwater. Further, once the small inventory of dissolved oxygen in the water is consumed, it is typically replaced very slowly by processes of molecular diffusion and small-scale turbulence; the transfer of oxygen in water is nearly 10,000 times slower than similar transfers in air. Consequently, storage under permanent water cover is perhaps the single most effective measure that may be taken to inhibit acid generation from sulphidic mine tailings.

Sediments recreate an environment stable to sulphide minerals even more effectively than a water cover, in part because of the low concentrations of dissolved oxygen but also because of a natural tendency for sediments to become chemically reducing. To understand why the sedimentary environment is an appropriate site for the storage of sulphidic mine tailings, it is first necessary to outline some of the natural chemical processes found in that environment.

Natural sediments typically contain a spectrum of components ranging from eroded rocks and soils of local origin to unique substances formed within the deposits. However, of all the components found in natural sediments, the remains of plants and animals (organic matter) is perhaps the most important as they are considered to be the fuel for almost all chemical reactions that occur after deposition. This is because organic matter (like sulphides) is unstable in the presence of oxygen; it has a natural tendency to decompose into its constituent elements (mostly simple molecules containing the elements carbon, nitrogen, phosphorus, sulphur and hydrogen). In other words, organic matter consumes or reacts with the oxidant oxygen to form carbon dioxide and a suite of simple, biological by-products. This reaction is accelerated by a host of bacterial species which catalyse the reaction to derive energy for their own needs. Because the concentration of oxygen in natural waters is initially low, it is often rapidly depleted within the surface layers of sediments. When oxygen is no longer available to react with the organic matter, secondary oxidants are utilized in its place by the bacterial community. They are in order of preference: nitrate, Mn-oxide, Fe-oxide, sulphate and carbon dioxide; once one secondary oxidant is consumed (i.e., nitrate) the next most favoured is consumed (i.e., Mn-oxide) until all are exhausted. Of particular importance is the consumption of sulphate, since the by-product of the reaction between sulphate and organic matter (in the absence of more favourable oxidants) is hydrogen sulphide, a natural analogue to metal sulphide minerals. Thus, the natural tendency in sediments is toward the creation of an environment in which sulphides form naturally, and sulphide-rich mine tailings are at their most stable in just such settings.

There are two principal ways in which to assess whether or not sulphidic mine tailings are reacting or releasing acid and metals to the subaqueous environment. The first is direct microscopic or petrographic observation of the submerged tailings particles. Thus far, in all cases where subaqueous sulphide tailings have been studied, no signs of oxidation have been observed. However, a far more sensitive, effective and elegant approach is to look for direct effects of sulphide oxidation such as a drop in pH, an increase in sulphate or the most direct indicator of all, an increase in dissolved metals. Since dissolved metals are the parameters of environmental concern and because they exist at very low concentrations naturally, measuring their distribution within sediment porewaters (the water surrounding the deposited sediment or tailings particles) yields a very sensitive indication of tailings reactivity as well as potential environmental impact.

The distribution of dissolved metals in porewaters has been determined by two proven approaches. Within the MEND projects, sampling of porewaters was accomplished utilizing the techniques of sediment coring and dialysis array (peeper). Sampling porewaters by core involves the collection of sediment with a specialized, light-weight, gravity corer. The porewaters are separated from the sediment solids by placing sequential slices of sediment into a centrifuge; the resulting fluid fraction is filtered and analysed for dissolved metals. Peepers sample porewaters much more passively. Peepers consist of an array of depressions or wells in a plexiglas plate. The wells are filled with ultra-pure water and covered with a filtration membrane. The peeper is inserted vertically into the sediments and allowed to equilibrate within the sediments for 10 to 14 days. During that period, dissolved metals move across the membrane into the sample wells while the solids are excluded. After 10 to 14 days, the water within the sample wells is chemically indistinguishable from that of the porewaters; the sample waters are removed from the wells and analysed for dissolved metals.

In order to avoid oxidizing the samples by allowing them to contact the atmosphere, all sample handling of both cores and peepers after collection is carried out in nitrogen-filled, plastic glove bags. Once the porewaters have been filtered (again, under nitrogen), they are "preserved" for subsequent analysis by the addition of a small amount of ultra-pure acid.

Release or consumption of dissolved metals results in the formation of adjacent zones of differing concentrations. The difference in dissolved metal concentration between a high and a low define a concentration gradient and results in net migration of dissolved metals from the zone of high concentration to the zone of low concentration. In sediments, this process occurs through the random motion associated with all dissolved molecules and is termed molecular diffusion. The amount of dissolved metals that migrates down a concentration gradient (from high to low concentration) is termed the flux and is proportional to the steepness of the gradient. In other words, a greater flux (i.e., a greater transport of dissolved metals) occurs where a very high concentration is immediately adjacent to a very low concentration.

If a concentration gradient extends across the sediment-water interface, metals can be said to be diffusing out of or into the sediments (to or from lake water) depending on the direction of the gradient. Lower concentrations of dissolved metals in porewater relative to the overlying lake water indicates a flux of metals into the sediments from lake water (Figure 1(b)). Conversely, higher concentrations in porewaters than lake water infers a flux in the opposite direction (Figure 1(a)).

In the majority of the MEND project work undertaken thus far, metals have been observed to diffuse into the sediments from the overlying lake water. This has occurred in part because some of the lakes contained elevated concentrations of dissolved metals, but more importantly because of the natural tendency for sediments to create the environment stable to sulphides as discussed above. When sulphate is utilised as a oxidant in the decomposition of organic matter within the sediments, a natural by-product is hydrogen sulphide. Hydrogen sulphide is highly reactive with most dissolved metals (such as Cd, Cu, Hg, As, Mo, Ni, Fe, Pb, Zn and others) resulting in rapid precipitation of those metals as insoluble, solid metal sulphides. Because sulphate reduction (sulphide formation) typically occurs at shallow depths within sediments, there is a commensurate zone of localized metal consumption with the establishment of a dissolved metal concentration gradient from lake water into the sediments. The result is a flux or transport of dissolved metals into the surface sediments from the overlying lake water with the tailings acting as a sink for dissolved metals rather than a source. The concentration profile characteristic of such a case is shown in Figure 1(b).

In some instances, dissolved metals have been observed to be released from sediment solids to the porewaters. At first glance, this might suggest that the tailings are releasing dissolved metals to the overlying lake water, particularly if the concentration gradient extends to the sediment-water interface. However, there are several complicating factors that must be considered when such profiles are observed. First, several metals (such as Cd, Cu, and Zn) are released to near-surface porewaters naturally as they are often associated with organic matter - they are not tailings-derived. As the organic material decomposes or oxidizes, those associated metals are released in dissolved form and may indeed migrate back into the overlying lake water. This most commonly occurs in sediments where oxygen has not been sufficiently depleted (or more specifically, where sulphide precipitation is absent). Such release is a natural phenomenon and accounts for much of the natural cycling of certain trace metals in many natural environments. The second factor is that even though there may be some release of metals from tailings to porewaters in certain cases, a process referred to as oxide blocking or oxide scavenging can intercept much of the upward flux of those metals before the dissolved species cross the sediment-water interface into the lake water.

Such scavenging involves oxides of iron and manganese, two of the secondary oxidants discussed above. Where dissolved oxygen is present, Fe and Mn oxides exist as solids whose surfaces strongly adsorb many trace metals. When they are utilized in subsurface sediments as secondary oxidants in the absence of oxygen, they revert to dissolved Fe and Mn creating concentration gradients. As dissolved Fe and Mn diffuse upward toward the sediment-water interface, they eventually encounter dissolved oxygen and revert back to their original solid, oxide form. Iron and manganese oxides are both efficient in adsorbing a broad range of dissolved metal ions. Thus, their continuous formation in the near-surface sediments results in the establishment of an effective "blocking mechanism" that inhibits dissolved metals from entering the water column.

One final barrier to all metal release from tailings within lake sediments is time. The burial of tailings by natural sediments or more recently deposited tailings occurs progressively with time and has a profound effect on the ability of even the most reactive substances to affect lake water quality. As the dominant transport mechanism of dissolved metals in sediments is diffusion, and because mass transport by diffusion is effective only over short distances (i.e., a few centimetres), accumulation of a relatively thin layer of sediments over an abandoned tailings deposit is sufficient to isolate tailings chemically from the water column. In this regard, subaqueous tailings disposal from a tailings reactivity stand-point is at worst a relatively short-term issue even though this "worst" condition has not as yet been observed. Nonetheless, once deposition has ceased and tailings have been buried by a few centimetres of natural sediment, they can for all intents and purposes be considered to be chemically secure for the foreseeable future.

This MEND project work to date has involved the study of tailings in natural lakes; little attention has been paid to the comparatively abiotic system of the man-made tailings ponds. This and future research will be directed toward such systems; however, several generalizations can be drawn from the MEND data which apply equally to both tailings ponds and natural lakes.

Firstly, the diminished concentration of oxygen dissolved in water is the single-most effective inhibitor to tailings oxidation; low concentrations of oxygen translate into low oxidation reaction rates. The presence of a permanent water cover not only minimizes the maximum concentration of oxygen to which the tailings may be exposed, but it also inhibits the rate at which that oxygen may be resupplied.

Secondly, even though tailings ponds are typically deplete in organic carbon, they still present conditions suitable to long-term storage of sulphide-rich material. Sulphide-bearing tailings themselves act as an analogue for natural sediments in that they progressively lower the concentration of oxidants, thus enhancing the potential for long-term stability.

Finally, time itself is an effective component in allowing the establishment of a physical barrier which prevents the release of metals to the overlying lake waters. The accumulation of a veneer of natural sediments (a few centimetres thick) effectively isolates the tailings. Subaqueous disposal is at worst a relative short-term risk that decreases with time to yield a stable, passive but effectively final control system.