Sulfidic waste rock and tailings encountered during mining and mineral processing operations (and some road construction or quarry operations), sulfidic sediment encountered during dredging and land reclamation work, and acid sulfate soils encountered in many coastal lowlands have much in common. All have environmental problems that arise when iron sulfides are exposed to air and moisture and oxidize to produce acidic metal contaminated discharges. However, there are often large differences in scale, rates of acid generation (reflecting differences in sulfide mineralogy composition and texture), and in the presence and reactivity of minerals with an acid neutralizing capacity. Furthermore, different land uses and values, ecological sensitivities and public accessibility of affected areas, and the financial resources available to organizations responsible for affected sites, mean that assessment, monitoring and management strategies adopted to address acid rock drainage (ARD) problems can differ widely from those applied to acid sulfate soils (ASS). This paper discusses similarities and differences between geochemical processes associated with ARD and ASS and between the assessment, monitoring and management strategies adopted for affected sites.

Although ARD generation associated with mining, mineral processing and construction usually involves higher sulfide mineral abundances and potentially hazardous trace metal loads than acid generation from ASS areas, it normally involves sulfidic material within a well defined area of no more than a few square kilometers. Hence, ARD management at mine sites and similar areas can often employ large-scale engineering operations (e.g. encapsulation or treatment of potential source rocks, or deposition of sulfidic material under water in abandoned mine pits) as the primary management strategy. In contrast, it is seldom technically or economically possible to apply such approaches to large ASS areas unless they involve high value land used for residential or commercial building construction. Furthermore, because it is seldom possible to completely prevent the release of contaminated leachates from affected sites, it is often also necessary to use a water treatment plant or permeable reactive barrier (PRB) to protect local catchments. However, such water treatment systems are only possible where the impacted area is small and well defined and where the contaminated water can be collected for treatment. These conditions are seldom met in ASS areas, which are typically very large (often hundreds of square kilometers) and are gradationally rather than sharply bounded both laterally and stratigraphically. Hence, except where very high value land is involved and sulfidic material can be excavated and disposed of, major engineering solutions are seldom economically viable even where they are technically feasible.

The most widely used remediation strategy for both ARD and ASS affected areas remains the application of liming agents although there are differences in the type of liming agent used, how it is applied and the length of time over which it remains effective; some of these differences will be discussed. More importantly, differences between sites affected by ARD vs. ASS mean that ASS remediation has focused on smaller scale engineering work designed to minimize the exposure of sulfidic material above the water table, the use of regular natural flushing by water that has some alkalinity, and improving site assessment and analytical procedures to define the nature, magnitude and distribution of ASS materials more precisely. These improved analytical and site assessment methods could usefully be adopted by the mining sector to focus remediation efforts more effectively and to minimize over- or under-treatment. In ARD management there has recently been growing success with the use of PRBs to treat flowing contaminated water and there is excellent potential for adapting such systems for preserving water quality in drains and creeks in ASS affected areas. Other benefits from increased information transfer between scientists and managers working in ARD and ASS affected areas will also be discussed.