Using Lime-Assisted Tidal Exchange to Remediate 700 ha of Extremely Acidified Acid Sulfate Soil in Tropical Australia.
Col R. Ahern, Doug Smith, Angus E. McElnea, Simon van Heel, and Michelle A. Martens. Queensland Department of Natural Resources and Mines, Block C, 80 Meiers Rd, Indooroopilly, Queensland, Australia
In the early 1970s, a 700 ha area of estuarine wetlands on the eastern side of Trinity Inlet opposite Cairns in north Queensland Australia was drained and developed for sugarcane farming. This was achieved by constructing a bundwall incorporating tidal floodgates to prevent tidal entry, and installation of massive pumps to assist drainage. In doing so, the agricultural developer and consultants were unaware that the East Trinity site contained substantial quantities of framboidal pyrite (up to 6.5% oxidizable sulfur). The development lowered the natural watertable, exposing the acid sulfate soils (ASS) to oxygen. This created a highly acidified landscape, releasing acid and toxic levels of iron, aluminium and other metals from the soil—firstly into the extensive on-site drainage network and then off-site into Trinity Inlet. Substantial fish kills were documented and the community expressed concern over potential impacts on the adjacent Great Barrier Reef. In 2000, following the unsuccessful sugarcane development, and various failed urban and marina development applications, the Queensland Government purchased the property. Rehabilitation of the East Trinity land by conventional lime treatment of soil would have been both costly and impractical. Using limited data from CSIRO, it was estimated that lime-treatment of the soil would cost $62M (not including the cost of labor, capital equipment, earthworks and extensive vegetation clearing)—an unacceptable cost. In discussions with the government, the Department of Natural Resources and Mines' (NR&M) Queensland Acid Sulfate Soils Investigation Team (QASSIT) was called upon to come up with an innovative, ‘low cost', environmentally conscious remediation program. The main strategy developed involved controlled tidal exchange assisted by addition of hydrated lime Ca(OH)2 across the site. Detailed soil survey and investigation of the geomorphic stratigraphy from more than 100 boreholes across the site revealed that in the upper metre of the profile, drainage and oxidation had led to most of the potential acidity (from pyrite) being converted to existing acidity (up to 690 mol H+.t-1). Jarosite and other relatively insoluble, acidic, hydroxy sulfate minerals were prevalent in these oxidised upper horizons. In the underlying reduced horizons, potential acidity from pyrite commonly equated to 1-2% oxidizable sulfur, but was as high as 6.5 %S in some areas of the site. Areas of the site associated with the original samphire flat vegetation communities were found to contain compensating neutralizing material in the form of calcitic Foraminifera tests in sufficient quantities to neutralize the potential acidity and hence be self-neutralizing. Remediation of the site involves progressively and cautiously replacing the existing acidified freshwater environment with a managed tidal wetland system—ironically, by modifying and managing the tidal gate infrastructure that contributed to the problem some 30 years ago. Mechanical application of a Ca(OH)2 slurry to buffer incoming tidal water (and if necessary outgoing tidal water), is an essential control mechanism to prevent off-site acid and metal discharge. The strategy was expected to: (a) neutralize acidified drain and creek waters and near surface soils; (b) hydraulically displace much of the acidic cations and salts deeper into the profile; (c) create reducing conditions by keeping soils saturated via daily tidal exchange to achieve some reversal of the acidification process; and (d) limit further oxidation of the massive store of pyrite and its potential to generate more acid and release metals. The extensive water monitoring network installed has shown that the hydrated lime water treatment has kept off-site discharges to a pH ³6, despite very acidic water (pH as low as 3) continuing to drain from upper parts of the site. Soil sampling and analysis show increases in soil pH of 1-2 units in upper profiles of the regularly inundated soils. Decrease in soil titratable actual acidity (TAA) and redox measurements also point to a change from oxidizing, acid producing conditions to more reducing conditions throughout the profile. The preliminary conclusions from this and other extensive datasets is that the acidity is being brought under control and the site should reach an equilibrium over the next few years where active lime treatment will no longer be necessary. An essential component of the future management strategy is the design and installation of 'smart' tidal gates to ensure a consistent daily tidal exchange height inside the site despite the highly variable tide heights outside the site that varies considerably within a month and throughout the year.The authors acknowledge scientists from other organizations for their assistance and input into this project.