Impact of soil consolidation and solution composition on the hydraulic properties of coastal acid sulfate soils Thi Minh Hue Le A , An Ninh Pham A,B , Richard N. Collins B , and T. David Waite A,C A School of Civil and Environmental Engineering, the University of New South Wales, Sydney, NSW 2052, Australia. B Centre for Water and Waste Technology, School of Civil and Environmental Engineering, The University of New South Wales, Sydney, NSW 2052, Australia. C Corresponding author. Email: d.waite@unsw.edu.au Abstract. Acid sulfate soils (ASS) are distributed worldwide on coastal floodplains, presenting a great challenge to coastal development and urbanisation. Upon oxidation, these soils become stratified with visibly distinguishable soil strata that are progressively less oxidised with depth. In this study, the geotechnical properties, quantified by hydraulic conductivity and consolidation coefficient, of an ASS profile from the Tweed River floodplain, north-eastern New South Wales, Australia, were investigated at a laboratory scale and compared with results obtained from the field. Measurements were conducted with a Rowe cell (or hydraulic consolidometer) by controlled compressive and pore water pressures. The results indicated that hydraulic conductivity and consolidation coefficient values gradually decreased with increasing consolidation pressure or decreasing void ratio, but were significantly higher for the more oxidised ASS horizons. These results suggest that controlled soil consolidation along ASS drainage banks may prove to be effective at reducing acid discharge. Passing low pH (pH 3) or high cation concentration (50 mM CaCl 2 ) solutions through intact consolidated potential ASS samples did not induce changes in the hydraulic conductivity or consolidation coefficient of this material indicating that ASS soil ripening involves more than acidification reactions, and the practice of flushing drains with high ionic strength estuarine tidal waters is unlikely to induce soil subsidence as a result of ASS structural change and clay flocculation. Additional keywords: Darcy’s law, acid sulphate soils, void ratio, hydraulic conductivity, consolidation coefficient, Rowe cell. Introduction Coastal acid sulfate soils (ASS) in Australia, and worldwide, were generally deposited as unconsolidated sediments during the Holocene and contain (or contained) sulfidic minerals commonly in the form of pyrite (FeS 2 ) (White et al. 1993, 2003; Glamore 2003). Extreme weather conditions or anthropogenic disturbances such as prolonged drought, drainage, dredging, or excavation enable oxygen to oxidise sulfides in these sediments (Glamore 2003; Smith et al. 2003; White et al. 2003). As a result, many of these soils typically have an upper oxidised and sulfuric soil horizon (actual ASS) and, at depth, a saturated or ‘potential’ ASS sulfidic horizon below the long-term average water table level. In north-eastern New South Wales, Australia, the unconsolidated, sulfidic, blue- grey clay potential ASS can extend to 10 m in depth depending on the original depositional environment (White et al. 1993; Glamore 2003; Smith et al. 2003). The oxidation and drying process (known as ‘soil ripening’) alters the physicochemical properties of the gel-like potential ASS, resulting in an oxidised sulfuric ASS (the so-called ‘acidic horizon’) with a uniform texture consisting of a fine, tortuous, heterogeneous pore network (Blunden and Indraratna 2000). The thickness of this horizon varies greatly depending largely upon watertable fluctuations. For agricultural land, this layer is often overlain by 0.10–0.30 m of organic topsoil. A transition zone of variable thickness (usually 0.20–0.80 m) lies between the actual ASS and potential ASS horizons. This zone is typically located at or just below the minimal height of the long-term average watertable level and contains the oxidation front (White et al. 1993; Smith et al. 2003). The potential ASS layer at the bottom of the profile, which is characterised by high proportion of swelling montmorillonite (60%) of total clay materials followed by kaolinite (30%) and illite (10%), has a gelatinous appearance, high volume water ratio, and high plasticity (van Oploo 2000; White et al. 2003). Due to the serious environmental and economic impacts of ASS, the mechanisms of acid production and its transport in ASS have been the focus of many research studies both in Australia and in other countries experiencing ‘broadacre’ acid problems. Two characteristics of the soil profile that are important in both the ease of passage of water through the soil and the subsequent transport of protons and acid-carrying species such as iron and aluminium cations are the hydraulic conductivity (k) and the consolidation coefficient (C) of the soil. These parameters govern the flow of water through a soil mass and subsequent change in soil volume (Holtz and Kovac 1981; Ó CSIRO 2008 10.1071/SR07119 0004-9573/08/020112 CSIRO PUBLISHING www.publish.csiro.au/journals/ajsr Australian Journal of Soil Research, 2008, 46, 112–121