Using Soil Material Identification to Rehabilitate Coalbed Methane Water Storage Ponds.

The expansion of a coalbed methane (CBM) industry in Australia has resulted in a large number of remote and isolated exploration and appraisal wells now requiring decommissioning. In association with these wells are earthen wastewater storage ponds that are also required by law to be rehabilitated back to the pre-existing landscape condition. BeneTerra has rehabilitated 25 sites comprising soils that were predominantly Vertisols, exhibiting strong shrink/swell properties and saline-sodic subsoils. Some Alfisols were also encountered, presenting numerous physical and chemical constraints in their natural condition.

All the ponds were originally constructed by pushing up four walls of soil from a central point. This construction technique tends to invert the soil profile as the walls are built, often leaving hostile subsoil as the surface layer and preventing pasture establishment on the pond bank. We found the soil materials within the walls of the ponds could be assessed and identified in order to optimise the schedule whereby various parts of the walls would be layered back into the pond cavity. By determining a baseline for the natural soil condition of the surrounding landscape using a set of soil pits excavated to approximately three meters, with this depth being deeper than the depth of the pond, a sequence of desirability of soil materials was developed. Analogous horizons, and mixtures of horizons, could then be identified within the pond walls. In order to make comparisons with the baseline soils, these were analysed for electrical conductivity and pH in a 1:5 soil to water extract (EC_1:5 and pH_1:5), and described for morphological features such as colour, texture and segregations. All the pond sites now have healthy pastures developing that match or improve upon the surrounding landscape.