Beneficial Reuse: A Field Trial to Remediate and a Bench-Scale Test to Revegetate Coal Seam Gas Dam Sediments from Queensland

Abstract

Coal seam gas (CSG) has become an increasingly common method of extracting methane from coal in Australia, with more than 20,000 wells expected to enter production by 2020. However, large quantities of "produced water" also come to the surface with gas, and these several thousand litres of water per day per well have to be managed sustainably. Furthermore, up to five percent of produced water is composed of suspended or dissolved solids, most typically present in the form of salty brines and a range of other elements, sometimes including benzene and other hydrocarbons like phenols. CSG solids therefore have a high pH and total alkalinity, and hence have elevated electrical conductivity. As a consequence, the settled solids from CSG extraction have no proven beneficial reuse value, and successful revegetation of dams and untreated sediments is limited to salt-tolerant grass species but is often impossible using any species under any condition. The purpose of this study is to investigate the remediation of CSG dam sediments from Queensland for the purposes of determining their potential beneficial reuse as "clean, usable soil" in revegetation projects. Experiment #1, a field trial conducted in the Bowen Basin, examined the impact of various additives to two different types of CSG dam sediments. Experiment #1 found that both types of sediment could be remediated, examples of which include decreases in pH from 10.0 to 7.4, chloride from 19,900mg/kg to 1,770mg/kg, cation-exchange capacity (CEC) from 23meq/100g to 4.0meq/100g, and sodium adsorption ratio (SAR) from 931meq/100g to 44meq/100g, and increases in total phosphorus from 27mg/kg to 855mg/kg and total nitrogen from 950mg/kg to 3,520mg/kg. These findings confirm that contaminated CSG sediments have beneficial reuse potential in dam decommissioning and revegetation projects. Experiment #2, a bench-scale test utilizing samples of treated sediments from Experiment #1, examined the revegetation potential of these remediated CSG sediments. Experiment #2 showed that both types of CSG dam sediment could be effectively revegetated using non-salt-tolerant grass species, while untreated sediments were not suitable for revegetation. However, the design and scale of this work need to be expanded, and variables such as sediment pH, CEC and SAR should be monitored and controlled more carefully before fully reliable conclusions can be made about the revegetation potential of treated CSG dam sediments.