Determining water quality requirements of coal seam gas produced water for sustainable irrigation

Coal seam gas production in Australia generates large volumes of produced water that is generally high in total dissolved solids and has a high sodium absorption ratio (SAR) which may affect soil structure, hydraulic conductivity, and crop production if used untreated for irrigation. By coupling major ion soil chemistry and unsaturated flow and plant water uptake, this study incorporates effects of salt concentrations on soil hydraulic properties and on root water uptake for soils irrigated with produced water featuring different water qualities. Simulations provided detailed results regarding chemical indicators of soil and plant health, i.e. SAR, EC and sodium concentrations. Results from a base scenario indicated that the use of untreated produced water for irrigation would cause SAR and EC values to significantly exceed the soil quality guide values in Australia and New Zealand (ANZECC). The simulations provided further useful insights in the type of coupled processes that might occur, and what the potential impacts could be on soil hydrology and crop growth. Calculations showed that the use of untreated produced water resulted in a decrease in soil hydraulic conductivity due to clay swelling causing water stagnation, additional plant-water stress and a reduction in plant transpiration. In case the produced water was mixed with surface water in a 1:3 ratio prior to irrigation, the calculated soil SAR values were much lower and generally acceptable for sandy to sandy-loam soil. The use of reverse osmosis treated produced water yielded an acceptable salinity profile not exceeding guide values for SAR and EC; the plant water stress was limited as there was no additional salinity stress associated with the low level of salts. Results further illustrated that accounting for coupled geochemical, hydrological and plant water uptake processes resulted in more accurate water balance calculations compared to an approach where such interactions were not implemented. Coupling unsaturated flow modelling with major ion chemistry solute transport using HYDRUS provides quantitative evidence to determine suitable water quality requirements for sustainable irrigation using coal seam gas produced water.