Laboratory Evaluation of Soil Amendments To Limit Structural Degradation under a Sequential Irrigation with Coal Seam Gas and Rain Water

Land amendment irrigation (LAI) has become an increasingly useful practice in addressing sodic and alkaline coal seam methane gas (CSG) water for irrigation purposes. However, there is a paucity of information pertaining to rapid dilution of soil solution under LAI management. This study investigated the ability of land amendment to buffer soils against structure degradation under sequential irrigation with CSG water (electrical conductivity [EC], 3 dS cm(-1); sodium adsorption ratio [SAR], 100 mmol(c) L-1; pH 8.4) and rainwater in a highly smectitic black Vertisol (BV) and kaolinitic Oxisol (OX) soil. Stoichiometric quantities of gypsum and sulfur amendments were applied for the LAI process to reduce sodicity and alkalinity to a target of SAR <= 20 and HCO3- alkalinity <= 100 mg L-1. Soils were irrigated with 10 ML ha(-1) CSG water and 9 ML ha(-1) rainwater in the sequence of 1 ML ha(-1) and 0.9 ML ha(-1) CSG water and rainwater events, respectively, using a 7.5 mm h(-1) rainfall intensity. Saturated hydraulic conductivity was determined and reported as a reduction from initial conductivity. The collected leachates were used for subsequent measurements of pH, EC, SAR, and alkalinity at each event. Results suggested that LAI was inadequate to protect soil structure from more than three of the intensity-frequency-duration events but sufficient for up to three events. This was considered positive evidence for LAI given the annual recurrence of such an event was one for the region. Additional gypsum application had no significant effect on protecting soil from further hydraulic reduction compared with LAI alone. The OX and BV behaved differently in terms of the observed hydraulic reduction, which was determined to be both a function of iron oxides (OX) and the effect of dilution, in contrast to the threshold electrolyte concentration. This highlighted that both the initial hydraulic conductivity and threshold electrolyte conditions are important in implementing LAI.