Evidence for connected water processes through smectite-dominated clays at Breeza, New South Wales

The identification of deep drainage beneath crops and pastures is a major challenge for determining the water balance beneath irrigated crops. This is particularly the case in smectite-dominated clay subject to swelling and cracking. This detailed study at an irrigation site applied hydrogeological and hydrochemical techniques to identify drainage associated with irrigation water sourced from a deep underlying aquifer. The hydrogeology of the site was characterised by permeability (K v) testing of cores, installation of nested piezometers (16, 34 and 54 m depth) and automated logging of groundwater levels, barometric pressure and fluid electrical conductivity (EC). In response to irrigation and rainfall of 390 mm at the surface, a 2.3 m rise in water pressure was observed in the aquifer at 16 m depth during the irrigation season, gradually subsiding thereafter. The barometric efficiency of the P16 piezometer was an estimated as 70 ± 25%, typical of confinement by a low permeability matrix (K v 1.3 × 10−10 to 5.3 × 10−9 m/s). Yet, evidence of fracture flow to a shallow pit and freshening of porewater in the aquifers at 16 and 34 m depth during the irrigation season (ΔEC 1.73 to 1.43 mS/cm and 2.0 to 1.58 mS/cm, respectively), indicated that rapid leakage had occurred. PHREEQC geochemical models indicated that a significant proportion of porewater in the shallow aquifer was replaced by recharge water during the irrigation season, with dissolution of NaCl, precipitation of carbonates, ion exchange and possibly evaporation all shown to be occurring. Clearly, these clays cannot be considered to isolate the aquifer from overlying irrigation and associated contamination.