The impact of controlled tidal exchange on drainage water quality in acid sulphate soil backswamps

Periodic opening of one-way tidal floodgates was undertaken on two coastal flood mitigation drains to promote exchange with estuarine water and improve drain water quality. The drains were located in areas with acid sulphate soils and their drainage water frequently had high acidity and low dissolved oxygen (DO). Tidal exchange via floodgate opening generally raised drain water pH levels through dilution and/or neutralisation of acidity. Increases in DO and moderation of extreme diurnal DO fluctuations were also observed. The magnitude and stability of the improved physico-chemical conditions was highly dependant on the volume and quality of tidal ingress water. Relatively rapid reversion (hours to days) in drain water pH and DO was observed once floodgates were closed again. The rate of reversion following floodgate closure was strongly related to outflow volumes, antecedent drain water quality conditions and groundwater levels. Floodgate opening caused changes in longitudinal drain water gradients and has potential to slow net drainage rates during non-flood periods. However, complex site specific interactions between drain water and adjacent groundwater can also occur. At one location, a 4-day floodgate opening event caused recharge of adjacent acid groundwater during the opening phase, raising the potentiometric groundwater level above local low tide minima. This was followed by tidally modulated draw down of acid groundwater and enhanced acid export in the period immediately following floodgate closure. There are also practical considerations, which limit the efficacy of floodgate opening as an acid management strategy. The low elevation (close to mean sea level) of some acid sulphate soil backswamps, combined with seasonal migration of the estuarine salt wedge, means there is considerable potential for saline overtopping of what is currently agricultural land. This constrains the magnitude and duration of controlled tidal exchange. Also, it is during wet periods that acid drainage outflow to the estuary is greatest. At such times the salinity and acid buffering capacity of estuarine water is often low, thus reducing the capacity of tidal exchange waters to neutralise acidity.