Tidal-forcing groundwater dynamics in a restored coastal wetland: implications of saline intrusion

Abstract

Tidal restoration projects are currently being undertaken throughout coastal Australia without a full understanding of the influence of tidal forcing on groundwater salinity. To determine the impact of restoring tidal flows on groundwater salinity levels, field investigations were undertaken at a study site near Berry, New South Wales. Fluctuations in groundwater and surface water chemistry (soluble chloride, pH, electrical conductivity) and hydrodynamics were measured over a 12 week period using multilevel piezometers and submersible data loggers spaced at discrete distances from a flood mitigation drain. Additional parameters, including saturated hydraulic conductivity (K_sat), were undertaken to determine baseline conditions and to provide initialisation data for a three-dimensional finite-element model. The finite-element model was calibrated within ±5% of field data and developed to simulate saline intrusion during a ‘worse-case’ scenario. Results from the model simulations indicate that saline intrusion at the site is limited even under extreme conditions. To quantify the influence of K_sat(h) levels on saline intrusion, material properties in the numerical model were altered to represent 1, 10 and 20 m/d. Model simulations showed that K_sat(h) values >10 m/d permitted saline intrusion in excess of ANZECC criteria. Based on the model findings a series of management criteria are proposed that detail acid sulfate soil remediation techniques, including tidal restoration, dependent on K_sat(h) values. The proposed management criteria suggest that tidal restoration projects are most suited in sites where the K_sat(h) values are <10 m/d.

Key Words:

• acid sulfate soils
• coastal wetlands
• hydraulic conductivity
• numerical modelling
• saline intrusion

Acknowledgements

We wish to acknowledge the University of Wollongong, Shoalhaven City Council, the New South Wales Department of Agriculture and local landholders for their contributions to this study. Further acknowledgement is necessary to Kylie Ford for her diligent field and laboratory assistance.