The hydrogeochemistry of an unconfined coastal aquifer - Merimbula, New South Wales, Australia : implications for the migration of treated effluent

Abstract

The subterranean estuary underlying the Merimbula-Pambula peninsula in southeast New South Wales sustains fragile coastal ecosystems. Recharge to the shallow unconfined system occurs via infiltration of meteoric waters through sand-dominated sediments within a coastal forest/sand dune habitat hosting both permanent and seasonal wetlands. Solute proportions in rainwater differ from groundwater indicating processes other than evapotranspiration play an important role in the chemical evolution of groundwater recharge. High-resolution soil water solute profiles completed through the unsaturated zone illustrate the importance of both abiotic and biotic processes. Aerosol dissolution and carbonate weathering are the dominant abiotic processes and account for higher proportions of Ca, Mg and SO42- in groundwater recharge. Nutrient cycling and soil biogeochemical cycling account for the lower proportions of K, NO3-, and in some locations Ca and SO42-. Solute additions from aerosol dissolution and solute retention by biochemical cycling have little net-effect on overall salinity, but exhibits a large effect on the relative solute proportions in recharge waters. The small net effect on salinity indicates that biotic solute retention and abiotic solute addition have similar relative influences on the chemical evolution of groundwater. Within the subterranean estuary, groundwater geochemical conditions vary considerably over a very small area. Approximately 500 metres inland from the coast, geochemical conditions are characterised by low dissolved oxygen (< 0.5 mg/l), elevated CO2 partial pressure, low pH (< 5) and an abundance of electron donors (principally Fe2+). The aquifer in this area is characterised by organic rich sediments and is seasonally saturated forming temporary wetlands. These anoxic and acidic conditions result in a lower abundance of SO42- (reduced to HS-) and HCO3- (converted to H2CO3 and CO2) and increased silicate weathering as evidenced from elevated Al3+ concentrations. Down gradient from this area (toward the coast) groundwater geochemical conditions become progressively more oxic and are characterised by higher pH values (> 7). As the acidic and reduced water flow toward the coast through low organic sands, the pH is buffered by carbonate mineral weathering and incorporation of dissolved oxygen result in a large pH-redox front ~250 metres from the coast. The hydrogeology and hydrochemistry of this subterranean estuary has been highly modified by anthropogenic recharge of treated effluent waters and over-extraction for domestic use. Recharge of treated effluent increases the water table, increases evapotranspiration and leads to both vegetation die-back and discharge of nutrients rich waters (NO3- > 10 mg/l and PO43- > 2 mg/l) into the Southern Pacific Ocean. Geochemical modelling indicates that installation of exfiltration ponds within the anoxic portion of the system would lead to substantially greater nutrient attenuation. In the distal portion of the peninsula, over-extraction of groundwater has resulted in seawater intrusion. Time-series chemical data collected over 12 hour pumping periods indicate the rate of seawater intrusion has increased over the period of this study (3yrs). Findings from this research have revealed the vulnerability of coastal groundwater systems to anthropogenic modification and highlight how greater planning, research and regulation could prevent/minimize these impacts in the future. Show more