Inferring groundwater dynamics in a coastal aquifer near wastewater infiltration ponds and shallow wetlands (Kwinana, Western Australia) using combined hydrochemical, isotopic and statistical approaches

A proposal to increase wastewater infiltration at the Kwinana Wastewater Treatment Plant (KWTP) to an unconfined aquifer near a groundwater flow-through wetland (The Spectacles) in Western Australia, prompted a study of the groundwater geochemistry and water levels to identify the water types and to develop a conceptual model for groundwater flow and geochemical evolution. Within the catchment, the water table has dramatically declined (e.g up to -70 to - 90 mm/yr over three decades) due to the drying climate and increasing demand for groundwater. Annual rainfall has declined by 9 mm/decade on average or about 1.4% of the annual mean over the last three decades Wastewater infiltration began in 1975 and has increased to the current rate of 4.7 thousand cubic meters per day. Groundwater-surface water interactions were interpreted using multivariate statistical analysis of major ion and stable isotope (delta O-18, delta D) data. Rainwater infiltrating through the aquifer interacts with carbonate minerals while flowing westward to produce a Ca-HCO3 type of water (Group 1). Below the wetland, groundwater has an evaporative signature (Group 2) that has higher concentrations of TDS (> 1 g/L) and distinctively higher delta O-18 (> 2.5 per mil) than groundwater derived from wastewater infiltration (Group 3). The ratio of ions (K:Cl) was used to understand the mixing of water types and to quantify the proportions of wastewater at the site. Water samples collected in 2014 reveal a high proportion ( > 34%) of wastewater within the uppermost 30 m of aquifer and generally decreasing proportions with distance away from the source. Further investigation of The Spectacles water balance and hydrochemistry is needed to optimally manage and prevent degradation of the water quality. Proposed increases in wastewater recharge may continue to help offset the impact of the drying climate on lake levels.