Irrigated horticulture, worth $62m annually, abstracts 14 GL/y from the superficial coastal limestone aquifer in the Myalup area, north of Bunbury WA. Salinity has been increasing in the aquifer. Previously it was thought to have resulted from; evaporation and recycling of irrigation return water, the application of fertiliser and/ or saltwater intrusion, however the relative role of these processes, or other potential processes, has not been assessed.
Licensee water quality data from 140 irrigated sites, collected since 2009, plus groundwater samples from an additional 61 targeted sites, with more comprehensive chemistry data were analysed to identify the sources of salinity and temporal trends.
Results showed aquifer salinity ranged from 400 to 5800 mg/L (median 1000 mg/L) similar to the historic median value. The chemical composition of groundwater salinity was also spatially variable. Around 75% of licensee sites had chloride:sulfate ratios <2; a marker for pyrite oxidation. There were no relationships between sulfate and fertiliser markers, nitrate and potassium, leading to the conclusion that pyrite oxidation and not fertiliser was the source of high sulfate in the aquifer.
There was an increasing trend in salinity (TDS) in 29 out of 140 sites while 6 had a decreasing trend in salinity. Of those increasing, at over half (n=18) pyrite oxidation was the dominate process. Recirculation or upconing was the dominant process at a third (n=9) of the sites and saline groundwater intrusion at 2 sites (7%).
The increase in salinity was predominantly driven by pyrite oxidation due to declining watertables attributed to climate change with decreasing rainfall. The pyrite oxidation was predominately from wetlands to the east of the irrigation areas. The pyrite oxidation has gone unrecognised due to neutralisation of acidity by limestone and is potentially the first instance of increasing groundwater salinity caused by pyrite oxidation in the Australian context.