Understanding sub-surface solute distributions and salinization mechanisms in a tropical coastal floodplain groundwater system

SummaryThe Burdekin coastal floodplain aquifer is a tropical groundwater system exploited for irrigated agriculture that has exhibited increases in groundwater salinity since regular monitoring was introduced in the 1960s. Evapotranspiration of irrigation water, displacement of unsaturated zone solutes, enhanced mixing with relict seawater and seawater intrusion are mechanisms considered to explain the observed increase. The floodplain aquifer is comprised of complex successions of Quaternary terrigenous and marine sediments hosting groundwater with Cl− concentrations from <50 mg/l to >50,000 mg/l. The lowest salinity groundwater is found adjacent to the Burdekin River and within palaeochannels dissecting the floodplain. Cl− concentrations <100 mg/l, together with NO3-/Cl mass ratios >0.1 indicate the fresh groundwater originates from vertical infiltration of rainwater and irrigation water entrained with agricultural nitrogen through the coarse-grained palaeochannel sediments. Na/Cl ratios similar to the Burdekin River indicate that lateral discharge from the river channel to the floodplain aquifer also accounts for the distribution of low-salinity groundwater. Tidal and near-shore marine sediments deposited during Pleistocene and Holocene high sea level stands host highly saline groundwater (Cl− 20,000–80,000 mg/l) up to 15 km inland. In many areas of the floodplain, nested piezometer locations reveal older, more saline water at depth. Na and Cl− concentrations of more than 8000 floodplain groundwater samples collected over a 40 yr period fall on a mixing line between river water/fresh groundwater and modern/relict seawater indicating that intermediate salinity waters have evolved over time through varying degrees of mixing between the two end-member compositions. Temporal trends in Na and Cl− concentrations indicate that increases in groundwater salinity across the floodplain are primarily due to this mixing process, and that evapotranspiration from irrigated crops and displacement of unsaturated zone solutes has had less of an effect. This salinization process, which includes displacement of relict seawater within the aquifer and seawater intrusion, is likely exacerbated by extraction from >1400 production bores and enhanced recharge across the floodplain. Halting and reversing the ongoing groundwater salinization will require a mixture of setting and meeting local and regional groundwater targets (water table depths and groundwater quality), and where needed implementing appropriate deep drainage and drainage disposal management strategies.