Tracing 36Cl in river water of Eastern Australia

Chlorine-36, generally expressed as a ratio against stable chlorine (³⁶Cl/Cl x10¹⁵), has a half-life of 301 kyr, thus it is a useful tracer for estimating residence time of old groundwater between 50 kyr - 1 Myr. An underutilised use of ³⁶Cl/Cl is as a sensitive tracer of catchment scale processes such as: identifying sources of salinity, weathering, delineating groundwater-surface water interactions, quantifying irrigation infiltration, and identifying anthropogenic inputs.

We highlight the utility of ³⁶Cl/Cl for gleaning insights into regional natural and anthropogenic processes in the Nogoa, Namoi and Murrumbidgee river catchments of eastern Australia. All catchments are within important agricultural regions and are regulated by one or more large reservoirs in their headwaters. The Nogoa River (Lat 23°S) flows east and forms part of the Fitzroy River that meets the Coral Sea near the town of Rockhampton. The Namoi (Lat 30°S) and Murrumbidgee (Lat 35°S) rivers form part of the Murray-Darling Basin and flow westwards (inland) before joining the Darling and Murray Rivers respectively to flow south towards the Southern Ocean. River water was sampled bi-monthly in several stations from the upper to middle reaches of each river during two years between 2017-2020. Sampling took place during drought conditions; 2019 being the driest in ~120 years of instrumental records in many areas. Climatic conditions favoured sampling of baseflow with flows mostly relying on reservoir releases in some cases (Namoi River) until total reservoir and river dryness.

At the ground surface ³⁶Cl can be produced via two main pathways. Typically, the dominant source of ³⁶Cl in surface water is atmospheric, which was produced in the troposphere and stratosphere via interaction of cosmic-ray protons and secondary neutrons with Ar. However, secondary cosmic-ray neutrons can produce ³⁶Cl when they collide with rocks and minerals. These reactions are modulated by the composition of the geological materials and their elevation. The Nogoa and Namoi Rivers have similar basic geological materials in their headwaters at relatively lower altitudes, while the Murrumbidgee has abundant mafic and felsic igneous rocks in the headwaters at higher altitudes.

In general, the Namoi River showed the higher ³⁶Cl/Cl ratios (~650-300) followed by the Murrumbidgee River (~500-200) and the lowest readings were recorded in the Nogoa River (~200-100). These results do not follow simple latitudinal or elevation trends. In this presentation we discuss plausible geological and anthropogenic processes that may account for the observations.