Using Chlorine-36 to understand the sources of solutes in rivers: A new use for an old tracer

The radioisotope ³⁶Cl, which has a half life of 301 ka, is traditionally used to estimate groundwater residence times of deep old groundwater in large basins. However, systematic variations in R³⁶Cl values in young shallow catchment waters permit its use in determining the sources of solutes in rivers. Elevated R³⁶Cl values in precipitation were recorded during the 1950s to 1970s due to the atmospheric nuclear tests. Some of this bomb-pulse ³⁶Cl is likely to still be present in shallow catchment waters. Additionally, as R³⁶Cl values of precipitation generally increase with distance from the ocean, groundwater older that ~7 ka that was recharged during periods of low sea levels in the Holocene is likely to have higher R³⁶Cl values than modern rainfall. Most of the water from within catchments that sustains streamflow (e.g., the shallower parts of the groundwater system, interflow, bank storage waters, riparian groundwater) is less than a few thousand years old. There is negligible decay of ³⁶Cl over those timescales, and thus R³⁶C values will reflect the initial R³⁶C values of those waters.

River water from the intermittent Avoca catchment in southeast Australia has R³⁶Cl values of 32 to 67 that are generally higher than those of modern rainfall (R³⁶Cl = 25 to 35) but similar to shallow (<50 m deep) near-river groundwater (R³⁶Cl = 51-61). These data indicate that much of the solute load is derived from the input of older waters (mean residence times of up to a few thousand years) stored within the catchment rather than evapotranspiration of recent rainfall. River water from the headwaters of the nearby perennial Barwon catchment has higher R³⁶Cl values (38-46) than local rainfall (14-20) and most of the shallow groundwater (21 to 31). These high R³⁶Cl values reflect the input of bomb-pulse ³⁶Cl from shallow catchment waters. Downstream, R³⁶Cl values of the river water decrease to 20 to 31, reflecting the inputs of solutes from groundwater that again has mean residence times of up to a few thousand years.

³⁶Cl has allowed the origins of solutes in these rivers to be better understood. In both cases, the volume of older groundwater contributing to these rivers is moderate to minor. However, due to much higher salinities, these minor groundwater inflows influence solute geochemistry. ³⁶Cl was particularly useful in distinguishing between evapotranspiration of recent rainfall and input from waters stored within the catchment as a source of stream river. In turn, this helps understand catchment functioning and solute fluxes within the catchments. Additionally, the palaeoclimate signal of initial R³⁶Cl values adds to the understanding of groundwater residence times and recharge processes in catchments.