THE GEOCHEMISTRY OF RIVERS IN IRELAND: A SNAPSHOT

We have analyzed the geochemical composition of 23 of the 26 largest rivers by flow and a number of low order, monolithologic streams in Ireland, sampled during the wet winter followed by a prolonged dry period in 2018. This was done to compare and contrast their compositions, to estimate the geochemical fluxes and elemental yields from the catchments, to investigate differences resulting from landcover and bedrock geology, and to compare individual river compositions in both wet and dry seasons. For this presentation, the focus will be on 9 of these rivers where previously published particulate matter fluxes exist. Rain-corrected riverine compositions were dominated by calcium and bicarbonate in all the rivers. Rivers originating in the Midlands of the country have, in general, higher magnesium and sulfate values but whether this is due to natural or anthropogenic input is unknown. Clearly the weathering of carbonate minerals is a major process in all the catchments, with silicate weathering yields being as much as three orders of magnitude lower than calcium weathering yields. When compared to the monolithologic, low order stream chemistry, the larger rivers represent a mix of inputs of different land covers and lithologies, even though most are dominated by carbonate dissolution. The role of loughs located along the lengths of the rivers appears also to have some control on the overall geochemistry of the large river systems, especially the River Shannon. We speculate that longer residence times of waters in the loughs helps to homogenize the river geochemistry. We have compared our rain-corrected total chemical denudation fluxes to the particulate matter flux data and demonstrate that chemical denudation is the primary mode of mass loss. This work presents an overall view of the geochemistry of rivers throughout most of Ireland and demonstrates the importance of both geochemical characteristics of the bedrock and the landcover on the riverine geochemistry.