Jasonsmith, JuliaMacDonald, BennettWhite, Ian2020-04-210167-6369http://hdl.handle.net/1885/203342Wybong Creek discharges salts into the agriculturally and industrially important Hunter River in New SouthWales, Australia. Abrupt increases in salinity occur periodically in the mid-Wybong Creek catchment. In order to understand the processes which cause these abrupt increases, changes in surface and groundwater were investigated. It is shown that salinity increases can be attributed to highly discrete groundwater discharge directly into the river from below. Hourly electrical conductivity data measured in the river showed regular, diurnal electrical conductivity fluctuations of up to 350 mu S cm(-1). These fluctuations could not be attributed to barometric pressure, temperature, or evapotranspiration. Instead, a similar periodicity in surface water electrical conductivity and groundwater height in nearby groundwater wells was found. Fluctuations were of similar periodicity to the orthotides which cause fluctuations in surface water height and are induced by Earth tides. The geology in the mid-catchment area indicates conditions are optimal for Earth tides to impact groundwater. The reporting of orthotidal changes in water chemistry in this article is believed to be the first of its kind in the scientific literature, with the large fluctuations noted having important implications for watermonitoring and management in the catchment. Further research investigating Earth-tide-induced phases of groundwater heights will better constrain the relationships between surface water chemistry and groundwater height.Project funding and support was provided by the Hunter Central Rivers Catchment Management Authority and the New South Wales Office of Water, in addition to ARC Linkage grants (number LP05060743 and LP100100567). Scholarship funding to JFJ was provided by the Australian National University Faculty of Science and the Research School of Earth Science.application/pdfen-AU© Springer International Publishing Switzerland 2017201710.1007/s10661-017-5880-z2019-11-25