Controls on 87Sr/86Sr ratios of groundwater in silicate-dominated aquifers: SE Murray Basin, Australia

87Sr/86Sr ratios of groundwater in the southeast Riverine Province of the Murray Basin, Australia are between 0.7107 and 0.7191. The 87Sr/86Sr ratios vary between different subcatchments and generally decline with distance northwards from the basin margins. There are few carbonates in this region, and Sr is primarily derived from silicate minerals. The spatial variation in 87Sr/86Sr ratios reflects the distribution of K-rich minerals, such as biotite and K-feldspar in the aquifers. However, major ion chemistry implies that silicate weathering is only a minor process. Sr isotope ratios are most probably controlled by exchange on clays derived from weathering of the silicate minerals. Ion exchange is promoted by the low groundwater flow rates in the Riverine Province and the clay-rich nature of many of the aquifers.