Groundwater discharge is a crucial component in supplying various metals and nutrients into the ocean. Here, we carry out the first systematic multi-isotopic analysis of groundwater in Cardiff, UK. Multiple isotopes including stable silicon, magnesium and strontium isotopes and radiogenic strontium isotopes were measured in groundwater, river and estuarine water samples, to investigate (1) processes controlling the groundwater geochemistry, and (2) the role of groundwater in supplying metals and nutrients, especially silicon to the ocean.

The ionic and isotopic data reveal that groundwater geochemistry is influenced by multiple factors. Potential processes that control the groundwater geochemistry include, firstly, the influence of ocean tides. The significant fresh water-seawater mixing trend revealed by the elemental compositions suggests the influence of seawater intrusion. Secondly, the occurrence of carbonate precipitation in groundwaters is indicated by reduced calcium concentrations, as well as the elevated Mg and Sr isotopes. Thirdly, the increased Si concentration and Si isotopes indicates an extra Si source with heavy Si isotopes entering the groundwater, which indicates possible wastewater contamination. Lastly, historic chemistry data show that, after the construction of Cardiff Bay barrage, groundwater chemistry in areas where dewatering actions were conducted have been changing over time.

87Sr/86Sr in estuary water is above the freshwater -seawater mixing line, suggesting a third source of Sr in the estuary, which is likely to be the groundwater. Our results reveal that the intricate circumstances could result in complicated groundwater chemistry, and highlight the significant role of groundwater in supplying metals and nutrients into the ocean.