Reactive solute transfer through two contrasting subterranean estuary exit sites of the Ria de Vigo, NW Iberian Peninsula

Abstract

Subterranean estuaries are highly reactive interfaces locus of the interaction between seawater and continental groundwater, actively mediating the transfer of solutes to surface waters through Submarine Groundwater Discharge (SGD). Using radioisotope tracers, Ibánhez et al. (2019) recently identified significant SGD loads into Baiona Bay (Ría de Vigo, NW Iberian Peninsula), locus of the two subterranean estuaries studied here (Panxón and Ladeira beaches). Porewater samples were collected from both subterranean estuaries with push-pull piezometers installed at different depths along the permanently saturated zone during February, May, July and October 2019. Porewater 222Rn, 226Ra and salinity were used as tracers of porewater flow within the sediment and to quantify resulting SGD. The different coastal aquifer influence, wave exposure and configuration of these subterranean estuaries seem to drive highly contrasting biogeochemical dynamics at different spatial and time scales. On one hand, Ladeira beach showed generally reduced porewaters throughout the studied period. Low nitrate (<1 μM) and high phosphate (>50 μM) concentration porewaters were observed there, with high ammonium peaks (>100 μM) at specific depths during May and October. Porewater ammonium appeared to be generally linked with phosphate distribution, except in July. Observations suggest that organic matter mineralization within the sediment could be a main source of nutrients to this subterranean estuary. On the other hand, oxygen levels in Panxón beach reached near saturation levels in depth at high 222Rn-low salinity porewaters. There, high nitrate levels were strongly linked to brackish waters, suggesting its continental origin. Large modulation of porewater nutrient levels was observed at both subterranean estuaries, significantly altering its composition prior to discharge to Baiona Bay