Integrated assessment of continental groundwater discharge in coastal crystalline basins
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Crystalline coastal basins are predominantly composed of hard, low-porosity rocks such as granites and gneisses. As such, they are often considered of low groundwater storage capacity and, therefore, irrelevant in terms of continental groundwater discharge to the coast. In these systems, groundwater storage capacity and movement largely depend on the extent of basement rock fracturing, weathering, and regolith thickness, creating complex and heterogeneous flow paths as well as diffusive discharges difficult to observe at the shore. An integrated study of continental groundwater discharge was conducted in a coastal inlet surrounded by a crystalline drainage basin (Ría de Vigo, NW Iberian Peninsula) during July 2023 (dry period) and March 2024 (wet period). For this, a multitracer (222Rn, 226Ra, stable isotopes of water, thermal imagery) and multianalyte study of embayment’s surface waters was complemented with a synoptic survey of local subterranean estuaries (8), wells (7) and rivers (2). A large enrichment in 222Rn (up to 1250 Bq m-3) was observed in the inner flanks of the embayment (San Simón and Baiona bays), attributable to continental groundwater discharge. These are tidally modulated, peaking during low tide, but also by the complex circulation patterns within the embayment. The water isotopic signature confirmed the presence of continental groundwater within the embayment and in the sampled subterranean estuaries, where discharge is observable through thermal imagery. Continental groundwater discharge is a significant portion of total freshwater input into the system; however, its composition drastically changes as it passes through local subterranean estuaries. These, predominantly anoxic in contraposition to the local oxic groundwaters, modulate the transfer of land-derived solutes such as nutrients, organic matter, metals or rare earth elements, which affects the local microbial community structure and composition. Results emphasize the need for interdisciplinary approaches to quantify and monitor these hidden yet impactful groundwater flows in crystalline coasts.