Links Between Land Cover and the Chemical Characteristics of Groundwater in a Tidal Crystalline Coastal Environment
Please login to view abstract download link
Continental groundwater represents a significant local source of nutrients and pollutants into coastal waters, affecting ecosystem functioning. Two rivers, 7 wells and 8 subterranean estuaries (STEs) located in the Ría de Vigo (NW Spain) were sampled in July 2023 and March 2024. Concentrations of dissolved oxygen, inorganic nutrients, and dissolved organic matter (DOM), optical indices of DOM and trace metals, together with the diversity and abundance of prokaryotes were measured. Land cover characterization around the sampling sites was undertaken using the SIOSE Land Use Information System. The 15 most representative uncorrelated land cover classes were selected, and a principal component analysis was performed to identify coherent clusters of samples. Two principal components (PC) explaining 27.7 % and 18.0 % of the variance were obtained. The first PC (PC1) was related to a gradient from highly anthropized to moderately artificialized areas dominated by natural cover classes. Land cover classes associated with agriculture activities positively correlated with the second principal component (PC2). The chemical composition and DOM optical indices of groundwater drawn from wells and rivers were closely associated with the land cover characteristics of the sampling area. Nitrate concentration, the ratios FDOMm/a325, FDOMc/a340 and a254/a365 and Cr, Cu, Zn, As, Cd and Pb concentrations, positively correlated with artificialized land cover whereas a254/DOC was related to forest-dominated areas. None of the chemical variables correlated with the PC2. By contrast, the relationship between the chemical variables and land cover in STEs was less clear; only silicate was positively correlated with PC1 and dissolved oxygen, Cu and As with PC2. These results highlight the uniqueness of STEs as reactive interfaces mediating the transfer of groundwater-borne solutes to the coast, relatively independent on the uses of the surrounding landscape. STEs, therefore, control the magnitude of land-derived solute transport via continental groundwater discharge to the coast.