Spatial variability of hydrochemistry and environmental controls in karst aquifers of the southern Iberian Peninsula: Implications for climate change impact assessment

Abstract

Carbonate aquifers are crucial water and carbon reservoirs globally, particularly in semi-arid climates. However, these systems are susceptible to the impacts of climate change, given their sensitivity to specific environmental factors. This study presents the hydrochemical (water temperature, pH, electrical conductivity, and major ions) and isotopic (δ13C) composition of 39 karst springs in the southern Iberian Peninsula, along with the parameterization of environmental factors (temperature, precipitation, recharge altitude, and vegetation cover quantified by the Normalized Differential Vegetation Index, NDVI) in their recharge areas. The spatial analysis revealed that the climatic and environmental factors follow a longitudinal pattern producing a notable west-east environmental gradient in the study area. Through a statistical analysis based on Principal Component Analysis (PCA), it was found that environmental factors control the spatial variability of groundwater hydrochemistry in these karstic aquifers. The δ13CDIC values in groundwater, ranging from −1.84 to −12.46 ‰, show a prevalence of C4 plants mainly in the more arid study sectors and indicate an origin of dissolved inorganic carbon (DIC) mainly in biological processes in the recharge area. In addition, a relationship between NDVI values, equilibrium partial pressure of CO2 (pCO2), and groundwater bicarbonate content was observed. Springs further west of the study area exhibited higher bicarbonate content (about 400 ppm), which was associated with higher pCO2 levels (about 10,000 and 15,000 ppm) and higher NDVI values (between 0.5 and 0.7). In contrast, aquifers located further to the east had lower bicarbonate levels (<200 ppm), with an average pCO2 of about 2000 ppm and the lowest NDVI values (<0.3). Furthermore, the spatial variability and the relationship between environmental factors and groundwater hydrochemistry allow for the assessment of potential climate change impacts in carbonate systems with a comparison between aquifers in wetter regions and those in semi-arid conditions.