Abstract
The Port del Comte Massif (SE, Pyrenees) contains one of the most important vulnerable and strategic karst aquifers for supplying freshwater to the city of Barcelona (Spain). It is a fragile system, whose possible environmental impact is highly conditioned by land use. To improve the hydrogeological knowledge of the system, between September 2013 and October 2015, a detailed fieldwork was carried out for the revision of the geological model, the inventory of water points, and the in situ physico-chemical characterization on major elements and isotopes of up to a total of 43 springs, as well as precipitation water. This paper focuses on the characterization of the geochemical processes that allow explanation of the observed chemical variability of groundwater drained by the pristine aquifer system to determine the origin of salinity. The results show that the main process is the dissolution of calcite and dolomite, followed by gypsum and halite, and a minor cation exchange-like process. Sulfur and oxygen isotopes from dissolved sulfate in the studied springs point out a geogenic origin related to the dissolution of gypsum from Triassic and Tertiary materials, and that the contribution from anthropogenic sources, like fertilizers, is lower. Nitrate in groundwater is not an important issue, with a few localized cases related with agricultural activities. The multidisciplinary approach has allowed the development of a consistent hydrogeological conceptual model of the functioning of the aquifer system, which can be replicated in other places to understand the geogenic character of the hydrogeochemistry.
Highlights
High mountain karst aquifers are strategic freshwater reservoirs to maintain dependent ecosystems downstream, many of which are in semiarid zones
The two springs of cluster D show rCa/rMg values equal to 0.18 (M-30) and 4.32 (M-41). These results suggest that, except for the four cited samples from cluster A that drain the upper part of the Port del Comte Massif (PCM), where the materials associated with the PPEc unit are richer in dolomite, the contribution of dolomite increases as the discharge altitude decreases
The groundwater is dominantly of the calcium bicarbonate and calcium– magnesium bicarbonate type, suggesting a dominant calcite dissolution process in agreement with the lithology associated with the Eocene carbonate materials conforming the main aquifer of PCM
Summary
High mountain karst aquifers are strategic freshwater reservoirs to maintain dependent ecosystems downstream, many of which are in semiarid zones. It is essential to characterize such mountain karst aquifers and protect them to avoid undesirable quality issues in the stored water resources, because karst aquifers have been shown to be very vulnerable, especially to pollution, given their inner structure, hydrogeological behavior, and limited self-depuration capacity [2]. This is true especially when these karst aquifers are unconfined and the focused recharge flows through the most conductive karst features. This will drive the aquifer-dominant recharge process to migrate from spatial diffuse to a focused one [5], while exposing the aquifer to eventual surface contamination by anthropogenic activities for longer [6]
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