Understanding karst massif physical characteristics from the analysis of springs natural responses: a combined methodological approach

Karstified carbonate formations are among the most complex geomorphological structures due to their high spatial heterogeneity and intricated internal structure. Studying these specific environments is highly challenging and usually requires sophisticated, costly instruments and procedures.

Therefore, one of the most applied methodologies is the analysis of the springs' discharge. Its detailed analysis is a valuable method for understanding aquifer hydrodynamics and inferring information about the internal structure of karst systems. Many previous studies stated that the analysis of single karst springs could provide very useful information as spring flow carries an imprint of the global karst massif hydrodynamics and, consequently, of its physical characteristics.

Understanding karst underground features from a hydrogeomorphological perspective is crucial due to the high vulnerability of karst aquifers to contamination and its enormous importance as a freshwater reservoir, particularly in the present climate change scenario under the Mediterranean climate. 

The study area is the northern sector of Sicó karst massif located in the central-western region of Portugal. The massif's structure comprises limestone mountains and plateaus supported by the well-karstified Middle Jurassic units. This part of the karst massif corresponds to a karst aquifer of about 100 Km², which essentially drains through several temporary and perennial springs at the western shore. The recharge area is dominated by a plateau with covered karst in the central part, where diffuse recharge is dominant, and exhumed karst in the west part, where concentrated recharge occurs.

This study aims to infer Sicó karst massif physical characteristics from their hydrogeological functioning by applying well-known methodologies focused on the springs' discharge time series. The combined methodological approach used in the present investigations includes (i) time series analysis on rainfall and discharge (correlation and spectral analysis), (ii) recession curve hydrograph analysis (quantitative parameters from Mangin method and recessional equations from Malík & Vojtková master recession curve method), and (iii) spring outflow physicochemical variations analysis (electrical conductivity, temperature, Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃^-, Cl^-, SO₄^-2, NO₃^- ). The data were logged at the main spring (Olhos d'Água do Anços) every 20 minutes by a multiparameter gauge station from 2010. More recently, water samples were collected and examined in the laboratory. 

The results revealed a bimodal and heterogeneous hydrogeological behavior with: (i) rapid circulation through highly transmissive channels in the vadose zone (important quick flow component and turbulent circulation), and (ii) high non-linearity, slow vadose circulation (delayed flow), and ample storage capacity (dominance of baseflow). In terms of physical characteristics, these outputs allow inferring the duality of the massif corresponding to (i) very karstified sectors and a structured drainage network that quickly drains the unsaturated zone and (ii) less karstified sectors (with lower hydraulic conductivity – small fractures, fissures, pores) and a thick and deep saturated zone retaining and storing large amounts of water.

Although some documented limitations of each method, this combined methodological perspective has proved to supply precious information related to a karst system's complexity, evolution, and physical properties.

Keywords: Karst; spring discharge analysis; hydrodynamic functioning; Sicó karst massif; Portugal