Classification, Discharge Statistics & Recession Analysis - global comparisons

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Classification, Discharge Statistics & Recession Analysis - global comparisons

Structural Classification, Discharge Statistics, and Recession Analysis from the Springs of the Gran Sasso (Italy) Carbonate Aquifer; Comparison with Selected Analogues Worldwide

Abstract

The relative importance of karst conduits and fractures in driving groundwater flow affects the discharge of springs and the long-term availability of water resources. Applying statistics to the hydrographs of the discharge and studying the recessions provide information on the degree of reliability and variability of the springs and, therefore, the flow regime within the saturated part of the carbonate aquifers. This approach was applied to six springs at the Gran Sasso aquifer in Central Italy. These springs were divided into three structural geological groups that determined the position of the permeability thresholds. The type of tectonic structures and the pattern of the permeability thresholds allow a correlation with the computed statistics. The studied springs were associated with the presence of thrusts, overturned drag folds, and a normal fault. The computed statistics describe a general scenario of reliability and steadiness for the springs. The Flow Duration Curves for the springs show limited groundwater flow through the conduits through a comparison with analogues in Slovakia. Joints and bedding plane fractures dominate the groundwater flow, fitting both the relative steadiness of the discharges and the pattern of the Flow Duration Curves. The recessions are also characterized by more gentle slopes with respect to nearby areas fitting a conceptual model of dominant fracture flow. This mathematical scenario depicts groundwater resources, which have limited exposure to episodes of summer droughts. The proposed approach is a holistic combination of structural geology and hydrologic elements and can be successfully exported to other tectonized carbonate areas for the sustainable management of groundwater resources worldwide.

Keywords: 

carbonate aquifers; groundwater sustainability; fractures; karst; discharge statistics; spring recessions

 

1. Introduction

Fractured aquifers of carbonate sedimentary origin, which are subjected to different degrees of karstification, underlie a land surface covering approximately 15% of the globe and supply about 25% of the world’s population with potable water (Figure 1a) [1]. Hence, groundwater flow occurs through conduits and bedding planes, joints and fault-related fracture corridors rather than via pores in lithified carbonate rocks [2,3,4,5,6]. The relative importance of conduits and fractures in driving the groundwater flow affects the discharge rates at spring outlets and, therefore, the availability of natural water resources during the hydrological year in karst environments. Applying robust statistics to the hydrographs of the discharge and studying the recession provide information on the reliability, degree of variability, and also the flow regime within the saturated part of the fractured and karst aquifers [7,8,9,10,11,12]. Due to the economic and civil importance of the karst water resources, a statistical study of the discharge regime started back in the 1920s with the introduction of the first statistical parameters [13]. After this first input, other parameters have been introduced to classify the discharge, e.g., [14,15,16]. Some of these attempts were relatively recent, and are summarized in the methodological paper [9].

A statistical study of the discharge time series and the analysis of the recessions are also useful to manage natural groundwater resources. A mathematical definition of a hydrologic scenario that is expected to vary over the next years is fundamental, due to the expected effects of climate change that affects the infiltration and the aquifer recharge dynamics [17,18,19,20,21,22,23]. Therefore, the proposed topic has received renewed interest in the last fifteen years in the karst hydrogeological literature, being an attempt to analyse exposure to water scarcity that can be exacerbated in the near future [24,25,26,27,28,29]. The study of the recession of the springs provides information on the risk of water scarcity; highly karstified systems are more exposed to such a risk due to a combination of either rapidly decreasing discharge rates or water availability during the spring and summer times [30].

The research presented also combines an analysis of the discharge time series and recession with a careful study of the structural geology settings of the karst springs. A study of the structural geology combined with statistics of discharge time series and recession represents the novelty of this work. The presented mathematical parameters and curves were compared only with topography and aquifer recharge dynamics by previous authors [13,14,15,16]. Here, we discuss new mathematical results with structural geology, topography, and aquifer recharge dynamics to contrast previous studies.

This attention aims to provide additional information on the groundwater flow to the springs that can guide the interpretation of the discharge time series. The selected area for the combination of a study of the structural setting, discharge, and recession of the springs is the Gran Sasso Massif in Central Italy (Figure 1b,c) [31]. The site was selected in the framework of the Karst Aquifer Resources Availability and Quality in the Mediterranean Area (KARMA) project that was funded by the European Union [32,33].

The peculiarity of this project site is related to the elevated levels of spring withdrawals for drinking water use and its location within the Gran Sasso and Laga Mountains National Park, reflecting the need to supply water to the human population without compromising the environment and ecosystems. As a result of these needs, the most recent hydrogeological literature focuses on the dynamics of the recharge of the aquifer in the Gran Sasso Range. Here, the infiltration rates of the aquifer system were recently constrained using precipitation, temperature, and snow depth data, and the different proportions of rainfall and snowmelt on the aquifer recharge were discussed in detail [34]. The high infiltration rates were found at the Campo Imperatore plain due to the direct input of snow to the aquifer recharge. Groundwater stable isotopes were used to assess the influence of snowpack on the recharge dynamics of some carbonate massifs of Central Italy, including the Gran Sasso [35,36]. By contrast, the proposed research offers a focus on the discharge points of the selected carbonate aquifer system. The research therefore advances the conceptualization of the aquifer to achieve a virtuous management of the groundwater resources at the Gran Sasso and Laga Mountains National Park.

The Gran Sasso ridge is dominated by dolostone and limestone of Meso-Cenozoic age, and large paleokarst features such as dolines with diameters of tens of meters, along with the absence of large caves [37,38,39]. The study site is located in the proximity of the Marche region. Here, recent studies from [40,41] applied the same model to the recessions, thus allowing for a comparison (Figure 1b).

Overall, our goal is to characterize both conceptually and mathematically the discharge regime of the springs at Gran Sasso to verify the exposure of the natural groundwater resources to potential stresses, such as the reduction of the infiltration rates or the increase of withdrawals. The specific objectives are as follows: (i) to classify the springs from a structural point of view in order to unravel the dynamics of groundwater flow, (ii) to statistically characterize the discharge time series, (iii) to highlight a link between the structural setting, and the computed statistics, and (iv) to study the spring recession by using the classical Maillet model [42].

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