Groundwater recharge assessment at local and episodic scale in a soil mantled perched karst aquifer in southern Italy
Introduction
Karst aquifers represent about 12% of the Earth’s continental area. About one quarter of the global population uses drinking water from these hydrogeological systems (Ford and Williams, 2007). Many European and Mediterranean countries are completely or partially dependent on groundwater resources of karst aquifers. In southern Italy, these aquifers are the main source for drinking water supplies (Celico, 1983, Celico et al., 2000, Allocca et al., 2007a, Allocca et al., 2014) and they play also a vital role in groundwater-dependent fluvial ecosystems.
The mean annual yield of karst aquifers in southern Italy has been estimated to be about 4100 × 106 m3 year−1, with an average specific yield varying from 0.015 to 0.045 m3 s−1 km−2 (Celico, 1983, Allocca et al., 2007a). This high productivity results from the high permeability of karst rocks, the occurrence of precipitation primarily during autumn and winter, and the presence of large summit endorheic or flat areas that favor infiltration and groundwater recharge processes (Manna et al., 2013a, Allocca et al., 2014). Another important feature of this region is the widespread existence of allochthonous soil mantles, comprising ash-fall pyroclastic deposits erupted by the main volcanic centers of the Campania region (De Vita et al., 2013). These surficial volcanic overburdens influence groundwater recharge, especially where they are relatively thick, by serving as temporary water storage that enhances evapotranspiration and modulates the percolation of water into carbonate bedrock. At the same time, these deposits, in combination with abundant vegetation, limit the migration of microbial cells from the land surface to groundwater bodies (Naclerio et al., 2008, Naclerio et al., 2009, Bucci et al., 2015a, Bucci et al., 2015b). The pyroclastic deposits also affect recharge processes by fostering the development of epikarst (Petrella et al., 2007, Celico et al., 2010).
Karst aquifers of southern Italy are characterized by a prevalent groundwater basal flow that emerges in major basal springs (Celico, 1978, Boni et al., 1986, Allocca et al., 2007b). By perched groundwater circulation, this basal flow feeds numerous high-altitude minor springs of importance for local water use.
To advance the techniques and understanding needed to assess groundwater recharge processes in different hydrogeological settings, and in particular to quantify and model the replenishment of groundwater by flow of infiltrated water through the unsaturated zone, is a fundamental and challenging issue (Lerner et al., 1990, Stephens, 1995, Scanlon et al., 2006, Healy, 2010). The importance of such advances is heightened by needs for sustainable management of groundwater resources, protection of fluvial ecosystems, assessing potential impacts of climate change on groundwater hydrology (VV.AA., 2007) (De Vita et al., 2012, Manna et al., 2013b, Hartmann et al., 2014a), and increasing demand for potable water (Wada et al., 2010). An important specific need is for hydrological studies on perched karst aquifers, as the related high-altitude spring outflows are a high-value water resource in mountainous areas.
Several direct and indirect methods have been developed for estimating groundwater recharge at regional to local spatial scales, and for annual to daily and episodic temporal scales (Andreo et al., 2008, Hartmann et al., 2014b, Allocca et al., 2014, Fiorillo et al., 2014, Guardiola-Albert et al., 2014, Nimmo et al., 2015). One widely used technique is the Water Table Fluctuation (WTF) method (Healy and Cook, 2002). It has been infrequently applied to karst aquifers, perhaps because the water table is generally too deep for convenient monitoring. Various implementations of the WTF method have been developed, each having its own advantages and limitations (Todd, 1980, Coes et al., 2007). Examples include the RISE (Rutledge, 1998) and Master Recession Curve (MRC) methods (Heppner and Nimmo, 2005, Heppner et al., 2007, Lorenz and Delin, 2007, Delin et al., 2007), and other graphical approaches (Risser et al., 2005, Delin et al., 2007). Recently, Nimmo et al. (2015) developed an advancement of the WTF method, known as the Episodic Master Recession (EMR) method, to estimate episodic recharge at the local scales and to associate each recharge episode with a causal rainfall event. The EMR method has been tested (Nimmo et al., 2015) in a fractured sandstone aquifer (Masser Site, Pennsylvania, USA) and a glacial moraine aquifer (Silstrup Site, Denmark), but so far not in karst aquifers.
Our research proceeded through steps: (i) to estimate the recharge generated in individual storm-generated episodes in a one-year data record for a perched karst aquifer, along with the rainfall associated with each episode, to calculate the RPR; (ii) to quantitatively relate RPR values to the intensity of recharge-generating rainfall and to antecedent soil water content; and (iii) to test the computed average of RPR values against an independent estimate based on actual evapotranspiration evaluated using a soil water budget.
Section snippets
Description of the research site
The Acqua della Madonna test area is part of the central-southern sector of the Terminio Mount karst aquifer (Campania region, southern Italy), whose total extent is about 167 km2 (Fig. 1a and b). The test area, of about 0.9 km2 extent, is located at an altitude of about 1200 m a.s.l. (Fig. 1b and d) and is characterized by a perched karst aquifer that is constituted mainly by a fractured and partially karstified Cretaceous limestone series. The area is covered by alkali-potassic ash-fall
Hydrological monitoring
For use in evaluating groundwater recharge episodes at the local scale, daily thermo-pluviometric time series recorded by meteorological stations of the Regional Civil Protection Agency were collected from January to December 2008 (Fig. 2a). For the same period, hourly groundwater levels were measured in the piezometer P1 by a pressure transducer (STS Inc., USA) and datalogger (Figs. 1b, c and 2a). In addition, volumetric soil water content (θ = Vw/Vt) data were measured hourly by a transducer
Aquifer response time
During 2008, water-table levels observed in the piezometer P1 and measured with respect to the considered datum (H0 = 1165 m a.s.l.) varied between +3.2 m and +9.4 m, with an average of +5.2 m (Fig. 2a). The minimum level was recorded on the 2nd of October, at the end of a major recession period that coincided with the summer season of little rainfall (2.4 mm in July and 6.2 mm in August). The maximum value was detected at the beginning of December.
During the recession periods, water-table levels
Discussion
This research is a first attempt to estimate groundwater recharge at the local and episodic scales in this area or similar areas. Related research has been carried out at the aquifer and mean annual scales for karst aquifers of southern Italy by estimating the Annual Groundwater Recharge Coefficient (Allocca et al., 2014). A soil cover exerts a fundamental influence on groundwater recharge processes by enhancing water losses to evapotranspiration. The studied case therefore pertains to recharge
Conclusions
The results of this study represent progress in the application and testing of the Episodic Master Recession (EMR) method. The application to a new hydrogeological framework, a heterogeneous perched karst aquifer of southern Italy, demonstrated the method’s value where the stratified subsurface includes sharp hydrogeologic contrasts within the zone of water table fluctuation. This is an important step beyond the few cases previously considered, in which the subsurface was effectively
Acknowledgements
This work was supported by the personnel short mobility and the doctorate programs of the University of Naples Federico II that funded scientific collaboration with the Unsaturated Zone Interest Group (UZIG) of the US Geological Survey. The authors are grateful to Dr. Erik Smith, USGS, Minnesota, for his useful suggestions.
The authors are grateful to the Editor P.K. Kitanidis, the Associate Editor B. Mahler, and the other two anonymous reviewers for their helpful comments.
The authors gratefully
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