Non-permanent shallow halocline in a fractured carbonate aquifer, southern Italy Emma Petrella a, * , Gino Naclerio a , Antonio Falasca a , Antonio Bucci a , Paolo Capuano b , Vincenzo De Felice a , Fulvio Celico a a Università degli Studi del Molise, Groundwater Research Center, Contrada Fonte Lappone, 86090 Pesche (IS), Italy b Università degli Studi di Salerno, Dipartimento di Matematica e Informatica, Via Ponte Don Melillo, 84084 Fisciano (SA), Italy article info Article history: Received 19 September 2008 Received in revised form 7 April 2009 Accepted 30 April 2009 This manuscript was handled by L. Charlet, Editor-in-Chief, with the assistance of Jiin-Shuh Jean, Associate Editor Keywords: Carbonate rock Fractured aquifer Halocline Groundwater summary We carried out hydrogeological, geophysical, hydrochemical, isotopic, and molecular biological investiga- tions in a fractured carbonate aquifer in southern Italy to verify if prolonged variations in groundwater salinity with depth can be due to mixing processes between fresh infiltration water and groundwater. All investigations revealed the formation of a non-permanent halocline at the experimental site, whose thickness and shape varies over time. Variations in thickness and shape are influenced by infiltration pro- cesses. Three main types of Electrical Conductivity (EC) profiles were found during the research period. In the high-flow period, the EC profile consisted of a transition layer and a nearly homogeneous higher salin- ity groundwater layer, and no mixed layer was observed. A nearly-homogeneous mixed layer was detected in the low-flow period, excluding the late recession when the EC profile was characterized by a higher salinity groundwater layer and a negligible transition layer. The EC profile through the transition layer was approximately symmetric and linear, probably due to a velocity shear across this layer. The velocity shear was probably caused by the difference in opening-porosity detected between the upper and lower carbonate bedrock through geophysical investigations. The investigated phenomenon is due to water that infiltrates very close to the observation well. In fact: (a) the isotopic composition of the lower salinity groundwater layer in early recharge 2007/2008 (d 18 O= 8.12‰; d 2 H= 49.92‰) is very close to the composition of rainwater collected in the same per- iod near the observation well (d 18 O= 8.19‰; d 2 H= 51.35‰); (b) the C DIC composition in the lower salinity groundwater layer (15,96‰) is very close to the composition (15.77‰) in the infiltration water collected at the bottom of the soil medium, close to the observation well; (c) the lower salinity groundwater layer is characterized by significant concentrations of acetate (up to 4.5 mg/L) as well as the saturated paste extracts obtained only from the soil medium collected close to observation well (up to 110 mg/L); (d) in the lower salinity groundwater layer acetate-assimilating bacteria belonging to Betaproteobacteria and Bacteroidetes were found, further supporting that this groundwater layer signif- icantly interacts with the soil medium described above. Similar mean 3 H contents in local rainwater (4.6 TU), in the lower salinity groundwater layer (4.5 TU), in the higher salinity groundwater layer (4.5 TU) and in spring water (4.4 TU) clearly show that the halocline formation is not influenced by dif- ferences in residence times. Ó 2009 Elsevier B.V. All rights reserved. Introduction In fractured aquifers, variations in groundwater salinity with depth have been observed in the proximity of more transmissive fractures (Morin et al., 1997; Cook et al., 1999). Thus, this type of geophysical logging, coupled with other field investigations, is usu- ally used to infer the geologic structure of a fractured aquifer and to detect the types and orientations of openings that have a major control on groundwater flow. From a theoretical point of view, variations in groundwater salinity with depth may also be due to mixing between fresh infil- tration water and groundwater. This should be similar to the effect produced by rainfall on the sea surface, where haloclines can be observed due to mixing between the fresh lens and the sea water (Price, 1979; Soloviev and Lukas, 1996; Wijesekera et al., 1999). Within a fractured aquifer, this type of halocline can be theoreti- cally produced by infiltration water that percolates diffusely through a well-developed opening network, and joins the water ta- ble with a salinity significantly lower than that of groundwater. The main goal of this study was to verify the effectiveness of this hypothesis, analyzing variations in groundwater salinity with depth at an observation well drilled at the Acqua dei Faggi 0022-1694/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2009.04.033 * Corresponding author. Tel.: +39 0874 404112; fax: +39 0874 404123. E-mail address: emma.petrella@unimol.it (E. Petrella). Journal of Hydrology 373 (2009) 267–272 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol