Hydrogeological modeling for sustainable groundwater management under climate change effects for a karstic coastal aquifer (Southern Italy)

Abstract

Seawater intrusion is a pervasive problem affecting coastal aquifer, where the concentration of population and the increasing water demand creates risks of overexploitation, especially in those areas where is the only resource of drinking and irrigation water. This phenomenon is more considerable for the coastal karst aquifers, as observed in many Mediterranean countries and in some Italian regions as Friuli, Sardegna, Sicilia, Lazio, Campania and Puglia. This note aims to describe a research activity finalised to define a numerical model as management tools for groundwater resource of Salento (South Italy) to reduce the quantitative and qualitative degradation risks. The numerical codes used was MODFLOW (McDonald and Harbaught, 1988) and SEAWAT (Guo and Langevin, 2002). The active domain of the study area (active cells) covered approximately 2,300 km2 with 45,925 cells. Vertically, to allow a good lithological and hydrogeological discretization, the area was divided into 12 layers, from 214 to -350 m asl. Thickness and geometry of layers was defined on the basis of the aquifer conceptualisation based on the 3D knowledge of hydrogeological complexes. On the basis of detailed geological and hydrogeological conceptualisation, the climate change effects were considered in terms of natural recharge variations from 1930 to 1999 (Cotecchia et al., 2005; Polemio and Casarano, 2008). To take account of anthropogenic activity, mainly due to tourism and agriculture, the discharging trend was assessed, focusing on late decenniums (eighties and nineties), in which the discharge increase was mainly observed. Models representing the natural steady-state condition (using data of thirties) and transient scenarios of late decenniums were realised. The purpose of this first model implementation was, besides validated model, to assess the groundwater availability and quality in a recent period of seventy years (Polemio and Romanazzi, 2012; Romanazzi and Polemio, 2013). Results emphasize an essential decrease of piezometric levels and a worsening of seawater intrusion. On these bases, six forecasting transient scenarios were implemented, referred to future periods of about twenty years (2000-2020, 2021-2040 and 2041-2060) with the aim to predicting the evolution of piezometric level and seawater intrusion. For forecast data about precipitation and temperature, among the many models in the literature, we referred to the model developed by Giorgi and Lionello (2008), in relation to the defined scenario A1B. The model predicts temperature variations (°C) and precipitation percentage variation for the period 2001-2100. It was considered an average temperature variation form 0.9 °C (2001-2020) to 2.4 °C (2040-2060). Precipitation shows a negative percentage change (referred to 1960-80) equal to -3.9, -5.9 and -9,0% respectively for 2000-2020, 2021-2040 and 2041-2060. These climatic data are in agreement with other climate change models (Garcia- Ruiz et al., 2011). For the three future scenarios new recharge and discharge were assessed. In terms of discharge, they are mainly due to irrigation. For this kind of future utilisation two hypotheses were considered. The first assumes that type and extension of cultivations will be steady and, as an effect of climate change, the pressure on groundwater resource will further rise as necessary to satisfy irrigation demand (Dragoni and Sukhjia, 2008; Goderniaux et al., 2008). In the second hypothesis the irrigation discharge will be steady and equal to those of the 1999 due the adaption of cultivation types and irrigation practices. In both cases the scenario results show a general decrease of the piezometric head and a deterioration of water quality caused by seawater intrusion (Romanazzi et al., 2013). The results call for new land and groundwater resources management criteria. Considering the Water Framework Directive (EC, 2000) and international and regional experiences (LaMoreaux, 2010; Jiménez-Madrid, 2010; Polemio et al., 2009, Polemio et al., 2010), the study area was subdivided in three zones. To define the zone boundary, the threshold criterion was used (Polemio and Limoni, 2001; Polemio et al., 2009). The threshold between pure fresh groundwater and any type of mixing between fresh and saline groundwater was defined equal to of 0.5 g/l. In the first zone, the coastal zone, salinity was always (in the past) above the threshold, a transition zone, where salinity was variable respect to the threshold, and a third zone or inland zone where salinity value was permanently below the threshold. These three zones were implemented in the model. Different combinations of discharge criterions applied to these zones suggest the best choices to be applied for management criteria able to safely considered the future effects of climate changes.