Evaluation of groundwater contamination in a coastal area of South-East Sicily.

Intensification of agricultural activities under arid and semi-arid conditions involves irrigation, fertilization, organic additives (manure...) and pesticides. Yet, like any enhanced productive activity, it leads to environmental contamination if it is improperly managed. The pollution risk of surface water and groundwater is a worldwide problem and great effort is directed to study the rate of recharge and the measures required to minimize the loads of contaminants seeping from agricultural areas towards the aquifers. Few data are available on groundwater solute concentrations, especially for land under semi-arid and arid conditions, where aquifer pollution can lead to serious consequences on agricultural production and generally on environmental bio-systems. The study aims to evaluate the groundwater resources contamination due to intensive agricultural practices (particularly greenhouses). The study-area is located in the coastal area of the Ragusa province (South-East Sicily), where numerous existing greenhouses may cause the contamination of groundwater systems (unconfined and confined aquifers) beneath the cropped land. The pollution risk is mainly related with the seepage process of micro-elements (N, P, K), held in the irrigation water and the massive use of fertilizers and pesticides, that may pass through the unsaturated zone of the soil profile. Moreover, the area is characterized by the presence of several wells (about 15 wells/km2) for agricultural use that cause the aquifer overexploitation and the consequent risk of seawater intrusion. The agriculture practices adopted in the study area (irrigation volumes, fertilizer concentrations, the use of pesticides…) were monitored since February 2009; moreover, the pollution risk of the aquifers was evaluated through the analysis of groundwater water samples collected (monthly) in the monitoring wells; in particular, nitrogen compounds, soluble phosphorous (PO4- -), potassium (K), as well as the main active ingredients of pesticides commonly used in the study area, were measured. The analysis of the groundwater physico-chemical characteristics showed that all the water samples have a pH in the range 5.9 ÷ 7.6. Concentration of suspended solids ranged between 0.2 and 144 mg/l. The high concentration values of total nitrogen and chlorides (5.0 and 560 mg/l respectively) detected during the monitoring could cause problems to the crops. Moreover, in 6 over 7 wells the electrical conductivity values were between 0.483 and 1.241 mS/cm, while in the well closest to the sea the measured electrical conductivity was up to 2.680 mS/cm. The analysis carried out to detect the presence of pesticides, showed that 1 over 7 wells is contaminated with chlorpyrifos-methyl (up to 0.11 g/l), iprodione (up to 1.75 g/l), furalaxil (up to 2.00 g/l), azoxystrobin (up to 0.69 g/l), clortalonil (up to 1.10 g/l), chlorpyrifos (up to 0.25 g/l), procymidone (up to 0.38 g/l) and metalaxyl (up to 0.27 g/l); the concentrations of cyprodinil, oxadixyl, terbuthylazine desethyl resulted lower than 0.05 mg/l. During the experimental campaign the water table fluctuations measured in the monitoring wells were between 0.3 and 2.8 m. Finally, the results of the monitoring campaign were used to evaluate the MODFLOW-SURFACT/MODHMS (MS) model, a fully integrated groundwater flow and solute transport code, based on U.S. Geological Survey MODFLOW, that actually is one of the most used groundwater flow numerical model in the world.