EFFECTS OF NATURAL RECHARGE AND IRRIGATION ON GROUNDWATER SALINITY IN A COASTAL UNCONFINED AQUIFER, LIDO DI DANTE, RAVENNA, ITALY

Around Ravenna (Italy), more than 6800 hectares are at risk for soil salinization. This is agricultural land near sea level. We have studied a 10km-long transect, perpendicular to the Adriatic coast. A quarterly hydrologic monitoring was conducted during 2010-2011 in ten wells for water table height, salinity of the groundwater and hydrochemical analysis. A detailed study was carried out on a small agricultural area (10ha), at 500m from sea, around a ditch which is filled with freshwater in summer to irrigate the nearby fields. Thickness and dimensions of the freshwater lens near this irrigation channel was studied with VES, in March and July of 2010. The scope of this study is to quantify the effect of irrigation on the salinity of the groundwater and to assess whether irrigation practices can help to counteract further groundwater and soil salinization. The hydrochemical analysis shows that the coastal aquifer contains brackish/salt Na-Cl type water up to 7 km from sea; with some fresh water at the top of the aquifer underneath the pine forest and near the gravel quarries and in the area of detailed study. Near the irrigation ditch the water type is Ca-HCO3 similar to river water. Rainfall from January to September 2011 was 400 mm while total irrigation of a tomato field was 600 mm, applied with a rate of 120 mm per month (April-August 2011). The infiltration rate of water into the aquifer, measured from drain gauges was 8 mm/m2 in May, 2 mm/m2 in June and 0 mm/m2 in the other months. In winter the water level in the irrigation channel is equal to the groundwater level but during growing season it creates a hydraulic gradient towards sea. Comparing the V.E.S. profiles from different seasons confirms that the fresh water lens increases in size during the irrigation season. This study shows that infiltration of excess irrigation water occurs only via the irrigation ditch and not from superficial irrigation