EFFECT OF FOREST FIRE ON COASTAL AQUIFER SALINISATION AND FRESHWATER AVAILABILITY

Forest fires have usually been studied for their impact on soil properties and consequent change in erosion hazard and runoff generation. Also the post-fire recharge and net infiltration can undergo some changes. This aspect is even more important in case of large vegetated areas growing over a coastal aquifer affected by saltwater intrusion. In the Ravenna coastal area (Italy), a dense pine forest grows on the remains of the natural coastal dune belt, overlying a sandy coastal phreatic aquifer, which is completely compromised by marine ingression. Three profiles, in different portion of the forest, were monitored (2008 and 2013) for groundwater level, physical and chemical parameters in order to highlight any change in groundwater quality, infiltration and freshwater availability occurring after a forest fire that devastated 56 hectares of the studied area in July 2012. All pre-fire groundwater parameters were similar among each other in the three monitored profiles, whereas a postfire decrease in salinity was recorded across the burnt forest along with an increase in redox potential, infiltration and freshwater lens thickness. By applying analytical solutions, infiltration rates were calculated and comparison between all transects were made possible. The estimated infiltration rates indicated an increase in the partly and completely burnt area (219 mm/y and 511 mm/y, respectively) compared to the pristine area (73 mm/y). In the vegetated zone the aquifer recharge is generally limited to the autumn and winter season, while during spring and summer the high evapotranspiration rate exceeds the infiltration amount. This work provided an example of how fire can positively affect the quantity of fresh groundwater resources in low land coastal aquifers.