A strategy to mitigate soil and water salinity in a coastal farmland at the southern margin of the Venice Lagoon: preliminary results from a 2021 recharge test

The agricultural production in coastal environments is challenging. In the low-lying farmlands along the Venice Lagoon, Italy, saltwater intrusion that naturally occurs in coastal environments is exacerbated by land subsidence, seawater encroachment along the main watercourses, peat oxidation, and peat-driven salinity. Sea level rise expected in the next decades will intensify seawater contamination, enhancing the threats for crop productions. In this context, mitigation strategies are fundamental to avoid the loss of agricultural land. To this end, a test of freshwater recharge through a ~200-m long buried drain was conducted in an experimental field located at the southern margin of the Venice Lagoon. The soil is mainly silt-loam with the presence of acidic peat and sandy drifts. The drain was installed in 2021 at 1.5 m depth along a sandy paleochannel crossing the area in southwest to northeast direction. It supplies the Morto Channel freshwater to the farmland taking advantage of the high hydraulic conductivity of the sandy soil and the 2-m elevation difference between the channel water level and the farmland surface. The drain was tested at the end of the 2021 maize growing season, from August, 2^nd until September, 7^th. Five monitoring stations were installed and equipped with a 2.5 m deep piezometer to monitor depth to the water table and electrical conductivity (EC_w) and TEROS 12 (METER Group, Inc., Pullman, WA, USA) soil moisture, electrical conductivity (EC_b), and temperature sensors installed at four depths (0.1, 0.3, 0.5, and 0.7 m). Three of those stations were placed along the paleochannel (S1, S2, S3), while S4 and S5 were placed outside about 30 m away. Moreover, six additional piezometers were placed at 5, 10, and 20 m from both sides of S2 station to monitor the lateral spread of freshwater supply. Stations S1 and S2 were also equipped with electrical resistivity tomography (ERT) lines crossing the recharging infrastructure. The ERT lines were 14.4 m long, electrode spacing was 0.3 m and the resistivity electrode array was dipole-dipole, obtaining a maximum depth of investigation equal to approx 2.5 m. Data were collected on five dates, two before (7/12 and 7/30) and three after the drain opening (8/10, 8/20, and 9/7). The freshwater supplied to the farmland caused an increase of resistivity at both S1 and S2, with higher resistivity differences between dates at S1, suggesting a certain effectiveness of the implemented recharge solution. The EC_w measurements carried out in the piezometers show a highly variable response during the test, however EC_w increased after drain closure at all piezometers. On the contrary, the effects on soil water content and EC_b was negligible. The effectiveness of the strategy will be tested more deeply during the 2022 maize growing season by monitoring the effects of freshwater supply on plant stress and final crop yield.