Saltwater contamination seriously affects water quality and land productivity of reclaimed farmlands along the Venice Lagoon, Italy. To characterize the hydrogeochemical dynamics involved in this phenomenon, a three-year study was carried out in an experimental field located at the southern margin of the Venice Lagoon. Soil matric potential, quality of soil pore water and groundwater, and soil physical and chemical properties were monitored at five monitoring stations. Relationships between Cl⁻, Na⁺, Mg^2+, Ca²⁺, K⁺, SO₄²⁻, Br⁻ ionic concentrations, and electrical conductivity of the water samples with the soil characteristics (e.g., texture, exchangeable cations) were investigated. Soil water flux direction was calculated and related to ion concentrations. Moreover, specific molar ratios (Mg/Ca, Na/Cl, Cl/Br, and SO₄/Cl) were calculated to identify the main drivers affecting salinity in the field. The study confirmed that the experimental site was strongly affected by soil and water salinity, and two major contamination dynamics were identified. The first one was mainly driven by seawater intrusion from the near lagoon and salty watercourses, while the second was derived by the interactions between the peaty soil and salts that were originally in place, since the area was only reclaimed a few decades ago. The latter highlighted the potentiality of the experimental field to become an acidic sulfate environment. Ionic ratios were implemented and proved to be an important tool for the identification of salinity origin.

Seawater intrusion (SWI) is threatening coastal aquifers and farmland productivity worldwide. Although this phenomenon naturally occurs in coastal areas, it is intensified by anthropogenic activities such as groundwater pumping and land reclamation that cause a lowering of the hydraulic head and land subsidence. Moreover, the consequences of climate change such as sea level rise, increase of the mean temperature and the shifting of rainfall events to tropical regimes, have strong negative effects on groundwater quality and agriculture. Countermeasures against SWI are needed to maintain agricultural productivity and protect the freshwater resources in coastal areas. In the low‐lying farmlands surrounding the southern Venice Lagoon, in northern Italy, SWI is exacerbated by land subsidence, the presence of sandy paleochannels connected to the lagoon subsurface, seawater encroachment into the river estuaries, the presence of fossil brine waters and peat deposits. This study provides a detailed hydrogeological and geochemical characterisation of an experimental agricultural field affected by SWI located in this area using a large dataset collected over the 4 years between 2019 and 2022. Furthermore, it presents the results of novel intervention established across the farmland in 2021 to mitigate saltwater contamination. This intervention involved a controlled discharge of freshwater supplied by a reclamation channel through a 200 m‐long drainpipe buried 1.5 m below the field surface along a well‐preserved sandy paleochannel. The interpretation of the collected data demonstrates that the freshwater recharge carried out in 2021 and 2022 effectively reduced the groundwater salinity along the paleochannel. Moreover, statistical analyses highlighted that a certain lateral spread of freshwater occurred too, although the variability of the monitored parameters in the sites located outside the sandy body was only partially explained by the drain activity.

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, 2nd until September, 7th. Five monitoring stations were installed and equipped with a 2.5 m deep piezometer to monitor depth to the water table and electrical conductivity (ECw) and TEROS 12 (METER Group, Inc., Pullman, WA, USA) soil moisture, electrical conductivity (ECb), 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 ECw measurements carried out in the piezometers show a highly variable response during the test, however ECw increased after drain closure at all piezometers. On the contrary, the effects on soil water content and ECb 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.

Soil and groundwater salinization due to seawater intrusion is among the most important problems in coastal farmlands. Inverse estimation of unsaturated soil hydraulic and solute transport properties represents a fundamental step to understand saltwater intrusion dynamics. A three-year study was conducted in a maize field bounding the southern Venice Lagoon. Volumetric water content θ, soil matric potential ψ, and apparent electrical conductivity (ECa) were monitored hourly by five automatic monitoring stations at four depths (0.1, 0.3, 0.5 and 0.7 m). Groundwater electrical conductivity (EC) and depth to the water table were measured in five wells. In addition, soil water and groundwater samples were collected and analyzed to determine the chemical composition. Soil hydraulic parameters for the van Genuchten-Mualen equations were determined using the inverse method in Hydrus-1D. The water flow was modelled based on the daily averages of θ at the four depths and the θ values measured in the lab at selected ψ on undisturbed soil cores extracted from the five monitoring stations. Precipitation, crop transpiration, soil evaporation and depth to the water table were used as time-variable boundary conditions. Root water uptake was estimated by using Feddes model. Finally, the major ion chemistry module of HYDRUS-1D was used to model solute transport and root water uptake reduction due to osmotic stress. The use of HYDRUS-1D to understand saltwater dynamics would enable the developing of mitigation strategies to limit its detrimental effect on farmland productivity and groundwater quality.