Driving and limiting factors of CH4 and CO2 emissions from coastal brackish-water wetlands in temperate regions

Coastal wetlands play a fundamental role in mitigating climate change thanks to their ability to store large amounts of organic carbon in the soil. However, degraded freshwater wetlands are also known to be the first natural emitter of methane (CH4). Salinity is known to inhibit CH4 production, but its effect in brackish ecosystems is still poorly understood. This study provides a contribution to understanding how environmental variables may affect greenhouse gas (GHG) emissions in coastal temperate wetlands. We present the results of over 1 year of measurements performed in four wetlands located along a salinity gradient on the northeast Adriatic coast near Ravenna, Italy. Soil properties were determined by coring soil samples, while carbon dioxide (CO2) and CH4 fluxes from soils and standing waters were monitored monthly by a portable gas flux meter. Additionally, water levels and surface and groundwater physical–chemical parameters (temperature, pH, electrical conductivity, and sulfate concentrations of water) were monitored monthly by multiparametric probes. We observed a substantial reduction in CH4 emissions when water depth exceeded the critical threshold of 50 cm. Regardless of the water salinity value, the mean CH4 flux was 5.04 g m−2 d−1 in freshwater systems and 12.27  g m−2 d−1 in brackish ones. In contrast, when water depth was shallower than 50 cm, CH4 fluxes reached an average of 196.98  g m−2 d−1 in freshwater systems, while non-significant results are available for brackish/saline waters. Results obtained for CO2 fluxes showed the same behavior described for CH4 fluxes, even though they were statistically non-significant. Temperature and irradiance strongly influenced CH4 emissions from water and soil, resulting in higher rates during summer and spring

Detecting small-scale variability of trace elements in a shallow aquifer

Groundwater samples collected from an unconfined shallow aquifer were analysed for major and trace element (TE) concentrations with the aim to investigate small-scale variations possibly linked to fertilizer residual products applied until 2004. The field site, located near Ferrara (Northern Italy), covers an area of 200 m2 and was a former agricultural field then converted into a park and equipped with a grid of 13 monitoring wells. Three monitoring campaigns were carried out in June 2007, March and June 2009 in order to detect spatial and temporal variations in water quality. Groundwater nitrate, chloride, bromide and sulphate concentrations decreased with time indicating that the fertilizer plume was slowly replaced by unpolluted groundwater. However, the groundwater composition showed values of TEs (Fe, Mn, Al, As and Hg) above the recommended international and national guideline values. Dissolved TE concentrations varied randomly in the three campaigns, while TEs in the solid matrix did not show particular enrichment factors induced by fertilizer use. The data indicated that the dominant factor involved in determining small-scale spatial variability of TE concentrations in this shallow aquifer was the sediment-water interaction, while the temporal variation of TEs was driven by the organic matter leaching from the topsoil and by water table oscillations, which in turn drove the groundwater redox status. This study emphasizes the need of small-scale TE spatial resolution to discriminate between anthropogenic non-point sources of pollution (like fertilizers) and background concentrations

Impact of climate variability on the salinization of the coastal wetland-aquifer system of the Po Delta, Italy

Deltaic coastal areas are constituted by a patchwork of brackish lagoons and freshwater bodies; these coastal wetland-aquifer systems are fragile ecosystems that usually respond quickly to climate changes. To understand the hydrological processes occurring within the lagoons and the groundwater system of the Po River Delta (Italy), the contribution of both evaporation and anthropogenic factors on groundwater salinization was assessed. A time series (2002-2015) of monthly average climatic data and a temperature-salinity dataset were used in three adjacent salinebrackish lagoons with the aim to identify the actual evaporation patterns and predict future trends using artificial neural networks (ANN). Moreover, the use of groundwater and surface water equivalent freshwater heads, along with the geological architecture, allowed linking the fluctuation of lagoon salinities with the degree of hydraulic connection between wetland and aquifer system. Results show that the less a lagoon is hydraulically connected with the aquifer, the higher is the salinity peak that could be reached at the end of the summer period. ANN forecasts highlight that in the near future this behaviour would be the rule rather than the exception. The increase in salinity of surface waters could be of serious concern, especially for aquaculture, sensitive to sharp salinity increases

Salinization and Freshening Processes in Coastal Aquifer of Ferrara (Italy)

Salinization due to seawater intrusion is one of the major threats to coastal aquifers. Understanding origin and dynamics of the fresh-saltwater interface is a prerequisite for effective management of available water resources. This study identifies salinization and freshening processes on short-term monitoring in the coastal aquifer of Ferrara, Emilia-Romagna Region (Italy). A detailed vertical characterization of the aquifer along a flow line was performed by straddle packer multilevel sampling (MSL) technique. Water types and geochemical processes occurring in the coastal system were described. The base exchange index for application to dolomitic systems (BEXD) was used to characterize the status of the coastal aquifer. The aqueous geochemical modelling code PHREEQC was used to calculate saturation indexes of calcite and dolomite for water samples collected. Results emphasize that the salinization affects almost the entire coastal aquifer with only shallow zones of freshening in proximity of both irrigation canals and topographic heights, such as palaeodunes, which constitute recharge zone where infiltrating precipitation produces freshening process. The aquifer displays a transition zone of brackish and brackish-salt water type (Cl- from 0.3 to 10 g/l) with a thickness ranging from 2 to 5 m. Saline (Cl- from 10 to 20 g/l) to hyperhaline (Cl- >20 g/l) water type is present in the remaining thickness of the aquifer. The main water types identified in the aquifer are brackish-salt and high-very high alkaline water, and hyperhaline and high-rather extreme alkaline water, dominated by Na+ and Cl- and salinization processes. Hyperhaline water is likely due to fossil seawater, relic of Holocene transgression, rather than modern seawater intrusion. The chemical composition of groundwater in this lowland coastal aquifer is mainly controlled by dissolution of evaporite salts formed during the Holocene transgression, cation-exchange processes, and calcite and dolomite mineral dissolution

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

Sediment composition, provenance, and Holocene paleoenvironmental evolution of the Southern Po River coastal plain (Italy)

Geochemistry, sediment provenance, and the Holocene paleoenvironmental evolution of the Southern Po River coastal plain (Italy) were examined based on the geochemical analysis of 279 sediment samples from 35 cores. The Holocene succession is subdivided into four major facies associations: back-barrier, transgressive barrier, shallow-marine, and beach-ridge deposits. Bulk chemical composition shows remarkable differences between facies associations, and the poly-modal behavior in the related frequency curves suggests distinct sources of sediment. Using MgO and Ni as geochemical tracers, changes in sediment provenance were assessed with the aim to reconstruct the Holocene paleoenvironmental evolution of the area. The Po River and the Apenninic rivers represented the main sources of sediment during the early stages of transgression and highstand sea-level conditions, whereas transgressive barrier and early shallow-marine facies associations exhibit a clear North Adriatic river (eastern Alpine) provenance. The Holocene paleoenvironmental evolution of the study area, between about 10 ky and the present, indicates the presence of a bypass zone in front of the Po River mouth between 9 and 6 ky BP. At that time, sediment supplied by the North Adriatic rivers was transported southward by the longshore drift, bypassed the weak Po River influx and accumulated close to the Apennines. Comparison between Holocene facies associations and current back-barrier and shallow-marine sediments confirms that the present-day basin circulation is similar to the one that acted during the last episode of sea-level rise

Forest fire effects on groundwater in a coastal aquifer (Ravenna, Italy)

We examined the fire-induced changes in groundwater recharge rate. This aspect is particularly important in the case of large forested areas growing over a coastal aquifer affected by saltwater intrusion. In the Ravenna coastal area (Italy), pine forests grow on coastal dune belts, overlying a sandy unconfined aquifer, which is strongly affected by marine ingression. Three groundwater profiles across the forest and perpendicular to the coastline were monitored for groundwater level, physical, and chemical parameters. The aims were to define groundwater quality, recharge rate, freshwater volume, and highlight change, which occurred after a forest fire with reference to pre-fire conditions. Analytical solutions based on Darcy Law and the Dupuit Equation were applied to calculate unconfined flow and compare recharge rates among the profiles. The estimated recharge rates increased in the partially and completely burnt areas (219 and 511\ua0mm\ua0year 121, respectively) compared with the pristine pine forest area (73\ua0mm\ua0year 121). Although pre-fire conditions were similar in all monitored profiles, a post-fire decrease in salinity was observed across the burnt forest, along with an increase in infiltration and freshwater lens thickness. This was attributed to decrease canopy interception and evapotranspiration caused by vegetation absence after the fire. This research provided an example of positive forest fire feedback on the quantity and quality of fresh groundwater resources in a lowland coastal aquifer affected by saltwater intrusion, with limited availability of freshwater resources. The fire provided an opportunity to evaluate a new forest management approach and consider the restoration and promotion of native dune herbaceous vegetation

Geomorphological analysis and classification of foredune ridges based on Terrestrial Laser Scanning (TLS) technology

Along the North Adriatic Sea coast (Italy), vulnerability to climate change is further aggravated by anthropogenic influences, such as strong subsidence rate due to deep groundwater and gas abstraction, tourism and industry impacts. In this context, conservation and restoration of coastal sand dunes become extremely important especially because of their importance in terms of \u2018natural\u2019 coastal defense. This paper proposes an innovative geomorphological approach based on Terrestrial Laser Scanning - TLS, which allows us to measure and monitor morphometric dune evolution with high precision and details. Several TLS surveys were performed along the Ravenna coast (Adriatic Sea, Italy) and the resulting Digital Elevation Models (DEMs) were analyzed in order to classify the foredune ridges in three geomorphological sub-zones. The topographic, areal and volumetric variations over time of geomorphological units were calculated by GIS tools in order to identify seasonal trends or particular pattern. Meteo-marine climate conditions were also analyzed and Principal Component Analysis (PCA) was performed to correlate changes in morphology with meteo-marine forcing factors, highlighting the ones that most influence dune evolution and dynamics

Geostatistical assessment of groundwater arsenic contamination in the Padana Plain

: Arsenic (As) in groundwater from natural and anthropogenic sources is one of the most common pollutants worldwide affecting people and ecosystems. A large dataset from >3600 wells is employed to spatially simulate the depth-averaged As concentration in phreatic and confined aquifers of the Padana Plain (Northern Italy). Results of in-depth geostatistical analysis via PCA and simulations within a Monte Carlo framework allow the understanding of the variability of As concentrations within the aquifers. The most probable As contaminated zones are located along the piedmont areas in the confined aquifers and in the lowland territories in the phreatic aquifers. The distribution of the As contaminated zones has been coupled with hydrogeological, geological, and geochemical information to unravel the sources and mechanisms of As release in groundwater. The reductive dissolution of Fe oxyhydroxides and organic matter mineralization under anoxic conditions resulted to be the major drivers of As release in groundwater. This phenomenon is less evident in phreatic aquifers, due to mixed oxic and reducing conditions. This large-scale study provides a probabilistic perspective on As contamination, e.g. quantifying the spatial probability of exceeding national regulatory limits, and to outline As major sources and drivers