Abstract
The rise of sea water from the mouth in the sections of the Po river is a natural phenomenon which, however, in recent years has accentuated its frequency and depth: the salt-wedge reached 40 km from the Adriatic sea in 2022 and 17 km in 2023, causing damage to the delicate ecosystems along the coast, to the various agricultural, livestock and industrial activities, as well as to the water supply. There are several factors that influence the rise of the saltwater intrusion and are related to climatic change and human activities: the natural and the anthropic subsidence, the lowering in the rainfall regime, the lower release of fresh water from mountain basins, the lowering of the bottom of the riverbed due to the withdrawal of materials for building activities and the lower contribution of natural sediments from the tributaries.
When The River Goes Back is a series that talks about the causes and problems linked to the advance of the sea in the reclaimed territories of the Po delta and focuses on the strategies in place to try to solve this delicate environmental problem.
WHEN THE RIVER GOES BACK
The saltwater intrusion in the Po delta river
The Causes of Marine Intrusion
When the waters of a river meeting the sea create a mixing zone that marks the passage between the two environments, the first flowing within a conduit limited by more or less parallel banks with increasingly smaller slopes, the other theoretically without borders, unlimited with important depths. It is precisely in this final stretch that fresh water meets salt water and with it forms a brackish environment.
Under normal conditions, the river flow slows down as it moves towards the mouth speed and no longer being able to counteract the rising sea, it leaves the latter the possibility of moving back upstream, insinuating itself into the watercourse and its deepest strata, forming a sort of wedge. The creation of the “salt wedge” is therefore a natural phenomenon but which in recent years has accentuated its depth, taking on a dimension that is difficult to manage. Increasingly persistent and deep rising sea waters have been recorded in the Po, the largest Italian river, reaching 40.1 km from the sea in 2022 and 17 km from the sea in 2023 (Source: Arpae 2023).
There are several factors determining this situation. On the one hand there are human activities linked to the fight against climate change (the absence of precipitation and the drought situations which are occurring more and more frequently the Italian latitudes) such as the enormous withdrawal of fresh water for agricultural, zootechnical and industrial needs and work activities that withdraw sediments from the river bed, thus resulting in a decrease in the flow of fresh water. On the other hand, there are direct anthropic causes, such as the methane extractions of the second half of the Twentieth century which contributed to lowering the land compared to sea level.
Natural Subsidence and Methane Extraction
The territories of the provinces of Rovigo, Ferrara and the municipality of Ravenna were affected by the exploitation of methane deposits from 1938 to 1964; the withdrawal of this water triggered an acceleration in the lowering of the ground tens of times higher than the levels caused by nature: at the beginning of the 1960s it reached peaks of over 2 metres, with an estimated speed of up to 25 centimeters per year. Subsequent measurements have shown that the lowering of the territory had peaks of over 3 meters from 1950 to 1980. Subsequent surveys have highlighted further lowering in the internal areas of the Po Delta. Since the 1930s and especially in the 1940s and 1950s until the suspension decided by the Italian government in 1961, billions of cubic meters of methane and natural gas were also extracted in the Po Delta area. The extraction took place from hundreds of wells (of which thirty were located in the delta area) which did not reach a depth of 1000 meters. Through concrete structures, partly still visible in the area, the gas was sent to the compression stations, while the salt water (1 cubic meter of water for every cubic meter of gas extracted) was discharged into ditches and drains. Despite the suspension of the extractions in 1961, the territory continued to decline significantly in the following 15 years; from the beginning of the 1950s to the mid-1970s the territory dropped on average by over 2 meters up to peaks of 3.5 metres.
A study by the National Research Center (CNR) confirmed that today «the Porto Tolle delta area is characterized by subsidence exceeding 20 mm/year» (Source: CNR, 2016). The consequences of subsidence, also from an economic point of view, are easily imaginable: the sinking ground also drags the embankments with it. This causes less thickness of their safety sides, greater water pressure, greater possibility of overflows forming, greater possibility of bank failures.
The Lands Under The Sea Level
Those who live in the Po delta inhabit a land located below sea level. Extending between the regions of Emilia Romagna and Veneto, the Po delta is a territory created both by the sedimentation activity of the great river and by the work of man who over the centuries has regulated the waters and reclaimed the land. The hydraulic safety of the area is guaranteed by the incessant drainage work carried out by the “Consorzio di Bonifica della Pianura di Ferrara” for the Emilian section of the delta and by the “Corsorzio di Bonifica Delta del Po” for the remaining Venetian section. In fact, these two actors have a dense network of pumps and drainage channels that transport fresh water from the accumulation areas to the drier ones, guaranteeing hydraulic safety and the regular carrying out of human activities.
The Po delta is made up of five main branches: Goro, Gnocca, Maistra, Tolle and Po Grande-Pila, in which the rise of salt water can be observed which mainly depends on the extent of the flow arriving from upstream, divided into different branches, and from sea level. Approximately 70% of the delta’s territory is made up of agricultural land, mainly represented by arable land. Added to these are the fishing valleys, which find an ideal environment in the marginal brackish waters, where the sea mixes with the fresh water of the Po. The agricultural vocation of this territory also constitutes an element of enormous vulnerability in the current context of climate changes and the occurrence of salt wedge intrusion phenomena.
The advance of the salt wedge reached worrying levels in the two-year period 2022-23 but the phenomenon has been known since the 1990s, when the Consorzio di Bonifica Delta del Po began installing barriers to limit branching within the various tributaries. But in those years the sea water entered the river for a few hundred meters, limiting only human activities near the mouth. These are complex installations made up of poles driven into the river bed perpendicular to the current to which unidirectional mobile fins are anchored, which remain open to accommodate the flow of water towards the sea and which close automatically, forming a barrier. When the sea water goes up the river. Despite the engineering effort, the various anti-salt barriers installed along the river branches of the delta are now obsolete as they are calibrated on a flow rate much greater than the current one of the Po which in low periods has reached 113 cubic meters per second.
The Consequences in Agriculture
The progressive decrease in snowfall and rain has led to an increase in the demand for water in some sectors and in particular in the agricultural sector. Every year in the district approximately 20 billion cubic meters of water are withdrawn for various uses, of which almost 75% is intended for irrigation uses (15 billion of which 4.4 billion intended for rice production alone), while the remaining part is intended to satisfy industrial and civil uses. Most of the volumes of water withdrawn come from surface sources, but an important contribution is also made by aquifers, especially to satisfy civil and industrial uses.
The primary consequence of the intrusion of the salt wedge in coastal agricultural areas is the salinization of the soil, which manifests itself with an accumulation of salts in the circulating solution. This process can occur both due to direct irrigation with brackish water and due to phenomena of capillary rising and lateral infiltration which, in the case of the Po delta, is facilitated by a countryside level below sea level and by a texture with high percentages of clay and silt.
Salinization has a significant negative impact on agricultural production, as the majority of cultivated species are sensitive to salt stress, which can irreversibly damage metabolic processes and compromise vegetative development.
«The limit salinity percentage for distributing water is one gram per liter. The problem is that even here, 30 km from the sea, last summer the value was 10 times higher than one gram per liter» explains Rodolfo Laurenti, Deputy Director of the Po Delta Reclamation Consortium. Salt water 10 times more than it should be is a serious risk for crops in the area: within two years the cultivated areas have reduced by 20%. Also at risk is the production of rice, an important crop typical of the area which offers high profitability for the land: only 550 hectares are dedicated to rice fields and last year production decreased by 20%.
Furthermore, salinisation alters the structure of the soils and the activity of the microorganisms that populate them, leading in the most extreme cases to a drastic reduction in the organic substance content and desertification phenomena.
Monitoring of the salt wedge and the aquifer
The problem of saline intrusion into the subsoil’s water tables makes the withdrawal of fresh water from the subsoil impracticable. All this causes immediate effects such as the suspension of water withdrawals for drinking water and for irrigation and long-term consequences such as the ideal living conditions in the environment. The presence of salt water in terrestrial habitats destabilizes them, firstly damaging vegetation and then animal fauna.
The monitoring of the ingression of the salt wedge into the coastal phreatic aquifer of the Ferrara area began in 2018 and is carried out by the Agenzia Regionale per l’Ambiente e l’Energia (Arpae) and by the Daphne Oceanographic Structure through seasonal measurements of groundwater levels carried out both on land and in water using a vessel.
The monitoring points are identified starting from the river mouths and are equidistant from each other by about 3 km going up the various branches of the delta. The measurements are performed along the entire water column using a multi-parametric probe with recording of the parameters of depth, temperature, salinity, dissolved oxygen, pH, conductivity and chlorophyll.
The monitoring network dedicated to this aquifer currently consists of 24 piezometers. The groundwater level and, for each meter of depth, the specific electrical conductivity are detected in each piezometer. The surveys take place twice a year, generally in spring when the groundwater level is maximum and in summer or autumn when the level is minimum.
