Seasonal geochemical and isotopic investigations for sustainable water resource managementa case study of the pordenone plain (ne italy)

Résumé

The availability and stability of water resources have profound implications for food security, rural pollution, urban energy production, industrial development, and overall economic growth. These resources are not only vulnerable to the direct impacts of human activities, such as pollution from agricultural and industrial practices, but also face additional vulnerability due to the impacts of climate change. To ensure the sustainability of water sources, it is crucial to conduct thorough surveys and analyses of groundwater resources to prevent their overexploitation. A multidisciplinary approach involving geological, hydrogeological, geochemical, and geophysical investigations is necessary to identify and characterize the sources and driving mechanisms. Understanding the responses of aquifers to both natural and human-induced stresses is essential, as it allows for the development of practical water resource management plans. Water geochemistry and isotopes play a crucial role in this process. In fact, they allow us to identify the primary processes of water-rock interaction (WRI) responsible for the formation of different geochemical facies, determine the origin of the water, evaluate the average residence time of water in the aquifers, model the transport routes of groundwater, evaluate groundwater quality, and contribute to the management of groundwater resources. To support the sustainable management of the water cycle and formulate planning strategies for responsible water resource use, this PhD project developed a comprehensive protocol for seasonal geochemical and isotopic investigations of complex alluvial aquifer systems at a watershed scale. The research was organized into three main phases: i) Protocol design, which involved an extensive review of existing literature, the selection of the study area, and the execution of a preliminary test in a delimited area to gather initial feedback and refine the protocol. ii) Geochemical and isotopic characterization of the test site: this phase aimed to develop a geochemical-isotopic conceptual model of the study site, enabling the identification of recharge areas, characterization of hydrofacies from a geochemical-isotopic perspective, and the study of their evolution over time. iii) Seasonal geochemical-isotopic monitoring: during this two-year phase, representative samples were collected from the hydrogeological system in the study area with the aim of understanding the influence of climatic changes and anthropogenic factors on local water bodies. The selected study area was the Pordenone Plain (PP) within the Friuli Plain, situated in the Friuli-Venezia Giulia region (NE Italy). The PP, like the Friuli Plain in general, can be divided into two main physiographic units: the High Plain, characterized by coarse and highly permeable deposits, and the Low Plain, where complex layered aquifer systems consisting of gravelly-sandy deposits interbedded with clays and silts are found. The two physiographic zones are divided by the resurgence belt, that represent a kind of "overflow". Unsustainable groundwater use in this area has led to a quantitative and qualitative decline in water resources, especially during the dry season, due to changes in precipitation patterns, increased water extraction for irrigation, and other uses. This research aims to provide a comprehensive characterization of the aquifers and establish the relationship between recharge areas and flow paths in the plain. It also seeks to enhance our understanding of groundwater dynamics and complex hydrogeochemical processes during seasonal variations in the PP. By integrating geological, hydrogeological, and anthropogenic factors, this PhD project provides valuable tools for sustainable water resource management and effective strategies to address future water challenges in the region. To validate the analysis protocol, a preliminary experimental study was conducted in a delimited study area. In this context, the methodological protocol was employed to assess the vulnerability of aquifers in the Fiume Veneto (PN) area. The goal was to evaluate the suitability or unsuitability of water resource use in the area. The methodological protocol allowed for the differentiation of various water types originating from both deep confined aquifers and natural or anthropogenic surface channels. The results confirmed that water from deep confined aquifers and water from the sewer system and surface channels shared geochemical and isotopic characteristics typical of deep, slow-recharge water. This information was crucial in identifying unsustainable water resource use, particularly the excessive use of domestic fluently-wells that continuously discharged water into surface channels and the sewer system. The preliminary study conducted in the Fiume Veneto area significantly contributed to the practical application of the geochemical and isotopic analysis protocol originally conceived in the early stages of the PhD project. Through the preliminary study, the protocol was revised and improved to ensure its effectiveness in the seasonal geochemical and isotopic characterization of the study area at the watershed level. The subsequent detailed geochemical-isotopic characterization of the test site, the PP, was essential in developing a geochemical-isotopic conceptual model. This model identified recharge areas, characterized various hydrofacies from a geochemical-isotopic perspective, and studied their evolution. The results obtained, combined with lithological and geochemical-isotopic data and relevant hydrogeological information, facilitated the development of a comprehensive hydrogeochemical conceptual model for the PP. This model highlighted three main groundwater flows differentiated by depth in the plain: i. Flow 1 - a superficial flow influencing unconfined or semi-confined/superficial aquifers in the High Plain, which are hosted in gravelly deposits. These waters exhibit a Ca(Mg)-HCO3 composition and are significantly influenced by local precipitation; ii. Flow 2 - an intermediate flow fed by the foothill zone, including foothill springs, unconfined deep aquifers in the High Plain, as well as semi-confined/surface aquifers (at a depth of 40-50 m) situated along the resurgence belt in the plain. The source of recharge for this water has a Ca-HCO3 composition, as identified through spring water analysis, but, through continuous interaction with the geological rocks and deposits in the plain, it acquires a Ca(Mg)-HCO3 composition; iii. Flow 3 - a deep flow originating from the mountainous recharge zone, which influences deep confined aquifers in the Low Plain. These waters are hosted in sandy lenses interbedded with clay layers and exhibit a Ca(Mg)-HCO3 composition with relatively high Na-K values. These data suggest that cation exchange with silty-clayey layers is responsible for the water composition variations. Additionally, these waters exhibit lower and uniform 18O values and very low tritium concentrations compared to other water samples, indicating slow groundwater flow. The development of a geochemical-isotopic conceptual model was crucial in understanding the hydrogeological dynamics of the study area. Finally, the seasonal geochemical and isotopic monitoring conducted during the 2021-2022 period revealed important climatic information. In 2022, a notable increase in temperatures was observed compared to the previous year. Simultaneously, precipitation and snowfall in 2022 were lower than in 2021. Piezometric variations in the unconfined aquifer of the High Plain highlighted a long-term decline in water levels, confirming trends observed in recent decades. This seasonal study phase specifically focused on the two main flows identified by the geochemical-isotopic conceptual model in the PP, namely Flow 2 and Flow 3. Flow 2 represents a complex and diversified component of the PP, with two distinct pathways. The first pathway, due to permeability contrasts resulting from regional foothill faults, feeds the foothill springs on the eastern slope of the Cansiglio Plateau. The second continues its flow in the plain and feeds various hydrogeological units in the PP, such as the High Plain and Low Plain. For the first pathway, seasonal variations closely mirror meteorological changes. Specifically, dilution effects were observed during the winter period due to autumn rains and an increase in chemical-physical parameters and ionic concentrations during summer periods characterized by reduced precipitation, especially in 2022. These results indicate an immediate response of foothill springs to meteorological variations, further confirmed by their reaction to the drought conditions of 2022, characterized by reduced snowfall and precipitation. Isotopic values of spring waters are closely related to rainfall and snowfall on the Cansiglio Plateau, showing an 18O depletion during the summer - attributed to spring snowmelt - and isotopic enrichment due to recharge induced by autumn precipitation in the winter period. Furthermore, it was observed that18O depletion was less pronounced during the summer of 2022 due to the 2022 drought, characterized by reduced snowfall in the foothill sector. In the second pathway, which crosses various hydrogeological units in the plain, different responses to seasonal fluctuations were observed, mainly influenced by the presence of the resurgence belt. Two main groups were identified: - Group 1 includes the unconfined aquifer of the High Plain and the semi-confined/shallow aquifers near the resurgence belt, especially north of it. For this group, an increase in chemical-physical parameters and ionic concentrations was observed during the summer period compared to the winter period. However, it should be noted that seasonal variations did not show consistent responses in both monitoring years. Furthermore, these variations affected different water types differently. For example, in the unconfined aquifers of the High Plain, seasonal variations of 18O values could be correlated with fluctuations in the water table level. These results emphasize the strong connection between minimum and maximum groundwater recharge periods and the geochemical evolution of unconfined aquifers. On the other hand, semi-confined/shallow aquifers exhibited seasonal variations of 18O values that followed the seasonal water table level, similar to the unconfined aquifer but with a regulated seasonal flow, indicating a more marked recharge during the winter period compared to the summer. This dampening effect could potentially be attributed to lithological changes along the resurgence belt zone, regulating seasonal flow. - Group 2 includes monitoring points in semi-confined/shallow aquifers located south of the resurgence belt within the complex hydrogeological system of the Low Plain. For this group, an increase in chemical-physical parameters and ionic concentrations during the winter period compared to the summer. The observed seasonal inversion was attributed to the hydrogeological context in which these aquifers are situated, with a significant permeability variation along the resurgence belt that could significantly slow down the flow. This phenomenon could lead to "delayed seasonal effects" of recharge recorded in the Low Plain compared to those in the High Plain. In this case as well, the seasonal stable isotope trends, particularly 18O, supported this hypothesis. Furthermore, samples from semi-confined/shallow aquifers, both in Group 1 and Group 2, given their geological location and shallow depth, were influenced by anthropogenic agricultural fertilizers. Conversely, the influence of agricultural activities was much less marked for unconfined aquifer samples. Flow 3, characterized by slow outflow, originates in a mountainous recharge area. Seasonal geochemical and isotopic analyses of deep confined aquifers located in the Low Plain did not reveal distinct seasonal trends, with limited variations in both chemical and isotopic composition between summer and winter over two years of monitoring. The geochemical and isotopic characteristics of the deep aquifers of the Low Plain, known for their low transmissivity, appear to be minimally influenced by the observed climatic and piezometric changes in the High Plain during the study period and remained unaffected by the current hydrogeological cycle of the PP. These aquifers, characterized by high water quality, low salinity, and the absence of NO3- and SO42-, represent a valuable source of drinking water that ensured conservation and effective management. In conclusion, this PhD project highlights the critical role of precipitation, both rain and snow, within the intricate hydrogeological system of the PP. It is evident that a reduction in precipitation and snowfall, as observed during the 2021-2022 period, can exert significant negative effects on the entire hydrogeological system of the Friulian Plain, particularly in the PP. The vitality of the system is closely linked to recharge areas situated in the mountains and foothill regions, where snowmelt and precipitation play a fundamental role in sustaining the entire hydrogeological system. Additionally, it is crucial to recognize anthropogenic influences on water use and quality, particularly in the context of excessive groundwater exploitation. The growing demands for domestic, agricultural, and industrial purposes, combined with the long-term decrease in precipitation, have put a strain on the hydrogeological system. The multidisciplinary approach of this PhD project, combining geochemical and isotopic analyses with lithological and hydrogeological investigations, not only enhances our understanding of complex alluvial systems but also provides valuable insights that can guide future efforts in the conservation and effective management of this essential natural resource. Addressing anthropogenic factors alongside climate change becomes imperative for sustainable water resource management. As climate models continue to evolve, understanding and preserving these critical components of the hydrogeological system are essential to ensure its long-term health and functionality. The geochemical and isotopic methodological protocol emerging from this doctoral research project serves as the fundamental framework for an expanded multidisciplinary exploration of intricate hydrogeological systems, with a specific focus on alluvial plains. This framework is of paramount importance in ensuring the production of robust and reliable findings, thereby contributing to efforts directed at the conservation and effective management of water resources to counteract the anthropogenic and climate change effects.