Hydrogeological and geochemical overview of the karst aquifers in the Apuan Alps (Northwestern Tuscany, Italy)

Apuan Alps, in north-western Tuscany (Italy), have a very complex geological structure. For this reason karst springs show very different behaviours according to the geological setting of aquifer systems. More than 80 springs are fed by carbonate aquifers; flow rates (Q) range from 10 to 1600 L/s, in average, temperatures range from 8 to 15 °C. Deep and very slow groundwater flow feed some low-thermal springs (20-30 °C). Major karst springs (Q > 100 L/s) are concentrated in two altimetry ranges, one from 200 to 300 m a.s.l. in the seaward side (SW-NW) and a second one from 500 to 600 m a.s.l. on the inner (NE-SE) side of the mountain range. Most of the springs are the final destination of large karst systems developed in meta-dolomite and marbles characterized by a very rapid flow. Some springs have a regular regime and are fed by bathyphreatic systems in metamorphic rocks or by carbonate aquifers with a major contribution of fissured drainage in non-metamorphic rocks. Large physical-chemical variations, both in space and time, are observed as a consequence of lithological heterogeneity, mixing processes and hydrodynamic conditions. Most waters are of the Ca-HCO3 type, but Ca-SO4 and Na-Cl facies are also present. A wide range of electrical conductivity is recorded, with values between 0.1 μS/cm and 10 μS/cm. Significant differences in the average isotopic signature (e.g. δ18O from -5.5 to -8.5‰) of the "base-flow" are registered due to the variability of hydrogeological basins dimension and their distribution in terms of altitude range and side. In some cases, springs with similar chemical features and located close to each other, point out very different isotopes signature, thus highlighting complicated flow path of groundwater. Furthermore, different seasonal evolutions of isotopic signatures are registered

Modelling tools for quantitative evaluations on the Versilia coastal aquifer system (Tuscany, Italy) in terms of groundwater components and possible effects of climate extreme events

The Versilia coastal plain hosts an important and strategic aquifer for water supply. Like all coastal aquifers, it is particularly vulnerable to the saltwater intrusion, which can be amplified not only by fresh water over-exploitation, but also by the effects of climate change, including the increase of extreme events. For an optimal management of this precious resource and for its protection both in quantitative and qualitative terms, an adequate knowledge of the aquifer system is necessary through the development of conceptual and mathematical hydrogeological models. The conceptual hydrogeological model was defined on the base of an integrated multidisciplinary approach with the elaboration of stratigraphic, hydrogeological and geochemical-isotopic data. Subsequently, groundwater flow mathematical models were created using the ModFlow code and Groundwater Vistas like graphical interface, subsequently transferred to the Freewat open platform. The models enabled acquiring further knowledge about this aquifer system and to identify and, where possible to quantify, the main processes and groundwater components involved, including the seawater ingression. An important groundwater component, both in terms of water quantity and quality,resulted widespread in the fan of the Versilia River and mainly fed by the river itself in the foothill zone. Although this component seems to be able to guarantee relative protection against marine ingression, in the summer season some piezometric depressions tied to groundwater exploitation tend to expand and move towards the coast, thus favouring the seawater intrusion process. These issues can be amplified by the extreme rainy events that frequently occur in the Apuan Alps region. The huge quantity of water that quickly flows by the river up to the sea during extreme events represents a lack of feeding respect to the aquifer, and consequently the mitigation role of the fan component towards seawater intrusion can be significantly weakened. Thanks to the water budget achieved by numerical model and considering real extreme events recently occurred in the Apuan-Versilian region it was possible to make considerations about possible effects these climate regimes on the aquifer system. As outcomes, we concluded that extreme events as those occurred in the area in the past, and awaited more frequently in the future, represent a concrete threat for the coastal aquifer system that over next decades could suffer more and more seawater intrusion. Given the reliance of local human activities on groundwater, far-sighted actions of water management (e.g. managed aquifer recharge) are recommended for mitigating such as climate effects

Impacts of a large wildfire on hydrologic behavior and water resources quality in the Pisano Mount area (northwestern Tuscany): preliminary results

Wildfires are recognized as one of the most effecting ecological agents, altering geomorphological processes, hydrologic cycles, and water quality (Shakesby, 2011; Smith et al., 2011; Moody et al., 2013). On average from 50,000 to 65,000 fires occur in Europe every year, burning approximately 500,000 ha of forested areas (Turco et al., 2014). Between September 2018 and February 2019 two large wildfires burned about 1,400 ha of forest and farmlands in the Pisano Mount area, mainly in the municipalities of Calci and Vicopisano (northwestern Tuscany) (Salbitano et al., 2020). The area is mainly formed of a hilly landscape, with extensive forest cover and peculiar geomorphological features. The main outcropping rocks are composed of quartzite, arenite, phyllite and quartz metaconglomerate. The burned area is of primary importance in the hydrologic recharge processes of the groundwater resources hosted in the alluvial fan of the Zambra stream and in the multi-layer aquifer of the Pisa plain (Del Tredici, 2019). The presented study is concerned with a characterization of the hydrological cycle and water resources quality in the burned area of the Pisano Mount. The recent wildfire impact on hydrological behaviour and water quality and quantity is being evaluated by the comparison between burned and unburned areas, selecting two major catchments in the former and two sub-catchments in the latter. The catchments were selected as similar as possible by evaluating morphometry, geology and pre-fire vegetation features. A network of meteorological stations was already present and was implemented. Five hydraulic sections of the main streams draining the area are currently monitored for hydraulic level and physico-chemical parameters. All the sections are equipped for monitoring water level, electrical conductivity, and temperature, whereas two probes register also pH, turbidity and oxidation-reduction potential. Monthly samples are collected for stream water and groundwater. In addition, four plate lysimeters will be installed to sample water infiltrated through the soils to evaluate the chemical exchanges between soil and rainwater. Samples are analysed for major anions and cations, total suspended solids, trace elements, water isotopes and organic compounds, to search chemical perturbation potentially arising by the wildfire. Finally, a survey of measurement of the hydraulic properties, permeability and sorptivity, is also scheduled, aiming to address changes of these properties induced by wildfires. The main knowledge about wildfire perturbation and the experimental design will be illustrated including the preliminary results of the work

Numerical modelling in the coastal aquifer between Burlamacca Canal and Bufalina Ditch, southern Versilia (Tuscany, Italy)

Groundwater numerical models are necessary instruments for management of water resources and for their protection, both in terms of quantity and quality. In this paper we present a study on an area located along the southern coastline of Versilia (Viareggio,LU) affected by marine intrusion. The main purpose of this work was to create a mathematical flow model based on a conceptual hydrogeological model defined by a multidisciplinary approach. For the realization of the numerical model the ModFlow code and the graphic interface Visual ModFlow was used. The procedure for defining the mathematical model involves a series of steps such as horizontal and vertical discretization of the space, the definition of the initial and boundary conditions, the assignment of the hydraulic properties of the cells and, finally, the definitions of external perturbations to the system (recharge, evapotranspiration, drainage and pumping wells). Implementation, development and calibration of the numerical flow model was performed both in steady and transient state. Both models were calibrated using the manual "trial and error adjustment" method using heads measurements. Moreover the model results gathered in transient state simulation were compared with the data continuously recorded by a piezometer of the monitoring network of the Regional Hydrological Service. There is a good correlation between the measured data and those calculated by the model which then turns out to be sufficiently representative and provides a solid basis for the development of a transport model that could be useful to control and manage the phenomenon of the salt water intrusion

Moisture sources and climatic effects controlling precipitation stable isotope composition in a western Mediterranean island (Pianosa, Italy)

The Mediterranean basin is indicated as a hot spot of climate change, which is an area whose climate is especially responsive to variations. The insular environment is one of the most threatened by the current climate change, especially in terms of drought events, with serious consequences for water scarcity and water stress. This issue is even enhanced in small islands, whose ecosystems are among more sensitive to climatic changes and water availability. The stable isotope composition of hydrogen (delta 2H) and oxygen (delta 18O) in precipitation is globally recognized as a powerful natural tracer in the water cycle and represents the starting point to investigate hydrological processes. The understanding of the prevailing factors that drive the isotopic variability of precipitation in the Mediterranean is therefore essential to unravel the hydrological processes and to ensure proper and sustainable management of potentially vulnerable resources to climate change. Here, we discuss the results of multi-year isotopic monitoring in the period 2014-2021 of monthly precipitation collected on Pianosa Island (Italy), a small island located in the northern Tyrrhenian (western Mediterranean). The lower slope and intercept of the Local Meteoric Water Line of the island compared to the Global Meteoric Water Line indicated warmer and drier climatic conditions, suggesting the existence of sub-cloud evaporation processes of raindrops during precipitation, especially in summer. The mean delta 18O of precipitation was lower with respect to other sites placed at higher elevation in this Mediterranean region, due to the lack of summer precipitation which were generally enriched in heavy isotopes. Temperature and amount effects may explain part of the delta 18O variability observed at the monthly and seasonal scale. An HYSPLIT-based moisture uptake analysis indicated the area between the western Mediterranean basin, Italy, and the Adriatic Sea as the region that supplied most of the humidity associated with monthly precipitation samples on Pianosa Island. Less moisture was picked from the northwestern areas of Europe, the North Atlantic Ocean, the proximal Atlantic Ocean, the Iberian Peninsula and North Africa. Consistently with the rainout effect, the higher the moisture fraction picked from the more proximal regions, the more positive the delta 18O of precipitation occurring on Pianosa Island; conversely, the higher the percentage of moisture sourced from more distal regions, the more negative the delta 18O. A multiple linear model was proposed to predict the delta 18O of monthly precipitation from temperature, precipitation amount and moisture origin data, which explained 45% of the delta 18O variability. The deuterium excess variability on the island was partly controlled by the local climatic variables, whose effect potentially modifies the original d-excess signature imprinted at the moisture source. No relationship was found between the precipitation deuterium excess and moisture sources, suggesting that more attention should be paid when using the deuterium excess as a tracer of moisture origin, especially in the Mediterranean

Estimating the Isotopic Altitude Gradient for Hydrogeological Studies in Mountainous Areas: Are the Low-Yield Springs Suitable? Insights from the Northern Apennines of Italy

Several prior studies investigated the use of stable isotopes of water in hydrogeological applications, most on a local scale and often involving the isotopic gradient (evaluated by exploiting the so-called altitude effect), calculated on the basis of rainwater isotopes. A few times, this gradient has been obtained using the stable isotopic contents of low-yield springs in a limited time series. Despite the fact that this method has been recognized by the hydrogeological community, marked differences have been observed with respect to the mean stable isotopes content of groundwater and rainwater. The present investigation compares the stable isotopic signatures of 23 low-yield springs discharging along two transects from the Tyrrhenian sea to the Po Plain of Italy, evaluates the different isotopic gradients and assesses their distribution in relation to some climatic and topographic conditions. Stable isotopes of water show that groundwater in the study area is recharged by precipitation and that the precipitation regime in the eastern portion of the study area is strongly controlled by a shadow effect caused by the Alps chain on the air masses from central Europe. Stable isotopes (in particular the d18O and deuterium excess (d-excess) contents together with the obtained isotopic gradients) allow us to identify in the study area an opposite oriented orographic effect and a different provenance of the air masses. When the windward slope is located on the Tyrrhenian side, the precipitation shows a predominant oceanic origin; when the windward slope moves to the Adriatic side, the precipitation is characterized by a continental origin. The main results of this study confirm the usefulness of low-yield springs and the need for a highly detailed survey-scale hydrological investigation in the mountainous context

Field testing of tube-dip-in water precipitation collectors used in isotope hydrology

The oxygen and hydrogen stable isotope composition in precipitation serves as a benchmark in most isotope atmospheric, (eco-)hydrological, and paleoclimatological applications. Several rain collectors have been designed for collecting monthly, daily or event-based precipitations aiming to prevent evaporation and associated isotope fractionation. Oil collectors have been the most widely used for many years and only recently they are being progressively replaced by free- oil Tube-dip-in water collectors, especially after their formal publication by Gröning et al. (2012) and the production of a commercial version (Palmex Ltd). Although the reliability of this precipitation collector has been proven, many doubts remain when dealing with small precipitation amounts (Michelsen et al., 2018). Field testing of precipitation collectors is therefore encouraged, which should be carried out under the same environmental conditions of areas where researchers want to undertake their studies. In this work, we tested the field performance of different precipitation collectors in preventing evaporation and isotope fractionation. Two main objectives were behind this study: i) to evaluate the reliability of tube-dip-in water collectors for very low precipitation amounts; ii) to test a homemade Tube-dip-in water collector for different water amounts. The experiment consisted of simulating the collection of small monthly precipitation samples in spring and summer when atmospheric conditions are more likely to promote evaporation. The experiment was carried out on the rooftop of the Earth Science Department of the University of Pisa from March 2022 to July 2022. Four different collector designs were tested simultaneously over four different periods (each lasting approximately one month): a Control collector with no anti-evaporative system; an Oil collector; a Palmex Tube-dip-in water collector; a homemade Tube-dip-in water collector. They were filled to 1.4% of their total volume (10 L) with water of known isotope composition. Since the diameter of 13.5 cm of most common funnels, this percentage corresponds to ~10 mm. Other two homemade Tube-dip-in water collectors were filled to 5% and 10%, corresponding to ~35 mm and ~70 mm. All the collectors were placed outside at the start of each period. Evaporative mass losses were determined gravimetrically and samples for isotope analyses were collected at the end of each period. On average, the Oil collector showed the smallest mass losses, and the isotope shifts were much lower than analytical errors. The Palmex collector failed, with even larger mass losses and isotope shifts (Δδ18O = 0.42‰ and Δδ2H = 1.6‰) than the Control collector. The home-made Tube-dip-in water collectors performed well and better than Palmex. Mass losses and isotope shifts tended to increase with increasing temperature and decreasing relative humidity

Last century rainfall variations in northern Tuscany (Italy)

Climate change is one of the main factors affecting groundwater resources, making an assessment necessary for the future exploitation. Recent past climate changes evaluation requires an extensive and distributed meteorological database. Some of the principal groundwater systems in Tuscany (particularly the karst aquifer systems of the Apuan Alps) are present in its northern part. Also, Tuscany has a densely distributed rain gauges network (some datasets longer than 100 years). Bartolini et al. (2014) and D’Oria et al. (2017) already identified precipitation reduction and temperature increase in the whole of Tuscany and its northern portion for the last century. In this work, we investigate the time evolution of rainfall in northern Tuscany in the last century both for quantity and dynamics. The number of rain gauges chosen for this analysis was heavily increased and deeply controlled for datasets continuity and homogeneity. We studied the precipitation data in terms of mean annual precipitation differences between the periods 1990/2019 – 1921/1950 and 1990/2019 – 1951/1980; detection of trends in the yearly and seasonal precipitation via the Mann-Kendall test; rainfall events variations; spatial distribution of the precipitation trend; changing in single precipitation events extension through the last seventy years. The precipitation reduction, identified in the previous studies, is confirmed and seasonally featured. We highlight variations in the rainfall events for different rain-depth intervals. We estimate the last century annual precipitation decrease to be around 10% of the Mean Annual Precipitation in the past thirty years. However, this variation seems not evenly distributed in the analysed area. The observed evolution in local hydrological regime represents a serious threat to the strategic karst groundwater of the area, which is sensitive to hydro-climate conditions (Doveri et al., 2019). The precipitation decrease and the change in distribution are already modifying the flow rate and regime in some major karst springs

Geothermal resources within carbonate reservoirs in western Sicily (Italy): A review

Abstract Low-to-medium temperature fluid reservoirs hosted in carbonate rocks are some of the most promising and unknown geothermal systems. Western Sicily is considered a key exploration area. This paper illustrate a multidisciplinary and integrated review of the existing geological, geochemical and geophysical data, mainly acquired during oil and gas explorations since the 1950s, specifically re-analyzed for geothermal purposes, has led to understanding the western Sicily geothermal system as a whole, and to reconstructing the modalities and particular features of the deep fluid circulation within the regional reservoir. The data review suggests the presence of wide groundwater flow systems in the reservoir beneath impervious cap rocks. We identified the main recharge areas, reconstructed the temperature distribution at depth, recognized zones of convective geothermal flow, and depicted the main geothermal fluid flow paths within the reservoir. We believe that our reconstruction of geothermal fluid circulation is an example of the general behavior of low-to-medium enthalpy geothermal systems hosted in carbonate units on a regional scale. Due to the recent technological developments of binary plants, these systems have become more profitable, not only for geothermal direct uses but also for power production