The Tiber basin has been extensively studied in recent decades from both the hydrogeological and hydrochemical points of view. However, a synoptic work on the hydrochemical and isotopic characteristics of the low water discharge of the river Tiber and its tributaries was still not available. The main aim of the present survey was to establish a relationship between the low water surface discharge and the groundwater circulation feeding it by assessing the chemical and isotopic composition of the surface water. During the period 2001-2002 four sampling campaigns were run on the water of the Tiber and its main tributaries: Chiascio, Topino, Paglia, Chiani, Nera, Velino, Treia and Aniene at some 40 sites. The survey showed how variation in the basic chemical-isotopic composition along the course of the Tiber displays no monotonic evolution from upstream to downstream as is found instead in other hydrographic basins such as the Arno, but instead changes suddenly at the main river junctions according to the composition and flow rate of the tributaries. This phenomenon is in perfect agreement with the hydrogeological characteristics of the Tiber, of which the groundwater contributions are huge and distributed in a spatially dishomogeneous way depending on the basin's geological-structural configuration. The influence of the tributaries is particularly strong at the point of entry of the river Nera owing to mixing with the mineralized water of the Montoro-Stifone spring system. The latter makes a particularly significant contribution to the surface flow of the Nera as it is characterized by a dominant Na-Cl-SO4 type water with stable flowrates of about 15 m3/s throughout the year, which modify the chemical characteristics of the Tiber downstream. The flow through the Latium volcanic region is furthermore marked by a gradual increase in the concentration of elements typical of volcanic rock, such as potassium and arsenic, while a significant increase in the calcium bicarbonate component is observed at the point of entry of the Aniene. Finally, seawater intrusion is perceived near the mouth. Moreover, the stable isotope composition (?18O and ?2H) varies substantially according to the physiographic characteristics of the basins fed by each tributary: the Nera-Velino system, characterized by a hydrographic basin with a higher average elevation contributes to the basic Tiber flow with isotopically impoverished water, while the Paglia-Chiani and the Treia, characterized by basins with lower average altitudes participate with isotopically enriched water; the Aniene and the Chiascio-Topino are characterized by isotopically intermediate values, compatible with the average altitude of their respective basins. Briefly, the experimental data point to the strong links between the hydrochemical facies of the individual tributaries and the geolithological configuration of the relative basin which in each case "labels" the overall composition of the Tiber until the next lateral contribution is received. The interpretation of the data was complicated by the heavy regulatory action of the numerous artificial barrages, above all in the Tiber and the Nera, which produce variations in the instantaneous flowrate over a 24 hour period and thus a mixing of the contributions that varies over time. It follows that the concentration of several parameters at a given site downstream from the dams can vary appreciably over a 24 hour period. This situation must be taken into account in the monitoring performed by the control bodies in order to ensure a correct interpretation of the environmental status of the hydrographic system.
INDAGINE SULLE CARATTERISTICHE IDROCHIMICHE E ISOTOPICHE DEL DEFLUSSO DI MAGRA NEL BACINO DEL TEVERE. RELAZIONI TRA DEFLUSSO SUPERFICIALE E CIRCOLAZIONE IDRICA SOTTERRANEA
Giuliano G;Preziosi E;Gherardi F;
2005
Abstract
The Tiber basin has been extensively studied in recent decades from both the hydrogeological and hydrochemical points of view. However, a synoptic work on the hydrochemical and isotopic characteristics of the low water discharge of the river Tiber and its tributaries was still not available. The main aim of the present survey was to establish a relationship between the low water surface discharge and the groundwater circulation feeding it by assessing the chemical and isotopic composition of the surface water. During the period 2001-2002 four sampling campaigns were run on the water of the Tiber and its main tributaries: Chiascio, Topino, Paglia, Chiani, Nera, Velino, Treia and Aniene at some 40 sites. The survey showed how variation in the basic chemical-isotopic composition along the course of the Tiber displays no monotonic evolution from upstream to downstream as is found instead in other hydrographic basins such as the Arno, but instead changes suddenly at the main river junctions according to the composition and flow rate of the tributaries. This phenomenon is in perfect agreement with the hydrogeological characteristics of the Tiber, of which the groundwater contributions are huge and distributed in a spatially dishomogeneous way depending on the basin's geological-structural configuration. The influence of the tributaries is particularly strong at the point of entry of the river Nera owing to mixing with the mineralized water of the Montoro-Stifone spring system. The latter makes a particularly significant contribution to the surface flow of the Nera as it is characterized by a dominant Na-Cl-SO4 type water with stable flowrates of about 15 m3/s throughout the year, which modify the chemical characteristics of the Tiber downstream. The flow through the Latium volcanic region is furthermore marked by a gradual increase in the concentration of elements typical of volcanic rock, such as potassium and arsenic, while a significant increase in the calcium bicarbonate component is observed at the point of entry of the Aniene. Finally, seawater intrusion is perceived near the mouth. Moreover, the stable isotope composition (?18O and ?2H) varies substantially according to the physiographic characteristics of the basins fed by each tributary: the Nera-Velino system, characterized by a hydrographic basin with a higher average elevation contributes to the basic Tiber flow with isotopically impoverished water, while the Paglia-Chiani and the Treia, characterized by basins with lower average altitudes participate with isotopically enriched water; the Aniene and the Chiascio-Topino are characterized by isotopically intermediate values, compatible with the average altitude of their respective basins. Briefly, the experimental data point to the strong links between the hydrochemical facies of the individual tributaries and the geolithological configuration of the relative basin which in each case "labels" the overall composition of the Tiber until the next lateral contribution is received. The interpretation of the data was complicated by the heavy regulatory action of the numerous artificial barrages, above all in the Tiber and the Nera, which produce variations in the instantaneous flowrate over a 24 hour period and thus a mixing of the contributions that varies over time. It follows that the concentration of several parameters at a given site downstream from the dams can vary appreciably over a 24 hour period. This situation must be taken into account in the monitoring performed by the control bodies in order to ensure a correct interpretation of the environmental status of the hydrographic system.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
