| Title | MICROINQUINANTI DI ORIGINE NATURALE NEGLI ACQUIFERI VULCANICI: UN APPROCCIO INTEGRATO QUANTITATIVO E QUALITATIVO NEL LAZIO SETTENTRIONALE |
| Abstract | Risk assessment in the management of drinking water is one of the aim of the GNDCI (National Group for the Defense from Hydrogeological Hazards), and in particular of MIMA project (Integrated Methodologies of Monitoring Aquifers), leaded by IRSA-CNR (Water Research Institute).
In this study the preliminary results of a research programme conducted in cooperation with the University of Rome "La Sapienza" are shown. The main objective is the quality assessment of groundwaters hosted in Northern Latium volcanic complexes (Sabatini, Vico and Vulsini volcanoes), which represent important water resources reservoirs at a regional scale.
In northern Latium (central Italy) anomalous concentrations of minor toxic elements, such as As and F, of natural origin in ground waters are known since several years (Brondi 1986, Dall'Aglio et al., 2001); in this area, volcanic alkaline-potassic rocks of quaternary age largely outcrop, hosting an important regional aquifer exploited also for human consumption. Concentrations in these elements are often above the threshold values for drinking waters established by the European Directive 98/83/EC (i.e.10 ?g/l for As and 1.5 mg/l for F).
In order to shed light on the hydrochemical processes leading to the enrichment in the above cited elements, a comprehensive study of the groundwater system was conducted in a representative area north of Rome. The aim is the characterization of the hydrodynamic and hydrochemical features of the groundwater system by integrating a quantitative approach (river discharge and piezometric head measures) with a qualitative one (study of the behaviour of chemical elements and their inter-correlation, analysis of hydrochemical facies). The relative variation of minor elements in groundwater has been considered a helpful tool to understand the local hydrogeological setting, to show the relationships between local and regional aquifers and to differentiate their contributions to the basal flow of rivers' discharge.
Northern Latium is characterized by a widespread outcrop of volcanic units of Quaternary age that overlay a sedimentary substratum (Baldi et al., 1974) represented by Mesozoic limestones, Cretaceous-Paleogenic flysch sequences and Plio-Pleistocene clay, sand and gravel deposits. The volcanic complex represents an important hydrogeological structure which discharges about 30 m3/s in an area of 3500 km2 (Boni et al., 1986). The basal unconfined aquifer, hosted in the volcanic units, is separated from the geothermal reservoir of the Mesozoic limestones beneath by Pliocene clays and flysch sequences.
Some of the volcano-tectonic depressions formed during the last phases of activity host now the main lakes of this area.
The study area is located about 100 km north of Rome between Vico lake and the Tiber valley. In this area volcanic products are mostly related to Vico volcano activity; the "Red Tuff with black scoria" is the mainly outcropping unit. The Pleistocene deposits underneath, represented by the sand and gravel deposits of the Paleo-Tiber (Alvarez, 1972), outcrop only at the eastern edge of the area.
In July and September 2002-2003 two hydrogeochemical surveys were conducted in drought periods; 60 private wells, 5 springs and 16 river sites were sampled using standard methodologies for sampling and for chemical analysis. Eh, pH, electric conductivity, alkalinity and piezometric levels of the boreholes were analysed in situ. At river sites discharge measures were performed in order to evaluate the contribution of groundwater to the rivers at different elevation: measuring sites were located possibly where the stratigraphic boundaries outcrop.
The samples were transported to the IRSA laboratory in refrigerated containers and analysed for major anions (F-, Cl-, PO43-, NO3-, SO42-) by ion chromatography, and for major cations (Na+, K+, Ca2+, Mg2+) and trace metals (28 elements) by inductively coupled plasma mass spectrometry (ICP-MS).
The basal aquifer flows eastward, moving from the highest part of the volcanic complex to the Tiber valley. Its piezometric surface has been reconstructed using water levels measured during field activity. In the east part of the area it circulates through the pyroclastic units of Vico volcano with an elevation of about 250-300 m a.s.l., and in the west part in the Paleo-Tiber deposits from 100 to 50 m a.s.l., where it shows a lower hydraulic gradient.The basal aquifer feeds several small rivers and springs: the main draining axis is the Rio Fratta with a measured discharge of about 0.4 m3/s in summer 2002-2003 (0.85 m3/s discharge has been recorded previously) at the confluence with the Tiber river. An important discharge augmentation (130-150 l/s) was recorded where the Paleo-Tiber deposits outcrop.
Moreover, local perched aquifers have been observed in the volcanic units overlaying the sedimentary complex and/or in the upper horizon of the last one. The perched aquifers are going to be abandoned because of their overexploitation during the last tens of years. Nowadays, the deeper aquifer represents the most interesting reservoir of groundwater resources in the area, used mainly for agricultural and drinking purposes.Chemical composition shows an alkaline-earth bicarbonate hydrofacies: a trend towards the alkaline facies is more evident for some samples from the volcanic units. A good linear correlation between As and F was observed (R2 = 0.64): it is probably due to a common process in their mobilization and in their enrichment in ground waters. Their occurrence in both volcanic and sedimentary aquifers clearly indicates the hydraulic connection of ground waters hosted in the different units. Because of this relation the two elements show a similar trend when compared to Ca concentration. The enrichment in Ca content and the relative reduction in the minor elements (As and F) suggest an evolution of the chemical facies of the waters flowing from the volcanic units towards the sedimentary ones as a consequence of leaching and/or mixing processes. The second hypothesis is the most probable because of the limited outcropping of Paleo-Tiber units. A similar chemical variation has been observed downstream along the rivers in relation to the contribution of the sedimentary aquifer. This inverse correlation is in agreement with the spatial distribution of samples.
Future research developments foresee a more detailed analysis of the hydrodynamic and hydrogeochemical setting of the area through the elaboration of the newly collected data (2003 survey). Modelling tool will allow to go on with the analysis of water-rock interaction processes (Vivona et al., 2004) and with the study of the solubilization and the enrichment processes of minor elements of toxicological importance. |
| Source | Ingegneria e geologia degli acquiferi (20), pp. 3–14 |
| Journal | Ingegneria e geologia degli acquiferi |
| Editor | Associazione georisorse e ambiente. Sezione italiana acque sotterranee, Torino, Italia |
| Year | 2005 |
| Type | Articolo in rivista |
| Authors | PREZIOSI E., VIVONA R., GIULIANO G. |
| Text | 67694 2005 MICROINQUINANTI DI ORIGINE NATURALE NEGLI ACQUIFERI VULCANICI UN APPROCCIO INTEGRATO QUANTITATIVO E QUALITATIVO NEL LAZIO SETTENTRIONALE PREZIOSI E., VIVONA R., GIULIANO G. CNR IRSA Risk assessment in the management of drinking water is one of the aim of the GNDCI National Group for the Defense from Hydrogeological Hazards , and in particular of MIMA project Integrated Methodologies of Monitoring Aquifers , leaded by IRSA CNR Water Research Institute . In this study the preliminary results of a research programme conducted in cooperation with the University of Rome La Sapienza are shown. The main objective is the quality assessment of groundwaters hosted in Northern Latium volcanic complexes Sabatini, Vico and Vulsini volcanoes , which represent important water resources reservoirs at a regional scale. In northern Latium central Italy anomalous concentrations of minor toxic elements, such as As and F, of natural origin in ground waters are known since several years Brondi 1986, Dall Aglio et al., 2001 ; in this area, volcanic alkaline potassic rocks of quaternary age largely outcrop, hosting an important regional aquifer exploited also for human consumption. Concentrations in these elements are often above the threshold values for drinking waters established by the European Directive 98/83/EC i.e.10 g/l for As and 1.5 mg/l for F . In order to shed light on the hydrochemical processes leading to the enrichment in the above cited elements, a comprehensive study of the groundwater system was conducted in a representative area north of Rome. The aim is the characterization of the hydrodynamic and hydrochemical features of the groundwater system by integrating a quantitative approach river discharge and piezometric head measures with a qualitative one study of the behaviour of chemical elements and their inter correlation, analysis of hydrochemical facies . The relative variation of minor elements in groundwater has been considered a helpful tool to understand the local hydrogeological setting, to show the relationships between local and regional aquifers and to differentiate their contributions to the basal flow of rivers discharge. Northern Latium is characterized by a widespread outcrop of volcanic units of Quaternary age that overlay a sedimentary substratum Baldi et al., 1974 represented by Mesozoic limestones, Cretaceous Paleogenic flysch sequences and Plio Pleistocene clay, sand and gravel deposits. The volcanic complex represents an important hydrogeological structure which discharges about 30 m3/s in an area of 3500 km2 Boni et al., 1986 . The basal unconfined aquifer, hosted in the volcanic units, is separated from the geothermal reservoir of the Mesozoic limestones beneath by Pliocene clays and flysch sequences. Some of the volcano tectonic depressions formed during the last phases of activity host now the main lakes of this area. The study area is located about 100 km north of Rome between Vico lake and the Tiber valley. In this area volcanic products are mostly related to Vico volcano activity; the Red Tuff with black scoria is the mainly outcropping unit. The Pleistocene deposits underneath, represented by the sand and gravel deposits of the Paleo Tiber Alvarez, 1972 , outcrop only at the eastern edge of the area. In July and September 2002 2003 two hydrogeochemical surveys were conducted in drought periods; 60 private wells, 5 springs and 16 river sites were sampled using standard methodologies for sampling and for chemical analysis. Eh, pH, electric conductivity, alkalinity and piezometric levels of the boreholes were analysed in situ. At river sites discharge measures were performed in order to evaluate the contribution of groundwater to the rivers at different elevation measuring sites were located possibly where the stratigraphic boundaries outcrop. The samples were transported to the IRSA laboratory in refrigerated containers and analysed for major anions F , Cl , PO43 , NO3 , SO42 by ion chromatography, and for major cations Na , K , Ca2 , Mg2 and trace metals 28 elements by inductively coupled plasma mass spectrometry ICP MS . The basal aquifer flows eastward, moving from the highest part of the volcanic complex to the Tiber valley. Its piezometric surface has been reconstructed using water levels measured during field activity. In the east part of the area it circulates through the pyroclastic units of Vico volcano with an elevation of about 250 300 m a.s.l., and in the west part in the Paleo Tiber deposits from 100 to 50 m a.s.l., where it shows a lower hydraulic gradient.The basal aquifer feeds several small rivers and springs the main draining axis is the Rio Fratta with a measured discharge of about 0.4 m3/s in summer 2002 2003 0.85 m3/s discharge has been recorded previously at the confluence with the Tiber river. An important discharge augmentation 130 150 l/s was recorded where the Paleo Tiber deposits outcrop. Moreover, local perched aquifers have been observed in the volcanic units overlaying the sedimentary complex and/or in the upper horizon of the last one. The perched aquifers are going to be abandoned because of their overexploitation during the last tens of years. Nowadays, the deeper aquifer represents the most interesting reservoir of groundwater resources in the area, used mainly for agricultural and drinking purposes.Chemical composition shows an alkaline earth bicarbonate hydrofacies a trend towards the alkaline facies is more evident for some samples from the volcanic units. A good linear correlation between As and F was observed R2 = 0.64 it is probably due to a common process in their mobilization and in their enrichment in ground waters. Their occurrence in both volcanic and sedimentary aquifers clearly indicates the hydraulic connection of ground waters hosted in the different units. Because of this relation the two elements show a similar trend when compared to Ca concentration. The enrichment in Ca content and the relative reduction in the minor elements As and F suggest an evolution of the chemical facies of the waters flowing from the volcanic units towards the sedimentary ones as a consequence of leaching and/or mixing processes. The second hypothesis is the most probable because of the limited outcropping of Paleo Tiber units. A similar chemical variation has been observed downstream along the rivers in relation to the contribution of the sedimentary aquifer. This inverse correlation is in agreement with the spatial distribution of samples. Future research developments foresee a more detailed analysis of the hydrodynamic and hydrogeochemical setting of the area through the elaboration of the newly collected data 2003 survey . Modelling tool will allow to go on with the analysis of water rock interaction processes Vivona et al., 2004 and with the study of the solubilization and the enrichment processes of minor elements of toxicological importance. Un approccio integrato quantitativo e qualitativo e stato applicato in un area campione del Lazio settentrionale per l individuazione dei processi naturali che causano concentrazioni elevate di elementi potenzialmente tossici, fra cui As e F, nelle acque sotterranee. La falda di base, utilizzata anche a scopo potabile, circola nelle unita vulcaniche vicane a monte e nelle alluvioni ghiaioso sabbiose del paleo Tevere a valle. Le variazioni relative osservate di F, As e Ca evidenziano un evoluzione delle facies idrochimiche in relazione a mixing di acque di diversa provenienza acquifero vulcanico acquifero sedimentario e/o all interazione acqua roccia lungo il percorso delle acque sotterranee. 20 Articolo su rivista 2005_preziosi_al_IGEA.tiff Articolo in rivista Associazione georisorse e ambiente. Sezione italiana acque sotterranee 1973 008X Ingegneria e geologia degli acquiferi Ingegneria e geologia degli acquiferi Ing. geol. acquiferi Ingegneria e geologia degli acquiferi. IGEA Groundwater geoengineering VIVONA RAFFAELLA GIULIANO GIUSEPPE GENNARO elisabetta.preziosi PREZIOSI ELISABETTA TA.P02.009.002 Effetti dei cambiamenti climatici sulla componente terrestre del ciclo idrologico |
