Fluid geochemistry and natural gas hazard in the urban area of Rome

Abstract

The city of Rome is located in the Roman Comagmatic Province, where large sectors experience a huge degassing both from soils and aquifers; gas composition is dominated by CO2, that can act as a carrier for other minor components such as N2, CH4, H2S and radon. Gases are produced at depth (throughout mantle degassing and/or decarbonation processes) and upraise towards surface through fault and fracture systems, thus characterising well defined enhanced permeability belts. Rising CO2 partly dissolves in shallow aquifers, pressurizing them when they are confined. Dangerous blowouts occur, even in urbanized areas, when these pressurized aquifers are reached by wells or excavations, at depths ranging from 350 to 10-15 m. Several human-triggered gas accidents occurred near Rome (Alban Hills volcano, Fiumicino) in the past ten years, creating a big concern for the local population; as a consequence, an evaluation of the level of Natural Gas Hazard (onwards NGH) to which these areas are exposed, with particular attention to urban areas, has to be done. Moreover, gases exhaled from soils and aquifers can enter houses, potentially reaching harmful indoor levels. Indeed, some gases (CO2, H2S) can have a short-term toxicity, while others (i.e. radon) can cause lung cancer at prolonged exposures. A first step to evaluate the NGH level is the identification of actively degassing areas; among other geochemical methods, this target can be achieved through the detailed knowledge of the distribution of dangerous gases in groundwater. By now, detailed information on CO2 and radon distribution in ground waters was assessed for the volcanic complexes around Rome and, partly, for the Fiumicino area. The study performed in groundwater could allow to discriminate areas where: i) analysis of indoor-gas levels has to be carried out and deepened, in order to lessen their impact on human health and ii) particular care has to be done in drilling new wells and or performing anthropic activities that could cause dangerous gas eruptions. Such a methodological approach is used to evaluate a possible NGH for the urban territory of the Italian capital, where about 2500000 people habitually live. In order to achieve the so far presented goals, the present research was aimed at performing an extensive study of both chemical and isotopic features of the groundwater circulating in the urban area of Rome. The main objectives were: i) the chemical classification of waters, also to determine their quality, and the definition of the main water-rock interaction processes; ii) to assess dissolved CO2 and radon distribution in groundwater, as well as isotopic characterization of total carbon (i.e. carbon dioxide) to determine its origin; iii) the comparison between the gas distribution and both lithological and structural setting of the investigated area and iv) a first evaluation of the Natural Gas Hazard potentially affecting a very populated area. These targets have been reached by considering specific isotopic analysis (He, 3H, Sr) done on selected samples as a support for the identification of the main water-rock interaction processes (3H, Sr) and the assessment the origin of the dissolved gases (i.e. He); all acquired data have been used to draw a final model of both shallow and deep groundwater circulations in the frame of the tectonic setting of the investigated area. Groundwater (192 samples) has been analysed for its chemical composition (major elements) and two main hydro-chemical facies have been recognised, each characterised by a different gas-water-rock interaction processes. Statistical approach allowed to divide waters into two main groups, referred to the lithological domains existing in the roman territory: volcanic (K-rich minerals) and sedimentary (alluvial, marls, clays). TDQ stands for Tor di Quinto, the third group of waters having chemical features completely different from the other ones and, consequently, considered as a separate entity. The dominant hydro-chemical facies is the bicarbonate-alkaline earth, with an evident differentiation of chemical components between the volcanic and sedimentary waters. The former show a general enrichment in alkali (Na and K) and high alkalinity, while the latter show a quite constant (Na+K/Ca+Mg) ratio and deeply different bicarbonate content and high (Cl + SO4)/HCO3. Water interaction with alkali-rich volcanites of the roman area fully justify the observed distribution, while CO2 dissolution accounts for high alkalinity. On the contrary, waters circulating in sedimentary layers become enriched in SO4 and Cl, moving towards the Ca-SO4 (Cl) quadrant. Some waters have a bicarbonate-alkaline chemistry, typical of prolonged interaction of CO2-rich waters with volcanic rocks (with reaction such as: 2K (Na)AlSi2O6 + 2 H2CO3 + H2O → 2K (Na)+ + 2HCO3- + 2SiO2 + Al2Si2O5(OH)4), and possible cationic exchange with clays that cause the drop in the Ca + Mg content. The importance of discriminating the Na abundance with respect to K has a special meaning in the roman groundwater due to the particular K-rich minerals contained in the volcanic rocks. Indeed, the volcanic waters have an average Na/K = 2, very similar to the Albani and Sabatini rock compositions, while the sedimentary ones show Na/K = 20 due to interaction with sands, marls and clays. These distribution fully justify the different evolution of these waters due to different types and extent of gas-water-rock interaction processes. TDQ waters show a large range in the Na/K ratio, with bicarbonate-alkaline earth samples having a particularly low Na/K, very similar to the volcanic waters. Dissolved CO2 plays an important role in defining the chemical evolution of groundwater. A general positive correlation between log PCO2 and salinity is observed. It is especially important for the CO2-rich waters belonging to the volcanic domain (south-eastern sector of Rome) that, apart few exceptions, have moderate saline content due to both the low solubility of silicate minerals and their low dissolution rate, even in acidic environment. Yet, CO2 increases the aggressiveness of the groundwater on the aquifer-hosting rocks, allowing the transfer of chemical elements into the solution. It is worthy of note the elevated log PCO2 and salinity of the TDQ waters. By taking K as representative of the vulcanite compositions, the degree of the water-rock-interaction can be evaluated. Volcanic waters are enriched in the main constituent of the mineral-forming rocks, either major, minor (SiO2, Sr) and trace (Ba, Rb). Same trend is observed in the sedimentary waters for Cl, Na and SO4 (interaction with clays and marls). Both TDQ and some volcanic waters with high salinity circulating in the southern sector of Rome probably represent a diluted and/or a mixed term of geothermal aquifers hosted in the deep carbonate platform. The B/Cl ratio, the strontium isotopes and the tritium content of these waters fully support our inference. CO2 distribution in groundwater, expressed as log PCO2, emphasised the presence of five sectors actively degassing in Rome: Cassia, Salaria, Tor di Quinto-Flaminio-Saxa Rubra in the north and Eur-Torrino and Appio-Tuscolano-Capannelle in the south. CO2 isotopic characterisation of these waters point out an inorganic (i.e deep) provenance of carbon dioxide, as recognised in the natural gas manifestations of Central Italy. Conversely, CO2-poor waters have an isotopic signature highlighting an organic (i.e. shallow) provenance of CO2. Radon-rich waters only partially mimic the CO2 distribution, mainly reflecting the outcrops of U and Ra-rich volcanites; they have been recognised in the southern-eastern (Appio-Tuscolano-Ardeatino) and eastern (Centocelle, Colli Aniene) sectors of Rome, where thick Alban Hills volcanic products largely outcrop. In the former huge sector, additional radon, beyond that produced by volcanic rocks, could be carried out toward surface by CO2. The other roman aquifers are hosted in the sedimentary layers, where a scarce presence of radon precursors have been found; as a consequence, average radon activity is low. Summarising, four CO2 and one huge radon (and partly CO2) degassing areas have been recognised in the urban area of Rome. CO2 is produced both in the mantle and in the structural highs (through decarbonation process) of the carbonate platform beneath Rome (Tor di Quinto, Eur-Torrino and Appio-Tuscolano-Capannelle), upraise through faults and fractures, dissolving into shallow aquifers. In particular, the role of deep CO2 as a sound marker of deep tectonic structures in this sector of central Italy, was emphasised. He isotopes were used to evaluate the mantle contribution: values span from 24 to 50% in CO2-rich waters, emphasising the existence of deep-rooted faults beneath the city of Rome; they have the role of tectonic-derived pathways for deep-originated fluids. Radon distribution is mainly controlled by the lithological setting of the investigated area. Indeed, high-activity radon waters circulate into the U and Ra-rich volcanites of the Alban Hills (south-eastern and eastern sectors), and only locally they receive an additional contribute from the huge flux of deep CO2, acting as carrier gas for radon. This research is only the first step for the evaluation of the NGH in Rome. In the actively degassing areas a detailed soil-gas survey is highly recommended in order to measure and quantify how much gas, dissolved in the shallow aquifers, can reach and permeate soils, where houses and dwellings are built. Quick exploratory surveys to determine the CO2 and radon indoor levels in the individuated NGH-prone areas must be promoted by local authorities, mainly in the most populated sectors, where the presence of gas in soil and/or indoor could represent a very big concern.