Fluoride enrichment mechanism and geospatial distribution in the volcanic aquifers of the Middle Awash basin,Northern Main Ethiopian Rift

Considering the anomalous concentration of fluoride in the ground waters of the Middle Awash basin, a comprehensive survey of the enrichment mechanism as well as its association with common hydrochemical variables was conducted using multivariate statistical methods, Hierarchal Cluster Analysis (HCA) and Principal Component Analysis (PCA). The HCA results indicate a marked heterogeneous spatial distribution of the fluoride concentration, the magnitude of which varies more gradually in the SSW–NNE direction along the Wonji Fault Belt (WFB) than it does in the E-W direction away from this belt. This is strongly associated with the geothermal anomaly that is prominent in the basin interior area. Furthermore, the PCA results show that the magnitude of the fluoride concentration is higher in the groundwater derived from non-calcium bearing aquifers, which are widely distributed in the rift floor. Hydrochemical processes involving a cation exchange reaction cause a systematic Ca²⁺ removal from solution from highland towards the rift floor. This geochemical reaction enhances the fluoride enrichment of groundwater along the same flow direction. In this regard, the geothermal belt of the basin interior area is a hydrogeological block marked by high loads of fluoride whereas the plateau areas are low-fluoride zones. The geospatial distribution of fluoride at the basin scale was estimated using the kriging procedure. Appropriate discrimination between local and regional aquifers is important in order to secure low-fluoride water supply for the community in the basin.     

Influence of hydrostratigraphy and structural setting on the arsenic occurrence in groundwater of the Cimino-Vico volcanic area (central Italy)

Arsenic occurrence in groundwater near the Cimino-Vico volcanoes (central Italy) was analysed considering the hydrostratigraphy and structural setting and the shallow and deep flows interacting within the Quaternary volcanics. Groundwater is the local source of drinking water. As documented in the past, arsenic in the groundwater has become a problem, and the European maximum allowable contaminant level was recently lowered to 10 μg/L. Chemical analyses of groundwater were conducted, sampled over an area of about 900 km², from 65 wells and springs representative of the volcanic aquifer and thermal waters. Considering the type of aquifer, the nature of the aquifer formation and its substratum, the hydrochemical data highlight that the arsenic content of the groundwater is mainly connected with the hydrothermal processes in the volcanic area. Thermal waters (54–60°C) fed from deep-rising fluids show higher arsenic concentrations (176–371 μg/L). Cold waters sampled from the volcanic aquifer are characterized by a wide variability in their arsenic concentration (1.6–195 μg/L), and about 62% exceed the limit of 10 μg/L. Where the shallow volcanic aquifer is open to deep-rising thermal fluids, relatively high arsenic concentrations (20–100 μg/L) are found. This occurs close to areas of the more recent volcano-tectonic structures.     

Karst hydrogeology and origin of thermal waters in the Codru Moma Mountains, Romania

 Two karst areas within Permian and Triassic carbonate rocks of the Codru Moma Mountains in the northwestern part of Romania yield thermal waters. Major karst springs occur where groundwater flow is intercepted by hydraulic barriers, which also results in the movement of water from deeper levels. At Moneasa, thermal groundwater rises along faults and fractures associated with a thrust, and at Vascau Town, water rises along faults marginal to the Beius Basin. Geochemistry suggests that the thermal component of the Moneasa groundwaters is derived from dolomites and that at least a proportion of the Vascau thermal waters originates from deeply buried Permian sandstones. Received, August 1999 / Revised, March 2000 / Accepted, March 2000     

The Chemical Composition of the Nitrogen Thermal Waters of the Khoja-Obi-Garm (Tajikistan) Balneoclimatic Resort

This paper presents new data on the contents of macro- and microcomponents from low-mineral nitrogen thermal water located within the Khoja-Obi-Garm resort (Tajikistan). It was found that these thermal groundwaters contain high contents of F, Si, Li, Al, Rb, Sr, and Cs. Detailed study of the geological and hydrogeological settings of the resort, along with thermodynamic simulations allows us reveal the sources of chemical elements in these waters. A comparison of thermal groundwater from Khoja-Obi-Garm spa with the thermal waters of other regions of Eurasia was carried out; new data on the rare-earth elements and yttrium (REY) concentrations in the studied thermal water are presented as well.     

Hydrochemistry and thermal activity of damt region,Yemen

The aim of this study was to identify the complex hydrogeological and hydrochemistry conditions of Damt region, through determining hydrochemical properties of groundwater in the study area. According to the results of hydrochemical analyses, sampled waters can be divided into three groups: cold, thermal, and mixed waters. Thermal waters in the area are characterized by Na–HCO₃, while the cold waters by CaHCO₃ facies. HCO₃ indiscriminate cation and/or Na-indiscriminate anion are present in many places in the region and indicate generally mixing water. Only three villages with dental fluorosis observed using water elevated in F⁻ for drinking. Agricultural and liquid waste disposal are the main sources of pollution, leads to increase of Na, Cl, NO₃, Cd, and Iron. The groundwater flow is from north, northwest, and northeast to the south. Within this regional trend, structural controlling groundwater flow along Wadis and it flows from upper reaches of tributaries toward the main channel, then downward to the south of the study area. The similarity of TDS and Cl concentration at Qa’a Al Haql and Al Nadirah between aquifers indicates hydraulic continuity between alluvial and the underlying volcanic, while at Damt no hydraulic continuity found between alluvial, volcanic and Sandstone aquifers. The temporal variation shows slight decrease in the concentration of nitrate and sulfate of thermal water indicating previously high gas content of nitrogen, hydrogen sulfide in the thermal active region. The developed conceptual model of water circulation indicates flood waters infiltrate slowly through the wadi bottoms in the East where Sandstone aquifer outcrops. These waters flow westward, following the westerly dip of the Sandstone through the effects of gravity, gains heat and dissolve materials as it comes in contact with the numerous dykes, which are the feeders to the overlying volcanoes and sputter cones. All thermal water samples from Damt region fall into immature water field in NA–K–Mg diagram. Therefore, the results obtained from the cation geothermometers should be taken into account as doubtful.     

Lake-groundwater relationships and fluid-rock interaction in the East African Rift Valley: isotopic evidence

The assessment of water resources in the Rift Valley environment is important for population, agriculture and energy-related issues and depends on a good understanding of the relationship between freshwater lakes and regional groundwater. This can be hampered by the amount of fluid-rock interaction which occurs throughout the rift, obscuring original hydrochemical signatures. However, O and H stable isotope ratios can be used as tracers of infiltration over sometimes considerable distances, while showing that the volcanic edifices of the rift floor have varying effects on groundwater flow patterns. Specific cases from Kenya and Ethiopia are considered, including Lakes Naivasha, Baringo, Awasa and Zwai.In addition to their physical tracing role, stable isotopes can reveal information about processes of fluid-rock interaction. The general lack of O isotope shifting in rift hydrothermal systems suggests a high water:rock ratio, with the implication that these systems are mature. Carbon isotope studies on the predominantly bicarbonate waters of the rift show how they evolve from dilute meteoric recharge to highly alkaline waters, via the widespread silicate hydrolysis promoted by the flux of mantle carbon dioxide which occurs in most parts of the rift. There appears to be only minor differences in the C cycle between Kenya and Ethiopia.     

Arsenic contamination in groundwater from parts of Ambagarh-Chowki block, Chhattisgarh, India: source and release mechanism

Arsenic contamination in tube-well water in Ambagarh-Chowki block, central India, is restricted to local areas confined within the N-S trending Dongargarh rift zone. Affected areas are preferentially located in acid volcanics, close to shear zones and also in granites. Dug-wells even in severely contaminated areas generally have As concentration ≤10 μg/l. But in Kaurikasa area, several tube-wells and dug-wells are severely polluted. Weathered rocks and soils are also enriched in As from severely contaminated areas. As preferentially occurs in iron-enriched soil and similarly altered biotite, chlorite in granite. As sorbed in hydrated iron oxide (HFO) that preferably occurs in acid-leachable fraction and possibly as coatings on kaolinite, illite and goethite in soil or as coatings and along cleavage traces on weathered biotite and chlorite. Reductive dissolution of HFO released sorbed As to groundwater and enriched it in Fe. Pyrite in volcanic and shear zone rocks, although locally As-bearing is a minor source of As in groundwater.     

Groundwaters of the Central Ethiopian Rift: diagnostic trends in trace elements, δ<Superscript>18</Superscript>O and major elements

The Ethiopian Rift (a major portion of the Great East African Rift) is characterized by a narrow elongated depression bounded by highlands from both sides. This topographic configuration leads to a monsoon redistribution which resulted in an arid rift floor and humid high rainfall highlands. The rifting and associated volcanism also caused a thinning of the crust and facilitates influx of CO₂ and other mantle gases as diffuse sources or along faults from deeper sources. Groundwaters in the rift floor are usually of high mineral content (high F, U, As and salinity) while those on the plateau are of low mineral content. Among many factors, groundwater availability and quality in the rift floor aquifers is the function of their connection to the aquifers in the high rainfall plateau and the residence time of groundwater prior to reaching the rift floor. This entails the need for addressing one basic hydrologic question in such a setting: at what depth and rate does recharge from the high rainfall highland reach the lowland rift aquifers? This study uses spatial variations in trace elements and relates them to ¹⁴C variations, thereby investigating the suitability of using trace elements as proxies for residence time estimation of groundwaters of relatively short (1,000–2,000 years) residence time. This work also investigates the behavior of trace element trends along the groundwater flow path in a rifted setting and compares them with such trends in sedimentary aquifers elsewhere. The comparison shows a clear difference in behavior of trace elements along the groundwater flow path when compared with such variations in big sedimentary basins with no prominent rifting and volcanism, suggesting the need of calibrating the relation between trace elements and any direct residence time indicators. An integrated use of major elements, trace elements, and environmental isotopes reveals that the main recharge of the aquifers originates from mountain blocks and that recharge takes place via fractures with no evidence of evaporation prior to recharge. Redox processes appear to play a limited role in trace element geochemistry of groundwaters in the region. Progressive trends in trace element composition along the groundwater flow path suggest continuous groundwater flow from the plateau.     

Groundwater origin and flow along selected transects in Ethiopian rift volcanic aquifers

The disruption of lithologies by cross-cutting faults and the variability in volcanic structures make the hydrogeology of the rifted volcanic terrain in Ethiopia very complex. Along two transects, selected due to their hydrogeologic characteristics, groundwater flow, depth of circulation and geochemical evolution have been conceptualized. The groundwater flow continuity between the high rainfall plateau bounding the rift and the rift valley aquifers depends principally on the nature of the bounding faults. Up to 50% of recharge to the rift aquifers comes from the plateau as groundwater inflow where the rift is cross cut by transverse fault zones. Recharge from the mountains is found to be insignificant where the rift is bounded by marginal grabens; channel loss and local precipitation are the principal sources of recharge to the rift aquifers in such cases. At a regional scale, there is a clear zonation in the geochemical compositions of groundwaters, the result of aquifer matrix composition differences. The environmental isotope results show that the majority of the aquifers contain modern groundwaters. In a few localities, particularly in thermal groundwaters representing deeper circulation, palaeo-groundwaters have been identified. Deeper groundwaters in the rift floor have a uniform ¹⁴C age ranging between 2,300 and 3,000 years.     

Hydrogeochemistry of thermal and mineralized waters in the Diyadin (Ağri) area,Eastern Turkey

The Diyadin Geothermal area, located in the eastern part of Anatolia (Turkey) where there has been recent volcanic activity, is favorable for the formation of geothermal systems. Indeed, the Diyadin geothermal system is located in an active geodynamic zone, where strike-slip faults and tensional cracks have developed due to N–S regional compression. The area is characterized by closely spaced thermal and mineralized springs, with temperatures in the range 30–64 °C, and flowrates 0.5–10 L/s. Thermal spring waters are mainly of Ca(Na)-HCO₃ and Ca(Mg)-SO₄ types, with high salinity, while cold groundwater is mostly of Ca(Na, Mg)-HCO₃ type, with lower salinity. High contents of some minor elements in thermal waters, such as F, B, Li, Rb, Sr and Cs probably derive from enhanced water–rock interaction.Thermal water samples collected from Diyadin are far from chemical equilibrium as the waters flow upward from reservoirs towards spring vents and possibly mix with cooler waters. The temperatures of the deep geothermal reservoirs are estimated to be between 92 and 156 °C in Diyadin field, based on quartz geothermometry, while slightly lower estimates are obtained using chalcedony geothermometers. The isotopic composition of thermal water (δ¹⁸O, δ²H, δ³H) indicates their deep-circulating meteoric origin. The waters are likely to have originated from the percolation of rainwater along fractures and faults to the deep hot reservoir. Subsequent heating by conduction due to the presence of an intrusive cupola associated with the Tendurek volcano, is followed by the ascent of deep waters to the surface along faults and fractures that act as hydrothermal conduits.Modeling of the geothermal fluids indicates that the fluid is oversaturated with calcite, aragonite and dolomite, which matches travertine precipitation in the discharge area. Likewise, the fluid is oversaturated with respect to quartz, and chalcedony indicating the possibility of siliceous precipitation near the discharge areas. A conceptual hydro-geochemical model of the Diyadin thermal waters based on the isotope and chemical analytical results, has been constructed.     

Hydrogeochemical study in the Main Ethiopian Rift: new insights to the source and enrichment mechanism of fluoride

The central Main Ethiopian Rift suffers a severe water quality problem, characterized by an anomalously high fluoride (F) content that causes an endemic fluorosis disease. The current study, conducted in the Ziway–Shala lakes basin, indicates that the F content exceeds the permissible limit for drinking prescribed by the World Health Organization (WHO; 1.5 mg/l) in many important wells (up to 20 mg/l), with even more extreme F concentration in hot springs and alkaline lakes (up to 97 and 384 mg/l respectively). The groundwater and surface water from the highlands, typically characterized by low total dissolved solids (TDS) and Ca (Mg)–HCO₃ hydrochemical facies, do not show high F content. The subsequent interaction of these waters with the various rocks of the rift valley induces a general increase of the TDS, and a variation of the chemical signature towards Na–HCO₃ compositions, with a parallel enrichment of F. The interacting matrixes are mainly rhyolites consisting of volcanic glass and only rare F-bearing accessory minerals (such as alkali amphibole). Comparing the abundance and the composition of the glassy groundmass with other mineral phases, it appears that the former stores most of the total F budget. This glassy material is extremely reactive, and its weathering products (i.e. fluvio/volcano-lacustrine sediments) further concentrate the fluoride. The interaction of these “weathered/reworked” volcanic products with water and carbon dioxide at high pH causes the release of fluoride into the interacting water. This mainly occurs by a process of base-exchange softening with the neo-formed clay minerals (i.e. Ca–Mg uptake by the aquifer matrix, with release of Na into the groundwater). This is plausibly the main enrichment mechanism that explains the high F content of the local groundwater, as evidenced by positive correlation between F, pH, and Na, and inverse correlation between F and Ca (Mg). Saturation indices (SI) have been calculated (using PHREEQC-2) for the different water groups, highlighting that the studied waters are undersaturated in fluorite. In these conditions, fluoride cannot precipitate as CaF₂, and so mobilizes freely without forming other complexes. These results have important implications for the development of new exploitation strategies and accurate planning of new drilling sites. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Origin of High Boron Contents of the Thermal Waters of Kizildere and Vicinity,Western Anatolia,Turkey

The thermal waters of Kizildere and environs in the Buyuk Menderes rift zone are marked by boron concentrations of up to 32 mg/l, and flow rates of ～250 l/s into the Buyuk Menderes River, thus increasing boron contents of the river water up to 4.4 mg/1l (e.g., in 1992), for a river-water flow rate of 2 m³/s. These high boron concentrations poison plants, particularly citrus fruits, in irrigated agricultural areas of the rift zone. High boron contents in the thermal waters can be attributed to: (1) unstable boron-bearing mineral phases (e.g., feldspars, muscovites, tourmalines, hornblendes, and biotites) in the metamorphic rocks, proven by experimental leaching tests of various rocks; and (2) a magmatic input, corroborated by isotope analyses of δ¹¹B, δ¹³C, and δ³⁴S of the thermal waters. Additionally, Neogene boron deposits in northwestern Turkey have to be taken into consideration as possible sources of boron, which may contribute to these anomalous concentrations by leaching of boron-bearing minerals. Meteoric input of boron into the thermal water reservoir is ruled out because the boron contents of groundwater from the Buldan horst in the northern part of the study area is below detection limits (0.01 mg/l). There is no single reason for the high boron concentrations measured in the thermal waters of Kizildere and environs; rather, a concurrence of several natural factors is likely.     

Hydrogeochemistry and arsenic contamination of groundwater in the Rayen area, southeastern Iran

High As contents in groundwater were found in Rayen area and chosen for a detailed hydrogeochemical study. A total of 121 groundwater samples were collected from existing tube wells in the study areas in January 2012 and analyzed. Hydrogeochemical data of samples suggested that the groundwater is mostly Na–Cl type; also nearly 25.62 % of samples have arsenic concentrations above WHO permissible value (10 μg/l) for drinking waters with maximum concentration of aqueous arsenic up to 25,000 μg/l. The reducing conditions prevailing in the area and high arsenic concentration correlated with high bicarbonate and pH. Results show that arsenic is released into groundwater by two major phenomena: (1) through reduction of arsenic-bearing iron oxides/oxyhydroxides and Fe may be precipitated as iron sulfide when anoxic conditions prevail in the aquifer sediments and (2) transferring of As into the water system during water–acidic volcanic rock interactions.     

Hydrogeochemical and isotopic characteristics of the Ilica geothermal system (Erzurum,Turkey)

In this paper, the hydrochemical isotopic characteristics of samples collected from geothermal springs in the Ilica geothermal field, Eastern Anatolia of Turkey, are examined and described. Low-temperature geothermal system of Ilica (Erzurum, Turkey) located along the Eastern Anatolian fault zone was investigated for hydrogeochemical and isotopic characteristics. The study of ionic and isotopic contents shows that the thermal water of Ilica is mainly, locally fed by groundwater, which changes chemically and isotopically during its circulation within the major fault zone reaching depths. The thermal spring has a temperature of 29–39 °C, with electrical conductivity ranging from 4,000 to 7,510 µS/cm and the thermal water is of Na–HCO₃–Cl water type. The chemical geothermometers applied in the Ilica geothermal waters yielded a maximum reservoir temperature of 142 °C according to the silica geothermometers. The thermal waters are undersaturated with respect to gypsum, anhydrite and halite, and oversaturated with respect to dolomite. The dolomite mineral possibly caused scaling when obtaining the thermal waters in the study area. According to the enthalpy chloride-mixing model, cold water to the thermal water-mixing ratio is changing between 69.8 and 75 %. The δ¹⁸O–δ²H compositions obviously indicate meteoric origin of the waters. Thermal water springs derived from continental precipitation falling on to higher elevations in the study area. The δ¹³C ratio for dissolved inorganic carbonate in the waters lies between 4.63 and 6.48 ‰. In low-temperature waters carbon is considered as originating from volcanic (mantle) CO₂.     

Source processes of the thermal waters from the Phlegraean Fields (Naples, Italy) by means of the study of selected minor and trace elements distribution

The geochemical characteristics of hydrothermal waters from the Phlegraean Fields (P.F.) (Naples, Italy) were analysed for minor and trace elements, selectively mobilised in hydrothermal systems such as B, F, Hg, As, Pb and Tl.The water samples, collected from a shallow aquifer likely to be fed by deeper fluids, showed various geochemical features, resulting from the mixing of three components: (1) surface waters of meteoric origin; (2) hot deep waters deriving from water-rock interaction and including deep waters of marine origin; (3) magmatic fluids rising from the local magma chamber, lying a few kilometres below the town of Pozzuoli.This setting, although very complex, provides a reliable means of studying the distribution of the investigated trace elements. In particular, within the Phlegraean area, high contents of B (0.1-48 mg/l), F (0.5-8 mg/l), As (16-6050 μg/l) and Hg (0.7-232 μg/l) were observed. The levels of thallium in the springs close to Solfatara (about 7 μg/l) were in line with those normally recorded in hydrothermal areas, whereas high levels of this element (up to 23.3 μg/l) were identified in other wells of the study area. Lead (1.3 to 29.1 μg/l) appears to be anomalous with respect to its normal content in groundwater (about 1 μg/l), owing to the presence of high-density brines at depth, which enhance the solubility of Pb in volcanic rocks under hydrothermal conditions.The distribution of the investigated trace elements in the Phlegraean Fields thermal area is probably related to the different ascent pathways of the fluids. Clearly, apart from the influence exerted by anomalous thermal conditions, each element shows a different behaviour, depending on its geochemical affinity with mineral phases and as a consequence of the different enthalpy values, which determine ion partitioning in gaseous phases.Based on geochemical evidences and on the distribution of minor and trace elements, the source processes of the investigated hydrothermal waters were defined. Five main groups were identified: (1) acid sulphate waters, resulting from mixing of meteoric water with magmatic gases (mainly H₂S); (2) high sulphate-chloride waters, from a deep reservoir located in the major upflow zone; (3) waters associated with significant degassing of magmatic CO₂; (4) waters from a deep geothermal neutral chloride reservoir, resulting from heating of marine water modified by water-rock interaction processes; (5) cold waters from the inner area, influenced by low-temperature, water-rock interaction processes.     

漳州盆地水热系统氚同位素研究

本文依据26个天然水样品的氚同位素测试数据,分析了漳州盆地地下热水及其它天然水的氚值特征及其形成条件;利用“活塞模型”方法计算了漳州盆地地下水和地下热水的年龄;为弄清全盆地地下水的补给、迳流和排泄的总体格局和揭示地下热水的成因提供了依据。     

Microbial and chemical characterization of a groundwater flow system in an intermontane basin of southern New Mexico,USA

Groundwater of the southern Jornada del Muerto Basin, an intermontane basin structure associated with the Rio Grande rift located in south-central New Mexico, USA, was analyzed chemically and microbially. A microbial phospholipid fatty acids (PLFA) analysis revealed a sparse microbial population consisting of relatively simple microorganisms with no major population changes along the flow system. A nucleic acid (DNA) analysis of the groundwater resulted in the identification of ten eubacterial and one archeal species. Chemical analyses revealed that sulfate along with calcium, magnesium, iron, and manganese is removed by about an order of magnitude in concentration from the recharge area to the discharge area. The removal of iron, manganese, magnesium, and to some extent calcium can be explained by oxidation reactions and the precipitation of dolomite. Sulfate and additional calcium are most likely removed by the precipitation of gypsum. Thiobacillus spp. are oxidizing metal sulfides that occur as subsurface sulfide mineral deposits to sulfuric acid, which subsequently reacts with calcium carbonate and water to precipitate gypsum. The presence of these sulfide deposits exposed to oxygenated water in the deep groundwater flow system significantly alters its chemical and bacteriological composition. Electronic Publication     

Groundwater flow dynamics in the complex aquifer system of Gidabo River Basin (Ethiopian Rift): a multi-proxy approach

Hydrochemical and isotope data in conjunction with hydraulic head and spring discharge observations were used to characterize the regional groundwater flow dynamics and the role of the tectonic setting in the Gidabo River Basin, Ethiopian Rift. Both groundwater levels and hydrochemical and isotopic data indicate groundwater flow from the major recharge area in the highland and escarpment into deep rift floor aquifers, suggesting a deep regional flow system can be distinguished from the shallow local aquifers. The δ¹⁸O and δ²H values of deep thermal (≥30 °C) groundwater are depleted relative to the shallow (<60 m below ground level) groundwater in the rift floor. Based on the δ¹⁸O values, the thermal groundwater is found to be recharged in the highland around 2,600 m a.s.l. and on average mixed with a proportion of 30 % shallow groundwater. While most groundwater samples display diluted solutions, δ¹³C data of dissolved inorganic carbon reveal that locally the thermal groundwater near fault zones is loaded with mantle CO₂, which enhances silicate weathering and leads to anomalously high total dissolved solids (2,000–2,320 mg/l) and fluoride concentrations (6–15 mg/l) exceeding the recommended guideline value. The faults are generally found to act as complex conduit leaky barrier systems favoring vertical mixing processes. Normal faults dipping to the west appear to facilitate movement of groundwater into deeper aquifers and towards the rift floor, whereas those dipping to the east tend to act as leaky barriers perpendicular to the fault but enable preferential flow parallel to the fault plane.     

Origin of sulfur species in acid sulfate-chloride thermal waters,northeastern Japan

Acid sulfate-chloride thermal water samples collected together with fumarolic gases from various volcanic areas in northeastern Japan were studied chemically and isotogdically. δ³⁴S (COT) values of sulfate and hydrogen sulfide from these volcanic hot springs range from +4.0 to +31 and from ?15.0 to ?2.0% respectively, with δ³⁴S_ys value of +2.5 to +31. The δ³⁴S of the sulfate in the more saline waters tends to become smaller with increasing ratio of SO₄ to Cl, although the chemical and isotopic composition of acid thermal water within some areas may be altered by secondary processes during the discharge of the thermal waters. This trend can be explained by the reaction of the volcanic gases, having S/Cl of 4 ~ 7 and total sulfur of ~0% in δ³⁴S, with ground water at 200°C, and/or the removal of sulfide phase depleted in ³⁴S from the acid thermal water formed by the disproportionation of volcanic sulfur. The sulfur species in acid sulfate-chloride thermal water are shown to be volcanic exhalations.     

Geostatistical techniques application to dissolved radon hazard mapping: An example from the western sector of the Sabatini Volcanic District and the Tolfa Mountains (central Italy)

Dissolved Rn was determined in 192 samples collected from cold shallow volcanic and sedimentary aquifers, deep thermal aquifers and from waters associated with bubbling gases in the western sector of the Sabatini Volcanic District and the Tolfa Mountains (central Italy). Shallow aquifers hosted in the Quaternary volcanic complexes show values ranging from 1.0 to 352 Bq/L (median value 55 and inter-quartile distance 62 Bq/L), while waters circulating within the permeable horizons of the sandy-to-clayey sediments of the Tolfa flysch have values from 1.0 to 44 Bq/L (median value 6.9 and inter-quartile distance 8.1 Bq/L). Thermal waters are hosted in the Mesozoic carbonate formations and move towards the surface along faults. Here, dissolved Rn values range from 0 to 37 Bq/L (median value 3.0 and inter-quartile distance 9.5 Bq/L). Waters associated with bubbling gases show dissolved Rn contents ranging from 2.0 to 48 Bq/L (median value 6.2 and inter-quartile distance 23 Bq/L). Those results suggest that lithology is the main factor affecting the Rn contents in shallow aquifers, due to the high levels of Rn progenitors U and Ra in the volcanic rocks relative to sedimentary units. The influence of other factors such as the presence of a fracture network, seasonal flow variations, type of discharge (from well or spring) was also investigated. Radon contents of thermal waters result from mixing with shallow waters (from both volcanic and sedimentary rock aquifers) and decrease of Rn solubility with temperature, while for bubbling pools the effects of strong degassing were also considered.In terms of health hazard from direct ingestion of Rn-rich waters, 20.8% of those circulating within the volcanic aquifer show values higher than the recommended value of 100 Bq/L, while none of those circulating within the sedimentary aquifers exceed the threshold value. Geostatistical techniques were used for the elaboration of contour maps by using variogram models and kriging estimation aimed at defining the areas where a potential health hazard due to the direct ingestion of Rn-rich waters and to inhalation of air following degassing of Rn from waters may be expected.