The Campanian Plain (CP) shallow aquifer (Southern Italy) represents a natural laboratory to validate geochemical methods for differentiating diffuse anthropogenic pollution from natural water-rock ...interaction processes. The CP is an appropriate study area because of numerous potential anthropogenic pollution vectors including agriculture, animal husbandry, septic/drainage sewage systems, and industry. In order to evaluate the potential for geochemical methods to differentiate various contamination vectors, 538 groundwater wells from the shallow aquifer in Campanian Plain (CP) were sampled. The dataset includes both major and trace elements. Natural water-rock interactions, which primarily depend on local lithology, control the majority of geochemical parameters, including most of the major and trace elements. Using prospective statistical methods in combination with the traditional geochemical techniques, we determined the chemical variables that are enriched by anthropogenic contamination (i.e. NO
3
, SO
4
and U) by using NO
3
as the diagnostic variable for detecting polluted groundwater. Synthetic agricultural fertilizers are responsible for the majority of SO
4
and U pollution throughout the CP area. Both SO
4
and U are present in the groundmass of synthetic fertilizers; the uranium concentration is specifically applicable as a tracer for non-point source agricultural fertilizer contamination. The recognition of non-geological (anthropogenic) inputs of these elements has to be considered in the geochemical investigations of contaminated aquifers.
Waters and dissolved gases collected along vertical profiles in the five basins (Main, Kabuno Bay, Kalehe, Ishungu, and Bukavu) forming the 485 m deep Lake Kivu (Democratic Republic of the Congo) ...were analyzed to provide a geochemical conceptual model of the several processes controlling lake chemistry. The measured horizontal and vertical variations of water and gas compositions suggest that each basin has distinct chemical features produced by (1) different contribution from long circulating fluid system containing magmatic CO2, responsible of the huge CO2(CH4)‐rich reservoir hosted within the deep lake water; (2) spatial variations of the biomass distribution and/or speciation; and (3) solutes from water‐rock interactions. The Kabuno Bay basin is characterized by the highest rate of magmatic fluid input. Accordingly, this basin must be considered the most hazardous site for possible gas outburst that could be triggered by the activity of the Nyiragongo and Nyamulagira volcanoes, located a few kilometers north of the lake.
On 17 January 2002, the city of Goma was partly destroyed by two of the several lava flows erupted from a roughly N‐S oriented fracture system opened along the southern flank of Mount Nyiragongo ...(Democratic Republic of Congo), in the western branch of the East African rift system. A humanitarian and scientific response was promptly organized by international, governmental, and nongovernmental agencies coordinated by the United Nations and the European Union. Among the different scientific projects undertaken to study the mechanisms triggering this and possible future eruptions, we focused on the isotopic (He, C, and Ar) analysis of the magmatic‐hydrothermal and cold gas discharges related to the Nyiragongo volcanic system, the Kivu and Virunga region. The studied area includes the Nyiragongo volcano, its surroundings, and peripheral areas inside and outside the rift. They have been subdivided into seven regions characterized by distinct 3He/4He (expressed as R/Rair) ratios and/or δ13C‐CO2 values. The Nyiragongo summit crater fumaroles, whose R/Rair and δ13C‐CO2 values are up to 8.73 and from −3.5‰ to −4.0‰ VPDB, respectively, show a clear mantle, mid‐ocean ridge basalt (MORB)‐like contribution. Similar mantle‐like He isotopic values (6.5–8.3 R/Rair) are also found in CO2‐rich gas emanations (mazukus) along the northern shoreline of Lake Kivu main basin, whereas the 13δC‐CO2 values range from −5.3‰ to −6.8‰ VPDB. The mantle influence progressively decreases in (1) dissolved gases of Lake Kivu (2.6–5.5 R/Rair) and (2) the distal gas discharges within and outside the two sides of the rift (from 0.1 to 1.7 R/Rair). Similarly, δ13C‐CO2 ratios of the peripheral gas emissions are lighter (from −5.9‰ to −11.6‰ VPDB) than those of the crater fumaroles. Therefore, the spatial distribution of He and C signatures in the Lake Kivu region is mainly produced by mixing of mantle‐related (e.g., Nyiragongo crater fumaroles and/or mazukus gases) and crustal‐related (e.g., gas discharges in the Archean craton) fluids. The CO2/3He ratio (up to 10 × 1010) is 1 order of magnitude higher than those found in MORB, and it is due to the increasing solubility of CO2 in the foiditic magma feeding the Nyiragongo volcano. However, the exceptionally high 40Ar*/4He ratio (up to 8.7) of the Nyiragongo crater fumaroles may be related to the difference between He and Ar solubility in the magmatic source. The results of the present investigation suggest that in this area the uprising of mantle‐originated f luids seems strongly controlled by regional tectonics in relation to the geodynamic assessment of the rift. These fluids are mainly localized in a relatively small zone between Lake Kivu and Nyiragongo volcano, with important implications in terms of volcanic activity.
Since 1938, Nyamulagira volcano (Democratic Republic of Congo) has operated as a classic pressurized basaltic closed system, characterized by frequent dike-fed flank eruptions. However, on June 24, ...2014, an active lava lake was observed in its summit, after a period of 76 years. The small lava lake is now exposed at the bottom of a pit-crater and is rising and growing. Based on satellite-derived infrared (IR) data, SO
2
fluxes and periodic field surveys, we provide evidence that the development of the lava lake was gradual and occurred more than 2 years before it was first observed in the field. Notably, this process followed the voluminous 2011–2012 distal flank eruption and was coeval with weakening of the central rock column below the summit. Hence, the opening and development of the pit-crater favoured the continuous rise of fresh magma through the central conduit and promoted the gradual “re-birth” of the Nyamulagira lava lake. Budgeted volumes of magma erupted, and magma degassed at depth indicate that the formation of the lava lake is due to the draining and refilling of a shallow plumbing system (1–2 km depth), probably in response to the rift-parallel 2011–2012 distal eruption. We thus suggest that the transition from lateral to central activity did not result from a substantial change in the magma supply rate but, more likely, from the perturbation of the plumbing system (and related stress field) associated with the distal eruption. The processes observed at Nyamulagira are not unique and suggest that rift-fissure eruptions, in addition to triggering caldera collapses or lava lake drainages, may also induce a progressive resumption of central vent activity. Current activity at Nyamulagira represents a tangible and major hazard for the population living at the base of its southern flank.
El Chichón is an active volcano located in the north‐western Chiapas, southern Mexico. The crater hosts a lake, a spring, named Soap Pool, emerging from the underlying volcanic aquifer and several ...mud pools/hot springs on the internal flanks of the crater which strongly interact with the current fumarolic system (steam‐heated pools). Some of these pools, the crater lake and a cold spring emerging from the 1982 pumice deposits, have been sampled and analysed. Water–volcanic gas interactions determine the heating (43–99°C) and acidification (pH 2–4) of the springs, mainly by H2S oxidation. Significantly, in the study area, a significant NH3 partial pressure has been also detected. Such a geochemically aggressive environment enhances alteration of the rock in situ and strongly increases the mineralization of the waters (and therefore their electrical conductivity). Two different mineralization systems were detected for the crater waters: the soap pool‐lake (Na+/Cl− = 0.4, Na/Mg>10) and the crater mud pools (Na+/Cl− > 10, Na/Mg < 4). A deep boiling, Na+‐K+‐Cl−‐rich water reservoir generally influences the Soap Pool‐lake, while the mud pool is mainly dominated by water‐gas–rock interactions. In the latter case, conductivity of sampled water is directly proportional to the presence of reactive gases in solution. Therefore, chemical evolution proceeds through neutralization due to both rock alteration and bacterial oxidation of ammonium to nitrate. The chemical compositions show that El Chichón aqueous fluids, within the crater, interact with gases fed by a geothermal reservoir, without clear additions of deep magmatic fluids. This new geochemical dataset, together with previously published data, can be used as a base line with which to follow‐up the activity of this deadly volcano.
A new and unusual eruptive event occurred on 29 February 2016 within the summit crater of the Mount Nyiragongo volcano. Based on field campaigns performed between July 2015 and September 2017, and ...building on a previously published buoyancy‐driven bidirectional magma flow model explaining the progression of Mount Nyiragongo lava lake level, we provide the first quantitative estimations of volumes of erupted lava outpouring from the new spatter cone. Besides matching field data of the lava lake level covering the period December 2002 to September 2017, numerical solutions of the model reveal that the most probable dike path is one originating from the shallow magma reservoir, and not from the lava lake or branching from its feeding conduit. According to these simulations, the reservoir and erupted lava volumes are respectively estimated to the order of 10 km3 and 20 M m3. Magma overpressure at the level of the shallow reservoir is estimated in the range 12 to 16 MPa, high enough to potentially initiate new erupting events.
Plain Language Summary
Mount Nyiragongo volcano contains the world's largest continuously active lava lake within its crater. This constitutes a major potential geological regional hazard to the Virunga volcanic region's inhabitants within the Democratic Republic of Congo and Rwanda, particularly the cities of Goma and Gisenyi with about 1.5 million people living at its foot. Within the last four decades, the lava lake has been twice (1977 and 2002) drained out catastrophically through a fracture network. A new and unusual eruptive event occurred on 29 February 2016 proximity to the lava lake, forming within a few months a small cone of about 30 m high. Based on a mathematical model incorporating fluid mechanics, we calculate using computer simulations the overpressure of the magmatic system due to the accumulation of magma circulating back from the lava lake into the feeding magmatic reservoir. Combined with the fractured nature of the edifice, we estimate this that overpressure renders the possibility of dikes reaching the surface inside the crater, as currently observed, and also outside the summit crater, in which case the threat of a future major flank eruption becomes real.
Key Points
Overpressure exerted in the shallow magmatic reservoir by buoyancy force is sufficient to initiate a dike feeding the new spatter cone
Geochemical and petrological analyses suggest that the feeding systems of the lava lake and spatter cone are of the same origin
The magmatic flow model tuned to fit field observations indicates 0.9 km3 of magma convected since 2002 for a reservoir volume of 10 M m3
Soil²²²Rn concentrations have been monitored during the 2009 autumn and Spring 2010 rainy periods, in an alluvial soil, resting on a pyroclastic substrate (locally reworked), in the Pietramelara ...Plain, in Southern Italy. The dry soil diurnal oscillations detected at the site in August 2009 have been found to be present also during the rainy periods of the year, provided, however, that soil moisture conditions are not affected by rainfall infiltration or post storm gravity-driven percolation in the soil depth interval considered. With significant rainfall, the characteristic dry soil diurnal cycle is altered. This rainfall-induced perturbation produces a spike-like signal featuring a sharp rise, with commencement of precipitation, then followed, with persistent and significant rainfall, by an equally sharp drop to very low levels. Both the sharp rise, resulting from capping, and the sharp drop (wash-out), resulting from infiltration, have been found to follow not previously reported second-order trends tied to cumulative rainfall, with very high statistical significance (r² > 0.97 for the sharp rise and r² > 0.95 for the sharp drop). Furthermore, the sharp drop appears to be tied to wetting front migration of²²²Rn-free, or²²²Rn-poor, rainwater and to commence when it reaches the soil probe depth, effectively flushing and transferring radon to lower levels of the profile, thus validating previous studies. Low rainfall events (<1–2 mm), however, do not appear to alter significantly the dry soil diurnal cycle. Post rainstorm recovery of soil²²²Rn concentrations, in the immediate aftermath of significant rainfall, appears to be very slow and limited. The spike-like pattern induced by rainstorm events poses significant questions as to the interpretation of soil²²²Rn concentration time series data in geophysical precursor studies. Likewise the extreme variations resulting from rainfall, and likely affected by rainfall duration, intensity, frequency and spatial variability, pose significant implications in environmental monitoring studies aimed at characterising representative soil²²²Rn concentration levels for the wet season.
222Rn concentrations have been measured in a well located on the edge of a large Pleistocene–Holocene fan and belonging to the shallow pyroclastic aquifer of the Pietramelara Plain, southern Italy. ...The aim of this study has been both to characterise the hydrological inputs that determine the influx of 222Rn to the shallow aquifer and to understand the correlations between 222Rn, major ions, physical–chemical parameters and rainfall. Results obtained from the time series indicate that the studied well shows a 222Rn variability that is inconsistent with a mechanism of pure hydrological amplification, such as described in Radon hazard in shallow groundwaters: Amplification and long term variability induced by rainfall (De Francesco et al., 2010a). On the contrary, in this well hydrological amplification appears to be mainly tied to the upwelling of alluvial fan waters, rich in radon, in response to pistoning from recharge in the carbonate substrate. This upwelling of alluvial fan waters occurs during almost the whole period of the annual recharge and is also responsible of the constant increase in 222Rn levels during the autumn–spring period, when both the water table level and weekly rainfall totals drop. Furthermore, a rapid delivery mechanism for 222Rn likely operates through fracture drainage in concomitance with the very first late summer–early autumn rains, when rainfall totals appear largely insufficient to saturate the soil storage capacity. Results obtained from this study appear to be particularly significant in both radon hazard zoning in relation to the shallow aquifer and possibly also for indoor radon, owing to possible shallow aquifer–soil–building exchanges. Moreover, both the spike-like events and the long wave monthly scale background fluctuations detected can also have potential significance in interpreting 222Rn time series data as seismic and/or volcanic precursors. Finally, 222Rn has proved to be an excellent tracer for hydrological inputs to the shallow aquifer when combined with major ions, physical–chemical data and geological and geomorphological controls.
► We describe the seasonal variability of radon in shallow groundwaters. ► We report the detection of two new mechanisms of hydrological amplification of radon in shallow groundwaters. ► These are significant for radon hazard evaluation and earthquake–volcanic precursor studies.
222Rn concentrations have been determined with a RAD7 radon detector in shallow groundwaters of the Pietramelara Plain, north-western Campania, southern Italy, where pyroclastic deposits, along with ...recent stream alluvial sediments, come in contact with Mesozoic carbonate reservoirs. The aim of this study has been to study the annual variation of
222Rn concentration in the shallow groundwaters, scarcely considered in the literature and of obvious relevance for radon hazard evaluation. Our results definitely show that
222Rn levels are characterized by a clear annual periodicity, strictly related to rainfall and water table levels, with a pronounced difference between the dry and the wet season. In this last case with concentrations increasing up to two orders of magnitude (up to two times the lower threshold given in the Recommendation 2001/928/EURATOM for public waters). In relation to this, experimental field data will be presented to demonstrate that this variability is due to purely hydrological mechanisms, mainly rinse out and discharge that control leaching efficiency. The detected cycle (Radon Hydrological Amplification Cycle, RHAC) has been generalized for the Mediterranean Tyrrhenian climate. The marked and seasonally persistent amplification in
222Rn levels poses the problem of evaluating the epidemiological risk brought up by this previously not yet reported mechanism. This mechanism, occurring in shallow groundwaters, very likely should strongly influence indoor radon levels via groundwater–soil–building exchange.
222Rn concentrations have been monitored during the dry season in August 2009 and August 2010, in a reworked alluvial-pyroclastic soil of the Pietramelara Plain, in Southern Italy, with the aim of ...determining the role of atmospheric factors in producing the quasi-periodic oscillations in soil 222Rn concentrations reported in the literature. In this study we present the results of a detailed analysis and matching of soil 222Rn concentrations, meteorological and solar parameters where the observed oscillations feature a characteristic behavior with second order build-up and depletion limbs, separated by a daily maximum and minimum. All these features are clearly shown to be tied to sunrise and sunset timings and environmental radiative flux regimes. Furthermore, a significant, and previously unreported, second order correlation (r2 = 0.73) between daily maximum hourly global radiation and the daily range of soil 222Rn concentrations has been detected, allowing estimates of the amplitude of these oscillations to be made from estimated or measured solar radiation data. The correlation has been found to be valid even in the presence of persistent patchy daytime cloudiness. In this case a daytime prolongation of the night-time build up stage and an attenuation or even suppression of daytime depletion is observed (a previously unreported effect). Neither soil cracking, nor precipitation, both suggested in some studies as causative factors for these oscillations, during the dry season appear to be necessary in explaining their occurrence. We also report the results of an artificial shading experiment, conducted in August 2009, that further support this conclusion. As soil 222Rn concentrations during the dry season show a characteristic daily cycle, radon monitoring in soils under these conditions necessarily has to be gauged to the timings of the daily maximum and minimum, as well as to the eventual occurrence of cloudiness and to its related effects, in order to avoid erroneous conclusions.
•We have monitored 222Rn concentrations in an alluvial-reworked pyroclastic soil.•222Rn levels have shown quasi-periodic daily oscillations.•Our results significantly affect radon monitoring in soils under dry season conditions.
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