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Chemical state of arsenic and copper in enargite: evidences from EPR and X-ray absorption spectroscopies, and SQUID magnetometry
No full textThe chemical state of elements in enargite, Cu(3)AsS(4), has been traditionally interpreted assigning the monovalent (+1) state to copper, pentavalent (+5) to arsenic, and divalent (-2) to sulphur. However, there are some evidences that make this interpretation not fully satisfying. These uncertainties reflect on the understanding of the oxidation process of enargite, that has important implications for both environmental issues and mineral processing. To clarify the issue, our study involves the combined use of X-ray absorption (XAS) and electron paramagnetic resonance (EPR) spectroscopies, and SQUID magnetometry. XANES spectra at the Cu K-edge for enargite and other copper sulphides show the expected affinities between the spectra of enargite and chalcopyrite (although the pre-edge feature typical of chalcopyrite is absent in enargite), reflecting the structural similarities of these two minerals; the spectra are, on the other hand, fairly similar to tetrahedrite, whose structure is different. The spectral shapes of bornite, chalcocite, and covellite are distinctly different from enargite. The edge position of enargite is consistent with the presence of monovalent copper, in spite of some small differences with respect to other sulphides where Cu is considered monovalent. At the As K-edge, the most striking feature is the clear difference in the edge position of enargite with respect to As(V) compounds such as calcium arsenate, and the similarity with As(III) compounds such as sodium arsenite. On the other hand, the edge position for enargite occurs at higher energies than those of tennantite, where As is regarded as trivalent, and of realgar (As(4)S(4)), thus supporting the view that in enargite arsenic is in a higher valence state. Enargite does not show a recognizable EPR spectrum in the X band region; however, the mineral shows a rather strong bulk magnetisation, equivalent (in the hypothesis of divalent Cu) to 0.05 unpaired electron per formula unit. The most likely explanation of this behaviour is the occurrence of small amounts of clustered paramagnetic impurities (e. g., divalent Fe). In summary, to the extent that nominal valence states can describe complex chemical bonding such as in sulphides, the results of this study rule out the occurrence of a significant fraction of Cu(II) in enargite, and support the "traditional" assignment of a monovalent state to Cu, and pentavalent to As
The Palaeocene palustrine carbonates in southwestern Sardinia (Italy): palaeoenvironmental significance
No full textFrom acid-rock drainage to acid-mine drainage: extreme contamination in waters at the Furtei Au-mine (Sardinia, Italy)
Full text linkThe weathering of sulfide-bearing mineral deposits, exposed either by natural erosion or by mining to
oxygen and water, can produce natural acid-rock drainage (ARD) and mining-related acid-rock drainage (acid
mine drainage, AMD), respectively. The composition and extent of acid drainage is highly variable, depending
on many factors, such as the composition of ore bodies and related host rocks, climate, mining operations and
processing (Nordstrom and Alpers, 1999). Due to the low pH and high concentrations of contaminants, acid
drainages can severely degradate surface and groundwater systems, as well as soils.
At Furtei (Sardinia, Italy), a high-sulfidation epithermal gold deposit consists of pyrite and enargite with
minor amounts of luzonite, tennantite, and chalcocite. The oxidized cap mainly consists of iron oxy-hydroxides,
gypsum, jarosite, halotrichite and scorodite. The gold deposit was exploited by open pit from 1997 to 2003.
The Au and Ag were recovered from oxidized ores by cyanidation. Sulfide ores were processed by flotation to
recover Cu concentrates. Total production was about 4 t of Au, 6 t of Ag, and 1500 t of Cu (Cidu et al., 2013).
Pre-mining conditions showed waters (flow <0.1 L/s) with pH down to 2.3, a typical example of ARD
characterized by high salinity (26 g/L) with predominant sulfate (20 g/L) and very high amounts of dissolved
contaminants (in mg/L): Al 2000, Fe 1700, Mn 35, Cu 29, Zn 11, Ni 4, Co 3, Cr 1, As 0.4, and Cd 0.1 (Cidu
et al., 1997).
During exploitation and after the mine closure, the number of sites with contaminated water increased.
Contaminated waters showed pH and chemical compositions similar to those observed under pre-mining
conditions, but much higher concentrations of Cu, Zn, As and Cd, respectively up to 180, 97, 5, and 1.7 mg/L
(Da Pelo et al., 2009; Cidu et al., 2013). The seepage from the tailings impoundment is collected in a well
and back pumped into impoundment. The highly contaminated water in the open pits and drainages from the
pyrite-rich waste dumps may flow downstream from the mine under heavy rain periods and pose a hazard to
agricultural areas
Innovative methodological approach for the modeling of seawater intrusion in the coastal plain of Muravera (Sardinia, Italy)
No full textMany coastal areas, especially low-lying deltaic areas, have high density populations and are often important
economic areas. At the same time, their aquifers are often overexploited and subject to seawater intrusion
phenomena. Climate change, including sea-level rise, will exacerbate the vulnerability of these sensitive areas,
so it is critical to provide the complex water resource management with tools capable to consider all the
variables involved to predict appropriate scenarios for action, balancing water demand of the population and
the natural water equilibrium. Hydrogeological models are as such powerful instruments though they need to
be calibrated with enough reliable hydrogeological data.
This study is applying an innovative hydrogeological modelling approach to simulate seawater intrusion in
the coastal plain of Muravera (south-east, Sardinia). This area has been studied since the 1950s due to seawater
intrusion phenomena that have led to various socio-economic and environmental problems. Thanks to the
geological, hydrogeological, and geochemical data collected in the recent years, it has been possible to develop
a 3D numerical groundwater model of the plain.
To simulate density dependent groundwater flow and coupled solute transport, by Deltares developed,
iMOD-WQ software is used. iMOD-WQ consists of the widely used software SEAWAT, which enables density
dependent fresh-salt groundwater modelling (Langevin & Guo, 2006), and the software RT3D, for reactive
transport modelling, which in not applied in this study. iMOD is a source code adapted for parallellization and
thus speeding up the computation up to two order of magnitudes depending on the available computer codes
(Verkaik et al., 2021) and it allows fast, flexible, and consistent sub-domain modelling techniques. Unlike other
groundwater modelling software, data with different cover extension can be implemented within the iMOD
without a pre-processing phase for fitting model-grid resolution but it regrids the datasets automatically to the
selected resolutions. The software is freely available and open source, can be implemented either using the
Graphical User Interface or with Python. In this study, the latter was preferred due to its ability to facilitate
working with groundwater models and for large datasets.
Based on the conceptual geological model described in Arras et al. (2019), the plain was divided into six
geological layers with different hydraulic conductivity values, which was then voxelized to a model with 45 model
layers. The recharge values were derived from the application of Soil Water Balance code (Porru et al., 2021). The
identification of the fresh water and saltwater interface was derived from direct measurements carried out with
a multiparameter probe in deep wells (20/30 m deep) in 2021. The model was then validated using piezometric
heads, electrical conductivity values and chemical and multi-isotopes data collected between 2020 and 2021.
The implemented 3D model is a preliminary valuable tool to support groundwater management. It can also
be used to simulate the effects of future sea water intrusion under different climate change driven sea-level and
natural recharge scenarios.
Arras C., Balia R., Buttau C., Cau P., Da Pelo S., Funedda A., Ghiglieri G., Loi A., Lorrai M., Melis M.T. & Testa M. (2019)
- Hydrogeological characterisation of the Flumendosa plain. Flowpath 2019, National Meeting on Hydrogeology, 6.
Langevin C.D. & Guo W. (2006) - MODFLOW/MT3DMS–based simulation of variable-density ground water flow and
transport. Groundwater, 44(3), 339-351.
Porru M.C., Da Pelo S., Westenbroek S., Vacca A., Loi A., Melis M.T., Pirellas A., Buttau C., Arras C., Vacca S., Mason
M.L., Lorrai M., Testa M. & Botta P. (2021) - A methodological approach for the effective infiltration assessment in a
coastal groundwater. Italian Journal of Engeneering Geology and Environment, 1, 183-193.
Verkaik J., Van Engelen J., Huizer S., Bierkens M.F.P., Lin H.X. & Oude Essink G.H.P. (2021) - Distributed memory
parallel computing of three-dimensional variable-density groundwater flow and salt transport. Adv. Water Resour.,
154, 10397
Nitrate occurrence in groundwater hosted in hard-rock aquifers: estimating background values at a regional scale
No full textThis study was aimed at assessing the occurrence of nitrate in poorly anthropized areas at a regional scale, as an attempt to estimate the nitrate background in areas far from intense agriculture, farming, industrial activities and urban areas.
Nitrate concentrations, together with physical-chemical parameters and major anions and cations, were determined in 49 spring waters flowing out of granitic and metamorphic rocks in Sardinia (Italy). Nitrate in the spring waters was in the range of <0.1 mg/L to 47 mg/L, a range similar to that observed in 404 rain samples collected in Sardinia, with highest values mostly occurring in spring waters located downstream villages.
The median nitrate might be considered an estimate of background values in the study area. The median nitrate in waters from metamorphic aquifers was 1.9 mg/L, close to the 1.3 mg/L median value observed in the Sardinian rain.
The median values of nitrate in waters from granitic aquifers was higher, i.e. 7.0 mg/L. These results were compared with an historical dataset consisting of 183 spring water samples collected between 2000 and 2012 in the same area.
Again, nitrate in spring waters from granitic rocks was higher (maximum value 80 mg/L, median: 7.6 mg/L) than nitrate in spring waters from metamorphic rocks (maximum value 48 mg/L, median 1.5 mg/L).
Geogenic factors may affect nitrate occurrence in groundwater and might explain the higher median values observed in springs flowing out of granitic rocks. In the study area, the soils developed on granitic rocks had higher permeability and lower cation exchange capacity, as compared to soils developed on metamorphic rocks. Such characteristics may favor a rapid leaching of nitrate to groundwater. Moreover, the redox potentials in granitic springs higher than those in metamorphic springs might have favored the persistence of nitrate in granitic environments. Overall results of this study showed that the granitic aquifers were more susceptible to nitrate contamination as compared to the metamorphic aquifers
Estimation of coastal aquifers vulnerability to seawater intrusion due to Climate Change
No full textSaltwater intrusion is a crucial environmental problem in coastal areas. The preservation and proper management of water resources is essential in these systems. Climate change can affect natural recharge of aquifers and sea level variation. However, the relative vulnerability of coastal aquifers to groundwater extraction and sea-level rise need to be considered in order to quantify which of these factors have the most significant impact and better manage groundwater to protect it from saltwater intrusion. The trend of saline intrusion will be simulated in the test area of the Muravera plain in Sardinia (Italy). In this area, the natural hydrodynamic equilibrium between groundwater, surface-water and seawater has been deeply modified by the construction of two dams across the Flumendosa river and the development of agriculture, tourism and aquaculture activities along the coast. In this work, the possible future scenarios of natural recharge of the aquifers in relation to climate change will be assessed through the use of Soil Water Balance (SWB) code. Moreover, the vulnerability of coastal aquifers to saltwater intrusion and variation in natural groundwater discharge caused by rising sea levels and in relation to abstraction will be examined. Monitoring systems of hydrogeological, geochemical and isotopic indicators will be designed to improve the qualitative and quantitative knowledge of groundwater in relation to the impact that climate change will have on the natural recharge of aquifers. These strategies could be extended at a regional level on coastal aquifers with similar hydrogeological behaviour
Ferruginous paleosols around the K/T boundary in central-southern Sardinia (Italy) as paleoclimatic and paleoenvironmental proxies
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