The geochemistry and origin of fluids in the carbonate structure of the Hranice Karst with the world\u27s deepest flooded cave of the Hranicka Abyss, Czech Republic

The origin of fluids in the Hranice Karst containing the deepest flooded abyss in the world has been investigated using hydrogeological, hydrogeochemical, and isotopic data. At least a part of the CO2 gas originates in the mantle as indicated by very enriched δ13C(DIC) values and from existing He isotope analyses. The origin of groundwater in the karstic aquifer which is exploited at the Teplice nad Bečvou Spa is meteoric with a recharge area about 200 m above the Bečva River valley as indicated by depleted values of δ2H and δ18O compared to the river water. Based on detectable tritium, the groundwater is from 20 to 50 years old. Water in the Hranicka Abyss and in the Zbrašov Aragonite Caves is a mixture of carbonate aquifer groundwater with the river and/or shallow groundwater comprising variable proportions of both end-members. Water in Death Cave Lake seems to be affected by agriculture contaminated shallow groundwater as indicated by increased nitrate concentration. Inverse geochemical modeling of aquifer geochemistry suggested two scenarios: (1) reaction of Mg-rich calcite with deep hypogenic CO2 (about 30 mmol/l) plus dissolution of trace amounts of halite and sylvite and cation exchange; (2) reaction of Mg-depleted calcite and Mg-silicate (talc) as a source of Mg together with deep CO2. Both scenarios were calibrated using δ13C(DIC) values and gave satisfactory results. A conceptual model of the site has been developed which includes a gravity-driven flow system where meteoric water which has recharged in the surrounding uplands is heated at depth and acquires large amounts of hypogenic CO2, which preferentially dissolves Mg-carbonates along the pre-existing tectonic features. The Miocene transgression followed by the later incision of the Bečva Valley played an important role in groundwater circulation and the origin of fluids

Geochemical and hydrological controls of arsenic concentrations across the sediment–water interface at Maharlu Lake, Southern Iran

The sediment–water transition zone in an aquatic system is key role to the distribution of contaminants between the surface water and sediment pore water. Maharlu Lake is a seasonal hypersaline lake in the central part of the Maharlu Basin in Southern Iran. Wastewater of various types produced within the basin is released into the seasonal freshwater rivers that ultimately drain into the lake. Samples were collected through one complete period of the lake water-level fluctuation. Lake surface water and shallow sediment pore water samples were collected three times at three piezometric stations at different distances from the river inflow points. Lake sediment samples were collected twice, and water samples from the inflowing rivers were collected five times. Changes in surface runoff and agricultural wastewater inflows were responsible for seasonal hydrochemical changes and changes in the As concentrations in the inflowing rivers. Data from the stations close to the river inflows indicated that the dissolved As concentrations across the sediment–water interface in Maharlu Lake are mostly controlled by evaporation and interactions between the surface water and shallow pore water. However, data from the station far from the freshwater inflows indicated that the brine and precipitated evaporites were at equilibrium and redox processes (e.g., iron (hydr)oxide dissolution and secondary sulphide precipitation) control the dissolved As concentration in pore water. The results confirmed the role of lake water evaporation in As enrichment and As sequestration by secondary sulphide minerals in sediment below the sediment–water interface. Climate change may alter the lake chemistry and As behavior in near future

Geochemistry and fluoride levels of geothermal springs in Namibia

© 2014 Elsevier B.V. A survey of groundwater from six geothermal springs in Namibia showed high concentrations of dissolved fluoride, with values up to 18.9mg/l. All values are higher than both the WHO limit and the Namibian guideline. High concentrations of fluoride are linked to Na-HCO3 or Na-SO4-HCO3 groundwater types, with increasing sulfate and chloride concentrations towards the south of Namibia. Values of δ2H and δ18O are more negative for the north of the country, and with increasing altitude of springs and distance from precipitation sources towards the southeast from the Indian Ocean. A shift of about 1‰ from the LMWL for Windhoek was observed for δ18O samples, which was probably caused by the exchange with reservoir rocks. Values of δ34S(SO4) reflect mixing of two principal sulfate sources, i.e., dissolution of gypsum originating from playas and interaction with sulfidic mineralization in tectonic bedrock zones. Values of δ13C(DIC) seem to be affected by a variable vegetation cover and mainly by the input of endogenous CO2. Estimated reservoir temperatures vary from 60°C to 126°C, with a maximum value at the Ganigobes site. The geothermal springs of Namibia in this study do not meet drinking water standards and thus their water can be used only for other purposes e.g. for thermal spas. Treatment would be necessary to decrease dissolved fluorine concentrations for drinking water purposes

Mineralogical study of arsenic-enriched aquifier sediments at Santiago del Estero, Northwest Argentina

Ashallowaquifer in an alluvial fan at Santiago del Estero, northwestern Argentina, is enriched in arsenic (As). Sediments from sites with high As concentration were collected by hand auger and studied by X-ray diffraction and by electron microprobe. X-ray diffraction confirmed the presence of quartz and albite. Concentration of total organic carbon (TOC) in soil is low, but concentration of total inorganic carbon (TIC) may be significant. The electron microprobe investigation found abundant glass particles with fluidal structure. The grains show significant weathering features such as voids and dissolution pits. Biotite grains present in sediments are also weathered. Iron oxyhydroxides occur in isolated spots on the surface of silicate minerals, but not as continuous coatings. Heavy minerals were represented by altered ilmenite, monazite, zircon, and garnet with predominant almandine component. The primary source of As could not be determined unequivocally, but volcanic glass and biotite are potential candidates. Ferric oxyhydroxides, which are important adsorbents of As, seem to have formed by precipitation of iron released from minerals like titano-magnetite, and ilmenite. However, the amount of precipitated ferric oxides and hydroxides is low and, furthermore, their As adsorption capacity depends on factors like pH and ionic strength of groundwater and on concentrations of species competing for adsorption sites

Spatial variation of groundwater arsenic distribution in the Chianan Plain, SW Taiwan: Role of local hydrogeological factors and geothermal sources

We present here major ion, trace element, stable and radioisotope data based on forty-six groundwater samples collected from various locations along few selected profiles across the Chianan Plain, southwestern Taiwan including the area affected by well known Blackfoot disease manifested by peripheral vascular gangrene. The objective of the study was to understand the role of local hydrogeology in terms of spatial variation of arsenic concentration in groundwater wells of the entire Chianan Plain and the foothill belt of the Central Mountain Range. An attempt has also been made to assess the contribution of nearby geothermal sources to the arsenic budget in groundwater of the Chianan Plain. Our study shows a gradual increase in all major and trace ion concentrations including total arsenic from foothill belt (arsenic: median=4μg/L, range=0-667.6μg/L, sample number n=16) to coastal zones (arsenic: median=42.74μg/L, range=0.14-348.6μg/L, n=15) of the plain. Inverse geochemical modeling shows that Ca may be exchanged on clays, and that the degree of the exchange increases from the foothill to the coastal zones. Inverse geochemical modeling further suggests that the oxidation of organic matter (CH2O) required in various east-west profiles across the plain to balance the total bicarbonate concentration and CO2 input from organic matters significantly increases from the foothill to the coastal zones with transfer coefficients ranging from 1.55�10- 2 to 1.69�10-5mol/L. High concentrations of tritium (mean=1.33±0.11 TU; n=4) in foothill groundwater and low concentration of tritium in groundwater of central zone suggest gradually increasing water-rock interaction from the foothill to the coastal part. Few elevated arsenic (median=171.8μg/L, maximum=667.60μg/L, minimum=24μg/L; n=6) hotspots are identified in the foothill belt. Available lithologs and aquifer test data suggest that the presence of impermeable clay around those pockets possibly inhibits vertical and lateral flushing of the aquifer and aids strong water-rock interactions subsequently leading to release of arsenic into groundwater. Using oxygen isotope and chloride mass balance method, we estimated that geothermal sources can recharge a maximum of 4 of groundwater in proximal aquifers and contribute <2 of average As concentration in the groundwater of Chianan Plain. Our preliminary observations thus show some arsenic enrichment in foothill aquifers, providing a necessity of detailed study of the aquifer systems in these understudied regions. Moreover, our research indicates that the contribution of arsenic from geothermal sources is insignificant, which stands in contrast to earlier studies suggesting that mud volcanoes and thermal springs in the Western Foothill Belt of the Central Mountain Range were potential sources of groundwater arsenic in the Chianan Plain aquifers