Applied Geochemistry (v.26, #1)

Geochemistry and arsenic behaviour in groundwater resources of the Pannonian Basin (Hungary and Romania) by Helen A.L. Rowland; Enoma O. Omoregie; Romain Millot; Cristina Jimenez; Jasmin Mertens; Calin Baciu; Stephan J. Hug; Michael Berg (1-17).
Elevated As levels in the Pannonian Basin are mainly present in very old (Palaeo) groundwater of methanogenic Pliocene/Quaternary aquifers, which is in contrast to Asian regions where arsenic-enriched groundwater is generally much younger.Display Omitted► Arsenic originates from Late Pliocene/Quaternary aquifers and some very old waters. ► Arsenic levels are controlled by both mobilisation and retention mechanisms. ► Mobilisation is caused by biogeochemical reductive dissolution. ► Sufficient sulfate supply triggers arsenic retention in sulfide precipitates. ► Nearly 500,000 people are exposed to elevated arsenic in their drinking water.Groundwater resources in the Pannonian Basin (Hungary, Romania, Croatia and Serbia) are known to contain elevated naturally occurring As. Published estimates suggest nearly 500,000 people are exposed to levels greater than the EU maximum admissible concentration of 10 μg/L in their drinking water, making it the largest area so affected in Europe. In this study, a variety of groundwaters were collected from Romania and Hungary to elucidate the general geochemistry and identify processes controlling As behaviour. Concentrations ranged from <0.5 to 240 μg/L As(tot), with As predominantly in the reduced As(III) form. Using cluster analysis, four main groups of water were identified. Two groups (1 and 2) showed characteristics of water originating from reducing aquifers of the area with both groups having similar ranges of Fe concentrations, indicating that Fe-reduction occurs in both groups. However, As levels and other redox characteristics were very different. Group 1, indicative of waters dominated by methanogenesis contained high As levels (23–208 μg/L, mean 123 μg/L), with group 2 indicative of waters dominated by SO 4 2 - -reduction containing low As levels (<0.5–58 μg/L, mean 11.5 μg/L). The remaining two groups were influenced either by (i) geothermal and saline or (ii) surface contamination and rain water inputs. Near absence of As in these groups, combined with positive correlations between δ 7Li (an indicator of geothermal inputs) and As(tot) in geothermal/saline influenced waters indicate that elevated As is not from an external input, but is released due to an in-aquifer process. Geochemical reasoning, therefore, implies As mobilisation is controlled by redox processes, most likely microbially mediated reductive dissolution of As bearing Fe-oxides, known to occur in sediments from the area. More important is an overlying retention mechanism determined by the presence or absence of SO 4 2 - . Ongoing SO 4 2 - -reduction will release S2−, removing As from solution either by the formation of As-sulfides, or from sorption onto Fe-sulfide phases. In methanogenic waters, As released by reductive dissolution is not removed from solution and can rise to the high levels observed. Levels of organic C are thought to be the ultimate control on the redox conditions in these 2 groups. High levels of organic C (as found in group 1) would quickly exhaust any SO 4 2 - present in the waters, driving the system to methanogenesis and subsequent high levels of As. Group 2 has much lower concentrations of organic C and so SO 4 2 - is not exhausted. Therefore, As levels in waters of the Pannonian Basin are controlled not by release but by retention mechanisms, ultimately controlled by levels of TOC and SO 4 2 - in the waters. δD and δ 18O analysis showed that groundwaters containing elevated As dated mostly from the last ice-age, and are sourced from Late Pliocene to Quaternary aquifers. The importance of TOC and retention capabilities of SO 4 2 - -reduction have only previously been suggested for recent (Holocene) sediments and groundwater, most notably those in SE Asia as these are the most likely to contain the right combination of factors to drive the system to a redox situation leading to high aqueous As concentrations. In contrast, it is shown here that a much older system containing As bearing Fe-oxides, also has the potential to produce elevated levels of As if the TOC is suitable for the microbial population to drive the system to the correct redox situation and SO 4 2 - is either absent or wholly consumed.

Mercury capture by selected Bulgarian fly ashes: Influence of coal rank and fly ash carbon pore structure on capture efficiency by Irena J. Kostova; James C. Hower; Maria Mastalerz; Stanislav V. Vassilev (18-27).
► Subbituminous coal-derived fly ash C traps more Hg than bituminous-coal-derived C. ► Subbituminous-derived C Hg capture increases with an increase in BET surface area. ► Subbituminous-derived fly ash Hg/C decreases with an increase in fly ash carbon. ► The latter suggests that some of the C is isolated from the flue gas stream.Mercury capture by fly ash C was investigated at five lignite- and subbituminous-coal-burning Bulgarian power plants (Republika, Bobov Dol, Maritza East 2, Maritza East 3, and Sliven). Although the C content of the ashes is low, never exceeding 1.6%, the Hg capture on a unit C basis demonstrates that the low-rank-coal-derived fly ash carbons are more efficient in capturing Hg than fly ash carbons from bituminous-fired power plants. While some low-C and low-Hg fly ashes do not reveal any trends of Hg versus C, the 2nd and, in particular, the 3rd electrostatic precipitator (ESP) rows at the Republika power plant do have sufficient fly ash C range and experience flue gas sufficiently cool to capture measurable amounts of Hg. The Republika 3rd ESP row exhibits an increase in Hg with increasing C, as observed in other power plants, for example, in Kentucky power plants burning Appalachian-sourced bituminous coals. Mercury/C decreases with an increase in fly ash C, suggesting that some of the C is isolated from the flue gas stream and does not contribute to Hg capture. Mercury capture increases with an increase in Brunauer–Emmett–Teller (BET) surface area and micropore surface area. The differences in Hg capture between the Bulgarian plants burning low-rank coal and high volatile bituminous-fed Kentucky power plants suggests that the variations in C forms resulting from the combustion of the different ranks also influence the efficiency of Hg capture.

Diel variation of selenium and arsenic in a wetland of the Great Salt Lake, Utah by Gennaro Dicataldo; William P. Johnson; David L. Naftz; Donald F. Hayes; William O. Moellmer; Theron Miller (28-36).
► Se and As concentrations varied on a diel cycle in a wetland. ► Se varied in phase with pH, dissolved oxygen, and water temperature. ► As varied opposite to Se, pH, dissolved oxygen and water temperature. ► Models showed that diel Se variations were driven by sorption and desorption due to pH and redox changes. ► As variations were linked to pH-driven sorption and desorption as well as co-precipitation and co-dissolution with Mn minerals.Diel (24-h) changes in Se and As concentrations in a freshwater wetland pond bordering the Great Salt Lake (GSL) were examined. Selenium concentrations (filtered and unfiltered) changed on a diel basis, i.e., were depleted during early morning and enriched during daytime over August 17–18. During the May 24–25, 2006 and September 29–30 diel studies, no significant 24-h trends were observed in Se concentrations compared to August, which showed daily maximums up to 59% greater than the daily minimum. Both filtered and unfiltered As concentrations also varied on a diel cycle, with increased concentrations during early morning and decreased concentrations during daytime. Filtered As concentrations increased 110% during the May 24–25, 2006 diel study. Selenium varied in phase with pH, dissolved O2 (DO), and water temperature (Tw ) whereas As varied opposite to Se, pH, DO and Tw . Changes in pH, DO and Tw showed a direct linear correlation (r  = 0.74, 0.75, and 0.55, respectively) to filtered Se. Also pH, DO and Tw were inversely correlated to filtered As concentration (r  = −0.88, −0.87, and −0.84, respectively). Equilibrium geochemical speciation and sorption models were used to examine the potential oxidation state changes in Se and As, and sorption and desorption reactions corresponding to the observed 24-h variations in pe and pH. In this wetland it was postulated that diel Se variation was driven by sorption and desorption due to photosynthesis-induced changes in pH and redox conditions. Diel variations of As were hypothesized to be linked to pH-driven sorption and desorption as well as co-precipitation and co-dissolution with mineral phases of Mn.

Arsenic distribution, concentration and speciation in groundwater of the Osijek area, eastern Croatia by Željka Romić; Mirna Habuda-Stanić; Brankica Kalajdžić; Mirko Kuleš (37-44).
► High As(tot) concentrations, up to 358 μg L−1 were found in Osijek’s groundwaters, eastern Croatia. ► Strong depth dependence of As(tot) concentrations in Osijek’s groundwaters were observed. ► Seasonal-dependencies of As(tot) were found and highest concentrations being found in the summer. ► The arsenic speciation is dominated by inorganic arsenic (III).Groundwater is the main source of drinking water for the population of nearly 200,000 people in eastern Croatia. The largest town in the region is Osijek whose citizens are supplied with drinking water obtained from groundwater from the “Vinogradi” well field. This study investigated and determined As occurrence in groundwater of the Osijek area. Groundwater samples were taken from 18 water wells and 12 piezometers with a depth ranging between 21 and 200 m. Over the 10-a period to 2007, a mean As concentration of 240 μg L−1 was found. There was no statistically significant secular change in concentration over that period, however small but significant seasonal variations were noted, with the highest seasonal As concentrations over the period May 2006–February 2007 being observed in summer. The predominant As species observed was As(III), constituting 85% and 93% of total As in piezometers and water wells, respectively. Higher concentrations of As tended to be found in deeper wells with the mean As concentration in shallow groundwater (<50 m) and deep groundwater (>50 m) being 27 μg L−1, and 205 μg L−1, respectively. Geochemically, the groundwaters show similarities to those in other parts of the Pannonian Basin. Arsenic(tot) is weakly correlated with pH and Fe, negatively correlated with Mn and has no significant correlation with any of EC, COD-Mn or alkalinity.

► Phosphate amendments do not suppress sulfide oxidation in coarse-grained wastes. ► MKP fertilizer inhibits sulfide oxidation in finely granulated wastes. ► Future field trials need to explore phosphate amendment on fine-grained mine wastes.The aim of the study was to determine whether the application of phosphate compounds (phosphorite rock, phosphate fertilizer) to polyminerallic waste rocks can inhibit sulfide oxidation and metal mobility (Cu, Pb, Zn, Cd, Ni, Mn, Mg). Waste rocks comprised sulfidic carbonaceous shales and were sourced from the Century Pb–Zn mine, NW Queensland, Australia. The acid producing, Pb–Zn rich rocks consisted of major quartz, muscovite/illite, dolomite, siderite and kaolinite as well as smaller amounts of sulfide minerals (e.g. galena, sphalerite, pyrite). Laboratory leach experiments were conducted on finely granulated phosphate-treated waste rocks (>2 to <30 mm) over 13 weeks, whereas phosphate amendment of coarsely granulated waste rocks (sand to boulder size) was investigated using heap leach piles at the mine site over an 11 months period. Results of the laboratory experiments demonstrate that the treatment of finely granulated waste rocks with phosphorite rock produced leachates with near-neutral pH values due to calcite dissolution. This in turn did not allow the leaching of apatite, formation of secondary phosphate phases and phosphate stabilization to occur. Metal mobility in these amended wastes was restricted by the dissolution of calcite and the resultant near-neutral pH conditions. By contrast, the application of the water-soluble phosphate fertilizer MKP (KH2PO4) to polyminerallic sulfidic waste rocks during the short-term laboratory experiments led to the formation of phosphate coatings and precipitates and inhibited acid and metal release (Cd, Mn, Ni, Pb, Zn). At least in the short term, the application of phosphate fertilizers proved to be an effective method. However, results of the long-term field trials demonstrate that coarsely granulated waste rocks were not coated by secondary phosphate phases and that amendment by phosphorite rock or superphosphate fertilizer did not improve leachate quality compared to the unamended waste. Thus, phosphate stabilization appears ineffective in suppressing oxidation of sulfides in coarsely granulated mine wastes.

Hydrotalcite-like layered bismuth–iodine–oxides as waste forms by James L. Krumhansl; Tina M. Nenoff (57-64).
► Hydrotalcite-like Bi–O–I phases are viable low-solubility radioiodine waste forms. ► Bi–O–I phases are a low cost alternative to radioiodine capture with Ag-zeolites. ► Bi–O–I radioiodine waste forms fabricated easily at room temperature.The effective capture and storage of radiological I (129I) remains a significant concern for safe nuclear waste storage and safe nuclear energy. Due to its long half-life (1.6 × 107  a) and concerns for involvement in human metabolic processes, durable waste forms are of great interest and research focus. Long term durability is mimicked in geological analogs. As a result, the authors have utilizing a facile, in situ process of synthesizing mineral analogs of layered (hydrotalcite-like) bismuth–iodine–oxide waste forms that does not require advanced separation and isolation of the I species from the aqueous solution. Specifically, the phases are crystallized and precipitated out of the waste stream at room temperature by the simple titration of an acidified Bi nitrate solution which precipitates layered oxide phases; phase composition and I weight loading is determined by the Bi:I ratio in solution. Products are designed to combine high I loading levels with chemical durability. Several characterization techniques were employed to better understand the relationship between waste forms, their abilities to encapsulate I, and their stability under possible repository conditions. They include solubility leach testing with elevated levels of common ground water anions Cl - , HCO 3 - , SO 4 2 - , thermal stability testing, elemental analysis, X-ray diffraction and microscopy studies.

Glass–iron–clay interactions in a radioactive waste geological disposal: An integrated laboratory-scale experiment by Guillaume de Combarieu; Michel L. Schlegel; Delphine Neff; Eddy Foy; Delphine Vantelon; Philippe Barboux; Stéphane Gin (65-79).
► A glass-iron-clay setup has been reacted at 90 °C for up to 18 months. ► The reacted interfaces were characterized with micro-spectroscopic techniques. ► Glass was altered at a rate close to the initial rate over a period of 18 months. ► P, Mo, and REE from glass precipitated as a thin layer. ► Oxidized Fe precipitated as a Fe-rich layer of siderite and Fe-phyllosilicate.Glass–iron–clay setups were reacted at 90 °C for 6–18 months to investigate the coupled interactions between glass alteration, Fe corrosion and clay transformation. The reacted interfaces were probed at the microscopic level using complementary characterization methods (scanning electron microscopy coupled with energy-dispersive X-ray analysis, micro-Raman spectroscopy, micro X-ray diffraction, micro X-ray fluorescence spectroscopy, and micro X-ray absorption near-edge structure spectroscopy). The 10-μm thick Fe foil was fully corroded within 10 months, exposing glass to the pore solution. Iron corrosion led to the formation of a layer containing mostly magnetite, siderite and Fe-rich phyllosilicates with one tetrahedral and one octahedral sheet (TO) or two tetrahedral and one octahedral (TOT) sheet per layer. The clay in contact with this corrosion layer was enriched in siderite (FeCO3). Glass alteration resulted in the formation of a gel layer whose thickness increased with reaction time (from 20 μm after 6 months to 80 μm after 18 months) and a thin layer of secondary precipitates that concentrated lanthanides, P, and Mo. Assuming conservative behavior of Zr, the Si molar concentration in the gel is about 57% that in the glass. Glass dissolution remained at a rate close to the initial dissolution rate r 0. The data are consistent with glass dissolution sustained by the uptake of dissolved Si and charge-compensating cations on secondary (corrosion) products, thus maintaining the gel porosity open and facilitating the leaching of easily soluble elements.

Trace metal behaviour in riverine sediments: Role of organic matter and sulfides by Adeline Charriau; Ludovic Lesven; Yue Gao; Martine Leermakers; Willy Baeyens; Baghdad Ouddane; Gabriel Billon (80-90).
Experimental and modelling approach on trace metal fate in anoxic sediments.Display Omitted► Experimental and modelling approach on trace metals fate in anoxic sediments. ► Organic matter and sulphides compete for the binding of trace metals. ► Efficient scavenging of trace metals in sulphide minerals. ► Dissolved organic matter increases the solubility of trace metals in pore waters. ► Similar lability of trace metals in pore waters and sediment particles.Three sediment cores were collected in the Scheldt, Lys and Spiere canals, which drain a highly populated and industrialized area in Western Europe. The speciation and the distribution of trace metals in pore waters and sediment particles were assessed through a combination of computational and experimental techniques. The concentrations of dissolved major and trace elements (anions, cations, sulfides, dissolved organic C, Cd, Co, Fe, Mn, Ni, Pb and Zn) were used to calculate the thermodynamic equilibrium speciation in pore waters and to evaluate the saturation of minerals (Visual Minteq software). A sequential extraction procedure was applied on anoxic sediment particles in order to assess the main host phases of trace elements. Manganese was the most labile metal in pore waters and was mainly associated with carbonates in particles. In contrast, a weak affinity of Cd, Co, Ni, Pb and Zn with carbonates was established because: (1) a systematic under-saturation was noticed in pore waters and (2) less than 10% of these elements were extracted in the exchangeable and carbonate sedimentary fraction. In the studied anoxic sediments, the mobility and the lability of trace metals, apart from Mn, seemed to be controlled through the competition between sulfidic and organic ligands. In particular, the necessity of taking into account organic matter in the modelling of thermodynamic equilibrium was demonstrated for Cd, Ni, Zn and Pb, the latter element exhibiting the strongest affinity with humic substances. Consequently, dissolved organic matter could favour the stabilization of trace metals in the liquid phase. Conversely, sulfide minerals played a key role in the scavenging of trace metals in sediment particles. Finally, similar trace metal lability rankings were obtained for the liquid and solid phases.

► NGTs show a 9.1 °C difference between the Last Glacial Maximum and the Holocene in Hungary. ► The 9.1 °C difference led to different C chemistry and 14C0 in the recharging waters. ► The elevated crustal He at the end of the flow path is attributed to presence of old formation water. ► Correction of 14C ages requires a more detailed knowledge about the hydrology.To establish the increase in temperature and the time span of the transition between the Late Glacial Maximum (LGM) and the Holocene, the noble gas content, 18O, 2H, 13C δ values, 3H and 14C activity and chemistry were studied in a groundwater flow system in Quaternary sediments in Hungary. The study area is a sub-basin of the Pannonian Basin, where the C isotope ratios are not influenced by carbonate reactions along the flow path, because the only water–rock interaction is ion exchange. The δ 18O and δ 2H values indicate a cold infiltration period, followed by warming, and, finally, warm temperature conditions. The noble gas data show that the average infiltration temperature was 3.3 °C in the cold, 12.9 °C in the warm, and intermediate in the transitional stage. Using the noble gas temperatures, geochemical batch modelling was performed to simulate the chemical processes. Based on the geochemical model, δ 13C and 14C0 (initial radiocarbon activity) in the recharging water were calculated. Transport modelling was used to simulate the distribution of chemical components, δ 18O, δ 2H and 14C0, along the flow path. It was found that the main processes determining the chemical composition of the groundwater were dissolution/precipitation of calcite and dolomite during infiltration near the surface, and ion exchange along the flow path. In the recharge area the δ 13C and 14C0 were controlled by dissolution and precipitation of carbonate minerals, C speciation, and fractionation processes. All these processes were influenced by the recharge temperature. NGTs calculated from the dissolved noble gas concentrations showed an average of 3.3 °C for cold, and 12.9 °C for warm infiltration, i.e. for the LGM and for the Holocene. The temperature difference was thus 9.1 ± 0.8 °C, which is one of the largest degree of warming detected by noble gases so far. The alkalinity indicates that carbonate reactions were unimportant along the flow path. Owing to the temperature dependence of the equilibrium constants, temperature conditions during infiltration have to be taken into consideration in radiocarbon age calculation. Dispersive transport along the flow path modified the chemical and isotopic composition of infiltrated water. The contribution of the old pore water, which was free of the 14C isotope, resulted in uncertainties in radiocarbon age determination. It was concluded that determination of the radiocarbon age or mean residence time requires detailed knowledge of the hydraulic conditions of groundwater.

Downward migration of Chernobyl-derived radionuclides in soils in Poland and Sweden by Gerald Matisoff; Michael E. Ketterer; Klas Rosén; Jerzy W. Mietelski; Lauren F. Vitko; Henning Persson; Edyta Lokas (105-115).
137Cs and 239,240Pu move down into the soil profile. ► 137Cs is from Chernobyl fallout; 239,240Pu is from stratospheric fallout from the 1960s. ► Solute transport and bioturbation models describe some but not all of the radionuclide profiles.Vertical profiles of 137Cs and 239,240Pu were measured in soils collected from two sites in southern Sweden and three sites in southern Poland and were modeled using both a solute transport model and a bioturbation model to better understand their downward migration. A time series of measured 137Cs profiles indicates that 137Cs from Chernobyl was found at the soil surface in 1986 but it has migrated progressively downward into the soil 4.5–25.5 cm since. However, because of dispersion during the migration and mixing following Chernobyl deposition and the much higher activities of 137Cs from Chernobyl, stratospheric fallout of 137Cs from the 1960s cannot be identified as a second 137Cs activity maximum lower in the soil column at any of the sites. Conversely, the 240Pu/239Pu ratio indicates that no Chernobyl-derived Pu is present in any of the cores with the exception of one sample in Sweden. This difference may be attributed to the nature of the release from Chernobyl. Cesium volatilized at the reactor temperature during the accident, and was released as a vapor whereas Pu was not volatile and was only released in the form of minute fuel particles that traveled regionally. Both the solute diffusion and the bioturbation models accurately simulate the downward migration of the radionuclides at some sites but poorly describe the distributions at other sites. The distribution coefficients required by the solute transport model are about 100 times lower than reported values from the literature indicating that even though the solute transport model can simulate the profile shapes, transport as a solute is not the primary mechanism governing the downward migration of either Cs or Pu. The bioturbation model uses reported values from the literature of the distribution coefficients and can simulate the downward migration because that model buries the fallout by placing soil from depth on top and mixing it slightly throughout the mixing zone (0.6–2% per year of mixing). However, mixing in that model predicts concentrations in the top parts of the soil profiles which are too high in many cases. Future progress at understanding the downward migration of radionuclides and other tracers will require a more comprehensive approach, combining solute transport with bioturbation and including other important soil processes.

Characterization of accumulated precipitates during subsurface iron removal by Doris van Halem; Weren de Vet; Jasper Verberk; Gary Amy; Hans van Dijk (116-124).
► Accumulated iron was not found to clog the well or aquifer after 12 years of subsurface iron removal. ► 56–100% of accumulated iron hydroxides were found to be crystalline. ► Subsurface iron removal favoured certain soil layers, either due to hydraulics or mineralogy. ► Other groundwater constituents, such as manganese and arsenic were found to co-accumulate with iron.The principle of subsurface iron removal for drinking water supply is that aerated water is periodically injected into the aquifer through a tube well. On its way into the aquifer, the injected O2-rich water oxidizes adsorbed Fe2+, creating a subsurface oxidation zone. When groundwater abstraction is resumed, the soluble Fe2+ is adsorbed and water with reduced Fe concentrations is abstracted for multiple volumes of the injection water. In this article, Fe accumulation deposits in the aquifer near subsurface treatment wells were identified and characterized to assess the sustainability of subsurface iron removal regarding clogging of the aquifer and the potential co-accumulation of other groundwater constituents, such as As. Chemical extraction of soil samples, with Acid-Oxalate and HNO3, showed that Fe had accumulated at specific depths near subsurface iron removal wells after 12 years of operation. Whether it was due to preferred flow paths or geochemical mineralogy conditions; subsurface iron removal clearly favoured certain soil layers. The total Fe content increased between 11.5 and 390.8 mmol/kg ds in the affected soil layers, and the accumulated Fe was found to be 56–100% crystalline. These results suggest that precipitated amorphous Fe hydroxides have transformed to Fe hydroxides of higher crystallinity. These crystalline, compact Fe hydroxides have not noticeably clogged the investigated well and/or aquifer between 1996 and 2008. The subsurface iron removal wells even need less frequent rehabilitation, as drawdown increases more slowly than in normal production wells. Other groundwater constituents, such as Mn, As and Sr were found to co-accumulate with Fe. Acid extraction and ESEM–EDX showed that Ca occurred together with Fe and by X-ray Powder Diffraction it was identified as calcite.

Zinc and nickel removal in simulated limestone treatment of mining influenced water by Andrew Miller; Linda Figueroa; Thomas Wildeman (125-132).
► Zinc and nickel removed in conjunction with iron and aluminum precipitates. ► Zinc and nickel removal depended on pH, alkalinity, and amount of iron and aluminum. ► Zinc removal may not be predicted from traditional surface complexation theory. ► Interplay between variables in treatment systems affects zinc/nickel behaviors.Mining influenced water (MIW) is often characterized by low pH (acid mine drainage) and high dissolved metal concentrations. Treatment of MIW is often required to mitigate these two characteristics. One option, which has traditionally been used only for pH neutralization, is limestone based treatment systems. However, there is field evidence that limestone systems are also effective at removing metals such as Zn and Ni. These field systems are often too complex to examine specific removal hypotheses, while certain modes of laboratory examination are too simplistic to be applied to field settings. Instead, the batch reactors used here were intentionally more complex to allow for the examination of how certain variables (pH, alkalinity, and primary metal concentrations) interact to affect Zn and Ni removal. The data herein suggest that one possible removal process for Zn and Ni is through surface interactions on the precipitated primary metals Fe and Al. The specific processes are complex and conditional, and were found to depend on pH, alkalinity, and total amount of primary metal present. Indeed the complex interplay between these variables led to an observed local maximum in Zn removal that would not be predicted from traditional surface complexation theory or observed from simpler experimental systems.

► We explore the relationship of U in groundwater to the hydrogeochemical environment. ► We integrate hydrogeochemical parameters to characterize the hydrogeochemical zones. ► U speciation and behavior in groundwater are different in different zones. ► U in groundwater is dissolved and transported as U(VI) in the oxidized zone. ► U in groundwater is precipitated and mineralized as U(IV) in the redox transition zone.This paper describes the occurrence and distribution of U within groundwater of the Ili basin in Xinjiang, China. Dissolved major ions, dissolved gases, pH, Eh and salinity in groundwater were employed to distinguish different hydrogeochemical environments and associated processes affecting U transport within the aquifer. The results show that dissolution and deposition of U by groundwater are closely related to reactions with organic C and minerals such as sulfides, carbonates and silicates along groundwater flow paths. Different chemical reactions predominate and account for changes in the occurrence of U in the rock and water of the oxidized zone, the redox transition zone, and the reduced zone. Knowledge of these associations between U and hydrogeochemical environments could enhance the understanding of relationships of groundwater circulation and U mineralization and could have implications for prospecting for sandstone-type U deposits.

Behavior of Gd-DTPA in simulated bank filtration by Peter Möller; Andrea Knappe; Peter Dulski; Asaf Pekdeger (140-149).
► Gd-DTPA and similar products are common pollutants in surface and groundwater and prove to be a reliable monitor for mixing of groundwater with surface water loaded with effluents from sewage treatment plants. ► Within a 30 m column filled with Pleistocene sand Gd-DTPA is reduced by 16% only within one month time by transmetallation. ► Artificial bank filtration is not very effective in reducing Gd-DTPA in infiltrating water.The behavior of Gd-DTPA during bank filtration was simulated in a 30 m column filled with Pleistocene sand and flushed by surface water from a lakeside in Berlin, Germany. The surface water is about a 1:1 mixture of river water and effluents from a sewage treatment plant. Throughout 34 days this water was continuously spiked with Gd-DTPA at a level of 60 μg/L. The broad plateau of the Gd-DTPA pulse declined by 15.4% within 34 days by transmetallation. Nine percentage of the total decline is caused by Y and rare earth elements; the remaining part is attributed to Cu2+ which is the most influential metal in surface water. All other metals also contributing to transmetallation are combined with Cu to Cu equivalents because only the rate constant of transmetallation of Cu2+ is known. The analytical results of the column effluents prove the pseudo-first-order kinetics of transmetallation based on reversible sorption of metals by pools in the column sediment and disprove biodegradation at noticeable levels. The mass ratio of water to tracer is <1010.