Applied Geochemistry (v.26, #5)
Oxygen influx and geochemistry of percolate water from reactive mine waste rock underlying a sloping channelled soil cover by Qing Song; Ernest K. Yanful (655-665).
► A channelled cover with preferential flow can still mitigate ARD to some extent. ► Oxygen ingress was more sensitive to the location of the channel than to Ks . ► The channel in the barrier layer was a major passage for O2 ingress. ► Actual flushing was an important factor when estimating O2 decay coefficient.An ideal engineered soil cover can mitigate acid rock drainage (ARD) by limiting water and gaseous O2 ingress into an underlying waste rock pile. However, the barrier layer in the soil cover almost invariably tends to develop cracks or fractures after placement. These cracks may change water flow and O2 transport in the soil cover and decrease performance in the long run. The present study employed a 10-cm-wide sand-filled channel installed in a soil barrier layer (silty clay) to model the aggregate of cracks or fractures that may be present in the cover. The soil cover had a slope of 20%. Oxygen transport through the soil cover and oxidation of the underlying waste rock were investigated and compared to a controlled column test with bare waste rock (without soil cover). Moreover, gaseous O2 transport in the soil cover with channel and its sensitivity to channel location as well as the influence of the saturated hydraulic conductivity of the channel material were modeled using the commercial software VADOSE/W. The results indicted that the waste rock underlying the soil cover with channel had a lower oxidation rate than the waste rock without cover because of reduced O2 ingress and water flushing in the soil cover with channel, which meant a partial soil cover might still be effective to some extent in reducing ARD generation. Gaseous O2 ingress into the covered waste rock was more sensitive to the channel location than to the saturated hydraulic conductivity of the material filling the channel. Aqueous equilibrium speciation modeling and scanning electron microscopy with energy dispersive X-ray analysis indicated that secondary minerals formed as a result of the oxidation of the waste rock included gypsum and goethite in the covered waste rock and schwertmannite and other Fe oxides in the uncovered waste rock. The findings of the study provided insight into the effect of channel flow on O2 transport and oxidation of the covered waste rock, which may help to improve soil cover design and construction to minimise the generation of preferential flow in the barrier layer.
Speleothems and pine trees as sensitive indicators of environmental pollution – A case study of the effect of uranium-ore mining in Hungary by Zoltan Siklosy; Zoltan Kern; Attila Demeny; Sebastian Pilet; Szabolcs Leel-Ossy; Ke Lin; Chuan-Chou Shen; Eva Szeles; Daniel Breitner (666-678).
► Stalagmites can preserve anthropogenic impact in the environment. ► Living pine (P. sylvestis) trees are also act as a chemoenviromental archive. ► A rise in uranium of the stalagmite suggested increasing amounts pollutants. ► Two different geochemical proxies as pollution recorders were highlighted.Four decades of U ore production in Hungary provides an opportunity to study the possible environmental effects of mining. The study reveals significant changes in chemical composition of a stalagmite (cave deposit). The good fit between U content changes in the studied deposit and the U ore production rate support the assumption of the relationship with mining activity. An independent chemoenviromental archive, living pine (Pinus sylvestis) trees were also investigated. Data on pine tree cores collected from the same region show different levels of pollution (Cu, Zn, Mn, U) after the 1950s and 1960s, linked to the opening of mines and subsequent dust fallout around the site. Elevated concentrations of detritally derived elements (Si, Al, Th) coupled with a rise in U concentration and change in δ234U values of the stalagmite suggest increasing amounts of mine-derived dust from 1 to 3 km distance that settled and washed into the karst system. The combined usage of different proxies not only provides historic records for the anthropogenic impact in the environment, but also allows the timing of U concentration increases within the stalagmite and the identification of elemental behavior from the pollution. This study shows that complementary geochemical archives such as stalagmites and tree rings used together can enhance understanding of past environmental contamination.
Quantitative assessment of radionuclide retention in the Quaternary sediments/granite interface of the Fennoscandian shield (Sweden) by Fidel Grandia; Clara Sena; David Arcos; Jorge Molinero; Lara Duro; Jordi Bruno (679-687).
► The release of radionuclides from a deep geological repository is investigated. ► We simulate the transport of radionuclides in a Quaternary sediment. ► The Quaternary sediment’s geochemistry is studied to select the reactive minerals. ► U, Sr, Cs and Ra were selected due to their contribution for the radioactive dose. ► The retention capacity of the Quaternary sediments was quantitatively evaluated.The Quaternary sediments representing the interface between the granite host rock and the Earth surface are of paramount importance when determining the potential cycling of anthropogenic and natural radionuclides in near-surface systems. This is particularly true in the case of high-level nuclear waste (HLNW) repositories placed in granite. In this work a modelling procedure is presented to quantitatively determine the retention capacity of a Quaternary till in the Forsmark area, which has been recently selected to host the deep geologic storage of HLNW in Sweden. Reactive transport numerical models have been used to simulate the intrusion of a deep groundwater carrying radionuclides potentially released from a repository into a Quaternary till. Four radionuclides (235U, 135Cs, 226Ra and 90Sr) have been selected according to their different geochemical behaviour and potential dose relevance to surface ecosystems. Numerical results indicate that repository-derived: (i) U will have a minor impact in the till, mainly due to the high natural concentration of U and its adsorption on ferrihydrite; (ii) Cs will be efficiently retained by cation exchange on illite; (iii) Ra will be retained via co-precipitation with barite; and although (iv) Sr will be retained via co-precipitation with calcite and cation exchange on illite, the retention capacity of the Quaternary till for Sr is limited.
Isotopic evidence for the source and fate of phosphorus in Everglades wetland ecosystems by Xin Li; Yang Wang; Jennifer Stern; Binhe Gu (688-695).
► Oxygen isotopic analysis of phosphate is a useful tool for studying source and degree of microbial cycling of phosphorus (P) in freshwater ecosystems. ► P was quickly cycled in the water column and the dissolved inorganic phosphate (DIP) pool consisted entirely of biologically cycled P in relatively pristine areas of the Everglades wetland ecosystem. ►In wetland areas highly impacted by agricultural runoff, biological cycling of P was not rapid enough to completely remove the fertilizer δ 18O signature. ►DIP pool in these areas consisted of biologically cycled P as well as fertilizer P, with fertilizer P accounting for about 15–100% of the total DIP.Phosphorus has historically been a limiting nutrient in the Florida Everglades. Increased P loading to the Everglades over the past several decades has led to significant changes in water quality and plant communities. Stormwater runoff that drains agricultural lands and enters the Water Conservation Areas (WCAs) are known to contain elevated levels of P, but the exact source of this P has not been fully determined. Here the results of an O isotope study of dissolved inorganic phosphate (DIP) in both polluted and relatively pristine (or reference) areas of the Everglades are reported. The data reveal spatial and temporal variations in the δ 18O signature of DIP, reflecting the source and the degree of cycling of P. The δ 18O values of DIP collected from the Everglades National Park were close or equal to the predicted δ 18O values of DIP formed in situ in equilibrium with ambient water, indicating that P is quickly cycled in the water column in oligotrophic ecosystems with very low P concentrations. However, most DIP samples collected from areas impacted by agricultural runoff yielded δ 18O values that deviated from the predicted equilibrium DIP–δ 18O values based on the δ 18O of water and water temperature, suggesting that biological cycling of P was not rapid enough to remove the fertilizer δ 18O signature in the DIP pool from areas receiving high P loading. The δ 18O signature of DIP in impacted areas reflects a mixing of fertilizer P and biologically cycled P, where the relative proportions of biologically cycled vs. fertilizer DIP are controlled by both biological (microbial activities and plant uptake) and hydrologic factors (loading rate and residence time). Using a two-end-member (i.e., fertilizer P and biologically cycled P) mixing model, fertilizers were estimated to contribute about 15–100% of the DIP pool in the highly impacted areas of the northern Everglades, whereas the DIP pool in the reference (i.e., relatively pristine) wetlands in the Everglades National Park was dominated by biologically cycled P. The study shows that O isotopic measurements of dissolved PO 4 3 - can be a useful tool for tracing the fertilizer P inputs to freshwater ecosystems.
Composition and solubility of precipitated copper(II) arsenates by Hanna Nelson; Andrey Shchukarev; Staffan Sjöberg; Lars Lövgren (696-704).
► By mixing solutions of Cu2+ and HAsO4 2− solid phases are formed in a wide pH range. ► Five different stoichiometric compositions were found. ► Two of the solid phases formed in 0.1 M NaCl contained Na+. ► Stability constants for all solid phases have been determined. ► Aqueous complexes containing Cu2+ and AsO4 3− ions could not be detected.Equilibrium reactions involving Cu(II) and As(V) have been studied with respect to formation of complexes in aqueous solutions as well as formation of solid phases. Potentiometric titrations performed at 25 °C (I = 0.1 M Na(Cl)) and at different Cu to As ratios gave no evidence for the existence of Cu(II) arsenate complexes in solution below the pH of the precipitation boundaries (pH ≈ 4), irrespective of the Cu to As ratio and pH. Mixing of solutions of Cu(II) and As(V) at different proportions and adjusting pH to values ranging from 4 to 9 resulted in precipitation of five different solid phases. The elemental composition of the solids was determined using X-ray Photoelectron Spectroscopy, and Environmental Scanning Microscopy–Field Emission Gun equipped with an energy dispersive spectroscopy detector. The average Cu/As ratio was determined by dissolving the solids. Total soluble concentrations of the components Cu(II) and As(V), as well as the basicity of the solid phases were determined by analysis of aqueous solutions. Based upon these experimental data the stoichiometric composition of the solid phases and their stability were determined. The resulting equilibrium model includes the solid phases Cu3(AsO4)2, Cu3(AsO4)(OH)3, Cu2(AsO4)(OH), Cu5Na(HAsO4)(AsO4)3 and Cu5Na2AsO4)4, where Cu5Na(HAsO4)(AsO4)3 and Cu5Na2(AsO4)4 have not been reported previously. In 0.1 M Na(Cl), Na+ was found to be a significant component in two of the solid phases. The Cu5Na2(AsO4)4 was formed in weakly alkaline conditions with pNa < 2.5. Stability constants for all solid phases have been determined. Distribution diagrams as well as predominance area (pNa–pH) diagrams are presented to illustrate stability fields of the different solid phases.
Geochemical characterization of arsenic-affected alluvial aquifers of the Bengal Delta (West Bengal and Bangladesh) and Chianan Plains (SW Taiwan): Implications for human health by Bibhash Nath; Jyoti Prakash Maity; Jiin-Shuh Jean; Gavin Birch; Sandeep Kar; Huai-Jen Yang; Ming-Kuo Lee; Rasmani Hazra; Debashis Chatterjee (705-713).
► Geochemical evolution of groundwaters in the Bengal Delta and Chianan Plains are mostly influenced by the complex interaction of several biogeochemical processes. ► Humic acid fractions of Chianan Plain sediments showed stronger aliphatic band and a higher resolved fingerprint area compared with Bengal Delta sediments. ► Association between dissolved humic substances and As and other metals (e.g. Fe and Zn) in Chianan Plain may be responsible for the high occurrence of peripheral vascular disease.Major ion and trace element analyses were performed on groundwater samples collected from the Bengal Delta (Chakdaha municipality, West Bengal and Manikgonj town, Bangladesh) and Chianan Plains (SW Taiwan) to compare geochemical characteristics. Results showed that concentrations of Na, K, Mg, Cl and SO4 were generally higher in Chianan Plain (CNP) groundwaters, while high Ca was observed in Bengal Delta Plain (BDP) groundwater. Measured As concentrations in groundwaters of BDP and CNP showed large variations, with mean As concentrations of 221 μg/L (range: 1.1–476 μg/L) in Chakdaha, 60 μg/L (range: 0.30–202 μg/L) in Manikgonj, and 208 μg/L (range: 1.3–575 μg/L) in CNP groundwater. The Fe-reduction mechanism was found to be the dominant geochemical process in releasing As from sediment to groundwater in Chakdaha, West Bengal, however the Mn-reduction process was dominant in groundwaters of Manikgonj, Bangladesh. In Chianan Plain groundwater, a combination of geochemical processes (e.g., bacterial Fe-reduction, mineral precipitation and dissolution reactions) controlled release of As. Fluorescence spectral patterns of the groundwater showed low relative fluorescence intensity (RFI) of dissolved humic substances in BDP groundwater (mean: 63 and 72 QSU, Chakdaha and Manikgonj, respectively), while high RFI was observed in CNP groundwater (mean: 393 QSU). The FT-IR spectra of the extracted humic acid fractions from sediments of Chianan Plain showed a stronger aliphatic band at 2850–3000 cm−1 and a higher resolved fingerprint area (from 1700 to 900 cm−1) compared with BDP sediments. The geochemical differences between the study areas may play a crucial role in the clinical manifestation of Blackfoot disease observed only in Chianan Plain, SW Taiwan.
Hydrochemistry, mineralogy and chemical fractionation of mine and processing wastes associated with porphyry copper mines: A case study from the Sarcheshmeh mine, SE Iran by Mehdi Khorasanipour; Majid H. Tangestani; Reza Naseh; Hamid Hajmohammadi (714-730).
► Mine and processing waste are the main sources of acid mine drainage (AMD). ► Secondary soluble minerals provide important control on the quality of AMD. ► Mineralogical compositions of wastes is the main factor that controls metal fractionation. ► Subsequent use of water is diminished by the presence of corrosive ions like chlorine. ► The environmental management of geochemical processes in the tailings dam is necessary.The Sarcheshmeh is one of the largest Oligo-Miocene porphyry Cu deposits in the world. Comparative hydrochemical, mineralogical and chemical fractionation associated with mining efflorescence salts and processing wastes of this mine are discussed. Hydrochemical results showed that rock waste dumps, reject wastes and old impoundments of tailings are the main sources of acid mine drainage waters (AMD) that contain potentially toxic metals such as Cd, Co, Cu, Mn, Ni and Zn as well as Al. Episodic fluxes of highly contaminated acidic waters were produced in a tailings dam over a short period of time. Secondary soluble minerals provide important controls on the quality of AMD produced, especially in old, dry tailings impoundments. Secondary sulfate minerals such as gypsum, magnesiocopiapite, hydronium jarosite, kornelite and coquimbite were found in rock waste drainages and in old weathered reject wastes. Highly soluble secondary minerals such as gypsum, eriochalcite, and bonattite are also observed in an evaporative layer on old tailings impoundments. Chemical fractionation patterns of potentially toxic elements showed that the geochemical behavior of metals is primarily controlled by the mineralogical composition of waste samples. Elements such as Co, Cr, Cu, Mn, Ni and Zn are readily released into the water soluble fraction from efflorescence salts associated with rock waste drainages, as well as from the evaporative layer of old tailings. Potentially toxic elements, such as As, Mo and Pb, are principally adsorbed or co-precipitated with amorphous and crystalline Fe oxides, but they may also be associated with oxidizing, primary sulfides and residual fractions. Following the development of the dammed tailings pond, the secondary minerals were dissolved, producing acidic waters contaminated by Al (154 mg L−1), Cu (150 mg L−1), Cd (0.31 m gL−1), Co (2.13 mg L−1), Mn (73.7 mg L−1), Ni (1.74 mg L−1), Zn (20.3 mg L−1) and Cl (1690 mg L−1). Therefore, the potential use of recycled water from the Sarcheshmenh dammed tailings pond is diminished by the presence of corrosive ions like Cl− in highly acidic fluids that promote corrosion of pipes and pumps in the water recycling system.
Groundwater electrical conductivity and soil radon gas monitoring for earthquake precursory studies in Koyna, India by D.V. Reddy; P. Nagabhushanam (731-737).
► It is the first hydrochemical precursory study in the Koyna region, India. ► Discrete conductivity measurements indicated progressive increase for 4 years. ► Strong precursory EC change observed 40 h before the M 5.1 earthquake. ► Precursory increase of soil Rn gas 20 days earlier than earthquakes M 4.7 & 5.1. ► On-line monitoring of these parameters may help in earthquake forecast.Hourly monitoring of electrical conductivity (EC) of groundwater along with groundwater levels in the 210 m deep boreholes (specially drilled for pore pressure/earthquake studies) and soil Rn gas at 60 cm below ground level in real time, in the Koyna–Warna region (characterized by basaltic rocks, >1500 m thick, and dotted with several sets of fault systems), western India, provided strong precursory signatures in response to two earthquakes (M 4.7 on 14/11/09, and M 5.1 on 12/12/09) that occurred in the study region. The EC measured in Govare well water showed precursory perturbations about 40 h prior to the M 5.1 earthquake and continued further for about 20 h after the earthquake. In response to the M 4.7 earthquake, there were EC perturbations 8 days after the earthquake. In another well (Koyna) which is located 4 km north of Govare well, no precursory signatures were found for the M 4.7 earthquake, while for M 5.1 earthquake, post-seismic precursors were found 18 days after the earthquake. Increased porosity and reduced pressure head accompanied by mixing of a freshwater component from the top zone due to earthquakes are the suggested mechanisms responsible for the observed anomalies in EC. Another parameter, soil Rn gas showed relatively proportional strength signals corresponding to these two earthquakes. In both the cases, the pre-seismic increase in Rn concentration started about 20 days in advance. The co-seismic drop in Rn levels was less by 30% from its peak value for the M 4.7 earthquake and 50% for the M 5.1 earthquake. The Rn anomalies are attributed to the opening and closing of micro-fractures before and during the earthquake. On line monitoring of these two parameters may be useful to check the entire chemistry change due to earthquake which may help to forecast impending earthquakes.
Cold seeps in Monterey Bay, California: Geochemistry of pore waters and relationship to benthic foraminiferal calcite by Joris Gieskes; Anthony E. Rathburn; Jonathan B. Martin; M. Elena Pérez; Chris Mahn; Joan M. Bernhard; Shelley Day (738-746).
► We describe the geochemistry of pore waters in the Clam Flats area of Monterey Bay. ► The geochemical data are compared with the δ13C chemistry of benthic foraminifera. ► Living foraminifera indicate little effects of pore water low δ13C (DIC) in the clam bed. ► This phenomenon and its implications are discussed in detail. ► Implications with regards to paleo-methane seepage are discussed.An extensive geochemical and biogeochemical examination of CH4 seeps in the Clam Flats area of Monterey Bay provides insight into the character of relationships between seep geochemistry and benthic foraminiferal geochemistry. The area is characterized by sulfide-rich fluids. Sulfide increases are associated with large increases in alkalinity, as well as small decreases in dissolved Ca and Mg. In addition, only small increases in NH4 are observed, but values of δ13C of dissolved inorganic C are as low as −60‰ at shallow depths (<3 cm). These observations indicate that all these processes are related to the bacterial oxidation of CH4, which is transported upward by slow seepage of pore fluids. The geochemistry of the pore fluids should be relevant to the geochemistry of the carbonate tests of living and dead foraminifera. However, a profound disequilibrium of approximately an order of magnitude occurs between the δ13C values of stained (cytoplasm-containing) foraminiferal carbonate and the C isotope values of ambient pore water dissolved inorganic C. Reasons are unclear for this isotopic disequilibrium, but have important implications for interpretations of foraminiferal carbonate as a paleoenvironmental proxy. Much fine scale work is needed to fully understand the relationships between the biogeochemistry of benthic foraminifera and the geochemistry of the pore waters where they live.
Effects of human-induced alteration of groundwater flow on concentrations of naturally-occurring trace elements at water-supply wells by Joseph D. Ayotte; Zoltan Szabo; Michael J. Focazio; Sandra M. Eberts (747-762).
► Human perturbation of aquifers mobilizes trace elements that can exceed human health benchmarks. ► Changes in geochemical conditions due to mixing of waters drives mobilization. ► Trace element concentrations in mixed waters can exceed those in end-member waters. ► An evaluation framework shows trace element mobilization potential and possible mitigation..The effects of human-induced alteration of groundwater flow patterns on concentrations of naturally-occurring trace elements were examined in five hydrologically distinct aquifer systems in the USA. Although naturally occurring, these trace elements can exceed concentrations that are considered harmful to human health. The results show that pumping-induced hydraulic gradient changes and artificial connection of aquifers by well screens can mix chemically distinct groundwater. Chemical reactions between these mixed groundwaters and solid aquifer materials can result in the mobilization of trace elements such as U, As and Ra, with subsequent transport to water-supply wells. For example, in the High Plains aquifer near York, Nebraska, mixing of shallow, oxygenated, lower-pH water from an unconfined aquifer with deeper, confined, anoxic, higher-pH water is facilitated by wells screened across both aquifers. The resulting higher-O2, lower-pH mixed groundwater facilitated the mobilization of U from solid aquifer materials, and dissolved U concentrations were observed to increase significantly in nearby supply wells. Similar instances of trace element mobilization due to human-induced mixing of groundwaters were documented in: (1) the Floridan aquifer system near Tampa, Florida (As and U), (2) Paleozoic sedimentary aquifers in eastern Wisconsin (As), (3) the basin-fill aquifer underlying the California Central Valley near Modesto (U), and (4) Coastal Plain aquifers of New Jersey (Ra). Adverse water-quality impacts attributed to human activities are commonly assumed to be related solely to the release of the various anthropogenic contaminants to the environment. The results show that human activities including various land uses, well drilling, and pumping rates and volumes can adversely impact the quality of water in supply wells, when associated with naturally-occurring trace elements in aquifer materials. This occurs by causing subtle but significant changes in geochemistry and associated trace element mobilization as well as enhancing advective transport processes.
Arsenic in sediments, groundwater, and streamwater of a glauconitic Coastal Plain terrain, New Jersey, USA—Chemical “fingerprints” for geogenic and anthropogenic sources by Julia L. Barringer; Pamela A. Reilly; Dennis D. Eberl; Alex E. Blum; Jennifer L. Bonin; Robert Rosman; Barbara Hirst; Marzooq Alebus; Kimberly Cenno; Miroslawa Gorska (763-776).
► Arsenic inputs to a Coastal Plain stream are mainly geogenic. ► Arsenic inputs from orchard pesticide residues were not clearly identified. ► The arsenic load in the stream is mainly particulate arsenic. ► Arsenic in groundwater discharge to the stream contributes to the arsenic load. ► Arsenic is released from minerals beneath the streambed by microbial activity.Glauconite-bearing deposits are found worldwide, but As levels have been determined for relatively few. The As content of glauconites in sediments of the Inner Coastal Plain of New Jersey can exceed 100 mg/kg, and total As concentrations (up to 5.95 μg/L) found historically and recently in streamwaters exceed the State standard. In a major watershed of the Inner Coastal Plain, chemical “fingerprints” were developed for streambed sediments and groundwater to identify contributions of As to the watershed from geologic and anthropogenic sources. The fingerprint for streambed sediments, which included Be, Cr, Fe and V, indicated that As was predominantly of geologic origin. High concentrations of dissolved organic C, nutrients (and Cl−) in shallow groundwater indicated anthropogenic inputs that provided an environment where microbial activity released As from minerals to groundwater discharging to the stream. Particulates in streamwater during high flow constituted most of the As load; the chemical patterns for these particulates resembled the geologic fingerprint of the streambed sediments. The As/Cr ratio of these suspended particles likely indicates they derived not only from runoff, but from groundwater inputs, because As contributed by groundwater is sequestered on streambed sediments. Agricultural inputs of As were not clearly identified, although chemical characteristics of some sediments indicated vehicle-related inputs of metals. Sediment sampling during dry and wet years showed that, under differing hydrologic conditions, local anthropogenic fingerprints could be obscured but the geologic fingerprint, indicating glauconitic sediments as an As source, was robust.
Mineralogic investigation into occurrence of high uranium well waters in upstate South Carolina, USA by Richard Warner; Jason Meadows; Scott Sojda; Van Price; Tom Temples; Yuji Arai; Chris Fleisher; Bruce Crawford; Peter Stone (777-788).
► Oxidative dissolution of uraninite in biotite granite is primary source of uranium in high-U well waters near Simpsonville, SC. ► Uranium is chiefly transported as mixed uranyl hydroxyl-carbonate complexes. ► Local reduction has resulted in secondary precipitation of uranium along fractures as coffinite. ► Dissolution of uraninite and precipitation of coffinite were geologically recent.High levels of U (up to 5570 μg/L) have been discovered in well waters near Simpsonville, South Carolina, USA. In order to characterize the mineralogical source of the U and possible structural controls on its presence, a deep (214 m) well was cored adjacent to one of the enriched wells. The highest gamma-ray emissions in the recovered core occur in coarse biotite granite at a depth just below 52 m. A slickenlined fault plane at 48.6 m and narrow pegmatite layers at depths of 113, 203 and 207 m also yield high gamma-ray counts. Thin sections were made from the above materials and along several subvertical healed fractures. Uraninite and coffinite are the principal U-rich minerals in the core. Other U-bearing minerals include thorite and thorogummite, monazite, zircon and allanite. Primary uraninite occurs in the biotite granite and in pegmatite layers. Secondary coffinite is present as tiny (<5 μm) crystals dispersed along fractures in the granite and pegmatites. Coffinite also occurs along the slickenlined fault plane, where it is associated with calcite and calcic zeolite and also replaces allanite. Coffinite lacks radiogenic Pb, hence is considerably younger than the uraninite.Dissolution of partially oxidized Ca-rich uraninite occurring in the surficial biotite granite (or secondary coffinite in fracture zones) is likely the main source for the current high levels of U in nearby area wells. The high-U well waters have a carbonate signature, consistent with pervasive calcite vein mineralization in the core. Aqueous speciation calculations suggest U transport as an uranyl (U6+) hydroxyl-carbonate complex. Later reduction resulted in secondary precipitation along fractures as a U4+ mineral (i.e., coffinite).
The application of in vitro gastrointestinal extraction to assess oral bioaccessibility of potentially toxic elements from an urban recreational site by Alexander Okorie; Jane Entwistle; John R. Dean (789-796).
► Potentially toxic elements (PTEs) determined in soil from an urban recreational site. ► In vitro gastrointestinal extraction used to assess oral bioaccessibility of PTEs. ► A generic quantitative risk assessment undertaken by comparison with SGVs/GAC. ► Concentrations of 3 of the PTEs could present a potential risk to site users.The use of in vitro gastrointestinal extraction to assess the oral bioaccessibility of 7 potentially toxic elements (PTEs) from soil derived from an urban recreational site, has been assessed. The pseudo-total concentration of the 7 PTEs was determined using microwave digestion followed by inductively coupled plasma mass spectrometry. The bioaccessible fraction was determined and the data compared to the pseudo-total concentrations to determine % bioaccessibility. A generic quantitative risk assessment (GQRA) was undertaken on the pseudo-total PTE concentration of the site by comparing the values with soil guideline (SGV) or generic assessment criteria (GAC) using the residential land use scenario. Based on the GQRA, concentrations of 3 of the PTEs investigated within the soils could present a potential risk to site users. Consideration of receptor exposure and bioaccessibility data allowed a more considered approach to human-health risk assessment at this site. Although the bioaccessibility data did not significantly alter the sites preliminary designation as contaminated (As and Pb concentrations exceeded the GAC even in the in vitro extracts) bioaccessibility data are clearly an additional tool towards furthering our understanding of human-health risk at contaminated sites and have the potential to act as a pragmatic decision-support tool.
Silver fluxes to the Gironde Estuary – Eleven years (1999–2009) of monitoring at the watershed scale by Laurent Lanceleur; Jörg Schäfer; Cécile Bossy; Alexandra Coynel; Aurélie Larrose; Matthieu Masson; Gérard Blanc (797-808).
► First Ag fluxes in the Gironde Estuary watershed. ► Geochemical background of Ag in SPM from the studied system expressed as Ag/Th = 0.026. ► Anthropogenic components in particulate Ag fluxes ranged from 24% to 99%. ► The estimated anthropogenic surface-specific Ag flux was 1.14–682 g/km2/a. ► The estimated anthropogenic population-specific Ag flux was 0.028–6.05 g/person/a.Although Ag has long been recognized as highly toxic to aquatic organisms and as an efficient tracer of urban wastewater inputs, there are very few data available on Ag levels and fluxes in rivers and estuaries. Due to anomalously high Ag concentrations in oysters from the estuary mouth, dissolved (AgD) and particulate Ag (AgP) concentrations in rivers of the Gironde Estuary watershed were studied. Using daily discharge, Suspended particulate matter (SPM) data and monthly measured AgD and AgP, the respective fluxes were estimated at the three main entries of the Gironde Estuary and at selected key sites at the outlets of different sub-watersheds for multiple years of the past decade. Dissolved and particulate Ag concentrations were 0.5 ng/L (detection limit) to 5.7 ng/L and 0.13–13.9 mg/kg in all rivers, except for the Riou-Mort River (up to 1260 ng/L and 261 mg/kg) that has been heavily polluted by former Zn ore treatment and remaining waste tailings. At all sites Ag transport mainly (73–100%) occurred in the particulate phase, i.e. strongly depended on hydrological parameters. Comparing Th-normalised (i.e. grain size independent) AgP levels at the different sites including one remote site showed that the regional background is best described by AgP/ThP = 0.026. The anthropogenic components in AgP levels and fluxes ranged from 24% to 90% at all sites, except for the Riou-Mort River (81–99%). Although this former ore treatment site still is an evident Ag source to the Lot-Garonne fluvial–estuarine system, its contribution to Ag fluxes at the main entry of the Gironde Estuary (i.e. 0.33–2.44 t/a at the La Reole site) is <11%. This clearly suggests that other sources such as rock weathering and erosion (0.057–0.817 t/a), urban wastewater inputs (0.042–0.057 t/a) and cloud-seeding (0.030–0.063 t/a) contribute the major parts of the fluvial Ag budget in this watershed. The estimated anthropogenic surface- and population-specific Ag fluxes in the studied system were 1.14–682 g/km2/a and 0.028–6.05 g/person/a, respectively. The results obtained suggest that the Gironde fluvial–estuarine system is contaminated by Ag inputs from various sources, but further work is necessary to (i) identify and quantify the major sources and (ii) produce comparable data for other systems, which would be useful for the evaluation of Ag contamination at a larger scale.
Stability of pyrochlores in alkaline matrices: Solubility of calcium antimonate by Geert Cornelis; Tom Van Gerven; Ruben Snellings; Bram Verbinnen; Jan Elsen; Carlo Vandecasteele (809-817).
► Calcium antimonate or romeite may control antimonate solubility in alkaline matrices. ► The effect of vacancies in the romeite structure has been poorly examined. ► The CaSb ( OH ) 6 + association affects antimonate solubility. ► Vacancies in romeite fill up at high pH and high calcium availability. ► Romeite causes lower antimonate solubility than previously assumed.The stability of pyrochlores as a function of solution composition is relevant in the context of hazardous waste immobilisation as this mineral family comprises minerals that can control actinide solubility in ceramic formulations as well as romeites or calcium antimonates that possibly control the solubility of Sb in cement pastes. However, to date, no thermodynamic model exists that can be used for long-term risk assessment of pyrochlores as stabilising minerals. In this work synthetic romeites were precipitated at pH 6.5 and pH 12 and at varying molar Ca:Sb ratio and analysed by XRD and Rietveld refinement which showed that, like naturally occurring romeites, synthesized romeites have a Ca1+ x Sb2O6OH2−2 x structure with less vacancies and a higher Ca-content as pH and Ca-availability were higher during synthesis. These romeites dissolve incongruently with preferential Ca leaching and antimonate is less soluble from romeites synthesized in alkaline conditions. Batch extractions of romeite in equilibrium with increasing Ca(NO3)2 additions up to 1 mol L−1 showed a minimum equilibrium Sb concentration at [Ca] = 0.01 mol L−1 that could only be explained by assuming the formation of a CaSb ( OH ) 6 + complex. Increasing the Ca concentration in romeite suspensions also resulted in a pH decline in the equilibrium solution, which may be explained by the increasing concomitant uptake of Ca2+ and OH− in the romeite structure as a function of increasing Ca(NO3)2 additions. Thermodynamic modelling supported these assumptions that may explain the incongruent dissolution behaviour of romeite, but the fitting of two equilibrium constants was required. Although the validity of these constants needs more experimental confirmation, this study suggests that despite possible CaSb ( OH ) 6 + formation, the solubility of Ca antimonate in alkaline conditions can be responsible for lower equilibrium Sb concentrations than previously assumed.
Water circulation control on carbonate-δ18O records in a low permeability clay formation and surrounding limestones: The Upper Dogger–Oxfordian sequence from the eastern Paris basin, France by Véronique Lavastre; Magali Ader; Stéphane Buschaert; Eddy Petit; Marc Javoy (818-827).
► Up. Dog./Oxf. sequence is investigated for radioactive waste disposal feasibilities. ► Marine carbonates suffered isotopic exchanges with meteoric water. ► Modelling shows that very low W/R ratio can explain isotopic changes in clay layer. ► Higher W/R ratio are needed to reach isotopic changes in carbonated layers. ► Confirmed barrier property of clay layer was probably reached during early burial.These features indicate advection in both aquifers. According to the history of the Paris basin, this hydrogeological behaviour could have been effective since Jurassic/Cretaceous transition times. Inversely, the carbonate-δ18O content trends observed for the Callovo-Oxfordian data show that changes were controlled by post-depositional fluid–rock interaction with water/rock ratio (0.02–0.15) similar to the present-day porewater/rock ratio. The 130 m thick Callovo-Oxfordian claystone appears remarkably homogenous regarding its hydrogeological properties. This study suggests an initial marine porewater replacement by meteoric water only after porosity was reduced to its present value, thus demonstrating that the Callovo-Oxfordian clay has mainly been isolated from advective meteoric water circulation. Only the upper 20 m of the Callovo-Oxfordian claystone Formation underwent heterogeneous water–rock exchange (W/R from 0.01 to 0.3), probably as a result of its mineralogical heterogeneity and proximity to the advective Oxfordian Limestone aquifer.This study of carbonate-δ18O confirms the hydrogeological barrier properties of the Callovo-Oxfordian clay and suggests that it has been a natural hydrological barrier since the earliest times of its diagenesis.
Assessment of MTBE biodegradation in contaminated groundwater using 13C and 14C analysis: Field and laboratory microcosm studies by Steven F. Thornton; Simon H. Bottrell; Keith H. Spence; Roger Pickup; Michael J. Spence; Nadeem Shah; Helen E.H. Mallinson; Hans H. Richnow (828-837).
► Carbon isotope fractionation for MTBE varies with dissolved oxygen concentration. ► Carbon isotope fractionation can underestimate MTBE biodegradation at plume fringes. ► Fractionation factors must be for specific biodegradation mechanisms and conditions. ► Specific microbial populations influence carbon isotope fractionation in groundwater.Radiolabelled assays and compound-specific stable isotope analysis (CSIA) were used to assess methyl tert-butyl ether (MTBE) biodegradation in an unleaded fuel plume in a UK chalk aquifer, both in the field and in laboratory microcosm experiments. The 14C-MTBE radiorespirometry studies demonstrated widespread potential for aerobic and anaerobic MTBE biodegradation in the aquifer. However, δ13C compositions of MTBE in groundwater samples from the plume showed no significant 13C enrichment that would indicate MTBE biodegradation at the field scale. Carbon isotope enrichment during MTBE biodegradation was assessed in the microcosms when dissolved O2 was not limiting, compared with low in situ concentrations (2 mg/L) in the aquifer, and in the absence of O2. The microcosm experiments showed ubiquitous potential for aerobic MTBE biodegradation in the aquifer within hundreds of days. Aerobic MTBE biodegradation in the microcosms produced an enrichment of 7‰ in the MTBE δ13C composition and an isotope enrichment factor (ε) of −1.53‰ when dissolved O2 was not limiting. However, for the low dissolved O2 concentration of up to 2 mg/L that characterizes most of the MTBE plume fringe, aerobic MTBE biodegradation produced an enrichment of 0.5–0.7‰, corresponding to an ε value of −0.22‰ to −0.24‰. No anaerobic MTBE biodegradation occurred under these experimental conditions. These results suggest the existence of a complex MTBE-biodegrading community in the aquifer, which may consist of different aerobic species competing for MTBE and dissolved O2. Under low O2 conditions, the lower fractionating species have been shown to govern overall MTBE C-isotope fractionation during biodegradation, confirming the results of previous laboratory experiments mixing pure cultures. This implies that significant aerobic MTBE biodegradation could occur under the low dissolved O2 concentration that typifies the reactive fringe zone of MTBE plumes, without producing detectable changes in the MTBE δ13C composition. This observed insensitivity of C isotope enrichment to MTBE biodegradation could lead to significant underestimation of aerobic MTBE biodegradation at field scale, with an unnecessarily pessimistic performance assessment for natural attenuation. Site-specific C isotope enrichment factors are, therefore, required to reliably quantify MTBE biodegradation, which may limit CSIA as a tool for the in situ assessment of MTBE biodegradation in groundwater using only C isotopes.
Karst springs as “natural” pluviometers: Constraints on the isotopic composition of rainfall in the Apennines of central Italy by A. Minissale; O. Vaselli (838-852).
► Isotopic compositions of karstic springs in central Italy have been reviewed. ► Isotopic gradients of rainfalls for elevations have been evaluated in an Alpine valley. ► Karstic drops have been calculated by using isotopic compositions of springs. ► Isotopic compositions of rainfalls in central Italy have been re-calculated using the isotopic compositions of karstic springs.This paper describes an indirect method to calculate the isotopic composition of rainfall by using the isotopic composition of karst springs fed by waters circulating in the most important regional aquifer of central Italy, i.e. the Mesozoic limestone sequence that forms the backbone of the Apennines. By using δ18O and δD data and the δ18O (and/or δD) average gradient for elevation, evaluated through the use of literature rainfall data and new measurements from a typical Alpine valley in northern Italy, the altitude of precipitation of their parent water has been re-calculated. Vertical descents of more than 2000 m, from recharge to discharge, have been assessed in some high flow-rate cold springs in the morphologically steep Adriatic sector of central Italy. A clear correlation between the vertical descents and more negative isotopic compositions at their relative emergence elevations is highlighted. In contrast, in the Tyrrhenian sector lower karstic drops (generally lower than 500 m) correlate with less negative isotopic composition of recharge areas.The δ18O iso-contour map of the “recalculated” parent rainfall in central Italy is more detailed than any possible isotopic map of rainfall made using pluviometers, unless large number of rainfall collectors were deployed on mountaintops. The data also show that the isotopic composition of rainfall depends on the source of the storm water. In particular, precipitation is isotopically heavier when originating in the Mediterranean Sea, and lighter when formed in the Atlantic Ocean. Consequently, the collision between air masses with such a different isotopic signature results in a relatively small latitudinal fractionation effect. The peninsular part of central Italy is very narrow, with several mountains and massifs more that 2000 m high, and any latitudinal variation in the isotopic composition between rainfall sourced in the Atlantic Ocean and in the Mediterranean Sea is much lower than that due to the isotopic fractionation due to elevation.
Anaerobic precipitation of manganese and co-existing metals in mine impacted water treated with crab shell-associated minerals by Mary Ann Robinson-Lora; Rachel A. Brennan (853-862).
► Crab shell can treat mine water under anaerobic and biologically limited conditions. ► Crab shell-associated minerals have significantly higher surface area than limestone. ► Alkalinity is enhanced by increased supersaturation and the release of organic compounds. ► Large anaerobic Mn removal can be achieved with crab shell particles.The chemical and physical treatment mechanisms by which crab shell removes metals from mine impacted water (MIW) were evaluated under anaerobic and biologically limited conditions in closed systems and kinetic tests. Raw (R-SC20) and deproteinized (DP-SC20) crab shell were tested and compared to limestone to quantify the contribution of chitin-associated minerals and proteins to alkalinity generation and metal precipitation. Single-metal closed systems (initial Mn and Fe = 0.18 mM and Al = 0.34 mM) containing 5 g/L of either R- or DP-SC20, yielded an increase in pH from 3 to 9.2–10.2, generation of 0.83–1.87 mM of alkalinity, and resulted in ⩾95% removal of metals within 72 h. In contrast, 5–125 g limestone/L only raised the pH to 7.8–8.3, produced lower alkalinity (0.56–0.63 mM), and resulted in less metal removal (⩽85%). In kinetic tests with 5 g-DP-SC20/L, removal of ⩾95% of the initial metal load was achieved after 0.5, 6, and 48 h for Al, Fe, and Mn, respectively. Geochemical calculations (PHREEQC) indicate that limestone-treated systems were close to equilibrium with calcite (CaCO3), whereas octacalcium phosphate (Ca4H(PO4)3) appears to be a controlling phase in systems treated with R- and DP-SC20. The probable mechanisms for Mn removal are the precipitation of rhodochrosite (MnCO3) and/or sorption. In the case of Al and Fe, geochemical calculations point to the precipitation of hydroxides; however, visual observations in Fe systems suggest the formation of green rust, a precursor of other, more stable phases like goethite or lepidocrocite. Several factors may account for the faster changes observed with R- and DP-SC20 compared to limestone: increased dissolution and degree of supersaturation, the presence of phosphates, the release of organic compounds, and a significantly larger surface area. These results are the first to verify and quantify the capacity of crab shell-associated minerals to treat MIW under biologically limited conditions.
Geochemical and isotopic evidence for palaeo-seawater intrusion into the south coast aquifer of Laizhou Bay, China by Dongmei Han; Claus Kohfahl; Xianfang Song; Guoqiang Xiao; Jilong Yang (863-883).
► Hydrochemistry, multi-isotopes, and hydraulic data were used to study saltwater intrusion. ► Salt sources and their contributions to the salinity distribution are identified. ► Groundwater age and mixing behavior are estimated. ► Hydrogeochemical evolution of different types of groundwater is determined. ► Multi-approach study can improve the understanding of reactive transport processes.This research aims to improve the current knowledge of groundwater salinisation processes in coastal aquifers using combined hydrochemical and isotopic parameters and inverse hydrochemical modelling. Field investigations were conducted in Laizhou Bay, which is the area most seriously affected by seawater intrusion in north China. During three sampling campaigns along a vertical transect in the Changyi-Liutuan area, 95 ground- and surface-water samples were collected for major ion and isotope analysis (2H/18O, 3H, 14C, 34S). The groundwater changes along the general flowpath towards the coast from fresh (<1 g/L), brackish (1–10 g/L), saline (10–100 g/L) to brine water (>100 g/L). Molar Cl/Br ratios are close to those of seawater in almost all groundwater samples, indicating that brines and deep seawater evolved from different events of palaeo-seawater intrusion. Depleted isotopic signatures of brines and deep saline water point to a former, initially depleted seawater reservoir due to runoff dilution. Tritium and 14C activities in deep saline water below confining units indicate isolation from modern precipitation and significant residence times. Brine water shows a wide range of 3H and 14C ages due to the complex conditions of mixing without isolation from modern groundwater. Sulphur-34 isotope ratios support seawater intrusion as a possible salt origin, although this parameter does not exclude gypsum dissolution. The combined use of Cl and 18O yields four different end-members of groundwater, and three different mixing scenarios were identified explaining the hydrochemical composition of groundwater samples with intermediate salinity in the different areas. To improve understanding of the various water types and their related processes in a spatial context, a conceptual model was developed integrating the results derived from the presented data in a vertical cross-section. Results of three inverse modelling simulations using PHREEQC-2 show that all hypothetical mixing scenarios derived from conservative components are thermodynamically feasible. In all scenarios, mixing, ion exchange, dissolution of dolomite and precipitation of gypsum and calcite account for the hydrochemical changes.
Geochemical evolution of groundwater in carbonate aquifers in Taiyuan, northern China by Rui Ma; Yanxin Wang; Ziyong Sun; Chunmiao Zheng; Teng Ma; Henning Prommer (884-897).
► Systematically assessed the hydrogeochemistry of karst groundwater at Taiyuan, northen China. ► Examined the geochemical processes that control karst groundwater evolution. ► Developed a mass transfer model that represents all major flow and reaction pathways.Thirty-nine samples of both cold and thermal karst groundwater from Taiyuan, northern China were collected and analyzed with the aim of developing a better understanding of the geochemical processes that control the groundwater quality evolution in the region’s carbonate aquifers. The region’s karst groundwater system was divided into three geologically distinct sub-systems, namely, the Xishan Mountain karst groundwater subsystem (XMK), the Dongshan Mountain karst groundwater subsystem (DMK) and the Beishan Mountain karst groundwater subsystem (BMK). Hydrochemical properties of the karst groundwaters evolve from the recharge zones towards the cold water discharge zones and further towards the thermal water discharge zones. In the XMK and the DMK, the hydrochemical type of the groundwater evolves from HCO3–Ca·Mg in the recharge – flow-through zone, to HCO3·SO4–Ca·Mg/SO4·HCO3–Ca·Mg in the cold water discharge zone, and further to SO4–Ca·Mg in the thermal water discharge zone. By contrast, the water type changes from HCO3–Ca·Mg to HCO3·SO4–Ca·Mg in the BMK, with almost invariable TDS and temperatures all along from the recharge to the discharge zone. The concentrations of Sr, Si, Fe, F− and of some trace elements (Al, B, Li, Mn, Mo, Co, Ni) increase as groundwater temperature increases. Different hydrogeochemical processes occur in the three karst groundwater sub-systems. In the XMK and the DMK, the geochemical evolution of the groundwater is jointly controlled by carbonate dissolution/precipitation, gypsum dissolution and dedolomitization, while only calcite and dolomite dissolution/precipitation occurs in the BMK without dedolomitization. The hydrogeochemical data of the karst groundwaters were used to construct individual geochemical reaction models for each of the three different karst groundwater sub-systems. The modeling results confirm that dedolomization is the major process controlling hydrochemical changes in the XMK and the DMK. In the thermal groundwaters, the dissolution rates of fluorite, siderite and strontianite were found to exceed those of the cold karst groundwater systems, which can explain the higher concentrations of F−, Fe and Sr2+ that are found in these waters.
Cadmium (II) distribution in complex aquatic systems containing ferrihydrite, bacteria and an organic ligand: The effect of bioactivity by Yantao Song; Simon Swift; Peter J. Swedlund; Naresh Singhal (898-906).
► Cd2+ and ferrihydrite did not inhibit H2Lp degradation by Comamonas spp. ► In systems lacking ferrihydrite, Cd2+ sorption onto live bacteria exceeded dead bacteria for pH > 6. ► With ferrihydrite present, no difference in Cd2+ sorption to live and dead bacteria was observed. ► Cd2+ sorption to ferrihydrite and bacterial was successfully modeled by neglecting H2Lp degradation.The distribution of Cd2+ in the presence of phthalic acid (H2Lp), ferrihydrite and bacteria (Comamonas spp.) was investigated in biologically active systems involving H2Lp biodegradation. Tests showed that Cd2+ sorption onto bacteria, ferrihydrite and bacteria–ferrihydrite mixture increased with pH in all systems, irrespective of H2Lp degradation or not. The use of bacterial growth medium, Bushnell Hass Broth modified for low phosphate, had negligible effect on Cd sorption. In the presence of ferrihydrite, no difference was observed between Cd2+ sorption in the ferrihydrite–live bacteria and in the ferrihydrite–dead bacteria systems as ferrihydrite proved to be the dominant sorption phase. Cadmium sorption to ferrihydrite and to bacterial cells was described using the diffuse layer model and a nonelectrostatic 4-site model, respectively, which were developed for systems lacking H2Lp degradation. For systems experiencing H2Lp degradation this modeling approach predicted the general trend of Cd2+ sorption-edge shift and gave good fits to the observed sorption data. The results obtained demonstrate that Cd2+ sorption in the biologically active system was reasonably estimated by a model developed for biologically inactive systems, although uncertainty exists due to processes involving H2Lp biodegradation products and changes in the bacterial population.
Submarine groundwater discharge in a subsiding coastal lowland: A 226Ra and 222Rn investigation in the Southern Venice lagoon by Julie C. Gattacceca; Adriano Mayer; Andrea Cucco; Christelle Claude; Olivier Radakovitch; Christine Vallet-Coulomb; Bruno Hamelin (907-920).
► Occurence/magnitude of submarine groundwater discharge investigated in Venice lagoon (Italy) using 226Ra and 222Rn isotopic tracers. ► Single box mass balance compared with multi boxes mass balance coupled with hydrodynamic model. ► Groundwater flux accounts for 1% of lagoon hydrological balance (1-3 times surface runoff) and 30-50% of tracers inputs. ► Necessary to assess this flux impact on nutrient budget in lagoon.Several recent studies have suggested that submarine groundwater discharge (SGD) occurs in the Venice lagoon with discharge rates on the same order or larger than the surface runoff, as demonstrated previously in several other coastal zones around the world. Here, the first set of 222Rn data, along with new 226Ra data are reported, in order to investigate the occurrence and magnitude of SGD specifically in the southern basin of the lagoon. The independent connection with the Adriatic Sea (at the Chioggia inlet), in addition to the relative isolation of the water body from the main lagoon, make this area an interesting case study. There is probably only minimal fresh groundwater flux to the lagoon because the surrounding aquifer is subsiding and mainly has a lower hydraulic head than seawater.The data show that the Ra and Rn activities are in slight excess in the lagoon compared to the open sea, with values on the same order as those observed in the northern and central basins. Taking into account the water exchange rate between the lagoon and adjacent seawater provided by previous hydrodynamic numerical modelling, it is shown that this excess cannot be supported at steady state by only riverine input and by diffusive release from the sediment interstitial water. High activities observed in groundwater samples collected from 16 piezometers tapping into the shallow aquifer over the coastal lowland substantiate that the excess radioactivity in the lagoon may indeed be due to the advection of groundwater directly into the lagoon bottom water through the sediment interface. However, the data show that the groundwater composition is extremely heterogeneous, with high Ra activities concentrated within a narrow coastal strip where the contact between fresh and saline water takes place, while Rn strongly decreases when approaching the lagoon shore across the 20 km coastal plain. Assuming that the average groundwater activities measured in the coastal strip are representative of the SGD composition, a SGD flux of 7.7 ± 3.5 × 105 and 2.5 ± 2 × 106 m3/d is calculated using a 226Ra and 222Rn budget, respectively, (i.e. about 1–3 times the surface runoff), substantially lower than in previous studies. The influence of all assumptions on SGD estimates (groundwater heterogeneity, diffusive sediment flux, one-box versus multi-boxes model calculations) is discussed, and a sensitivity analysis of the influence of imperfect exchange and mixing at the lagoon outlets that affects the lagoon composition is provided. Finally, the results confirm that the SGD flux, calculated with these assumptions, is largely (∼80%) composed of saline lagoon water circulating through the sediment under the lagoon margin, and that the fresh water discharge associated with SGD is at most a minor term in the lagoon hydrologic balance.
Experimental determination of struvite dissolution and precipitation rates as a function of pH by T. Roncal-Herrero; E.H. Oelkers (921-928).
► Struvite was dissolved and precipitated in closed-system reactors. ► Experiments were performed at 25 °C and 7.76 < pH < 10.65. ► Aqueous fluids rapidly equilibrate with struvite. ► Struvite solubilities and dissolution and precipitation rates were retrieved. ► Results support use of struvite as a renewable fertilizer.Struvite (MgNH4PO4·6H2O) was dissolved and precipitated in closed-system reactors at 25 °C and 7.76 < pH < 10.65. Reactive fluid Mg and P concentrations attained a stationary-state after ∼10 h in all experiments. The temporal variation of these concentrations was evaluated to retrieve struvite solubilities, and dissolution and precipitation rates. Resulting struvite solubilities are consistent with an equilibrium constant for the reaction: MgNH 4 PO 4 · 6 H 2 O ⇔ Mg 2 + + NH 4 + + PO 4 3 - + 6 H 2 O of 10−13.24±0.5, which is similar to values previously reported in the literature. Measured struvite dissolution and precipitation rates (r) are found to be consistent with r = r + 1 - exp A 2 RT where r + symbolizes the forward dissolution rate, A refers to the chemical affinity of the dissolution reaction, R designates the gas constant, and T represents absolute temperature. Values of r + are found to be independent of pH and equal to 10−12.85±0.15 mol/cm2/s in initially NH4-free reactive fluids. The presence of 0.06 mol/kg NH4 in the initial reactive fluid led to non-stoichiometric element release interpreted to stem from the precipitation of a NH4-rich phosphate phase. Use of measured rates in geochemical modeling calculations indicate that both undersaturated and supersaturated aqueous fluids will rapidly equilibrate with struvite, supporting its potential use as a renewable fertilizer.