Applied Geochemistry (v.27, #4)

► Determination of mercury (Hg) fluxes using speciation of sulfur and organic carbon. ► The majority of the flux is particulate bound, from sulfide weathering. ► Atmospherically-deposited Hg is a minor (<5%) contributor to the total flux. ► Most of the Hg flux is of low bioavailability.The Mackenzie River Basin (MRB) in NW Canada is one of the least human-impacted large watersheds in the world. The western and eastern sub-basins of the MRB are also marked by contrasting geology, geomorphology, hydrology, and biology. These remarkable differences in a remote river basin provide a unique opportunity to probe the biogeochemical processes governing the sources, transport, and bioavailability of Hg at the terrestrial-marine interface and ultimately in the Arctic Ocean. Based on a large dataset of the concentration and speciation of Hg, S and organic matter in surface sediments across the MRB, a source-apportioned budget was constructed for Hg in the MRB. The results indicate that the Hg flux in the basin originates primarily from the weathering of sulfide minerals in the western mountainous sub-basin (∼78% of the total), followed by the erosion of coal deposits along the mainstream of the Mackenzie River (∼10%), with the remainder split between atmospheric inputs and organic matter-bound Hg (6% and 5%, respectively). Although the Hg flux from the eastern peatland sub-basin only accounts for ∼10% of the total riverine Hg flux, Hg in this region correlates strongly with labile organic matter, and will likely have a much stronger influence on local biota.

Anomalous change in atmospheric radon concentration sourced from broad crustal deformation: A case study of the 1995 Kobe earthquake by Yumi Yasuoka; Yusuke Kawada; Yasutaka Omori; Hiroyuki Nagahama; Tetsuo Ishikawa; Shinji Tokonami; Masahiro Hosoda; Tetsuo Hashimoto; Masaki Shinogi (825-830).
Display Omitted► The pre-seismic radon in air was observed as one exhaled from the regional area. ► The regional area had a large displacement up to 30 cm due to the co-seismic event. ► Mean radon exhalation rates are considered to increase up to five times higher. ► The regional area was highly strained in the order of 10−6.Before the Kobe earthquake, an anomalous increase in atmospheric Rn concentration was observed. By separating the measured concentration of atmospheric Rn into three components according to the distance from the monitoring station, the variation of Rn exhalation rate can be estimated for the respective area using the daily minimum and maximum concentrations. The mean rate of Rn exhalation gradually increased in an area of 20 km around the monitoring station, becoming five times higher than normal in the period between October 1994 and the date of the earthquake. This area had a large co-seismic displacement of up to 30 cm, which roughly corresponds to the crustal strain of 10−6-order, and it is considered the main source for the atmospheric Rn prior to the Kobe earthquake. Analyses revealed that the pre-seismic change in the atmospheric Rn concentration exhibited an anomalous pattern which would yield information on the spatial distribution of the mechanical response of the ground.

► Evaluated the correlation between meteorological factors and δ 15 N – NO 3 - . ► Source changes are not the only factors to bring about δ 15 N – NO 3 - oscillation. ► Air pollutants and meteorological conditions lead to changes in the δ 15 N – NO 3 - . ► Reactions involving SO2 lead to negative correlation between δ 15 N – NO 3 - and SO2.The characterization of N isotopes in NO 3 - is an effective method to determine NO 3 - sources and NO X transformation mechanisms. Xi’an city is located in a semi-arid to arid region and is representative of most cities in the north and NW of China. Few studies have been carried out on N isotopes in wet deposition of this region. In this study, 29 wet deposition samples were collected from Xi’an city in northwestern China between March 2008 and May 2009 and analyzed for their volume-weighted NO 3 - and NH 4 + concentrations and N isotopic compositions. Volume-weighted mean concentrations were 3.9 μmol L−1 for NO 3 - and 13.5 μmol L−1 for NH 4 + . The wet deposition samples showed pronounced seasonal variation in the N isotopic composition of NO 3 - with δ 15 N – NO 3 - ranging from −2.0‰ to +9.2‰ over the course of a year, with a mean value of +2.6 ± 2.6‰. Higher δ 15 N – NO 3 - values were observed in the cool season (November–May, +3.8‰) than in the warm season (June–October, +1.0‰). In autumn and winter in particular, the mean δ 15 N – NO 3 - values were clearly different (+0.9‰ and +6.7‰, respectively). Various factors are shown to lead to variation in the δ 15 N – NO 3 - values in wet deposition. One effect is changing ratios of N derived from agricultural/soil sources (low δ 15 N – NO 3 - , dominant in the warm season) and fossil fuel-derived NO X (high δ 15 N – NO 3 - , relatively more important in the cool season). However, in the cool season δ 15 N – NO 3 - unexpectedly shows a strong negative correlation with SO2 concentration, which should correlate with fossil fuel combustion inputs. This may be explained by interactions of N-species with other atmospheric pollutants. The results also suggest that temperature may be a significant factor affecting δ 15 N – NO 3 - . The NO X reacts in different ways depending on temperature, leading to a correlation of δ 15 N – NO 3 - with temperature. Moreover, the effects of dilution cannot be neglected here, as there is clear seasonal variation in precipitation. When combined with meteorological parameters, the volume-weighted concentrations of NO 3 - and NH 4 + and air pollutant precursor data, the δ15N of NO 3 - in wet deposition can be used as a reliable tool for the precise identification of NO X transformation mechanisms in the environment and the fate of NO X emissions.

Delta Chromium-53/52 isotopic composition of native and contaminated groundwater, Mojave Desert, USA by John A. Izbicki; Thomas D. Bullen; Peter Martin; Brian Schroth (841-853).
► Cr isotopes do not uniquely define anthropogenic Cr near plume margins. ► Mixing and reductive fractionation control Cr concentrations within plumes. ► Apparent fractionation factors for Cr reduction lower than lab derived factors. ► Difference in fractionation factors may be a useful tool to understand processes.Chromium(VI) concentrations in groundwater sampled from three contaminant plumes in aquifers in the Mojave Desert near Hinkley, Topock and El Mirage, California, USA, were as high as 2600, 5800 and 330 μg/L, respectively. δ 53/52Cr compositions from more than 50 samples collected within these plumes ranged from near 0‰ to almost 4‰ near the plume margins. Assuming only reductive fractionation of Cr(VI) to Cr(III) within the plume, apparent fractionation factors for δ 53/52Cr isotopes ranged from ε app  = 0.3 to 0.4 within the Hinkley and Topock plumes, respectively, and only the El Mirage plume had a fractionation factor similar to the laboratory derived value of ε  = 3.5. One possible explanation for the difference between field and laboratory fractionation factors at the Hinkley and Topock sites is localized reductive fractionation of Cr(VI) to Cr(III), with subsequent advective mixing of native and contaminated water near the plume margin. Chromium(VI) concentrations and δ 53/52Cr isotopic compositions did not uniquely define the source of Cr near the plume margin, or the extent of reductive fractionation within the plume. However, Cr(VI) and δ 53/52Cr data contribute to understanding of the interaction between reductive and mixing processes that occur within and near the margins of Cr contamination plumes. Reductive fractionation of Cr(VI) predominates in plumes having higher ε app, these plumes may be suitable for monitored natural attenuation. In contrast, advective mixing predominates in plumes having lower ε app, the highly dispersed margins of these plumes may be difficult to define and manage.

Influence of upwelling on the shallow water chemistry in a small wetland riparian zone (Basque Country) by M. Martínez-Santos; E. Ruíz-Romera; M. Martínez-López; I. Antigüedad (854-865).
► The fractured bedrock is at least as dynamic as the upper clayed deposits. ► The presence of more fractured zones allow the upwelling of deeper groundwater. ► The hydrochemistry study helps in locating areas influenced by hydraulic windows.Many hydrologic conceptual models in riparian areas assume that the alluvial deposits zone is hydraulically more active than the fractured bedrock below. Therefore, these models undervalue the possible contribution of deeper groundwater from the fractured bedrock system. A hydrochemical study, under various hydrological conditions, has been carried out in a small riparian zone of the Salburua wetland (Basque Country) in order to highlight the conceptual model. This wetland is included in a wide Quaternary aquifer, which has been declared a Nitrate Vulnerable Zone. The results of this study suggest that the fractured bedrock is at least as dynamic as the upper clayey deposits. The presence of more fractured zones, which act as hydraulic “windows”, allow the upwelling of deeper groundwater and, consequently, make the upper alluvial deposits and the fractured bedrock water systems to be cross-connected. Nevertheless, this upwelling is limited to some small areas in the riparian zone. As a result of this local interaction, several chemical reactions have been observed and the hydrochemical characteristics of shallow groundwater undergo seasonal variations. The study shows that a hydrologic conceptual model, which does not consider the hydraulic activity of the fractured bedrock, can be too simplistic. The presence of hydraulic windows could be considered throughout the entire Quaternary aquifer, knowledge of which could help the managers of the Vulnerable Zone and the wetland to take more effective measures for regulation and conservation.

Groundwater recharge environments and hydrogeochemical evolution in the Jiuquan Basin, Northwest China by Jianhua He; Jinzhu Ma; Peng Zhang; Liming Tian; Gaofeng Zhu; W. Mike Edmunds; Qinghuan Zhang (866-878).
► We use multiple environmental tracers (chemicals and isotopes). ► We determine the dominant geochemical processes of the aquifers and the quality of the groundwater. ► We identify the recharge sources, recharge environment and the residence time of the groundwater. ► The confined groundwater was recharged as palaeowater during the late Pleistocene and Holocene under a cold climate.The groundwater recharge environments and hydrogeochemical characteristics in the Quaternary aquifer of Jiuquan Basin was investigated using a combination of chemical indicators, stable isotopes, and radiocarbon dating. The d-excess values of winter precipitation and surface water revealed that the meltwater from snow and ice played the dominant role in the basin’s surface water supply. The unconfined groundwater showed gradual enrichment of heavy isotopes along the flow path, but δ 18O and δ 2H values were similar to those of surface water, suggesting recent recharge as a result of rapid seepage along rivers combined with the effects of high evaporation. The 14C (pmc) values of unconfined groundwater was between 71.5% and 90.9%, and since 80% modern carbon probably represents the upper limit of initial 14C activity, this suggests that the groundwater is relatively young. The confined groundwater was depleted in heavy isotopes; coupled with low 14C values (∼20–53%), indicating that the groundwater was mainly recharged as palaeowater during the late Pleistocene and Holocene epochs under a cold climate. The surface water and most groundwater samples were fresh rather than saline, with TDS <490 and <1000 mg L−1, respectively. The chemistry of unconfined groundwater changed from HCO 3 - -dominated to no dominant ions and then to SO 4 2 - -dominated moving along the flow path from the Jiuquan-Jiayuguan Basin to the Jinta Basin, and the confined water was SO 4 2 - -dominated. The results have important implications for groundwater management in the Basin, where a high proportion of the water being used is in effect being mined (i.e., extracted faster than its replacement rate); thus, significant changes are urgently needed in the regional water-use strategy.

► Speciated pore-water and sediment-bound As, U, and Fe as a function of depth. ► Intermittent fluvial zones promote hydrological and geochemical transport. ► Seasonal variations change redox boundaries, influence sediment pore-water geochemistry. ► Localized biogeochemical conditions cycle As and U between pore and surface waters. ► Fe(II) production suppressed thioarsenite formation and promoted As and U release.The extent of historical U mining impacts is well documented for the North Cave Hills region of Harding County, South Dakota, USA. While previous studies reported watershed sediment and surface water As and U concentrations up to 90× established background concentrations, it was unclear whether or how localized changes in sediment redox behavior may influence contaminant remobilization. Five pore-water equilibration samplers (peepers) were spatially and temporally deployed within the study area to evaluate seasonal solid–liquid As and U distributions as a function of sediment depth. Pore-water and solid phase As and U concentrations, Fe speciation, Eh and pH were measured to ascertain specific geochemical conditions responsible for As and U remobilization and transport behavior. At a mine overburden sedimentation pond adjacent to the mine sites, high total aqueous As and U concentrations (4920 and 674 μg/L, respectively) were found within surface water during summer sampling; however pond dredging prior to autumn sampling resulted in significantly lower aqueous As and U concentrations (579 and 108 μg/L, respectively); however, both As and U still exceeded regional background concentrations (20 and 18 μg/L, respectively). At a wetlands-dominated deposition zone approximately 2 km downstream of the sedimentation pond, pore-water geochemical conditions varied seasonally. Summer conditions promoted reducing conditions in pore water, resulting in active release of As(III) to the water column. Autumn conditions promoted oxidizing conditions, decreasing pore-water As (Aspw) 5× and increasing Upw 10×. Peak U pore-water concentrations (781 μg/L) were 3.5× greater than determined for the surface water (226 μg/L), and approximately 40× background concentrations. At the Bowman–Haley reservoir backwaters 45 km downstream from the mine sites, As and U pore-water concentrations increased significantly between the summer and autumn deployments, attributed to increased Fe reduction processes. Geochemical modeling suggests solid-phase Fe reduction promotes the liberation of pore-water As and U via suppressing the formation of thioarsenite. Intermittent hydrological processes facilitate As and U transport and deposition throughout the watershed, while biogeochemical-influenced redox changes cycle As and U between pore and surface water within localized environments.

Speciation and spatial distribution of solid-phase iron in surface sediments of the East China Sea continental shelf by Mao-Xu Zhu; Xiao-Chen Hao; Xiao-Ning Shi; Gui-Peng Yang; Tie Li (892-905).
► Seven Fe pools and their spatial distribution are investigated. Clay fraction strongly controls size and spatial distribution of main Fe pools. Iron partitioning in the Yangtze River particles exerts control on Fe speciation in the sediments. High content of poorly-reactive Fe is likely due to neoformation of Fe-rich clay phases.Speciation and reactivity characterization of solid-phase Fe in marine sediments are of significance to understanding its heterogeneous mineralogy and crystallinity, the diagenetic cycling of Fe and its regulating roles on many other elements in sediments. In this study, a combination of sequential and single-step extractions was used for the determination of seven Fe pools in surface sediments of the East China Sea (ECS) continental shelf: (1) carbonate associated Fe (Fe(II)carb) plus acid volatile sulfide-Fe (Fe(II)AVS), (2) easily reducible amorphous/poorly crystalline Fe oxides (Feox1), (3) reducible crystalline Fe oxides (Feox2), (4) magnetite (Femag), (5) poorly reactive sheet silicate Fe (FePRS), (6) pyrite-Fe (Fepy), and (7) unreactive silicate Fe (FeU). Total Fe (FeT) in the sediments is largely determined by terrestrial aluminosilicate particles as indicated by a great similarity of the FeT with that of the Yangtze River and global riverine particulates. The size of FePRS is found to be the largest pool, followed by FeU, Feox2, Femag, Fe(II)AVS+carb, Feox1 and Fepy. The large FePRS may result from neoformation of Fe-rich clay minerals via reverse weathering and subsequent ageing. The small sizes of Fe(II)AVS+carb and Fepy pools is believed to be the result of low SO4 reduction due to generally low labile organic matter together with the oxic/suboxic, dynamic environments of the surface sediments. The occurrence of Feox1, Feox2 and FePRS in the sediments is closely associated with the clay fraction as indicated by a high spatial correlation between the former and the latter. Highly reactive Fe(FeHR) in the sediments is comparable to that in global marine sediments, but apparently lower than in the Yangtze River and global riverine particulates due probably to sequestration in the Yangtze Estuary. The ratios of FeHR/FeT, FePR/FeT and FeU/FeT in the ECS surface sediments consistently show more similarity to those in the Yangtze River particulates than in the global continental margin or deep-sea sediments. The surface sediments maintain a high level of buffering capacity toward sulfidation suggested by a large fraction of highly reactive Fe(III) oxides (Fe(III)HR) in FeHR.

Metal speciation in rivers affected by enhanced soil erosion and acidity by Miriam I. Nystrand; Peter Österholm; Maria E. Nyberg; Jon Petter Gustafsson (906-916).
► Total concentrations of several elements are highly elevated. ► Elements occur significantly as colloids and particles due to soil erosion. ► Acid sulphate soils increase dissolved metal concentrations. ► Toxicity may be overestimated in conventional monitoring and speciation modelling.Dissolved (<1 kDa), colloidal (1 kDa–0.45 μm) and particulate (>0.45 μm) size fractions of 30 elements were determined for four rivers (Sirppujoki, Laajoki, Mynäjoki and Paimionjoki), including 12 low-order inflow streams, largely affected by soil erosion and acidity in SW Finland. In addition, geochemical modelling was used to predict the formation of free ions and complexes in these rivers. Total metal concentrations were relatively high but most of the elements occurred mainly in a colloidal or particulate form and even elements expected to be very soluble occurred to a large extent in colloidal form. According to geochemical modelling these patterns could be explained by in-stream metal complexation/adsorption only to a limited extent. Instead there were strong indications that the high metal concentrations and dominant solid fractions were largely caused by erosion of metal bearing phyllosilicates. A strong influence of acid sulphate (AS) soils, known to exist in the catchment, could be clearly distinguished in Sirppujoki river as it had very high concentrations of dissolved metals, while in the two nearby rivers (Laajoki and Mynäjoki) the influence of AS soils was largely masked by eroded phyllosilicates. In Paimionjoki river the colloidal and particulate fractions dominated very strongly, indicating that total metal concentrations are almost solely controlled by erosion of phyllosilicates. Consequently, rivers draining clay plains sensitive to erosion, like those in SW Finland, have generally high “background” metal concentrations due to erosion of relatively non-toxic colloidal/particulate phyllosilicates. Thus, relying on only semi-dissolved (<0.45 μm) concentrations obtained in routine monitoring and/or speciation modelling can lead to a great overestimation of the water toxicity in this environment.

► Estuarine DOM composition was found to vary on spatial and temporal scales. ► Hydrological transport and primary productivity control DOM composition. ► Hurricane disturbance did affect DOM composition locally but not on a bay-wide scale.The environmental dynamics of dissolved organic matter (DOM) were characterized for a shallow, subtropical, seagrass-dominated estuarine bay, namely Florida Bay, USA. Large spatial and seasonal variations in DOM quantity and quality were assessed using dissolved organic C (DOC) measurements and spectrophotometric properties including excitation emission matrix (EEM) fluorescence with parallel factor analysis (PARAFAC). Surface water samples were collected monthly for 2 years across the bay. DOM characteristics were statistically different across the bay, and the bay was spatially characterized into four basins based on chemical characteristics of DOM as determined by EEM-PARAFAC. Differences between zones were explained based on hydrology, geomorphology, and primary productivity of the local seagrass community. In addition, potential disturbance effects from a very active hurricane season were identified. Although the overall seasonal patterns of DOM variations were not significantly affected on a bay-wide scale by this disturbance, enhanced freshwater delivery and associated P and DOM inputs (both quantity and quality) were suggested as potential drivers for the appearance of algal blooms in high impact areas. The application of EEM-PARAFAC proved to be ideally suited for studies requiring high sample throughput methods to assess spatial and temporal ecological drivers and to determine disturbance-induced impacts in aquatic ecosystems.

Bonfires as a potential source of metal pollutants in urban soils, Galway, Ireland by Ligang Dao; Liam Morrison; Chaosheng Zhang (930-935).
► Bonfires have been identified as a significant source of metal pollutants in urban soils. ► Concentrations of Zn, Cu and Pb in soils are elevated at bonfire locations. ► Strong spatial variations of bonfire influenced metals in soils are observed.Soils are recipients of contaminants from the burning of metal-containing materials in bonfires. In order to obtain a better understanding of the impacts of bonfires on soils, a total of 218 surface soil samples were collected from a traditional bonfire site in Galway City, Ireland. Concentrations of 14 elements including Al, Ca, Co, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sc, Ti and Zn in 30 selected samples were determined using ICP-OES. Strong variations were observed for these elements. Concentrations of Zn, Cu and Pb were elevated, indicating that these metals may be influenced by human activities. Therefore, concentrations of these three metals were further measured using a portable X-ray fluorescence (P-XRF) analyser (n  = 218) and their spatial distribution was investigated using a Geographical Information System (GIS). Soils with elevated metal concentrations, as evident from the spatial distribution maps, coincided with the locations of traditional festival bonfires. The results of this study provide useful information for the management of bonfire sites including the enforcement of a control policy regarding the burning of materials in bonfires.