Migration and dispersion of trace elements in the rock–soil–plant system in areas underlain by black shales and slates of the Okchon Zone, Korea

The Okchon black shale in Korea provides a typical example of natural geological materials enriched with potentially toxic elements. The Chung-Joo, Duk-Pyung, Geum-Kwan, I-Won, Bo-Eun and Chu-Bu areas are underlain by these black shales and slates of the Guryongsan Formation or the Changri Formation, which are parts of the Okchon Group in the central part of the southern Korean Peninsula. In order to investigate the enrichment levels and dispersion patterns of potentially toxic elements in the rock–soil–plant system, environmental geochemical surveys were undertaken in the above six study areas in the Okchon Zone. After appropriate preparation, rock and soil samples were analyzed for potentially toxic elements by instrumental neutron activation analysis (INAA) and inductively coupled plasma atomic emission spectrometry (ICP-AES), and plant samples by atomic absorption spectrometry (AAS). In particular, Ba, Cd, Mo, V and U in Okchon black shales are highly enriched, and their mean concentrations are significantly higher than those in black slates. These elements are geochemically associated, and might be enriched simultaneously. The highest mean concentrations of 42.0 μg g⁻¹ As, 2100 μg g⁻¹ Ba, 10.9 μg g⁻¹ Cd, 213 μg g⁻¹ Mo, 83 μg g⁻¹ U, 938 μg g⁻¹ V and 394 μg g⁻¹ Zn are found in black shales from the Duk-Pyung area. Mean concentrations of As, Mo and U in soils overlying black shales occurring in the Duk-Pyung area (30 μg g⁻¹ As, 24 μg g⁻¹ Mo and 50 μg g⁻¹ U) and Chu-Bu area (39 μg g⁻¹ As, 15 μg g⁻¹ Mo and 27 μg g⁻¹ U) are higher than the permissible level. Enrichment index values of the six study areas decrease in the order of Duk-Pyung > Chu-Bu > Bo-Eun > Chung-Joo > Geum-Kwan = I-Won areas. Relationships between trace element concentrations in soils and plants are significantly correlated, and the biological absorption coefficients (BAC) in plants are in the order of Cd > Zn = Cu > Pb, which suggests that Cd is more bioavailable to plants than the other elements. Cadmium concentrations in plant species decrease in the order of chinese cabbage > red pepper > soybean = sesame > rice stalk > corn > rice grain. From the result of sequential extraction analysis of soils, relatively high proportions of Cu, Pb and Zn are present as residual fractions, and that of Cd as non-residual fractions. Cadmium occurs predominantly as exchangeable/water-acid soluble phase in soils, and this is in agreement with the findings of high Cd concentrations in plants.     

Gold, silver and other elements in aquatic bryophytes from a mineralised area of North Wales, U.K.

Concentrations of Au, Ag, As, Cd and Sb in aquatic bryophytes collected from the Dolgellau Mineral Belt, North Wales, U.K. are reported. One aquatic liverwort, Scapania undulata (L) Dum. and two mosses, Fontinalis squamosa Hedw. and Racomitrium aciculare (Hedw.) Brid. were collected from sites upstream and downstream of the recently reopened Gwynfyndd Au mine. There was little inter-species variation in metal contents for these three bryophytes, but Scapania undulata appeared the most sensitive to changes in water concentrations of Ag, As and Sb. Gold concentrations varied little between the contaminated and control sites. Concentrations in the range < 4–18 ng Aug g⁻¹ D.W. were typical background levels, while bryophytes collected immediately below the mine contained 6–45 ng Au g⁻¹. Silver and Sb both showed more pronounced ( 5–10 fold) elevations above control concentrations in samples collected downstream of the mine. Background concentrations for these elements were 5–85 ng Ag g⁻¹ and 0.15–1.3 μg Sb g⁻¹.Arsenic concentrations downstream of the mine (160–1080 μg g⁻¹) greatly exceeded the background range of 9–32 μg g⁻¹. It is suggested, therefore, that As may be an ideal ‘pathfinder’ element when prospecting for auriferous deposits using aquatic bryophytes.     

Environmental impact of an urban landfill on a coastal aquifer (El Jadida, Morocco)

The El Jadida landfill is one among many uncontrolled dumping sites in Morocco with no bottom liner. About 150 tons/day of solid wastes from mixed urban and industrial origins are placed directly on the ground. At the site of this landfill, the groundwaters circulate deeply (10–15 m) in the Cenomanian rock (calcareous–marl), which is characterised by an important permeability from cracks. The soil is sand–clay characterized by a weak coefficient of retention.The phreatic water ascends to the bottom of three quarries, which are located within the landfill. These circumstances, along with the lack of a leachate collection system, worsen the risks for a potential deterioration of the aquifer.To evaluate groundwater pollution due to this urban landfill, piezometric level and geochemical analyses have been monitored since 1999 on 60 wells. The landfill leachate has been collected from the three quarries that are located within the landfill. The average results of geochemical analyses show an important polluant charge vehiculed by landfill leachate (chloride = 5680 mg l⁻¹, chemical oxygen demand = 1000 mg l⁻¹, iron = 23 000 μg l⁻¹). They show also an important qualitative degradation of the groundwater, especially in the parts situated in the down gradient area and in direct proximity to the landfill. In these polluted zones, we have observed the following values: higher than 4.5 mS cm⁻¹ in electric conductivity, 1620 and 1000 mg l⁻¹ respectively in chlorides and sulfate (), 15–25 μg l⁻¹ in cadmium, and 60–100 μg l⁻¹ in chromium. These concentrations widely exceed the standard values for potable water.Several determining factors in the evolution of groundwater contamination have been highlighted, such as (1) depth of the water table, (2) permeability of soil and unsaturated zone, (3) effective infiltration, (4) humidity and (5) absence of a system for leachate drainage. So, to reduce the pollution risks of the groundwater, it is necessary to set a system of collection, drainage and treatment of landfill leachates and to emplace an impermeable surface at the site of landfill, in order to limit the infiltration of leachate.     

Distribution of copper, zinc, lead and cadmium concentrations in stream sediments from the Mapocho River in Santiago, Chile

The Mapocho river, which crosses downtown Santiago, is one of the most important rivers in contact with a population of about six million inhabitants. Anthropogenic activities, industrialization, farming activities, transport, urbanization, animal and human excretions, domestic wastes and copper mining have affected the river, contaminating it and its sediments with heavy metals. Concentration and distribution of Cu, Zn, Pb and Cd were studied with the purpose of determining their bioavailability and their relation with the characteristics of the sediments. Freshly deposited seasonal sediments were collected from 0–8 cm depths from 6 locations (S₁ to S₆) along the 30-km long channel length, in the four seasons of year on the following dates: May 2001 (D₁, autumn); August 2001 (D₂, winter); October 2001 (D₃, spring) and January 2002 (D₄, summer). The dried samples were sifted to obtain the < 63-μm sediment fraction, since it has been shown that large amounts of heavy metals are bound in the fine-grained fraction of the sediment. Cu and Zn were analyzed by atomic absorption spectrophotometry and Pb and Cd by square wave anodic stripping voltammetry. The highest concentrations of Cu (2850 μg g^− 1) were found in the northern part of the river (S₁, average D₁–D₄), near the mountains and a copper mine, and then decreased downstream to 209 μg g^− 1 (S₆). Total Zn showed an irregular variation, with higher values at S₁ (1290 μg g^− 1) and high values in some winter sampling (1384 μg g^− 1 S₄, S₅–D₂). Pb showed different trends, increasing from S₁ to S₆ (17 to 61 μg g^− 1), with the highest values in the summer samples (83 μg g^− 1, S₄–S₆, D₄), and total Cd increased slightly from mean values of 0.2 and 0.5 μg g^− 1. Partition into five fractions was made using Tessier's analytical sequential extraction technique; the residue was treated with aqua regia for recovery studies, although this step is not part of the Tessier procedure. The results show that Cu, Zn and Pb in the sediments were dependent on the sampling places along the river, and variation in two years was low (D₁–D₄). The highest values of total organic matter, carbonate and conductivity were found in S₆, which has the smallest size particles, while at S₁ the sediments were predominantly sand and contain larger amounts of silica. Cu associated with carbonate decreased gradually from 58% (1771 μg g^− 1, S₁) to 16% (32 μg g^− 1, S₆); Cu bonded to reducible fraction was almost constant (33% to 37%), and Cu associated with oxidizable fraction increased from 7% (S₁) to 34% (S₆), but copper content was lower (214 to 68 μg g^− 1). Zn had a similar fractionation profile. However, Pb bound to oxidizable fraction did not show significant percent variation along the river (20% to 19%), but the amount bounded was 4 to 12 μg g^− 1. The residual fraction increased from 24% to 41% (5 to 25 μg g^− 1, S₁ to S₆). The distribution of Cd in the sediment was almost independent of the sampling stations and was bound to carbonate, reducible and residual fraction in similar proportion. Cu and Zn at S₁ were mainly bound to carbonates and reducible phases with 91% and 73% (2779 and 965 μg g^− 1, respectively), and with a change in the pH and/or the redox potential of the sediment–water system, these contaminants could easily enter the food chain. In S₆ the amount of Cu and Zn in these phases was 50% and 53% (100 to 313 μg g^− 1, respectively).     

Production of dissolved organic carbon in forest soils along the north–south European transect

The aim of this study is to estimate the C loss from forest soils due to the production of dissolved organic C (DOC) along a north–south European transect. Dissolved organic matter (DOM) was extracted from the forest soils incubated at a controlled temperature and water content. Soils were sampled from forest plots from Sweden to Italy. The plots represent monocultures of spruce, pine and beech and three selected chronosequences of spruce and beech spanning a range of mean annual temperature from 2 to 14 °C. The DOM was characterized by its DOC/DON ratio and the C isotope composition δ¹³C. The DOC/DON ratio of DOM varied from 25 to 15 after 16 days of incubation and it decreased to between 16 and 10 after 126 days. At the beginning of incubation the δ¹³C values of DOC were 1‰ or 2‰ less negative than incubated soils. At the end of the experiment δ¹³C of DOC were the same as soil values. In addition to DOC production heterotrophic respiration and N mineralization were measured on the incubated soils. The DON production rates decreased from 30 to 5 μgN gC⁻¹ d⁻¹ after 16 days of incubation to constant values from 5 to 2 μgN gC⁻¹ d⁻¹ after 126 days at the end of experiment. The DIN production rates were nearly constant during the experiments with values ranging from 20 to 4 μgN gC⁻¹ d⁻¹. DOC production followed first-order reaction kinetics and heterotrophic respiration followed zero-order reaction kinetics. Kinetic analysis of the experimental data yielded mean annual DOC and respiration productions with respect to sites. Mean annual estimates of DOC flux varied from 3 to 29 g of C m⁻² (1–19 mg C g⁻¹ of available C), corresponding to mean DOC concentrations from 2 to 85 mg C L⁻¹.     

Cu and Fe associated with humic acids in sediments of a tropical coastal lagoon

The amount of Cu and Fe associated with humic acids was estimated in five sediment cores from a tropical coastal lagoon (Piratininga Lagoon, Rio de Janeiro, Brazil). Core samples were analysed for humic acid contents, total Fe and Cu content. Fe and Cu associated with humic acids were also measured. Results show amounts of humic acids ranging from 0.7 to 21.7% of the dry weight of sediment (average 4.6%, standard deviation 4.4%). Concentrations of Fe and Cu ranged from 0.3 to 6.0% (average 2.2%, S.D. 1.2%) and from <1.0 to 65.0 μg g⁻¹ (average 28.6 μg g⁻¹, S.D. 16.4 μg g⁻¹), respectively. The results of strongly bound metals show that while humic acids are the main carrier for Cu, Fe does not seems to be significantly associated with this organic matter.     

Molecular biomarker evidence of origins and transport of organic matter in sediments of the Pearl River estuary and adjacent South China Sea

Surface sediments from the subtropical Pearl River estuary and adjacent South China Sea were investigated by molecular organic geochemical methods to determine the composition, distribution and origin of extractable lipids (n-alkanes, n-alkanols and sterols). The absolute and organic C normalized concentrations of total alkane, n-alkanol, and sterol ranged from 0.16 to 2.67 μg g⁻¹ and 0.9 to 12.3 μg g⁻¹ OC, 24.4 to 427.3 ng g⁻¹ and 63.2 to 1966.7 ng g⁻¹ OC, and 9.0 to 493.5 ng g⁻¹ and 58.4 to 1042.4 ng g⁻¹ OC, respectively. The spatial distributions of these biomarkers indicated that terrestrial-derived molecular biomarkers such as long-chain n-alkanes, n-alkanols and plant-derived sterols were higher at the river mouth and along the coastline, suggesting that a higher proportion of terrestrial particulate organic matter was deposited there. Relatively lower amounts of marine-derived biomarkers such as short-chain n-alkanes, algal sterols at the river mouth reflected the lower primary productivity due to high turbidity. The spatial patterns of these biomarkers were partially related to the estuarine processes and conditions, evidencing an increased terrestrial signal from the Pearl River mouth to the inner estuary, and enhanced marine conditions further offshore.     

Geochemical characteristics of shallow groundwater in Datong basin, northwestern China

Located in semi-arid regions of northwestern China, Datong basin is a Quaternary sedimentary basin, where groundwater is the most important source for water supply. It is very important to study groundwater characteristics and hydrogeochemical processes for better management of the groundwater resource. We have identified five geochemical zones of shallow groundwater (between 5 and 80 m) at Datong: A. Leaching Zone (Zone I); B. Converging Zone (Zone II); C. Enriching Zone (Zone III); D. Reducing Zone (Zone IV); E. Oxidizing Zone (Zone V). In Zones I, II, and V and some parts of Zones III and IV, hydrolysis of albite/K-feldspar/chalcedony system and/or albite/K-feldspar/quartz system enhanced concentrations of Na⁺, K⁺, HCO₃⁻ and silicate. In Zone I, dissolution of carbonate and hydrolysis of feldspar generally controlled the groundwater chemistry. Infiltration of meteoric water promoted the formation of HCO₃⁻ in the water. In Zone II, the main geochemical processes influencing the groundwater chemistry were dissolutions of calcite and dolomite, ion exchange and evaporation. In Zones III and IV, in addition to ion exchange, evaporation and precipitation of calcite and dolomite, leaching of NaHCO₃ in saline–alkaline soils dominated the water quality. Zone IV was under anoxic condition, and reduction reactions led to the decrease of SO₄²⁻, NO₃⁻ and occurrence of H₂S, with the highest arsenic content (mean value of 366 μg/L), far exceeding Maximum Contaminant Level (MCL). Abnormal arsenic in the groundwater resulted in endemic disease of waterborne arsenic poisoning among local people. Zone V overlapped Zone I was intensively affected by coal mining activities. Sulfide minerals, such as pyrite, would have been oxidized when exposed to air due to coal mining, which directly added sulfate to groundwater and thus increased SO₄²⁻ concentration. Oxidization of sulfide minerals also decreased pH and promoted dissolutions of calcite and dolomite.     

Silicon transfers in a rice field in Camargue (France)

We conducted a study of the biogeochemical cycle of silicon in a rice field in Camargue (France) in order to evaluate the role of biogenic silicon particles (BSi) in the cycle. Opal-A biogenic particles (phytoliths, diatoms…), which dissolve more rapidly than other forms of silicate usually present in soils, are postulated to represent the easiest bioavailable Si for rice. We found 0.03–0.06 wt.% of BSi in soils (mainly phytoliths). This value is lower than other values from the literature. Each year, the exportation of BSi from rice cultivation is 270 ± 80 kg Si ha^− 1. We show that BSi input by irrigation is mostly composed of diatoms and we estimate it at 100 kg Si ha^− 1 year^− 1. This value is more than a third of the annual Si need for rice. The budget of the dissolved silicon (DSi) fluxes gives the following results: the atmospheric and irrigation inputs represents 1% and roughly 10%, respectively, of the annual need for rice; the drainage and infiltration outputs represent 17 ± 14 and 12 ± 9 kg Si ha^− 1 year^− 1, respectively; the balance of our budget shows that at least 170 kg Si ha^− 1 year^− 1 are exported from the soil. If we consider the soil BSi as the only source of dissolved silicon, this stock could be exhausted in 5 years.     

Geochemical evidence for Se mobilization by the weathering of pyritic shale, San Joaquin Valley, California, U.S.A.

Acidic (pH 4) seeps issue from the weathered Upper Cretaceous-Paleocene marine sedimentary shales of the Moreno Formation in the semi-arid Coast Ranges of California. The chemistry of the acidic solutions is believed to be evidence of current reactions ultimately yielding hydrous sodium and magnesium sulfate salts, e.g. mirabilite and bloedite, from the oxidation of primary pyrite. The selenate form of Se is concentrated in these soluble salts, which act as temporary geological sinks. Theoretically, the open lattice structures of these hydrous minerals could incorporate the selenate (SeO₄⁻²) anion in the sulfate (SO₄⁻²) space. When coupled with a semi-arid to arid climate, fractional crystallization and evaporative concentration can occur creating a sodium-sulfate fluid that exceeds the U.S. Environmental Protection Agency limit of 1000 μg l⁻¹ for a toxic Se waste. The oxidative alkaline conditions necessary to ensure the concentration of soluble selenate are provided in the accompanying marine sandstones of the Panoche and Lodo Formations and the eugeosynclinal Franciscan assemblage. Runoff and extensive mass wasting in the area reflect these processes and provide the mechanisms which transport Se to the farmlands of the west-central San Joaquin Valley. Subsurface drainage from these soils consequently transports Se to refuge areas in amounts elevated to cause a threat to wildlife.     

Chemical and isotopic (B, Sr) composition of alluvial sediments as archive of a past hydrothermal outflow

The geochemical and isotopic signature of Quaternary alluvial sediments filling a post-orogenic basin along the Tyrrhenian coasts of Italy (Cornia Plain, Tuscany) was investigated to unravel possible interactions with geothermal fluids from the Larderello geothermal field. Two cores located in the upper (UCP) and lower (LCP) sector of the plain were sampled to depths of up to 80 m. A third core in a neighbouring area not affected by geothermal activity was also sampled (Arno plain at Pisa), and its sediment composition was used as reference. The Cornia sediments (fraction < 65 μm) show high B, Cs and Sb concentrations related to a peculiar chemical enrichment of the clay fraction. They also show remarkable enrichments in As (up to 1000 μg g^− 1) reflecting a contribution from local ore deposits.⁸⁷Sr/⁸⁶Sr ratios, ranging from 0.71022 to 0.71698, reveal the nature of the weathered mother rocks of the alluvial sediments, whereas the boron isotopic composition, varying from − 20‰ to − 10‰, suggests an interaction between the clay fraction and boron-rich fluids at temperatures greater than 50 °C. This implies that hydrothermal fluids widely circulated within the Cornia basin in the past, ultimately leading to the geochemical anomalies currently recorded in local sediments.Although natural (geogenic) in origin, these anomalies cause severe problems to the regional water management (groundwater exploitation) through leaching of trace elements into circulating groundwater, a phenomenon which has to be carefully studied and monitored.     

Seasonal variation of gold and arsenic in biogeochemical samples from a disseminated gold deposit in the Northern Cordillera

An orientation survey was conducted over the Quartz Mountain, Oregon, hot-spring type disseminated gold deposit to address three questions critical to successful exploration of the area: What is the relationship between bedrock geology and the trace-element content of trees and soils; do these relationships change seasonally; are these relationships similar in the two tree species which discontinuously blanket the area?Twig, needle and wood samples were collected four times during the year from both ponderosa pine (Pinus ponderosa) and white fir (Abies concolor). Soils were collected once. All samples were analyzed for Au and As and the wood samples were analyzed also for Sb.Arsenic was the best pathfinder element, having little analytical, spatial, or seasonal variation in the twig and needle samples. Anomalous levels were 150 μg/g (ppm) As in pine and 30 μg/g in fir. Gold showed analytical variation of 20–30%, sample duplicate variation up to 90%, spatial changes, and seasonal variation ranging from winter-summer background [10–20 ng/g (ppb) Au] to anomalies of 300–400 ng/g (ppb) Au in pine and 100 μg/g (ppb) Au in fir in the spring and fall. Antimony could not be completely evaluated as a pathfinder element because it was only determined in wood samples.Needles and twigs from both species proved to be viable sample media for exploration of the area. Wood was not a suitable medium because of low, erratic values, perhaps due in part to analytical technique. Needles had 2 to 20 times higher As concentrations than did twigs. Twigs had a slightly higher Au content [25 ng/g (ppb)] than did needles. The pine samples were higher in both elements than were the fir samples. The data, normalized by species, could be treated as one homogeneous data base.The soil Au and As data outlined the mineralization clearly with a central Au anomaly [100 ng/g (ppb) and greater] surrounded by As anomalies [100 μg/g (ppm) and greater] over a distance of 594 m. Neither spring nor fall tree Au anomalies were coincident with the soil Au anomaly. Consistent throughout the year, tree As anomalies coincided with the soil As anomalies, but covered a smaller area. Both the Au and As anomalies in trees appeared to be related to bedrock contact zones rather than to the soil the trees were growing in.     

Elemental composition of sediment cores from a mangrove environment using neutron activation analysis

The geochemistry of Sepetiba Bay was studied in four sediment cores using a multi-element approach. Two cores were sampled in the more contaminated eastern part of the bay and two cores were sampled in the western region. The aim was to determine whether less common elements like the rare earths or the actinides are associated with contaminant metals like zinc in the Bay. Samples were analysed by instrumental neutron activation analysis that permits the quantification of total concentrations of metals (Ba, Co, Cr, Cs, Fe, Hf, Rb, Sc, Zn), rare earth elements (Ce, Eu, La, Lu, Sm and Yb), actinides (Th, U), non-metals and semi-metals (As, Br). Organic carbon and total sulphur were also analysed. The results show very strong zinc contamination in the top layers (more than 1000 μg g⁻¹) and background concentrations in the bottom (15 μg g⁻¹). Elements like chromium which would be expected to be released by the heavy industries of Sepetiba Bay, did not show a contamination profile, and concentrations remained close to those of natural environments. No evidence of any association between the zinc and other potential contaminant elements could be identified in this work.     

Oxidation of As-bearing gold ore—a comparison of batch and column experiments

This study deals with the experimental determination of the rate of weathering and As release from Au-bearing sulfide ores of the Mokrsko-West deposit (Czech Republic). The elevated As contents in the ore and the possible As mobilisation must be taken into account due to the close proximity of the deposit to the Vltava River, a source of drinking water for Prague. Crushed and homogenized ore material was divided into four grain-size fractions showing uniform chemical and mineralogical composition and subjected to batch and column leaching experiments. The most important reactions controlling metal leaching include (i) oxidation of sulfides, (ii) carbonate equilibrium and (iii) precipitation of hydrated ferric oxides (HFO). Hydrated ferric oxides were found to be important sorbents of mobilized As. The highest concentration of As in the leachates was obtained from the 64–1000 μm fraction. The oxidation rate in the agitated batch experiment calculated on the basis of sulfate concentrations is an order of magnitude higher (3.8×10⁻⁹ to 4.4×10⁻⁸ mol SO₄²⁻ h⁻¹ g⁻¹) than in the static column experiment (9.6×10⁻¹⁰ to 7.4×10⁻⁹ mol SO₄²⁻ h⁻¹ g⁻¹). The higher oxidation rate of batch experiments is explained by abrasion of oxidation products (mainly HFO) and by further exposition of reaction surfaces of the sulfide grains.     

Uranium in groundwater near Uaroo, Western Australia: Distribution and relevance to mineralization

Groundwater samples were collected from stock wells and bores during September 1979, June 1980, November 1980 and November 1981, and were analyzed for uranium by XRF and fluorimetry. It was found that: (1) High U concentrations were generally confined to the immediate vicinity of granitoid bodies; and (2) with the exception of two open, shallow wells, concentrations of U did not vary more than ±25 μg l⁻¹ over the sampling period, despite very heavy rainfall (rainfall events > 100 mm day⁻¹) in 1980.     

Groundwater recharge history and hydrogeochemical evolution in the Minqin Basin, North West China

The Minqin Basin is a type area for examining stress on groundwater resources in the Gobi Desert, and has been investigated here using a combination of isotopic, noble gas and chemical indicators. The basin is composed of clastic sediments of widely differing grain size and during the past half century over 10 000 boreholes have been drilled with a groundwater decline of around 1 m a⁻¹. Modern diffuse recharge is unlikely to exceed 3 mm a⁻¹, as determined using unsaturated zone profiles and Cl⁻ mass balance. A small component of modern (<50 a) groundwater is identified in parts of the basin from ³H–³He data, probably from irrigation returns. A clear distinction is found between modern waters with median δ¹⁸O values of 6.5 ± 0.5‰ and most groundwaters in the basin with more depleted isotopic signatures. Radiocarbon values as pmc range from 0.6% to 85% modern, but it is difficult to assign absolute ages to these, although a value of 20% modern C probably represents the late Pleistocene to Holocene transition. The δ¹³C compositions remain near-constant throughout the basin (median value of −8.1‰ δ¹³C) and indicate that carbonate reactions are unimportant and also that little reaction takes place. There is a smooth decrease in ¹⁴C activity accompanied by a parallel increase in ⁴He accumulations from S–N across the basin, which define the occurrence of a regional flow system. Noble gas temperatures indicate recharge temperatures of about 5.6 °C for late Pleistocene samples, which is some 2–3 °C cooler than the modern mean annual air temperature and the recharge temperature obtained from several Holocene samples. Groundwaters in the Minqin Basin have salinities generally below 1 g/L and are aerobic, containing low Fe but elevated concentrations of U, Cr and Se (mean values of 27.5, 5.8 and 5.3 μg L⁻¹, respectively). Nitrate is present at baseline concentrations of around 2 mg L⁻¹ but there is little evidence of impact of high NO₃ from irrigation returns. Strontium isotope and major ion ratios suggest that silicate reactions predominate in the aquifer. The results have important implications for groundwater management in the Minqin and other water-stressed basins in NW China – a region so far destined for rapid development. The large proportion of the water being used at present is in effect being mined and significant changes are urgently needed in water use strategy.     

Correlation between dissolved He concentration and Cl in groundwater at Äspö, Sweden

Tunnel excavation at Äspö Island, Sweden, has caused severe groundwater disturbance, gradually extending deeper into the tunnel as present-day Baltic seawater intrudes through fractures connecting to the surface. However, the paleo-hydrogeochemical conditions have remained in the deep highly saline waters that have avoided mixing. A correlation has been observed between dissolved ⁴He concentration and Cl⁻ ion concentration, measured every two years from 1995 to 2001 at Äspö. Groundwater mixing conditions can be examined by the correlations between 1/Cl, ³⁶Cl/Cl, and ³H concentrations. Subsurface production is responsible for the majority of the ³⁶Cl and excess dissolved ⁴He of interstitial groundwater in fractures. The secular equilibrium ratio of ³⁶Cl/Cl in rock was theoretically estimated to be (5.05 ± 0.82) × 10⁻¹⁴ based on the neutron flux intensity, a value comparable to the measured ³⁶Cl/Cl ratio in rock and groundwater. The degassing crustal ⁴He flux was estimated to be 2.9 × 10⁻⁸  1.3 × 10⁻⁶ (ccSTP/cm²a) using the HTO diffusion coefficient for the Äspö diorite. The ⁴He accumulation rate ranges from 6.8 × 10⁻¹⁰ (for the in situ accumulation rate) to 7.0 × 10⁻⁹ (ccSTP/(g_water · a) considering both ⁴He in situ production and the degassing flux, assuming ⁴He is accumulated constantly in groundwater. By comparing the subsurface ³⁶Cl increase with ⁴He concentrations in groundwater, the ⁴He accumulation rate was determined from data for groundwater arriving at the secular equilibrium of ³⁶Cl/Cl. The ⁴He accumulation rate was found to be (1.83 ± 0.72) × 10⁻⁸ ccSTP/(g_water · a) without determining the magnitude of degassing ⁴He flux.     

Pollution par les nitrates des nappes phréatiques sous environnement semi-urbain non assaini: example de la nappe de Yeumbeul, Sénégal

High nitrate concentrations, above the WHO guideline of 50 mg l⁻¹, were observed in samples of shallow wells reaching the Yeumbeul suburb (Senegal) area groundwater. This groundwater is exploited by 7000 houses and therefore there are health implications. Correlations between parameters such as nitrate content (NO₃⁻) in the groundwater and soil water, the distance between shallow wells and family latrines, and soil water chloride (Cl⁻) and colon bacillus content led to two possible sources of groundwater pollution: first, contamination by non impervious and shallow latrines; and second, the leaching of soil NO₃⁻ from waste organic matter carried in groundwater.     

Gold and other metals in big sagebrush (Artemisia tridentata Nutt.) as an exploration tool, Gold Run District, Humboldt County, Nevada

Big sagebrush — a cold-desert species that dominates the terrain over large parts of western United States — was sampled along several traverses that crossed thermally metamorphosed limestone, phyllitic shale, and schist of the Middle and Upper Cambrian Preble Formation that host skarn-, disseminated gold and silver-, and hot springs gold-type mineral occurrences. Patterns of detectable levels of gold (8 to 28 ppb or ng g⁻¹) in ash of new growth were consistent with areas affected by known or suspected gold mineralization. Soils collected along one of the traverses where a selenium-indicator plant was common contained no gold above background levels of 2ppb, but were consistently high in As, Sb, and Zn, and several samples were unusually high in Se (maximum 11 ppm or μg g⁻¹). Sagebrush along this traverse contained Li at levels above norms for this species. We also found a puzzling geochemical anomaly at a site basinward from active hot springs along a range-front fault scarp. Sagebrush at this site contained a trace of gold and an unusually high concentration of Cd (13 ppm) and the soil had anomalous concentrations of Cd and Bi (3.2 and 6 ppm, respectively). The source of this anomaly could be either metal-rich waters from an irrigation ditch or leakage along a buried fault. Despite the limited nature of the study, we conclude that gold in sagebrush could be a cost-effective guide to drilling locations in areas where the geology seems favorable for disseminated and vein precious metals.     

High levels of uranium and radium in groundwaters at Canada's Underground Research Laboratory, Lac du Bonnet, Manitoba, Canada

High concentrations of U and²²⁶Ra, and elevated²³⁴U/²³⁸U activity ratios have been measured in groundwater samples collected from water supply wells and exploratory boreholes in the area surrounding the Underground Research Laboratory (URL) of Atomic Energy of Canada Limited, in southeastern Manitoba. All groundwaters come from the Lac du Bonnet granite batholith or sediments overlying the batholith.Uranium concentrations attain almost 1 mg/l in some shallow, low-salinity groundwaters, whereas²²⁶Ra tends to be high (up to 38 Bq/l) in deeper, saline waters. The U concentrations are some of the highest observed in global groundwaters, yet no significant ore body or mineralization is known in the area. Analyses of unaltered rock samples of the Lac du Bonnet granite show slight U enrichment over average Canadian Shield granites (6.5 μg/g vs 4 μg/g), and altered wall rock in fracture zones is enriched in U by up to an order of magnitude compared to adjacent bedrock. Low²³⁴U/²³⁸U activity ratios in this altered rock indicate active and recent leaching of U by groundwater.The key control on U concentration appears to be redox potential. Concentrations of U in rock, residence time and groundwater composition are of lesser importance. Geochemical modelling of the shallower, oxidized waters indicates that U speciation consists mainly of anionic carbonate complexes of the uranyl ion. This is supported by the remarkable efficiency of an anionic filter developed to remove high levels of U from drinking water in the area.In more reducing groundwaters, U concentrations are similar to those determined in recent experimental work on uraninite solubility in the pH range 7–8.5. Colloidal U is <10% of total U and organic complexation is unlikely to be significant because of low dissolved organic concentrations. The results emphasize the significance of redox potential in controlling U mobility in both oxidizing and reducing environments and indicate the usefulness of U concentration in estimating groundwater Eh.