Applied Geochemistry (v.22, #5)
Annual Index (e1-e44).
Aged anthropogenic iodine in a boreal peat bog by S. Maillant; M.I. Sheppard; G. Echevarria; S. Denys; G. Villemin; P. Tekely; E. Leclerc-Cessac; J.L. Morel (873-887).
This in situ study assesses the long term sorption of I in a natural peat bog, a matter that is scarcely addressed but required for safety studies such as for radioactive waste disposal. Fifteen years after the artificial contamination of a boreal peat bog, the groundwater (piezometers), the peat and the vegetation were resampled to determine I distribution with comparison to the initial situation (1989). Spectroscopic analyses (SS-NMR, electronic microscopy and EDX spectroscopy) were carried out on the peat solids to identify the sorption processes. Over the past 15 a the I has been spreading mostly outwards and possibly upwards in the groundwater. Sorption of I is higher at the surface of the bog (K d = 37.6 L kg−1) than at the bottom (K d = 5.1 L kg−1), and this is attributed to the oxic/anoxic conditions of the peat layers. The average surface K d values showed more than a 2-fold increase after 15 a. TEM–EDX analyses of the surface peat showed here for the first time that I was only associated with natural polyphenolic substances contained in humified plant tissues. Plants growing in the bog have not taken up much I with the exception of sedge species ([I]sedge leaves is up to 283 mg kg−1).
Temporal and spatial variability of geochemical backgrounds in the Windmill Islands, East Antarctica: Implications for climatic changes and human impacts by Massimo Gasparon; Katharina Ehrler; Jörg Matschullat; Martin Melles (888-905).
To establish a natural background and its temporal and spatial variability for the area around Casey Station in the Windmill Islands, East Antarctica, the authors studied major and trace element concentrations and the distribution of organic matter in marine and lacustrine sediments. A wide range of natural variability in trace metal concentrations was identified between sites and within a time scale of 9 ka (e.g., Ni 5–37 mg kg−1, Cu 20–190 mg kg−1, Zn 50–300 mg kg−1, Pb 4.5–34 mg kg−1). TOC concentrations are as high as 3 wt.% at the marine sites and 20 wt.% at the lacustrine sites, and indicate highly productive ecosystems. These data provide a background upon which the extent of human impact can be established, and existing data indicate negligible levels of disturbance. Geochemical and lithological data for a lacustrine sediment core from Beall Lake confirm earlier interpretation of recent climatic changes based on diatom distribution, and the onset of deglaciation in the northern part of the Windmill Islands between 8.6 and 8.0 ka BP. The results demonstrate that geochemical and lithological data can not only be used to define natural background values, but also to assess long-term climatic changes of a specific environment. Other sites, however, preserve a completely different sedimentary record. Therefore, inferred climatic record, and differences between sites, can be ascribed to differences in elevation, distance from the shore, water depth, and local catchment features. The extreme level of spatial variability seems to be a feature of Antarctic coastal areas, and demonstrates that results obtained from a specific site cannot be easily generalized to a larger area.
Influence of parent sediments on the concentration of heavy metals in urban and suburban soils in Turku, Finland by Veli-Pekka Salonen; Kirsti Korkka-Niemi (906-918).
The purpose of this study was to examine the influence of parent sediment material on soil geochemistry and to locate possible pollution sources in the area of Turku town in southwestern Finland. The study area is glaciated terrain and subsurface samples were chosen to represent five different parent sediment materials: glacial till, glaciofluvial or littoral sand, clay, peat and marine mud. Elemental concentrations of surface (100 samples) and background soil (50 samples) were analysed by ICP-AES (Al, Ba, Ca, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, S, Sr, Ti, V) and by graphite furnace AAS (As, Cd, Mo, Se). Loss-on-ignition was determined as an indicator of organic matter. Analytical results were examined using multivariate statistical methods. The results indicate that the studied soils are not heavily contaminated; there were only few sample sites with elevated heavy metal concentrations. High Pb concentrations were observed along main traffic routes, and also in places where there has been small-scale use of antifouling paints. However, concentrations of heavy metals in topsoil were in general significantly lower than those measured in large cities elsewhere. The parent material strongly affects the geochemical character of the soils. Clays naturally have a significantly higher concentration of heavy metals than sands or till material. This is also reflected in concentrations in organic topsoil. When assessing target levels for polluted soil, the primary sediment appears to be a more important factor than the amount of organic matter.
Potential health risk in areas of high natural concentrations of thallium and importance of urine screening by Tangfu Xiao; Jayanta Guha; Cong-Qiang Liu; Baoshan Zheng; Graham Wilson; Zengping Ning; Libin He (919-929).
There is a lack of information in the literature regarding Tl exposure from naturally occurring Tl enrichment. This paper draws attention to the potential health risk posed by high concentrations of naturally occurring Tl in the environment. The inhabitants of a rural area in SW Guizhou Province, China, reside within a natural Tl accumulated environment resulting from Tl-rich sulfide mineralization, and they face ongoing severe Tl exposure. High Tl concentrations were detected in urine of the local residents. Urinary Tl concentrations are as high as 2668 μg/L, with most subjects surpassing the accepted world urinary Tl concentration at <1 μg/L for “non-exposed” humans. The urinary Tl concentrations show significant differences among three communities (n = 21, p = 0.001), but no significant difference in either sex or age groups (n = 21, p = 0.7806). However, there is a positive statistical relationship between the extent of Tl exposure from Tl concentrations in soil and crops in the immediate environment and the concentrations of Tl detected in urine. A majority of the volunteer subjects from the communities have urinary Tl concentrations above 4.5–6 μg/L, implying early adverse health effects, and some of them have over 500 μg/L urinary Tl, considered to be at/about the level of clinical intoxication. This study has been able to identify that the elevated urinary Tl concentrations are mainly attributable to Tl accumulation in locally grown vegetables, which acquire Tl from the soil. This study also shows that Tl in urine of the local population represents a steady-state condition with long-term exposure, and that urinary Tl concentrations can be taken as a bio-marker of total dose based upon total daily dietary intake. This study demonstrates that natural sources of elevated Tl pose a potential health risk to the population, and that monitoring the urinary Tl concentration is a reliable and accurate way of bio-marking Tl exposure.
Assessing the concentration, speciation, and toxicity of dissolved metals during mixing of acid-mine drainage and ambient river water downstream of the Elizabeth Copper Mine, Vermont, USA by Laurie S. Balistrieri; Robert R. Seal; Nadine M. Piatak; Barbara Paul (930-952).
The authors determine the composition of a river that is impacted by acid-mine drainage, evaluate dominant physical and geochemical processes controlling the composition, and assess dissolved metal speciation and toxicity using a combination of laboratory, field and modeling studies. Values of pH increase from 3.3 to 7.6 and the sum of dissolved base metal (Cd + Co + Cu + Ni + Pb + Zn) concentrations decreases from 6270 to 100 μg/L in the dynamic mixing and reaction zone that is downstream of the river’s confluence with acid-mine drainage. Mixing diagrams and PHREEQC calculations indicate that mixing and dilution affect the concentrations of all dissolved elements in the reach, and are the dominant processes controlling dissolved Ca, K, Li, Mn and SO4 concentrations. Additionally, dissolved Al and Fe concentrations decrease due to mineral precipitation (gibbsite, schwertmannite and ferrihydrite), whereas dissolved concentrations of Cd, Co, Cu, Ni, Pb and Zn decrease due to adsorption onto newly formed Fe precipitates.The uptake of dissolved metals by aquatic organisms is dependent on the aqueous speciation of the metals and kinetics of complexation reactions between metals, ligands and solid surfaces. Dissolved speciation of Cd, Cu, Ni and Zn in the mixing and reaction zone is assessed using the diffusive gradients in thin films (DGT) technique and results of speciation calculations using the Biotic Ligand Model (BLM). Data from open and restricted pore DGT units indicate that almost all dissolved metal species are inorganic and that aqueous labile or DGT available metal concentrations are generally equal to total dissolved concentrations in the mixing zone. Exceptions occur when labile metal concentrations are underestimated due to competition between H+ and metal ions for Chelex-100 binding sites in the DGT units at low pH values. Calculations using the BLM indicate that dissolved Cd and Zn species in the mixing and reaction zone are predominantly inorganic, which is consistent with the DGT results. Although the DGT method indicates that the majority of aqueous Cu species are inorganic, BLM calculations indicate that dissolved Cu is inorganic at pH < 5.5 and organic at pH > 5.5.Integrated dissolved labile concentrations of Cd, Cu and Zn in the mixing and reaction zone are compared to calculated acute toxicity concentrations (LC50 values) for fathead minnows (Pimephales promelas) (Cd, Cu and Zn) and water fleas (Ceriodaphnia dubia) (Cd and Cu) using the BLM, and to national recommended water quality criteria [i.e., criteria maximum concentration (CMC) and criterion continuous concentration (CCC)]. Observed labile concentrations of Cd and Zn are below LC50 values and CMC for Cd, but above CCC and CMC for Zn at sites <30 m downstream of the confluence. In contrast, labile Cu concentrations exceed LC50 values for the organisms as well as CCC and CMC at sites <30 m downstream of the confluence. These results suggest that environmental conditions at sites closest to the confluence of the river and acid-mine drainage should not support healthy aquatic organisms.
Naturally occurring arsenic in the Miocene Hawthorn Group, southwestern Florida: Potential implication for phosphate mining by Olesya Lazareva; Thomas Pichler (953-973).
To understand the mineralogical association, concentration, and distribution of arsenic (As) in the Hawthorn Group, the chemical and mineralogical composition of 362 samples that were collected from 16 cores in southwestern Florida were examined in detail. In the study area, the Hawthorn Group consisted primarily of a basal carbonate unit (the Arcadia Formation) and an upper siliciclastic unit (The Peace River Formation). The Peace River Formation contains appreciable amounts of phosphate and is currently being exploited for phosphate ore. Samples were taken from cores of each formation at intervals of 7.5 m. In addition, to the interval samples, sections likely to have high As concentrations, such as zones with pyrite crystals, hydrous ferric oxides, green clays, and organic material, were collected and analyzed. Bulk As concentrations were determined by hydride generation-atomic fluorescence spectrometry (HG-AFS) after digestion with aqua regia (3:1 HCl and HNO3). The elements Fe, Al, Si, Mg, Ca, S, and P were measured on the same solutions by inductively coupled plasma optical emission spectrometry (ICP-OES). The identification of discrete minerals was aided by scanning electron microscopy (SEM) and chemical compositions within the sample matrix and in individual minerals were obtained by electron-probe microanalysis (EMPA).This detailed mineralogical and geochemical study demonstrated that: (1) As in the Hawthorn Group varied from formation to formation and was mostly concentrated in trace minerals, such as pyrite; (2) average As concentrations significantly changed from 8.8 mg/kg (σ = 8.6 mg/kg) in the Peace River Formation to 3.0 mg/kg (σ = 3.7 mg/kg) in the Tampa Member of the Arcadia Formation. Arsenic concentrations for all Hawthorn samples varied from 0.1 to 69.0 mg/kg; (3) pyrite, with one exception, occurred as framboids and was unevenly distributed throughout the Hawthorn Group; (4) pyrite framboids were located inside a francolite (carbonate fluorapatite) matrix with As concentrations as high as 3730 mg/kg and as a trace mineral in the sediment matrix concentrations varying from <1 to 8260 mg/kg; (5) hydrous ferric oxides (HFO) contained As concentrations as high as 540 mg/kg; (6) francolite, organic material, and clays contained substantially less As than pyrite; (7) thus, the release of As from pyrite could pose a potential problem for the phosphate industry.
The limited role of aquifer heterogeneity on metal reduction in an Atlantic coastal plain determined by push-pull tests by Brian J. Mailloux; Stephanie Devlin; Mark E. Fuller; T.C. Onstott; Mary F. DeFlaun; Keun-Hyung Choi; Maria Green-Blum; Donald J.P. Swift; John McCarthy; Hailiang Dong (974-995).
Sixty push-pull experiments were conducted to determine the factors controlling Fe(III) and Mn(IV) reduction in a well-characterized, shallow, coastal plain aquifer near Oyster, VA, USA. The five multi-level samplers each equipped with 12 ports sampled a heterogeneous portion of the aquifer from 4.4 to 8 m-bgs. Each multi-level sampler (MLS) was injected with groundwater that contained NO 3 - and Br− along with: (1) just groundwater (control treatment), (2) humics, (3) lactate (conducted twice) and (4) lactate plus humics. Microbially mediated Fe(III) reduction caused the aqueous FeTot concentrations to increase at every depth in the lactate treatment with significant increases within 1 day even while NO 3 - was present. Little change in the FeTot concentrations were observed in the control and humics treatment. Humics may have acted as an electron shuttle to increase Fe(III) reduction in the lactate plus humics treatment. The amount of Mn(IV) reduction was significantly lower than that of Fe(III) reduction. Geochemical modeling indicated that gas formation, sorption on reactive surfaces, and mineral precipitation were important processes and that Fe(III) and S O 4 2 - reduction were co-occurring. Conditions were favorable for the precipitation of Fe-carbonates, Fe-sulfides and Fe-silicates. In the lactate treatment protist concentrations increased then decreased and planktonic cell concentrations steadily increased, whereas no change was observed in the control treatment. Correlations of Fe(III) reduction with physical and chemical heterogeneity were weak, probably as a result of the abundance of Fe(III) bearing minerals relative to electron donor abundance and that the push-pull test sampled a representative elemental volume that encompassed the microbial diversity within the aquifer. This work indicates that stimulating metal reduction in aquifer systems is a feasible method for remediating heterogeneous subsurface sites contaminated with metals and radionuclides.
The use of tracers to assess leakage from the sequestration of CO2 in a depleted oil reservoir, New Mexico, USA by Arthur W. Wells; J. Rodney Diehl; Grant Bromhal; Brian R. Strazisar; Thomas H. Wilson; Curt M. White (996-1016).
Geological sequestration of CO2 in depleted oil reservoirs is a potentially useful strategy for greenhouse gas management and can be combined with enhanced oil recovery. Development of methods to estimate CO2 leakage rates is essential to assure that storage objectives are being met at sequestration facilities. Perfluorocarbon tracers (PFTs) were added as three 12 h slugs at about one week intervals during the injection of 2090 tons of CO2 into the West Pearl Queen (WPQ) depleted oil formation, sequestration pilot study site located in SE New Mexico. The CO2 was injected into the Permian Queen Formation. Leakage was monitored in soil–gas using a matrix of 40 capillary adsorbent tubes (CATs) left in the soil for periods ranging from days to months. The tracers, perfluoro-1,2-dimethylcyclohexane (PDCH), perfluorotrimethylcyclohexane (PTCH) and perfluorodimethylcyclobutane (PDCB), were analyzed using thermal desorption, and gas chromatography with electron capture detection. Monitoring was designed to look for immediate leakage, such as at the injection well bore and at nearby wells, and to develop the technology to estimate overall CO2 leak rates based on the use of PFTs. Tracers were detected in soil–gas at the monitoring sites 50 m from the injection well within days of injection. Tracers continued to escape over the following years. Leakage appears to have emanated from the vicinity of the injection well in a radial pattern to about 100 m and in directional patterns to 300 m. Leakage rates were estimated for the 3 tracers from each of the 4 sets of CATs in place following the start of CO2 injection. Leakage was fairly uniform during this period. As a first approximation, the CO2 leak rate was estimated at about 0.0085% of the total CO2 sequestered per annum.
The sources, pathway, and preventive measures for fluorosis in Zhijin County, Guizhou, China by Shifeng Dai; Weiwei Li; Yuegang Tang; Yong Zhang; Peng Feng (1017-1024).
The F content of several materials from households of Zhijin County, one of the most severe endemic fluorosis-stricken areas in western Guizhou, was determined using the pyrohydrolysis/fluoride ion-selective electrode method to determine the sources of F and the pathways causing the disease. The average F content of the coal burned is 237 μg/g, the clay used as a binder for fine coal is 2262 μg/g, with 828 μg/g in the mixed coal and clay, while the F content of corn is 1419 μg/g, with 110 μg/g in chilies. The binder clay is found to be the main F source for the endemic fluorosis. Fluorine in the clay not only occurs in apatite and hornblende, but also is associated with illite and mixed-layer clay minerals of smectite and illite. About 80% of the F is released during combustion of the mixture. The F content of corn and chilies is about 1000 and 110 times higher, respectively, than the permitted level of F in foodstuffs according to the Chinese Standard GB 4809-84. Most of the F in corn and chilies probably occurs either in smoke dusts attached to the surface or is adsorbed onto the outer peel, rather than being absorbed by the inner part. Thus, in addition to developing F-sequestration technologies, changing the living habits of the residents in the endemic area, for example, washing corn and chilies before cooking and peeling corn before being pulverized into corn flour, will remove a large proportion of the F, and thus it can play an important role in endemic fluorosis prevention.
Temporal trends of dissolved weathering products released from a high Arctic coal mine waste rock pile in Svalbard (78°N) by Jens Søndergaard; Bo Elberling; Gert Asmund; Claus Gudum; Karl Martin Iversen (1025-1038).
It is well known that oxidation of sulphide-containing coal mine waste has considerable environmental impacts due to generation of acid mine drainage (AMD) containing high dissolved metal concentrations. This study is the first to evaluate seasonal trends in the release of AMD from high arctic coal mine waste rock. Runoff from an abandoned coal mine waste pile in Svalbard (78°N) was studied during the entire 3–4 month period with running water in 2005. Temporal variation in concentrations and fluxes of dissolved elements were quantified based on daily water sampling and used to evaluate weathering processes and estimate element budgets on a daily, seasonal and annual basis. Apart from alkali- and alkaline earth metals; Fe, Al, Mn, Zn and Ni were found to be the most abundant metals in the runoff. Element concentrations were highly correlated and suggest that the processes of sulphide oxidation, ion exchange and silicate weathering occurring within the waste pile were linked throughout the measuring period. Observed pH values varied from 2.8 to 5.2 and SO4 concentrations from 21 to 1463 mg L−1. Manganese and Al concentrations were observed above phytotoxic levels (up to 4 and 23 mg L−1, respectively) and were considered the most critical elements in terms of environmental impact. Throughout the summer a total dissolved quantity of 58 kg Mn, 238 kg Al and 13,700 kg SO4 was released from the pile containing approximately 200,000 m3 of pyritic waste material (<1% FeS2). The highest concentrations of metals, lowest pH values and a very high daily release of H2SO4 (up to twice as high as the following month) were observed during the first week of thaw. This is considered a result of an accumulation of weathering products, generated within the waste pile during winter and released as a pollution-flush during early spring. Similar accumulation/flush sequences were observed later in the summer where rain events following relatively long dry periods caused high daily metal fluxes and on some occasions also elevated dissolved metal concentrations. Despite highly variable weather/climate conditions during the rest of the summer the investigated waste rock pile acted like a relative constant pollution-source during this period. Future investigations regarding the environmental impact of mine waste in the region should include measurements of bioavailable metals in order to provide further details on the seasonal trends in environmental impact.
Removal of arsenic from aqueous solution by natural siderite and hematite by Huaming Guo; Doris Stüben; Zolt Berner (1039-1051).
Batch and column experiments were conducted to examine the capability of naturally formed hematite and siderite to remove As from drinking water. Results show that both minerals were able to remove As from aqueous solutions, but with different efficiencies. In general, each material removed arsenate much more efficiently than As–DMA (dimethylarsinic acid), with the lowest adsorption efficiency for arsenite. The best removal efficiency for As species was obtained using a hematite, with a grain size range between 0.25 and 0.50 mm. The adsorption capacity for inorganic As(V) reached 202 μg/g. The pH generally had a great impact on the arsenate removal by the Fe minerals studied, while arsenite removal was slightly dependent on the initial pH of between 3 and 10. The presence of phosphate always had a negative effect on arsenate adsorption, due to competitive adsorption between them. A column packed with hematite in the upper half and siderite in the lower half with a grain size range of 0.25–0.5 mm proved to be an efficient reactive filter for the removal of all As species, causing a decrease in As concentration from 500 μg/L (including 200 μg/L As(V) as arsenate, 200 μg/L As(III) as arsenite and 100 μg/L As(V) as DMA) to less than 10 μg/L after 1055 pore volumes of water were filtered at a flow rate of 0.51 mL/min. After 2340 pore volumes passed through the column filter, the total inorganic As in the effluent was less than 5 μg/L. The total As load in the column filter was estimated to be 0.164 mg/g. Results of μ-synchrotron X-ray fluorescence analysis (μ-XRFA) suggest that coatings of fresh Fe(III) oxides, formed on the surface of the siderite grains after two weeks of operation, greatly increased the adsorption capacity of the filling material towards As.
New isotopic evidence for the origin of groundwater from the Nubian Sandstone Aquifer in the Negev, Israel by Avner Vengosh; Sharona Hening; Jiwchar Ganor; Bernhard Mayer; Constanze E. Weyhenmeyer; Thomas D. Bullen; Adina Paytan (1052-1073).
The geochemistry and isotopic composition (H, O, S, Osulfate, C, Sr) of groundwater from the Nubian Sandstone (Kurnub Group) aquifer in the Negev, Israel, were investigated in an attempt to reconstruct the origin of the water and solutes, evaluate modes of water–rock interactions, and determine mean residence times of the water. The results indicate multiple recharge events into the Nubian sandstone aquifer characterized by distinctive isotope signatures and deuterium excess values. In the northeastern Negev, groundwater was identified with deuterium excess values of ∼16‰, which suggests local recharge via unconfined areas of the aquifer in the Negev anticline systems. The δ 18OH2O and δ 2H values (−6.5‰ and −35.4‰) of this groundwater are higher than those of groundwater in the Sinai Peninsula and southern Arava valley (−7.5‰ and −48.3‰) that likewise have lower deuterium excess values of ∼10‰. Based on the geochemical differences between groundwater in the unconfined and confined zones of the aquifer, a conceptual geochemical model for the evolution of the groundwater in the Nubian sandstone aquifer has been reconstructed. The isotopic composition of shallow groundwater from the unconfined zone indicates that during recharge oxidation of pyrite to SO4 (δ 34SSO4 ∼−13‰; δ 18OSO4 ∼+7.7‰) and dissolution of CaCO3 (87Sr/86Sr ∼0.70787; δ 13CDIC = −3.7‰) occur. In the confined zone of the aquifer, bacterial SO4 reduction removes a significant part of dissolved SO 4 2 - , thereby modifying its isotopic composition (δ 34SSO4 ∼−2‰; δ 18OSO4 ∼+8.5‰) and liberating dissolved inorganic C that contains little or no radiocarbon (14C-free) with low δ 13CDIC values (<−12‰). In addition to local recharge, the Sr and S isotopic data revealed contribution of external groundwater sources to the Nubian Sandstone aquifer, resulting in further modifications of the groundwater chemical and isotopic signatures. In the northeastern Negev, it is shown that SO4-rich groundwater from the underlying Jurassic aquifer contributes significantly to the salt budget of the Nubian Sandstone aquifer. The unique chemical and isotopic composition of the Jurassic groundwater (δ 34SSO4 ∼ +14‰; δ 18OSO4 ∼ 14‰; 87Sr/86Sr ∼0.70764) is interpreted as reflecting dissolution of Late Triassic marine gypsum deposits. In the southern Arava Valley the authors postulate that SO4-rich groundwater with distinctively high Br/Cl (3 × 10−3) low 87Sr/86Sr (0.70734), and high δ 34SSO4 values (+15‰) is derived from mixing with underlying brines from the Paleozoic units. The radiocarbon measurements reveal low 14C activities (0.2–5.8 pmc) in both the northeastern Negev and southern Arava Valley. Taking into account dissolution of carbonate rocks and bacterial SO4 reduction in the unconfined area, estimated mean residence times of groundwater in the confined zone in the northeastern Negev are on the order of 21–38 ka, which suggests recharge predominantly during the last glacial period. The 14C signal in groundwater from the southern Arava Valley is equally low but due to evidence for mixing with external water sources the residence time estimates are questionable.
Arsenic in groundwater from mineralised Proterozoic basement rocks of Burkina Faso by P.L. Smedley; J. Knudsen; D. Maiga (1074-1092).
This study describes the hydrogeochemistry and distributions of As in groundwater from a newly investigated area of Burkina Faso. Groundwaters have been sampled from hand-pumped boreholes and dug wells close to the town of Ouahigouya in northern Burkina Faso. Although most analysed groundwaters have As concentrations of less than 10 μg L−1, they have a large range from <0.5 to 1630 μg L−1. The highest concentrations are found in borehole waters; all dug wells analysed in this study have As concentrations of <10 μg L−1. Skin disorders (melanosis, keratosis and more rare skin tumour) have been identified among the populations in three villages in northern Burkina Faso, two within the study area. Although detailed epidemiological studies have not been carried out, similarities with documented symptoms in other parts of the world suggest that these are likely to be linked to high concentrations of As in drinking water. The high-As groundwaters observed derive from zones of Au mineralisation in Birimian (Lower Proterozoic) volcano-sedimentary rocks, the Au occurring in vein structures along with quartz and altered sulphide minerals (pyrite, chalcopyrite, arsenopyrite). However, the spatial variability in As concentrations in the mineralised zones is large and the degree of testing both laterally and with depth so far is limited. Hence, concentrations are difficult to predict on a local scale. From available data, the groundwater appears to be mainly oxic and the dissolved As occurs almost entirely as As(V) although concentrations are highest in groundwaters with dissolved-O2 concentrations <2 mg L−1. The source is likely to be the oxidised sulphide minerals and secondary Fe oxides in the mineralised zones. Positive correlations are observed between dissolved As and both Mo and W which are also believed to be derived from ore minerals and oxides in the mineralised zones. The discovery of high As concentrations in some groundwaters from the Birimian rocks of northern Burkina Faso reiterates the need for reconnaissance surveys in mineralised areas of crystalline basement.