Applied Geochemistry (v.19, #9)

Hydrogeochemical assessment of 40 saline waters and brines from 20 locations within the lower (southern) and middle regions of the Benue-Trough, Nigeria are presented and discussed in terms of genesis of the primary salinity and subsequent hydrochemical evolution. The total dissolved ions range from 5263 to 88,800 mg/L and 5148 to 47,145 mg/L in the lower and middle region, respectively.The saline waters and brines are characteristically Na–Cl type enriched in Ca and Sr on the one hand and depleted in Mg and SO4 on the other, relative to the seawater evaporation trend. Ionic ratios, Na–Cl–Br systematic and divalent cations suggest two likely sources of primary salinity: a fossil seawater source and dissolution of halite. However, water–rock interaction involving Mg uptake by clay minerals and possibly dolomitization during diagenesis appear to be responsible for further modification of the primary chemistry. A conceptualized hydrogeological/flow model for the brines is presented.

Engelmann spruce (Picea engelmannii) is the dominant tree species in many abandoned mine areas of the Rocky Mountains. It is long-lived, and therefore, may act as a long term biological monitor of changes in soil chemistry caused by past mining activity. In this study, laser ablation inductively coupled mass spectrometry (LA-ICPMS) was used to analyze individual tree rings of Engelmann spruce for Fe, Zn, Cu, Cd, Mn, Pb and Sr concentrations. Cores were obtained from trees growing in tailings-impacted and control (non-tailings impacted) sites near the Waldorf mine (Waldorf, CO, USA). Zinc, Cu, Fe, Cd, Pb and Sr concentrations remained low and consistent over time in the control tree rings. However, in the tailings impacted cores, concentrations of Zn, Cu, Fe and Cd increase significantly in post-mining rings. In addition, Zn, Cu, Fe, and Cd concentrations in pre-mining rings of both the control and tailings impacted cores are similar, indicating that present day soil concentrations of these elements in the control area are a reasonable estimation of background for this area. Lead and Sr concentrations in control and tailings-impacted rings remained similar and relatively constant through time and are not useful in determining changes in soil chemistry due to past mining activity.

Control mechanisms for dissolved phosphorus and arsenic in a shallow lake by Kathryn L Linge; Carolyn E Oldham (1377-1389).
This study investigated P and As sediment remobilisation in Lake Yangebup, a shallow lake with an overlying floc layer that covers the consolidated sediment. This floc is frequently resuspended into the water column, a process that was postulated to produce high P and As lakewater concentrations. Rate investigations using deionised water showed that P and As remobilisation reached steady state after 20 h in the consolidated sediment and within 1 h for the floc. Floc resuspension in lakewater showed no net release of either P and As, indicating that the floc was in constant equilibrium with the water column. A protocol to distinguish between desorption and dissolution was applied to both sediments and the response of remobilisation to varying slurry density and As addition measured. For the consolidated sediment, the concentration of Fe(II), P and As were unaffected above a slurry density ∼30 g L−1 and added arsenate (10–100 μg L−1) did not significantly change As and P remobilisation. It is shown that these results do not fit an adsorption/desorption equilibrium formulation for P and As remobilisation. Instead, the evidence suggests that the solubility of a thin, non-stoichiometric FeP x FeAs y oxyhydroxide surface coating determined the remobilisation process. Data scatter lead to some uncertainty in the floc results but suggest that dissolved P is controlled by dissolution, while dissolved As is controlled by adsorption/desorption. The results conclusively show that P and As remobilisation was lower from the floc than from the consolidated sediment and that the removal of the floc would not lower P and As lakewater concentrations. Implications of these results for the management of As in Lake Yangebup are outlined.

Colloid formation in groundwater by subsurface aeration: characterisation of the geo-colloids and their counterparts by Anke Wolthoorn; Erwin J.M Temminghoff; Willem H van Riemsdijk (1391-1402).
Subsurface aeration is used to oxidise Fe in situ in groundwater to make the water potable. In a groundwater system with pH > 7, subsurface aeration results in a non-mobile Fe precipitate and mobile Fe colloids. Since originally the goal of subsurface aeration is to remove Fe in situ, the formation of non-mobile Fe precipitate is the desired result. In addition to this intended effect, subsurface aeration may also strongly enhance the microbiological removal of NH4 in the purification station. A hypothesis is that mobile Fe colloids may be the link between subsurface aeration and the positive effect on the microbiological removal of NH4. The objective of this study is to characterise the mobile Fe colloids and to derive a synthetic substitute for the naturally formed Fe colloids in order to be able to apply the Fe colloids as a management tool to enhance the removal of NH4 in the process of producing drinking water from groundwater. At a purification station in The Netherlands natural Fe colloids from an aerated well were sampled. Furthermore, eight synthetic Fe colloids were prepared by oxidising synthetic solutions differing in elemental composition. The colloids were analysed using chemical analysis and electron microscopy (SEM and SEM-EDAX). The Fe colloids sampled in the field contained Fe, Ca, Na, PO4 and Mn. Also in the synthetic Fe colloids PO4, Ca, Na and Mn were the most important elements next to Fe. Phosphate and dissolved organic C strongly influenced the morphology of the synthetic Fe colloids. When both the elemental composition and the morphology of the Fe colloids are taken into account, the synthetic Fe colloids formed in the synthetic solution containing Fe, Mn, PO4, SiO4 and dissolved organic matter best match the Fe colloids from the field.

Formation of secondary Fe-oxyhydroxide phases during the dissolution of chlorite – effects on uranium sorption by E Krawczyk-Bärsch; T Arnold; H Reuther; F Brandt; D Bosbach; G Bernhard (1403-1412).
The formation of Fe-oxyhydroxide colloids resulting from the simulated weathering of chlorite (CCa-2) was studied in batch experiments. The detected colloids showed a particle diameter of 30–80 nm and were composed of Fe and O. On the basis of their chemistry and their appearance, they were identified as ferrihydrite colloids. Mössbauer spectroscopy measurements were used to quantify the increase in Fe(III) which formed during the batch experiment, i.e. the chlorite platelets in contact with water for 2 months. The detected increase in Fe(III) was attributed to the formation of the newly formed ferrihydrite. Further SEM investigations revealed that the colloids were preferentially attached to the {h k 0} edge surfaces and only to a minor extent to the {0 0 1} basal plane surfaces as isolated Fe-colloids.Additional sorption experiments were conducted to study the effect of newly forming secondary mineral phases on the sorption behavior of U(VI) on chlorite. Two different chlorites with different tendencies for forming secondary phases were studied. The first one was a hydrothermally altered chlorite (Grimsel chlorite) and the second one was an unaltered chlorite (CCa-2 chlorite). The Grimsel chlorite was found to be more resistant to weathering than the CCa-2 chlorite which tends to form a secondary phase as a result of weathering reactions. The U(VI) sorption on the CCa-2 chlorite showed a maximum at pH 6.3–7.5 with up to 87.9 % of the initially added U (1 × 10−6 M U(VI)) adsorbed. In contrast the Grimsel chlorite showed a sorption maximum at pH 5.8–6.9 with a maximum of 70% U(VI) adsorbed. This greater sorption capacity of the CCa-2 chlorite was attributed to newly forming secondary phases, in particular Fe-oxyhydroxides, emerging during the sorption experiments as a result of weathering reactions. Since adsorbed ferrihydrite particles are well-known environmental sinks for migrating heavy metals, they may enhance the adsorption capacity of the chlorite, and the U transport may thus be significantly retarded. On the other hand, mobile ferrihydrite colloids, which are also a product of chlorite weathering, may enhance U migration.

In Bangladesh, concentrations of boron in groundwater reach 2.1 mg L−1and are high regionally in alluvial aquifers of Late Pleistocene/Holocene age. Concentrations exceed 0.5 mg L−1 across approximately 6700 km2 of the deep aquifer (>150 m depth) and 3000 km2 of the shallow aquifer (<150 m depth). In the Carboniferous sandstone and shale aquifers in Ingham County, Michigan, concentrations reach 6.1 mg L−1 and high values are widespread in the NE of the county. These concentrations exceed the regulatory guideline values for human consumption of 0.5 mg L−1 (WHO) or 0.9 mg L−1 (USA). The boron has desorbed from mineral surfaces as freshwater flushing displaces saline waters from the aquifers. In Bangladesh, desorption is driven by decreasing ionic strength, the equilibrium re-adjustment of mineral sorption sites to the low boron concentration in freshwater, and competitive exchange with HCO3/CO3. In deep Bangladesh aquifers in Barisal District (>250 m deep), boron enrichment delineates a buried estuary marking a previous course of the Ganges and/or Brahmaputra rivers. Boron enrichment is accompanied by ion-exchange that depletes Ca and enriches Na in the flushing freshwater. The patterns of enrichment and depletion indicate the direction of water quality change, in terms of salinization or freshening, with greater sensitivity than absolute chemical parameters.

Modelling the interaction of hyperalkaline fluids with simplified rock mineral assemblages by A.R Hoch; C.M Linklater; D.J Noy; W.R Rodwell (1431-1451).
Designs for geological disposal facilities for radioactive wastes often envisage the extensive use of cementitious materials. After closure, the repository will saturate with groundwater and cement porewater will migrate into the geosphere to form an alkaline (pH 12.5–13) plume. This plume will react with the components of the surrounding host rock leading to mineralogical, chemical and physical changes which will be complex in nature. Coupled computer models of geochemistry and hydrogeological transport will be needed to scope these changes. In the recent past, a series of laboratory column experiments were carried out by the British Geological Survey in order to test the capabilities of coupled models to predict the evolution of outflow fluid compositions and product solids. These experiments reacted single minerals (i.e., quartz, albite, calcite and muscovite/quartz) and potential host rock lithologies (i.e., Borrowdale Volcanic Group fault rock, Äspö granite and Wellenberg marl) with cement pore fluids. The objectives of the present work were to develop the understanding of these experiments and to improve simulations of the data by computer models. To this end, a systematic approach was adopted in which dissolution rates for individual minerals were calibrated to the results of the single mineral columns and the resulting parameter values applied to the more complex columns. It was found that for the systems studied, reasonable agreement between the experimental data and the calculated results could be obtained. However, the calibrated dissolution rate constants used in the models first vary between experiments by up to half an order of magnitude and secondly differ from literature values by up to one order of magnitude, with the calibrated values being generally slower than those found in the literature.

Geochemical mapping in Hokuriku, Japan: influence of surface geology, mineral occurrences and mass movement from terrestrial to marine environments by Atsuyuki Ohta; Noboru Imai; Shigeru Terashima; Yoshiko Tachibana; Ken Ikehara; Takeshi Nakajima (1453-1469).
The authors have created terrestrial and marine geochemical maps of the Hokuriku region, Japan, and examined the background distribution of elemental concentrations. The terrestrial geochemical maps strongly reflect the surface geology and mineral occurrences. An analysis of variance (ANOVA) reveals the relationships between the distribution of surface geology and geochemical maps. Granite, metamorphic and felsic volcanic rocks, and sedimentary rock in accretionary complexes give rise to high concentrations of alkali metal, Be, Y, Ba, REE (except Eu), Tl, Th and U. The distributions of MgO, Al2O3, P2O5, CaO, 3d transition metals (except Cu), Ga, Sr and Eu are controlled by mafic volcanic and ultramafic rocks. The watersheds with above threshold levels for Cu, Zn, As, Cd, Mo, Sn, Sb, Hg, Pb and Bi strongly relate to the mineral occurrences, but ANOVA demonstrated that they are partly affected by surface geology.The marine geochemical maps exhibit control by 3 factors: calcareous sediments on the continental shelf, clay minerals on the continental slope and ocean floor, and sediments supplied from rivers. Many elemental concentrations are quite low in continental shelf sediment because of dilution by calcareous materials. The continental slope and deep valley sediments are rich in most elements except for MgO, CaO and Sr. Coarse sediments supplied from large rivers contribute to local enrichments of several elements such as K2O, Cr and Zn. The spatial distribution patterns of Cr and Zn concentrations show that gravity moves the river sediments over 60 km offshore along a deep-valley.

Ground-water chemistry data from coastal plain environments have been examined to determine the geochemical conditions and processes that occur in these areas and assess their implications for aquifer susceptibility. Two distinct geochemical environments were studied to represent a range of conditions: an inner coastal plain setting having more well-drained soils and lower organic carbon (C) content and an outer coastal plain environment that has more poorly drained soils and high organic C content. Higher concentrations of most major ions and dissolved inorganic and organic C in the outer coastal plain setting indicate a greater degree of mineral dissolution and organic matter oxidation. Accordingly, outer coastal plain waters are more reducing than inner coastal plain waters. Low dissolved oxygen (O2) and nitrate (NO3 ) concentrations and high iron (Fe) concentrations indicate that ferric iron (Fe (III)) is an important electron acceptor in this setting, while dissolved O2 is the most common terminal electron acceptor in the inner coastal plain setting.The presence of a wide range of redox conditions in the shallow aquifer system examined here underscores the importance of providing a detailed geochemical characterization of ground water when assessing the intrinsic susceptibility of coastal plain settings. The greater prevalence of aerobic conditions in the inner coastal plain setting makes this region more susceptible to contamination by constituents that are more stable under these conditions and is consistent with the significantly (p<0.05) higher concentrations of NO3 found in this setting. Herbicides and their transformation products were frequently detected (36% of wells sampled), however concentrations were typically low (<0.1 μg/L). Shallow water table depths often found in coastal plain settings may result in an increased risk of the detection of pesticides (e.g., alachlor) that degrade rapidly in the unsaturated zone.

Biogeochemistry and natural attenuation of nitrate in groundwater at an explosives test facility by Harry R. Beller; Vic Madrid; G. Bryant Hudson; Walt W. McNab; Tina Carlsen (1483-1494).
An interdisciplinary study was conducted to characterize the distribution and fate of NO3 in groundwater at Lawrence Livermore National Laboratory (LLNL) Site 300, a high-explosives test facility in the semi-arid Altamont Hills of California. Site 300 groundwater contains NO3 concentrations ranging from <0.5 to >200 mg NO3 /L. Several lines of evidence strongly suggest that denitrification is naturally attenuating NO3 in the confined, O2-depleted region of the bedrock aquifer under study (Tnbs2): (a) both NO3 and dissolved O2(DO) concentrations in groundwater decrease dramatically as groundwater flows from unconfined to confined aquifer conditions, (b) stable isotope signatures (i.e., δ 15N and δ 18O) of groundwater NO3 indicate a trend of isotopic enrichment that is characteristic of denitrification, and (c) dissolved N2 gas, the product of denitrification, was highly elevated in NO3 -depleted groundwater in the confined region of the Tnbs2 aquifer. Long-term NO3 concentrations were relatively high and constant in recharge-area monitoring wells (typically 70–100 mg NO3 /L) and relatively low and constant in the downgradient confined region (typically <0.1–3 mg NO3 /L), suggesting a balance between rates of NO3 loading and removal by denitrification. Chemolithoautotrophic denitrification with pyrite as the electron donor is plausible in the Tnbs2 aquifer, based on the low dissolved organic C concentrations (<1.5 mg/L) that could not support heterotrophic denitrification, the common occurrence of disseminated pyrite in the aquifer, and the trend of increasing SO2− 4 as groundwater flows from aerobic, unconfined to anoxic, confined aquifer conditions. Nitrate sources were investigated by experimentally determining the δ15N and δ18O signatures of NO3 from three potential anthropogenic sources of NO3 at Site 300: Ba(NO3)2 (mock explosive), HNO3, and photolysis of the explosive RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine). The isotopic signatures of these potential NO3 sources were markedly different than those of NO3 in Tnbs2 groundwater samples, suggesting that other sources must contribute significantly to the NO3 loading at Site 300. In particular, NO3 and NO2 resulting from RDX photolysis reflected dramatically depleted δ15N (ca. −7.4‰) and δ18O (ca. −25.7‰) values.

Distribution and mineralogical controls on ammonium in deep groundwaters by David A.C Manning; Ian E Hutcheon (1495-1503).
Compositional data from published sources, environmental monitoring and new analyses demonstrate that for a wide range of water types (oilfield water, coal mine water, landfill leachate) NH4 + is present in amounts up to 2200 mg/L. Oilfield waters from Alberta, Canada contain 1–1000 mg/L NH4 +, coal mine water (UK) surface discharges 1–45 mg/L NH4 +, and landfill leachates (UK) up to 2200 mg/L NH4 +. Ammonium contents generally show a positive correlation with K, and increase with increasing salinity. Geochemical modelling of sufficiently complete data using SOLMINEQ88 demonstrates that NH4 + activities vary systematically, and are consistent with a mineralogical control. Sodium–K exchange divides the entire sample suite into at least 4 groups, controlled by reaction temperature and reaction with either albite/K-feldspar or illitic clay minerals. In contrast, comparison of NH4 + and K divides the sample suite into 2 groups. On the basis of geological setting, these correspond to K–NH4 + exchange involving illitic (illite-muscovite) clays (and possibly feldspars) for samples from natural sources, and to exchange involving smectitic clays for samples from landfill sites. This study demonstrates the importance of NH4 + as a constituent of natural groundwaters, requiring that this reservoir of N is taken into account in detailed discussion of hydrological components of the N cycle.