Applied Geochemistry (v.17, #10)

Assessing mine drainage pH from the color and spectral reflectance of chemical precipitates by David J. Williams; Jerry M. Bigham; Charles A. Cravotta III; Sam J. Trainaa ; John E. Anderson; John G. Lyon (1273-1286).
The pH of mine impacted waters was estimated from the spectral reflectance of resident sediments composed mostly of chemical precipitates. Mine drainage sediments were collected from sites in the Anthracite Region of eastern Pennsylvania, representing acid to near neutral pH. Sediments occurring in acidic waters contained primarily schwertmannite and goethite while near neutral waters produced ferrihydrite. The minerals comprising the sediments occurring at each pH mode were spectrally separable. Spectral angle difference mapping was used to correlate sediment color with stream water pH (r 2=0.76). Band-center and band-depth analysis of spectral absorption features were also used to discriminate ferrihydrite and goethite and/or schwertmannite by analyzing the 4T16A1 crystal field transition (900–1000 nm). The presence of these minerals accurately predicted stream water pH (r 2=0.87) and provided a qualitative estimate of dissolved SO4 concentrations. Spectral analysis results were used to analyze airborne digital multispectral video (DMSV) imagery for several sites in the region. The high spatial resolution of the DMSV sensor allowed for precise mapping of the mine drainage sediments. The results from this study indicate that airborne and space-borne imaging spectrometers may be used to accurately classify streams impacted by acid vs. neutral-to-alkaline mine drainage after appropriate spectral libraries are developed.

Many countries are considering options for long-term management of nuclear waste. One common aspect among deep geological disposal options in granitic host rock is the use of clay-based buffer materials to limit radionuclide migration in case of container failure. The isothermal test (ITT) involved placing ∼2.4 m3 of clay-based buffer in a borehole at the 240 m level of AECL's Underground Research Laboratory to study the response of buffer to resaturation by groundwater over a 6.5-year period. Results are reported here on measurements taken at the end of the test for microbial, redox and organic characterization of the buffer. Results from enumerations and biomass determinations suggested that the viable population of cells in the buffer was several orders of magnitude larger than could be cultured. It is postulated that, due to the constrictive and nutrient-poor buffer environment, viable and active cells became stressed during burial and lost activity and culturability but not viability. Culturable microbial populations at interfaces in the ITT were about an order of magnitude larger than in comparable bulk buffer samples, suggesting that interfaces may be preferred sites for microbial activity and transport. The presence of culturable SO4-reducing bacteria and an increase in solid sulphide concentrations in the buffer suggested SO4 reduction, which appeared to be very variable locally. Only about 0.02–0.5% of SO4 was converted to sulphide, suggesting that SO4 reduction was not (yet) a dominant process. No methanogens could be enumerated from the ITT, and phospholipid fatty acid (PLFA) profiles did not suggest their presence. Gas analysis of samples recovered from the ITT suggested some reduction in O2 near the top of the experiment, but deeper samples did not show a significant decrease in O2 and had only a small increase in CH4 and H2 levels. This suggested that microbial processes were depressed in the buffer but may have been more active near the concrete/buffer interface. The suggestion of low microbial activity in the buffer was corroborated by the results from the PLFA analysis, which indicated low biomass turnover rates and starvation biomarkers. The combination of enumerations, PLFA and gas analysis results suggested that no significant evolution towards reducing conditions occurred during the duration of the ITT. Fulvic acids made up the largest fraction of water-leachable humic substances but accounted for only about 2% of the total C inventory of the buffer material. The complexing capacity of these humic substances, based on carboxylic functional groups, ranged from 24 to 32 meq/g dissolved organic C. This may provide buffer porewater with considerable complexing capacity for radionuclides.

A study on the effects of drying conditions on the stability of NaNd(CO3)2·6H2O and NaEu(CO3)2·6H2O by Craig Fannin; Robert Edwards; Jack Pearce; Eugene Kelly (1305-1312).
The effect of different drying conditions on the stability of NaNd(CO3)·6H2O and NaEu(CO3)·6H2O and the identity of the decomposition product have been investigated. The rate of decomposition and the nature of the altered phases are dependant on the drying conditions used. When the phases are oven dried at 120 °C, the decomposition is immediate and the phase completely alters to Nd2(CO3)3 or Eu2(CO3)3 respectively. Under less severe drying conditions, the Na rare earth carbonate phases alter to Nd2(CO3)3·8H2O and Eu2(CO3)3·8H2O over a period of 24–48 h, but they can be kept indefinitely in a water saturated environment. The implications for using Nd and Eu as actinide analogues are discussed.

Hectometer wide cryptokarsts in Paleozoic limestone from Southern Belgium have been studied, to determine to what extent U, Th, Pb and rare earth elements (REE) have been mobilized in the karst sedimentary filling, during a Miocene weathering event. The weathering process resulted in the massive halloysite/kaolinite formation at the karst wall. As with most fossil systems, data on weathering fluid chemistry are lacking, hence it is difficult to quantify relevant parameters such as pH, Eh, and to address solution chemistry. However, on the basis of both field studies of more recent systems, and of geochemical modeling, it is proposed that moderately acid fluids percolated through a multi-layer sedimentary filling, in near-surface conditions and in a temperate/warm climate. Special attention is paid to the trace element immobilization/trapping processes, in newly crystallized REE phosphates, at the karst wall. Analytical methods used include major/trace element geochemistry (emission ICP, ICP–MS) and mineralogy (XRD, SEM, TEM, microprobe). The results suggest that both the sandy sediments that are in contact with the karst carbonate wall, and the carbonate wall itself acted as a kind of geochemical “barrier”. Mineralization cells settled there, at the decimeter to meter scale. This results in sequential trace element (Pb, Th, REE, U) trapping, according to the affinity of these elements for the aqueous solution. At the end of the sequence, minute U-rich automorphic (Ce, Nd) monazite crystals (from 3 nm upwards) formed on kaolinite flakes. Though the analogy between the studied cryptokarst and planned surface-based repositories for low-level radioactive waste (LLW) in argillaceous context is far from complete, the results outlined here are relevant because they show that even in natural—i.e. intrinsically uncontrolled and unmonitored—systems, “pollutant” radionuclide (U, Th, REE, Pb) migration paths are often limited in space. Various processes converge towards trapping of these elements, that are present in the radioactive waste.

Fluid mixing in carbonate aquifers near Rapolano (central Italy): chemical and isotopic constraints by A Minissale; O Vaselli; F Tassi; G Magro; G.P Grechi (1329-1342).
Chemical (major and trace elements) and isotopic compositions (δD and δ18O in waters and δ13C in CO2 and 3He/4He in gases) of natural thermal (11) and cold (39) fluids (spring waters and gases) discharging from a tectonic window of Mesozoic limestones in central Italy, have proved to be the result of mixing processes inside the limestone formations. The limestones provide a preferential route for subsurface fluid migration and they gather both descending cold, Ca-HCO3, B-depleted groundwaters and rising convective Ca-SO4(HCO3), CO2-saturated, B-rich thermal waters. Atmospherically-derived descending gas components (N2, Ne, He), dissolved in rainfall that infiltrates the limestone system mix with N2, Ne, He-depleted hot rising waters. Boron in the liquid phase and N2 and Ne in the gas phase are the most useful elements to trace the mixing process. The deeper gas samples recognised in the area are associated with the hotter waters emerging in the area. In spite of being depleted in Ne and He and light hydrocarbons they have the higher measured 3He/4He ratios, suggesting a contribution of mantle 3He to the gas phase. This contrasts with deep circulation in the crust which would lead to increased concentration of 4He in the deeper gases. Paradoxically, there is more relative concentration of 4He in the more air-contaminated gas samples than in the deeper gas samples. A similar paradox exists when the δ13C of CO2 in the deeper gas samples is considered. The shallower air-contaminated gas samples, although they should be affected by the addition of soil-13C depleted organic C, have δ13C in CO2 more positive than the deeper gas samples recognized. Since any deep hydrothermal source of CO2 should generate CO2 with more positive values of δ13C than those measured at surface, a multiple (single) calcite precipitation process from hydrothermal solutions, with C isotopic fractionation along the rising path inside the Mesozoic limestone formations, is proposed.

Major ion chemistry of groundwaters in the Continental Terminal water table of southwestern Niger (Africa) by Françoise Elbaz-Poulichet; Guillaume Favreau; Christian Leduc; Jean Luc Seidel (1343-1349).
Numerous chemical analyses undertaken during the last 4 a in the Niamey area of the Continental Terminal groundwaters (SW Niger, Africa) reveal large seasonal variations and high NO3 content (up to 2.87 meq/l). The positive correlation between conductivity and depth to the water table indicates that the chemical change is associated with the recharge. In the shallow wells, this highly mineralised recharge reflects the rain composition, variably concentrated by evaporation processes.

The aim of this experimental study was to investigate the usefulness of non-radioactive mono-isotopic metal tracer “spikes” for studying metallic contamination in the environment. These spikes are commonly used in Earth Sciences. Because of their unique non-natural isotopic composition, these tracers are readily distinguishable from the metals initially present in an organism and hence, the evolution of both the “natural” and the “added” metals can be quantified during the same measurement by ICP-MS. The concentrations used in this study allow us to simulate conditions similar to those found in nature, but without the potential dangers associated with the use of radioactive tracers. Mussels Mytilus galloprovincialis, commonly used in environmental surveys, were collected in a lagoon (Thau, S. France) and kept for 14 days in clean seawater tanks, to which 66Zn (20 μg l−1), 111Cd (1 μg l−1), 207Pb (1 μg l−1) tracers were added. The concentrations of these metals and their isotopic compositions were regularly analyzed in the mantle, gills and digestive gland of the mussels. Even though the tracer concentrations were relatively low in the tanks, a significant uptake by the 3 organs was observed from the first day. The dynamics of this uptake are similar in all 3 organs but specific for each metal. The spikes are found to be superimposed on metals initially present and could be stored or excreted by the mussels. At the end of the experiment, the tracer concentrations reached 17% of the total Zn, 67% of the Cd, and 36-58% of Pb in the studied organisms. The differing reponse of the mussels to each metal contamination suggests the possibility of using various biomonitoring strategies in environmental surveys.

Sulfur hexafluoride was injected as a tracer gas into the air stream during air-drilling of a borehole in the unsaturated zone of a partially-welded, fractured tuff at Apache Leap, Arizona. One-meter intervals were later isolated at multiple depths and pumped to purge drilling air from each interval. The volume of air purged (at 1 atm, 20 °C), ranged from a low of 0.3 m3 in an unfractured interval, to a high of 252 m3 in a highly fractured interval. The concentration of SF6 remained high throughout the purge in all intervals and often increased over time. Measurements of δ13C, 14C and CO2 concentration indicated that atmospheric air was eventually drawn into several of the intervals in spite of the fact that SF6 concentrations remained high. Possible explanations include mixing of atmospheric air drawn through fractures with partially-purged matrix air, and delayed removal of SF6 relative to atmospheric gases due to adsorption of SF6 within the tuff matrix, dissolution into pore water, or diffusion from dead-end pores with restricted openings. In this system, following a long delay between drilling and purging, it was found that the risk of contamination from surface air by over-purging was substantially greater than the risk of contamination from residual drilling air by under-purging.