Applied Geochemistry (v.18, #7)

This review highlights the major progress over the last decade on characterization of geochemically heterogeneous soil/sediment organic matter (SOM) and the impacts of SOM heterogeneity on sorption and desorption of hydrophobic organic contaminants (HOCs) under equilibrium and rate limiting conditions. Sorption and desorption by soils and sediments are fundamental processes controlling fate and transport of less polar and nonpolar organic pollutants in surface aquatic and groundwater systems. Recent studies have shown that soils and sediments exhibit an array of HOC sorption phenomena that are inconsistent with an early partition model based on an assumption of homogeneous gel-like SOM. Increasing data have revealed that isotherm nonlinearity, varied sorption capacity, sorption–desorption hysteresis, and slow rates of sorption and desorption are characteristics for HOC sorption by soils and sediments. These phenomena have been shown to result from different types of condensed SOM that exhibit capacity limiting sorption processes. Recent findings of glass transition phenomena and the nonlinear HOC sorption by humic acids provide a scientific foundation for drawing an analogy between humic acids and synthetic organic polymers that supports a dual mode model for sorption by soils and sediments. Humic acid is glassy or rigid at temperatures lower than its glass transition temperature and exhibits relatively nonlinear sorption isotherms for HOCs. Fractionation and quantification of SOM indicate that soils and sediments contain significant amounts of black carbon and kerogen of different origins. These particulate organic materials have rigid 3-dimensional structures and are often less polar compared to humic substances. Limited studies show that black carbon and kerogen exhibit nonlinear sorption for HOCs and may dominate the overall nonlinear sorption by soils and sediments.

Metals in the sediments of Ensenada de San Simón (inner Rı́a de Vigo), Galicia, NW Spain by Graham Evans; Richard J Howarth; M.A Nombela (973-996).
The Ensenada de San Simón is the inner part of the Rı́a de Vigo, one of the major mesotidal rı́as of the Galician coast, NW Spain. The geochemistry of its surface sediments, and the river sediments which drain into it from a granitic and metamorphic hinterland, are described. Multivariate statistical analysis of the sediment compositions (using ridge regression and mixture-modelling) enabled the major- and trace-element distributions to be accounted for in terms of both natural and anthropogenic sources: Between 60 and 80% of the Cr, Cu, Ni, Pb and Zn concentrations of the bottom sediments of the Ensenada can be explained by sediment input from the combined R. Oitaben and R. Verdugo, the R. Redondela and faecal matter from mussel rafts, but there is additional enrichment towards the mouth of the Ensenada. This enrichment is attributed to marine water entering the Ensenada from the polluted industrial areas of the adjacent Rı́a de Vigo. It is suggested that these metals are carried landwards in solution by the tidal incursion of marine water (the volume of which, on an annual basis, greatly exceeds that contributed by the rivers). Although the incoming marine waters may also be important in supplying Pb to the outer Ensenada, this element may also be delivered by land run-off, or by windborne vehicular emissions to the Ensenada as a result of the surrounding network of roads and a road bridge over the Estrecho de Rande.

Paleoclimatic interpretation of the past 30 ka from isotopic studies of the deep confined aquifer of the North China plain by Chen Zongyu; Qi Jixiang; Xu Jianming; Xu Jiaming; Ye Hao; Nan Yunju (997-1009).
The δ 18O and δD values in the deep confined aquifer beneath the North China Plain which is located at 112°30′E–119°30′E and 34°46′N–40°25′N, reflect differences in paleoclimatic conditions between the Holocene and the late Pleistocene. Groundwater samples whose 14C ages are between 12 and 25 ka B.P have ranges of −9.4 to −11.7‰ for δ 18O and −76‰ to −85‰ for δD values. These very negative δ 18O and δD values reflect the cold and arid climate in the last glacial period. The temperature estimated in this period is 6–9 °C cooler than that of the present. The entire ranges of δ 18O and δD values for samples with 14C dating from 7 ka B.P to present are −7.7‰ to −10.2‰ and −63‰ to −73‰, respectively. The greater δ 18O and δD enrichments of these samples indicate a period of relatively humid and warm climate in the Holocene. However, the wide ranges of δ 18O (−9.0‰ to −11.1‰) and δD (−66‰ to −80‰) values for samples with 14C age ranging from 12 to 7 ka B.P. imply an unstable climatic condition of rapidly increasing temperature, which marks the transition from the Pleistocene to the Holocene.

Diagenetic mobility of trace elements in sediments covered by a flash flood deposit: Mn, Fe and As by Alfonso Mucci; Bernard Boudreau; Constance Guignard (1011-1026).
The vertical distribution and concentrations of Fe, Mn, and As were measured in porewaters and sediments recovered with box-cores taken each year since the 1996 flood along the main axis of the Saguenay Fjord and in the Baie des Ha! Ha!. Time-series analyses reveal that Mn remobilization was not significantly affected by the accumulation of the flood deposit. Authigenic Mn oxi-hydroxides present at the original sediment–water interface and in the flood material were progressively reduced, and much of the Mn(II) diffused to the current interface, where it was oxidized and precipitated. First-order rate constants for the reduction and precipitation of the authigenic Mn oxi-hydroxides were estimated by fitting the time-series data to a diagenetic model. On the basis of these rate constants and given that the maximum reactive solid Mn remains concentrated in the thin, oxic layer near the sediment-water interface, an upper limit to the biodiffusion coefficient, D B, was also evaluated. In contrast to Mn, Fe associated with the authigenic oxi-hydroxides at the original interface was mostly trapped as acid volatile sulfides (i.e., AVS) under the SO4-reducing conditions established rapidly after the flood. Most of the As associated with the authigenic oxi-hydroxides at the original (buried) interface also appears to be trapped with the authigenic sulfides, thus limiting its diffusion through the flood deposit.

Evidence for chlorine recycling—hydrosphere, biosphere, atmosphere—in a forested wet zone on the Canadian Shield by G.M Milton; J.C.D Milton; S Schiff; P Cook; T.G Kotzer; L.D Cecil (1027-1042).
The ability to measure environmental levels of 36Cl by Accelerator Mass Spectrometry and 3H by 3He-ingrowth Mass Spectrometry has made it possible to use the pulses of these two isotopes released into the atmosphere during nuclear weapons testing as tracers of Cl and water movement in soils and groundwater. The authors have investigated the movement of these tracers below a forested wet zone, and have found that both are retarded to a differing extent in the near surface because of vegetative uptake and recycling. Adsorption by clay particles, followed by slow release to the groundwater, may also be significant. The data accumulated in this region of near vertical recharge have gone a considerable distance towards explaining the anomalously low concentrations of 36Cl measured in the 5 Laurentian Great Lakes, as well as indicating possible mechanisms for large scale Cl recycling in the atmosphere and biosphere. Identification of the near-term non-conservative behaviour of the Cl is significant, since such a phenomenon could introduce errors in many watershed calculations, e.g. water residence times, evaporation rates, and mixing calculations.

The Memphis aquifer in southwestern Tennessee is confined to a semi-confined unconsolidated sand aquifer and is the primary municipal water source in the Memphis metropolitan area. Past studies have identified regions in the metropolitan area in which the overlying upper Claiborne confining unit lacks significant clay and provides a hydraulic connection between the shallow aquifer and the Memphis aquifer. In this study, major solute chemistry, 3H, and 3H/3He groundwater dating are used to investigate the extent and chemical effects of leakage through the confining unit to the Memphis aquifer in the vicinity of a municipal well field. The 3H/3He dates and geochemical modeling of the chemical data are used to constrain mixing fractions and the timing of modern recharge. Tritium activities of as much as 2.8 TU are observed in shallow production wells, but deeper production wells have 3H activities that approach the detection limit. Trends in water chemistry indicate vertical mixing in the aquifer of shallow Na–SO4–Cl-rich water and deeper Ca–Mg–HCO3-rich water. Water chemistry does not vary consistently with seasonal pumping, but 3H activity generally decreases during low use periods. Stable O and H isotopes show little variation and are not useful groundwater tracers for this study. The 3H-bearing, Na–SO4–Cl-rich water is interpreted to reflect recharge of modern water through the upper Claiborne confining unit. The 3H/3He dates from 5 production wells indicate modern recharge, that infiltrated 15–20 a ago, is present in the shallow production wells. Geologic data and hydrologic boundary conditions suggest that the most likely source for continued leakage is a nearby stream, Nonconnah Creek. Geochemical reaction modeling using the NETPATH computer code suggests that proportions of shallow aquifer water leaking into the Memphis aquifer range from 6 to 32%. The 3H/3He dating and NETPATH modeling results correlate well, suggesting that these complementary analytical tools provide an effective means to evaluate proportions of modern water leaking into semi-confined aquifers. These results also indicate a need to carefully consider connections between surface water and semi-confined groundwater resources in wellhead protection programs.

The reactive behavior of supercritical CO2 under conditions relevant to geologic storage and sequestration of C is largely unknown. Experiments were conducted in a flexible cell hydrothermal apparatus to determine the extent of fluid–rock reactions, in addition to carbonate mineral precipitation, that may occur in a brine aquifer–aquitard system that simulates a saline aquifer storage scenario. The system was held at 200 °C and 200 bars for 59 days (1413 h) to approach steady state, then injected with CO2 and allowed to react for another 80 days (1924 h). In addition to magnesite precipitation, silicate minerals (quartz, plagioclase, microcline and biotite) in the aquifer and the aquitard display textures (etch pits, mineralization) indicating significant reaction. Changes in elemental abundances in the brine following addition of CO2 include pH decrease and enrichment in Cl, partly due to supercritical CO2 desiccation of the brine. Geologic sequestration systems have potential for geochemical reactions that extend beyond simple aqueous dissolution of CO2 and precipitation of carbonate. These reactions may produce geochemical and geotechnical consequences for sequestration and provide important characteristics for monitoring and evaluation of stored CO2. An understanding of multi-phase equilibrium relationships between supercritical CO2 and aquifer–brine systems also raises new questions for a variety of geologic systems. Multi-phase fluid equilibria may, for example, account for the large amounts and heterogeneous distributions of calcite cement in a wide variety of geologic systems, particularly in sedimentary basin sandstones.

Potential for travertine formation by John Malusa; Steven T Overby; Roderic A Parnell (1081-1093).
Chemical analyses of water emanating from Fossil Springs in Central Arizona were conducted to predict changes in travertine deposition related to changes in stream discharge caused by diversion for hydroelectric power generation. During spring of 1996, water was sampled at 15 locations during normal seepage flow in a 6.7 km reach below Fossil Springs and at full baseflow during turbine maintenance. Analyses resulted in a rate of 11,923 kg d−1 of CaCO3 precipitated from 1218 l s−1 of water emanating from the springs, while flows of 5.6 l s−1 that seep past a diversion dam produced 46 kg d−1 of CaCO3 precipitation. Active travertine dams currently occur predominantly below the Irving hydroelectric powerplant with partial return of diverted flow back into the natural channel. The lower reach resulted in 519 kg d−1 of CaCO3 precipitated from the return of 56.6 l s−1 with a reduced rate of precipitation during surface runoff conditions due to a dilution effect. Artificial substrates were located at sites in the lower reach for comparison with mass transfer rates derived from changes in water chemistry. Comparison between actual precipitation rates and overall mass transfer rates suggested preferential deposition was occurring at dam locations. Rates of mass transfer for high flow were greater than lower flow. Overall, mass transfer rates for the upper reach were proportional to the flow velocities with total mass transfers for both flows being approximately equal.

The mobility of radium-226 and trace metals in pre-oxidized subaqueous uranium mill tailings by A.J Martin; J Crusius; J Jay McNee; E.K Yanful (1095-1110).
The exchange of 226Ra and trace metals across the tailings-water interface and the mechanisms governing their mobility were assessed via sub-centimetre resolution profiling of dissolved constituents across the tailings–water interface in Cell 14 of the Quirke Waste Management Area at Rio Algom's Quirke Mine, near Elliot Lake, Ontario, Canada. Shallow zones (<1.5 m water depth) are characterized by sparse filamentous vegetation, well-mixed water columns and fully oxygenated bottom waters. Profiles of dissolved O2, Fe and Mn indicate that the tailings deposits in these areas are sub-oxic below tailings depths of ∼3 cm. These zones exhibit minor remobilization of Ra in the upper 5 cm of the tailings deposit; 226Ra fluxes at these sites are relatively small, and contribute negligibly to the water column activity of 226Ra. The shallow areas also exhibit minor remobilization of Ni, As, Mo and U. The release of these elements to the water cover is, however, limited by scavenging mechanisms in the interfacial oxic horizons. The presence of thick vegetation (Chara sp.) in the deeper areas (>2 m water depth) fosters stagnant bottom waters and permits the development of anoxia above the benthic boundary. These anoxic tailings are characterized by substantial remobilization of 226Ra, resulting in a relatively large flux of 226Ra from the tailings to the water column. The strong correlation between the porewater profiles of 226Ra and Ba (r 2=0.99), as well as solubility calculations, indicate that the mobility of Ra is controlled by saturation with respect to a poorly ordered and/or impure barite phase [(Ra,Ba)SO4]. In the anoxic zones, severe undersaturation with respect to barite is sustained by microbial SO4 reduction. Flux calculations suggest that the increase in 226Ra activity in the water cover since 1995 (from <0.5 to 2.5 Bq l−1) can be attributed to an increase in the spatial distribution of anoxic bottom waters caused by increased density of benthic flora. The anoxic, vegetated areas also exhibit minor remobilization with respect to dissolved As, Ni and Zn. The removal of trace metals in the anoxic bottom waters appears to be limited by the availability of free sulphide. Collectively, the data demonstrate that while the water cover over the U mill tailings minimizes sulphide oxidation and metal mobility, anoxic conditions which have developed in deeper areas have led to increased mobility of 226Ra.