Applied Geochemistry (v.17, #7)

Glacier meltwater hydrochemistry by Giles H. Brown (855-883).
Glacierised areas present an ideal environment in which to study water-rock interaction, since chemical weathering rates are high and anthropogenic impacts are often minimal. Detailed investigations of meltwater quality variations have suggested the importance of these environments in estimates of terrestrial chemical erosion and global biogeochemical cycles. Most notably, the role of meltwaters in CO2 sequestration during episodes of deglaciation has attracted considerable attention, since this may impact on climate at glacial-interglacial timescales. However, there is still considerable uncertainty surrounding estimates of CO2 drawdown by meltwaters which remains to be resolved. Water flow through glaciers exerts an important control on ice mass dynamics, and influences the quantity and quality of water delivered to environments downstream of glacierised basins. Thus, the study of the configuration and dynamics of subglacial drainage systems is important not only to enhance scientific understanding, but also to allow effective water resource utilisation in glacierised headwater catchments. Bulk meltwater quality characteristics draining terrestrial ice masses also offer the potential to provide unique information on hydrological and hydrochemical processes operating in the inaccessible subglacial environment. Here, significant advances have been made in understanding the controls on chemical weathering reactions, based on the identification of key dissolved indicator species. This has allowed water quality variations to be exploited as a tool for both subglacial hydrochemical and hydrological investigations. As a result, this area of glaciology has received considerable attention in recent years, utilising an increasing range of dissolved ions, and integrating field and laboratory studies. However, uncertainty still surrounds certain areas of meltwater quality science, including the role of microorganisms in a system which to date has largely been viewed as abiotic. A better understanding of the processes and rates of chemical weathering in glacierised environments has the potential to enhance our understanding of the environment, and to facilitate the exploitation of water quality variations for both scientific and applied objectives. In this paper the development and current state of meltwater quality science as a tool for investigating subglacial hydrology and geochemistry is detailed, and problems and future directions identified. This will hopefully stimulate wider interest in an important area of aquatic chemistry with significant applied implications.

Geochemistry and mineralogy of saprolite in Finnish Lapland by M.R Islam; V Peuraniemi; R Aario; S Rojstaczer (885-902).
An ancient saprolite has developed on the Palaeoproterozoic granulite, granite gneiss and amphibolite bedrock of the Vuotso–Tankavaara area of central Finnish Lapland. The present day climatic regime in Finnish Lapland lies within the northern boreal zone and so the saprolite there can be regarded as fossil. Cores of saprolite were collected from 4 sections (42 samples) and analyzed chemically and mineralogically. In the study area, progressive weathering of the rocks has been marked by gradual enrichment in Al, Fe and Ti; and depletion of Na, K and Ca. The higher concentration of Fe(III) and water and reduced Na and Ca in weathered bedrock in the 4 sections are indicative of oxidation, hydration and leaching processes involved during weathering. The primary minerals in the saprolite are plagioclase feldspar, K-feldspar, quartz, garnet (almandine) and hornblende; the common secondary minerals are kaolinite, halloysite, and vermiculite in addition to minor amounts of sericite. Intense weathering is indicated by: (1) the presence of kaolinite and halloysite in 4 sections of different bedrock types, and (2) the comparatively lower SiO2/Al2O3 (wt.%) ratio (2.30) of weathered granulites (3 sections) as compared to fresh granulite (4.33) and that of weathered amphibolite (2.68) as compared to fresh amphibolite (3.56). In general, kaolinite and halloysite have formed through the weathering of feldspars, garnet, and biotite. Vermiculite is the most probable alteration product of biotite. The formation of kaolinite and halloysite in Finnish Lapland indicates wetter and warmer climatic conditions during the time of their formation than at present. The possible time for formation of the saprolite is early Cretaceous–early Tertiary into Middle Miocene.

Farm waste stores such as cattle slurry lagoons are widespread in the UK and many overly important aquifers. Stores can be serious risks to water quality because they are important sources of N species, organic C and pathogenic microbes. At two sites on the Chalk aquifer of southern England, inclined boreholes were drilled and cored to obtain aquifer material from directly beneath unlined slurry stores. Vertical boreholes were also drilled adjacent to the slurry stores to determine any lateral movement of contaminants. Interstitial porewaters were analysed for major and minor ions and S isotopes. At the second site, unsaturated zone gases were sampled from the inclined hole. Infiltration of slurry into the unsaturated zone caused significantly elevated concentrations of metals such as Cu and Ni at both sites. Sulphate reduction was occurring at Site 1, as evidenced by SO4 concentrations decreasing from 150 to 50 mg/l and enhanced ratios of δ34S–SO4 and δ18O–SO4. Ammonium-N also leaches along with dissolved organic C which were found 17 m below ground surface at concentrations up to 400 and 260 mg/l, respectively. Contaminant concentrations were similar in the porewaters from both the inclined and vertical boreholes. At Site 2, higher contaminant concentrations were found in the inclined borehole compared with the vertical borehole. Organic C concentrations were considerably lower than at Site 1, ranging from 10 to 70 mg/l. Ammonium–N concentrations reached a maximum concentration of 25 mg/l, however NO3-N concentrations were up to 500 mg/l and SO4 concentrations were generally higher than Site 1. Data for N2/Ar and δ15N–N2 from the gas samplers show a peak of 102 and 2.2‰, respectively, at 14 m below ground level indicating denitrification was taking place. Evidence from δ34S–SO4 and δ18O–SO4 suggest that some SO4 reduction was taking place simultaneously. From CH4 and NH3 detected at depth it is suggested that slurry contamination, emanating from early use of the store, has passed through the top 18 m of the unsaturated zone at Site 2. The presence of high concentrations of NO3 and lower concentrations of organic C suggests that this lagoon has formed a relatively impermeable seal at its base within the first few years of its lifetime. The anoxic conditions at both sites may have mobilised U from N–P–K fertilisers. Both sites are continuing to impact on the porewater chemistry and pose a risk of groundwater contamination.

The accumulation and storage of trace metals in coastal sediments is an environmental concern. It is, therefore, important to understand better how these metals are bound or released under different redox conditions. This study of Fe and trace metal fixation under continuously anoxic conditions in the bottom sediments and the lower water column of the Nordåsvannet fjord in western Norway contributes further to such understanding. It allows investigation of both an end member redox state and one important mechanism of Fe and trace metal accumulation in sediments, the pyritization of Fe and trace metals. Pyrite formation occurs both in the water column and in the sediments of the Nordåsvannet fjord and favours the fixing of Fe and trace metals in the bottom sediments of the fjord. Thus, these sediments act as a continuous sink for Fe and trace metals. The DOP, and the degrees of trace metal pyritization for Mo, Ni and Cr correlate with organic matter content. While it is generally thought that Fe is the factor limiting pyrite formation in anoxic environments, this study found that degrees of pyritization of Fe (DOP) are clearly below 100%, and the availability of metabolizable organic matter is limiting pyrite formation. This is an important finding, because it indicates that increased supply of organic and mineral matter by higher runoff from land would further enhance the fixation of these metals in the fjord sediments, as would higher organic matter availability from increased productivity due to higher nutrient supply. The metals stored in the bottom sediments could be released into the biogeochemical cycle if redox conditions were to change from anoxic to suboxic or oxic. The fjord would then become a source rather than a sink for these metals.

Assessment of models for estimation of land-derived nitrogen loads to shallow estuaries by Ivan Valiela; Jennifer L. Bowen; Kevin D. Kroeger (935-953).
The performance of several models used to estimate land-derived N loads to shallow receiving estuaries are compared. Models included in the comparison differed in complexity and approach, and predicted either loads or concentrations in estuary water. In all cases, model predictions were compared to measured loads or concentrations, as appropriate. Measured N loads to 9 estuaries on Cape Cod, MA, were obtained as the product of mean concentrations in groundwater about to seep into estuaries multiplied by the annual recharge of groundwater. Measured annual mean N concentrations in estuaries were obtained by extensive sampling surveys. The validity of this procedure to measure loads was verified by comparison against seepage meter data. Responsiveness of model predictions was generally good: predictions increased significantly as measured values increased in 8 of the 10 models evaluated. Precision of predictions was significant for all models. Three models provided highly accurate predictions; correction terms were calculated that could be applied to predictions from the other models to improve accuracy. Four of the models provided reasonable predictive ability. Simulations were run with somewhat different versions of two of the models; in both cases, the modified versions yielded improved predictions. The more complex models tended to be more responsive and precise, but not necessarily more accurate or predictive. Simpler models are attractive because they demand less information for use, but models with more comprehensive formulations, and emphasis on processes tended to perform better. Different models predicted widely different partitioning of land-derived N loads from wastewater, fertilizers, and atmospheric deposition. This is of concern, because mitigation options would be based on such partitioning of predictions. Choice of model to be used in management decisions or for research purposes therefore is not a trivial decision.