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Aquatic Geochemistry (v.5, #2)

Geochemical Study of a Granitic Area – The Margeride Mountains, France: Chemical Element Behavior and 87Sr/86Sr Constraints by Philippe Négrel (pp. 125-165).

A small watershed (89 km2) underlain by granite or granite-gneiss in the Margeride mountains, southern Massif Central (France), has been studied using the chemical and isotopic composition of its dissolved load, bed sediments and soils. Dissolved concentrations of major ions (Cl, SO4, NO3, HCO3, Ca, Na, Mg, K, Al and Si), trace elements (Rb and Sr) and strontium isotopes have been determined for three different hydrologic periods in the main stream of the Desges river and its tributaries.The aim was to characterize the chemical and isotopic signatures of each reservoir in the watershed; signature changes are interpreted as fluctuations in the different influencing components: rainwater, weathering products and anthropogenic addition. In the study area, as in other watersheds in granite environments, the only source for input of chemical species into the dissolved load at high altitude is chemical weathering and atmospheric deposition, whereas at lower altitude, human influence plays a non-negligible role.As precipitation is a major vehicle for the addition of dissolved chemical species into the hydrosystem, a systematic rainwater study using an automatic collector was carried out over one year in order to better constrain rain elemental input. Corrections for rainwater addition, using chloride as an atmospheric-input reference, were computed for selected elements and for 87Sr/86Sr ratios. After these correction, the geochemical budget of the watershed was determined and the role of anthropogenic addition was evaluated based on strontium isotope relationships.For particulate matter, we used the normalization of chemical species relative to parent rocks and the element ratios which reflect the depletion or enrichment in soils and sediments. Both the immobile- and mobile-element approaches have been tested, using Ti/Zr and La/Ce ratios for the former and Ca/Sr, K/Rb, and K/Fe ratios for the latter.

Keywords: France; Massif Central; crystalline environments; strontium isotopes; dissolved load; bed sediments; soils

Geochemistry of Suspended Particulate Matter (SPM) in the Murray-Darling River System: A Conceptual Isotopic/Geochemical Model for the Fractionation of Major, Trace and Rare Earth Elements by G.B. Douglas; B.T. Hart; R. Beckett; C.M. Gray; R. L. Oliver (pp. 167-194).

A conceptual isotopic/geochemical model is presented to explain the variation of major, trace and rare earth element (REE) geochemistry and Sr isotope systematics in suspended particulate matter (SPM) as a function of particle/colloid size. This conceptual model is an extension of a previous investigation of the origin of SPM in the Murray-Darling River system (MDRS) that utilised Sr isotope systematics to examine aspects of SPM (particle/colloid) origin, structure and mineralogy. The geochemical processes that give rise to the often coherent trends in major, trace and REE geochemistry and Sr isotopic signature as a function of particulate (<1 μm) and colloidal (>1 μm) size in the MDRS have been identified using an enhanced SPM size fractionation technique as a basis to not only obtain a broad range of particle/colloid size ranges, but also to provide sufficient material for subsequent geochemical and isotopic analysis. The conceptual isotopic/geochemical model proposed here contains three major components: (i) the differential weathering of micas and alkali (K-) feldspars to form the majority of the particulate (<1 μm) fractions (high 87Sr/86Sr ratio), which have a geochemical and Sr isotopic signature that closely resembles precursor mineralogies, (ii) the differential weathering of Na, Ca-feldspars (plagioclase) which decompose to form clay minerals in the colloidal (>1 μm) fractions (low 87Sr/86Sr ratio), with a range of geochemical signatures related to the relative proportions of inorganic and organic constituents, and (iii) the presence of natural organic matter as coatings on the particulate (<1 μm) and colloidal (>1 μm) matter and possibly as organo-colloids which exert an increasing influence in particular on bulk colloid geochemistry with decreasing colloid size. This conceptual isotopic/geochemical model also accounts for the distinct variation in major, trace and REE geochemistry and Sr isotopic systematics between the particulate (<1 μm) and colloidal (>1 μm) fractions, the variation being primarily a function of the distinctly different precursor mineralogies of the SPM fractions and geochemical fractionation during the weathering and transport. Additionally, this model explains a systematic fractionation of REE apparent within colloidal (>1 μm) fractions. Statisitcal (hierachical cluster) analysis of two particulate and three colloidal fractions from 23 samples from the MDRS is used as a basis to investigate geochemical and mineralogical associations within the particulate and colloidal size fractions and to provide additional supporting evidence for the conceptual isotopic/geochemical model.

Keywords: suspended particulate matter; particulate; colloid; Sr isotope; fractionation; Murray-Darling River system

On Equilibrium Constants for Aqueous Geochemical Reactions in Water Unsaturated Soils and Sediments by Michael Zilberbrand (pp. 195-206).

Usually, equilibrium constants for aqueous geochemical reactions in the unsaturated zone are assumed to be equal to those for free water solutions (at atmospheric pressure) considered in classical water chemistry. This paper shows that high negative pressures in pore water may essentially change these constants in dry soils and sediments.The influence of negative capillary pressures on equilibrium constants for some important reactions occurring in the upper part of the unsaturated zone is analyzed. It is shown that values of these constants at low water contents may differ from those normally used by orders of magnitude. Sediment drying usually decreases the equilibrium constant for salt dissolution-precipitation reactions (makes precipitation easier) and for silicate weathering (delays it), whilst in the case of dedolomitization, orthoclase-albite transition and some types of cation exchange the equilibrium constant grows and these processes in dry soils and sediments have to be enhanced.

Keywords: equilibrium constant; water unsaturated soil; high negative pressure; aqueous geochemical reactions

The Interaction of Natural Organic Matter with Iron in a Wetland (Tennessee Park, Colorado) Receiving Acid Mine Drainage by Stefan Peiffer; Katherine Walton-Day; Donald L. Macalady (pp. 207-223).

Pore water from a wetland receiving acid mine drainage was studied for its iron and natural organic matter (NOM) geochemistry on three different sampling dates during summer 1994. Samples were obtained using a new sampling technique that is based on screened pipes of varying length (several centimeters), into which dialysis vessels can be placed and that can be screwed together to allow for vertical pore-water sampling. The iron concentration increased with time (through the summer) and had distinct peaks in the subsurface. Iron was mainly in the ferrous form; however, close to the surface, significant amounts of ferric iron (up to 40% of 2 mmol L-1 total iron concentration) were observed. In all samples studied, iron was strongly associated with NOM. Results from laboratory experiments indicate that the NOM stabilizes the ferric iron as small iron oxide colloids (able to pass a 0.45μm dialysis membrane). We hypothesize that, in the pore water of the wetland, the high NOM concentrations (>100 mg C L-1) allow formation of such colloids at the redoxcline close to the surface and at the contact zone to the adjacent oxic aquifer. Therefore, particle transport along flow paths and resultant export of ferric iron from the wetland into ground water might be possible.

Keywords: natural organic matter; ferrous iron; ferric iron; wetland; colloidal iron; acid mine drainage; pore water

A Continuous and Mechanistic Representation of Calcite Reaction-Controlled Kinetics in Dilute Solutions at 25°C and 1 Atm Total Pressure by Takeshi Arakaki; Alfonso Mucci (pp. 225-225).

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Aquatic Geochemistry (v.5, #2)

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Aquatic Geochemistry (v.5, #2)