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Aquatic Geochemistry (v.6, #4)
Hydrogeochemical Processes in the Kafue River upstream from the Copperbelt Mining Area, Zambia by Ulf T. Pettersson; Johan Ingri; Per S. Andersson (pp. 385-411).
Frequent sampling during an annual cycle of dissolved(<0.45 μm) and suspended (>0.45 μm) elementshas been conducted in the Kafue River at Raglan'sFarm, upstream from the mining activities within theCopperbelt Province, Zambia. Additional sampling ofsediment and interstitial pore water was conductedduring low water discharge. The presence of carbonateswithin the drainage basin naturally gives rise to highelement concentrations in the dissolved phase(Ca = 626, Mg = 494, Na = 360 and K = 24 mmol l-1).During the rainy season the relative composition ofthe dissolved elements indicated a wash out ofaccumulated weathering products and mineralisedorganic material from the unsaturated zone of the soilprofile. High concentrations of dissolved Al, Fe andMn were measured during high water discharge. At lowwater discharge the sediment was a major source of Fe,Mn and associated Co and Cu to the water column.Enhanced concentrations of dissolved and suspended S,Co and Cu during the rainy season indicated thatatmospheric deposited particles from the mining areawere washed out into the river. Autochthonousformation of particles rich in Si indicated diatomproduction during low water discharge.
Keywords: hydrogeochemistry; element ratios; 87Sr/86 ; Ba/Sr; weathering; semi-arid; Kafue River; Zambezi River
Size Fractionation (Dissolved, Colloidal and Particulate) of Trace Metals in the Thur River, Switzerland by Laura Sigg; Hanbin Xue; David Kistler; René Sshönenberger (pp. 413-434).
The distribution of some trace metals (Cu, Zn, Ni, Co,Fe, Mn) and of DOC over a particulate (> 1 μm),a colloidal (size < 0.45 μm and molecular weight > 10 kD) and an ultrafiltered fraction (< 10 kD)was determined at several sites on the Thur River,Switzerland, at various times of the year. Thecomplexation of Cu by strong ligands in theultrafiltrate and in the conventional filtrate (<0.45 μm) was compared using a ligand-exchange/CSV method.The <0.45 μm concentrations of Cu (from anaverage of 7 nM to 24 nM), Zn (<5–23 nM), Ni (5–13 nM),Co (1.5–3 nM) and Mn (7–92 nM)increased downstream. The major part of Cu, Zn, Niand Co usually occurred in the ultrafiltratefraction at all sites, whereas Fe and Mn were mostlyin the particulate fraction, under conditions of lowsuspended matter content (< 10 mg L-1) in theriver. The percentage of metal in the colloidalfraction, with respect to the 0.45-μm filtrate,decreased in the order: Cu (median 11%) > Zn ≈Ni(median 5–6%) > Mn ≈ Co (median < 5%). DOCalso consisted mostly of molecules in the < 10 kDrange.Cu was strongly complexed by natural organic ligandsin all filtrate and ultrafiltrate samples. A largepart of the strong Cu binding ligands consisted ofcompounds in the < 10 kD range, but colloidalligands with similar properties also occurred. Cu wasdistributed among the dissolved and the colloidalligands, roughly in proportion to organic carbon.The colloidal fraction (as defined here) did notincrease in its proportional amount downstream and wasonly of limited significance in transporting traceelements in the Thur River under low discharge conditions.
Keywords: river water; trace metals; colloids; Cu complexation
Chemical Weathering Rates, Erosion Rates and Mobility of Major and Trace Elements in a Boreal Granitic Till by Magnus Land; Björn Öhlander (pp. 435-460).
Chemical weathering rates and erosionrates of granitic till in northern Sweden have beenestimated. The present-day chemical weathering rate iscompared with the long-term average weathering ratesince the last deglaciation approximately 8,700 yearsago. Also, the present-day release rates of major andtrace elements due to chemical weathering are comparedwith the mobility of these elements in a spodosolprofile as shown by soil water samples from the vadozezone. The estimation of the past weathering rate isbased on elemental depletion trends in a soil profile(typic haplocryod), whereas the present weatheringrate is based on elemental input/output budgets in asmall catchment (9.4 km2). The long-term averagechemical erosion rate, expressed as the sum of majorelement oxides (SiO2, Al2O3, CaO,Fe2O3, K2O, MgO, MnO, Na2O,P2O5, TiO2), was estimated to be4.9 gm-2 yr-1. The long-term base cation(Ca2+, Mg2+, Na+, K+) depletionwas 0.325 keq ha-1 yr-1. The currentchemical erosion rate was estimated to be2.4‐3.0 g -2 yr-1, which is at least an order ofmagnitude higher than the rate of physical erosion,and the base cation flux due to chemical weathering is0.356–0.553~keq ha-1 yr-1. However,0.074 keq ha-1 yr-1 of this flux may be related tocation exchange processes induced by atmospheric inputof acid rain. There is no evidence for any recentlyincreased weathering rate of silicates in this area.The inputs of Cd, Cu, Ni and Zn exceed the outputs,and hence, these elements are currently accumulatingin the soil.There is a distinct seasonal variation in thechemical composition of the soil water. Results fromthe soil water samples show that Na, followed by Siand S (Cl was not measured), are the most mobileelements in the spodosol profile between the E-horizonand the C-horizon, and that Al and Fe were the leastmobile elements. However, there is no simplerelationship between the mobility of an element in thespodosol profile and the current release rate due toweathering of that element. This fact may haveimplications for the validity of the comparisonbetween the past and present weathering rates asperformed in this study.
Keywords: Weathering rates; till; spodosol; soil water; vadoze zone; mobility
Seasonal Cycles of Dissolved Cd, Mn and Zn in River Water Caused by Variations in pH Induced by Biological Activity by Alain C. M. Bourg; Monika A. M. Kedziorek; Cathrine Crouzet (pp. 461-471).
pH, dissolved oxygen, Zn, Cd and Mnwere measured once a month for 2.5 years in the LotRiver, France. The pH is controlled by biologicalactivity (photosynthesis and respiration). Dissolvedoxygen variations are due to a combination ofbiological activity and of the temperature dependenceof the oxygen solubility. pH is the master variablefor concentrations of aqueous Zn, Cd and Mn, thelowest concentrations being associated with thehighest pH values, which occur during the summer. pH-related processes rather than redox phenomenaexplain variations in dissolved Mn. Variations indissolved Zn and Cd are also controlled bypH-dependent reactions (possibly related to thedissolutive behavior of solid Mn).
Keywords: Cd; Mn; Zn; dissolved heavy metals; river water; seasonal cycling; biological activity