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Aquatic Geochemistry (v.3, #1)

Changes in Surface Morphology of Calcite Exposed to the Oceanic Water Column by Paul J. Troy; Yuan-Hui Li; Fred T. Mackenzie (pp. 1-20).

Reactions occurring on the surfaces of biogenic carbonate minerals can have important consequences for the biogeochemical cycle of carbon. In this study, carbonate mineral surface reactions with ambient seawater were investigated by atomic force microscopy (AFM). A sampling method was developed in which calcite surfaces were hung at discrete depths on a sediment trap array line for a three-day deployment period in subtropical North Pacific waters. Changes in surface morphologies were examined at nanometer resolution and evaluated using as a constraint the depth profile of calcite saturation in these waters. Evidence suggests that: (1) organic films which develop on carbonate surfaces exposed to shallow seawater may be responsible for the oversaturated state of the upper oceanic water column, (2) dissolution of carbonate minerals within the shallow warm layer of the ocean could be responsible for part of the alkalinity anomaly observed in the North Pacific.

Keywords: carbonates; calcite; dissolution; upper ocean; organic film; in-situ experiment

A Seasonally Anoxic Mountain Lake with an Active Fe Cycle in Tropical Taiwan by J. K. Wann; C. T. A. Chen; B. J. Wang (pp. 21-42).

Variations of temperature and oxygen indicate that the 2150 m high Great Ghost Lake in tropical Taiwan is seasonally stratified and anoxic. Low concentrations of ions and nutrients and the similarity in the distribution of the reduced species in the water column in the early stages of stagnation indicate that the production of the reduced species occurs mainly in the sediments with a subsequent release into the overlying water. In addition, the high organic matter content in the sediments and the short period of water overturn make Fe(II) the most abundant reduced species in the sediments and in the water column. As a result, the annual Fe cycle plays one of the most important roles in regulating the water chemistry and in driving the reduced species from the sediments to the anoxic hypolimnion. The distribution of reduced species during the stagnation is in agreement with the thermodynamic sequence. The accumulation rates of Fe2+, Mn2+, NH4+, sulfide and CH4 in the water column during the early stages of stagnation are about 5.07, 0.059, 1.69, 0.015 and 1.65 mmol m-2 day-1, respectively.

Keywords: anoxic; subalpine lake; Fe redox cycle; Taiwan

Permeabilities and Chemical Properties of Water in Crystalline Rocks of the Black Forest, Germany by Ingrid Stober (pp. 43-60).

Investigations were carried out to determine the hydraulic and hydrochemical properties of crystalline rocks in the Black Forest of Germany and neighbouring regions. Rock permeabilities (K) were determined to a depth of 3500 m. These parameters range from K = 3.5 × 10-10 ms-1 to K = 8.7 × 10-5 ms-1; and can increase up to an order of magnitude which is typical for porous aquifers. It is shown that on an average, granites are more pervious than gneisses and only the permeabilities of gneisses decrease with depth. The geochemistry of natural waters in crystalline rocks is not constant, but varies with depth and location. The concentration increases with depth and the water-type changes from a Ca–-Na–-HCO 3-type (or Na–-Ca–-HCO3–-) at shallow depths to a Na–-Cl-type at greater depths. Thermal springs are found only in granitic rocks with on average higher permeabilities than in gneisses. Thermal waters are welling up in valleys at the bottom of steep mountains. The chemical composition of thermal spring water is identical to that of water found at greater depths. Using geothermometers it is found, that the depth of the deposits of thermal spring water in the crystalline basement rocks of the Black Forest is some 1000 m below the surface. The topographic relief in the mountains induces a deep circulation of infiltrating rain-water with an upwelling as thermal springs in the valleys.

Keywords: crystalline rocks; hydraulic properties; hydrochemical properties; Black Forest

Oxyanion Concentrations in Eastern Sierra Nevada Rivers – 2. Arsenic and Phosphate by Kevin H. Johannesson; W. Berry Lyons; Suey Huey; Georgia a. Doyle; Eric E. Swanson; Ed Hackett (pp. 61-97).

Water samples were collected from the Truckee River-Pyramid Lake system, the Walker River-Walker Lake system, and the Carson River, all located in eastern California and western Nevada, U.S.A., at three different times (i.e., summer 1991, spring 1992, and autumn 1992) over a two year period. The concentrations of As, Na, Cl, ΣPO4, and pH were measured in these river samples and the associated terminal lakes. Arsenic values ranged from below 13 nmol/kg near Truckee, California to 160 nmol/kg at Nixon, Nevada in the Truckee River, from 40 nmol/kg in the headwaters of both West and East Walker Rivers to 270 nmol/kg below Weber Reservoir on the main branch of the Walker River, and from <27 nmol/kg to 234 nmol/kg for the lower Carson River system. Arsenic concentrations in Steamboat Creek (0.91 μmol/kg–1.80 μmol/kg) in the Truckee River catchment are above the U.S. EPA drinking water maximum contaminant level of 0.67 μmol/kg, as are the As concentrations in both Pyramid Lake (1.33 μmol/kg–1.57 μmol/kg ) and Walker Lake (13.7 μmol/kg–18.7 μmol/kg). Sources of As for all three rivers include weathering of As-rich rocks and/or regolith and input of high-As geothermal spring waters, both processes primarily, although not exclusively, adding As to the headwater regions of these rivers. Steamboat Hot Springs (29 μmol/kg ≤ As ≤ 54.5 μmol/kg), for example, is identified as a source of As to the Truckee River via Steamboat Creek. The high As concentrations in Pyramid and Walker Lakes are likely due to (1) desorption of arsenate from aquatic particulate matter in these high pH waters (9.0 ≤ pH ≤ 9.5), (2) limited biologic uptake of arsenate, and (3) evaporative concentration of the lake waters. Evaluation of molar ΣPO4}/As ratios of river waters and geothermal spring waters (e.g., Steamboat Hot Springs), indicates that phosphate is substantially enriched in Steamboat Creek as well as the mid to lower reaches of the Walker and Carson Rivers. These regions of each river are dominated by agricultural interests and, additionally, in the case of Steamboat Creek, residential areas and golf courses. Our data strongly imply that phosphate-rich agricultural return flow has likely added P to these streams and, consequently, increased their respective P:As ratios.

Keywords: arsenic; phosphate; oxyanions; eastern Sierra Nevada rivers; surface waters; Lake Tahoe; Pyramid Lake; Walker Lake; closed-basin lakes

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Aquatic Geochemistry (v.3, #1)

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Aquatic Geochemistry (v.3, #1)