Redox-sensitive Species Research Articles

Influence of aquatic macrophytes on phosphorus and sediment porewater chemistry in a freshwater wetland

The influence of the emergent aquatic macrophyte, Menyanthes trifoliata L. on sediment interstitial porewater chemistry, particularly on the distribution of soluble reactive phosphorus (SRP), was investigated at Silver Lake in southwestern Washington (Cowlitz County). Menyanthes trifoliata and many other wetland species create an oxygenated rhizosphere by translocation of oxygen to the roots. Close interval diffusion sampling showed that SRP, total soluble phosphorus (TSP), and concentrations of other redox-sensitive species such as TFe, Fe 2+, and TMn were reduced in interstitial waters when macrophytes were present. Total alkalinity and pH also were lower and oxidation-reduction potentials were higher in sediments with plants than those in which the plants were removed. Rhizosphere oxidation appears to provide wetland species with a mechanism for sequestering phosphorus and for creating favorable concentration gradients within the root zone.

Temperature, pressure and redox conditions governing the composition of the cold CO 2 gases discharged in north Latium (Central Italy)

Gases rich in CO 2 are produced over the northern Latium area by metamorphic reactions involving carbonate formations or by magma degassing. The gas migrates towards the surface and accumulates in sedimentary permeable structures which act as gas traps. In that environment, the PCO 2 is a variable externally fixed and the redox conditions at which the gases equilibrate are governed by the C CO 2 buffer. Temperature in the interval 100–180°C and PCO 2 from few bar to tens of bar have been estimated by means of the ratios between the redox sensitive gas species and CO 2. The composition of gases from two geothermal wells located in the study area and of other hydrothermal fluids suggest that the redox potential of low-medium temperature carbonate hosted hydrothermal systems may be fixed by the C CO 2 buffer.

The chromium cycle in a seasonally anoxic lake

The Cr cycle was investigated in seasonally anoxic Greifensee, Switzerland. Cr(III), Cr(VI), and total dissolved (<0.45 µm) Cr and total Cr concentration profiles were measured from January until November 1989 together with other redox sensitive species of N, Mn, Fe, and S. Cr(VI) was the predominant species even under anoxic conditions. Concentrations of Cr(VI) ranged from 1 to 4 nM. Reduction of Cr(VI) could be observed only in extremely reducing conditions in the presence of S(‐II) and Fe(II). It was concluded that a suitable reducing agent had to be present for the reduction of CR(VI) to Cr(III) to take place and that organic reductants most probably played a minor role. In addition, calculations indicated the occurrence of a slow removal process, with a half‐time of 100–230 d, that reduced Cr(VI) concentrations by either adsorption or reduction in both oxic and suboxic waters. Dissolved free Cr(III) species were not detected, which was explained by the strong tendency of this species to bind with surfaces and organic ligands and colloids. Even in oxic conditions, the sediments did not appear to be a source of Cr(VI).

Redox species in a reducing fjord: equilibrium and kinetic considerations

In June 1977 ten redox sensitive chemical species were determined in the oxidizing and reducing waters of Saanich Inlet, British Columbia. An oxygen-hydrogen sulfide interface was found at a depth of 130 m. The concentrations of the oxidized species: oxygen, nitrate, iodate, and chromate, and the reduced constituents: hydrogen sulfide, ferrous iron, and ammonia reached background or near background levels at this depth. Three reduced constituents, manganese (II), chromium (III), and iodide existed metastably in the oxic layer. Thermodynamic calculations of p e indicate that the redox couples in the anoxic waters approach equilibrium with each other while those in the oxygenated waters are far from equilibrium. A time-dependent diffusion model is used to determine the removal rate constants for Mn 2+ and Cr(III) in the linear temperature-salinity region surrounding the oxygen-hydrogen sulfide interface (130 to 115 m). In this region I − is conservative within the error of our measurements over the period of anoxia in the inlet (estimated to be 3 to 6 months). The residence times of Cr(III) and Mn 2+ in the oxic layer are one to three weeks and about two days, respectively. We argue that the dominant manganese removal mechanism is by oxidation, resulting in a calculated oxidation rate constant for Mn 2+ five orders of magnitude greater than published laboratory values and widely different from previous field estimates. The rapid oxidation kinetics in Saanich Inlet are probably caused by bacterial catalysis near the oxygen-hydrogen sulfide interface.

Ground‐Water Pollution Potential of a Landfill Above the Water Tablea

ABSTRACTA study of the character and movement of landfill leachate through unsaturated soil was begun in 1967 at the State College (Pennsylvania) Regional Sanitary Landfill which has operated since 1962 employing the trench method of waste disposal. The landfill occupies a gently sloping underdrained valley with a water table more than 200 feet below land surface. Precipitation averages about 37 inches as rain per year. Residual sandy‐clay to sandy‐loam soils range from a few feet to greater than 70 feet in thickness on a sandy dolomite bedrock. Soil moisture samples were extracted at different depths from beneath two of the refuse cells using suction lysimeters. Water samples were also bailed from these cells and pumped from a water table well beneath the landfill. Monthly or less frequent analyses performed on water samples included Eh, pH, temperature, specific conductance, BOD, Cl, SO4, total alkalinity, NH3, NO2, NO3, PO4, Ca, Mg, Na, K, and total Fe. Soil samples from beneath the refuse were subjected to particle size and x‐ray analysis, and chemical analysis of soil pH, soluble salt content, exchangeable Ca, Mg, Na, and K, cation exchange capacity, and extractable P content.The study showed that the quality and quantity of leachate beneath a landfill varies considerably with the topographic setting of landfill trenches or cells. Leachates 2 feet under an upslope cell which received only direct precipitation, had the following maximum values 3‐13 months after refuse burial: specific conductance 8445 μmhos, Cl 1890 mg/1, BOD 3300 mg/1, NH3‐N 540 mg/1, and total Fe 225 mg/1. Upon reaching a depth of 14.5 feet after about 21/2 years or more, maximum values of these species in the leachate had been reduced by 83%, 80%, >99%, >99%, and 98% respectively. In contrast, more water, including precontaminated surface and subsurface runoff from adjacent upslope cells; infiltrated a downslope cell, saturating the refuse. Even after moving downward in the soil to a depth of 36 feet in 7 years, the leachate beneath this cell had a conductance of 6600 μmhos, 600 mg/1 Cl, over 9000 mg/1 BOD, 40 mg/l NH3‐N, and 100 mg/1 total Fe. A practically continuous depletion of inorganic species in the refuse as indicated by the quality of leachate from both cells has occurred with time. However, concentrations of BOD and redox sensitive species such as Fe and NH3 in the leachate have fluctuated in response to changes in the moisture content and temperature of the refuse.Leachate beneath instrumented cells is moving downward in the subsoil at the rate of 6‐11 ft/yr. Observed mechanisms of leachate renovation during this downward percolation, along with supporting evidence for each listed parenthetically, include: dilution and dispersion (decrease in Cl with depth); oxidation (Eh and pH measurements, decrease in BOD and Fe with depth); chemical precipitation (decrease in soil extractable P after leachate percolation); cation exchange (increase in percent base saturation of clays affected by leachate, and depletion of NH3 under reducing conditions—bacterial growth may also retard or remove NH3); and anion exchange (decrease in SO4 with depth). Removal of bacteria, suspended ferric oxyhydroxides and other particulates by soil filtration undoubtedly also occurs. Although renovation takes place, it is incapable of preventing highly contaminated leachate from moving to depths of 50 feet or more in soils beneath downslope cells. Ground‐water contamination has occurred, probably by leachate channelled down fractures in locally shallow bedrock, or by leachate contaminated runoff from heavy storms which has entered sinkholes or infiltrated along the valley bottom. The study shows that improper design of landfills emplaced above the water table in relatively permeable soils and bedrock such as are found in many carbonate‐rock terranes, can result in serious ground‐water pollution.