Physical, chemical, and mineralogical characteristics of a reservoir sediment delta (Lake Powell, U.S.A.) and implications for water quality during low water level.

Lake Powell is a large reservoir in Utah and Arizona that has experienced large changes in water level during a recent drought. As a first step in assessing the connection between hydrologic and chemical changes at Lake Powell, we characterized the particle size and solid-phase bulk concentrations for 31 elements and 25 minerals in sediment from the inflow region and some shoreline locations by using laser diffractometry, X-ray fluorescence, elemental analysis, and X-ray diffraction Our results are consistent with previous results that show a negative correlation between particle size and concentrations of most elements and most minerals other than quartz and some feldspars. In our samples, however, solid-phase iron, rather than particle size or organic carbon, is the best predictor variable for the solid-phase concentrations of elements and minerals. Sediment characteristics vary on a scale of tens of kilometers, with fine sediment that is enriched in trace elementsnearer to the dam. These trends allow formulation of an algorithm for determining a water-level threshold below which sediment resuspension may alter water chemistry in a generic reservoir with a long and narrow sediment delta.

Hydrous manganese oxide doped gel probe sampler for measuring in situ reductive dissolution rates. 2. Field deployment.

In situ rates of reductive dissolution in submerged shoreline sediments at Lake Tegel (Berlin, Germany) were measured with a novel hydrous manganese (Mn) oxide-doped gel probe sampler in concert with equilibrium gel probe and sequential extraction measurements. Rates were low in the top 8 cm, then showed a peak from 8 to 14 cm, with a maximum at 12 cm depth. This rate corresponded with a peak in dissolved porewater iron (Fe) at 11 cm depth. Below 14 cm, the reductive dissolution rate reached an intermediate steady value. Lower rates at depth corresponded with increases in operationally defined fractions of carbonate-bound and organic- and sulfide-bound Mn and Fe as detected by sequential extraction. Observed rates of reductive dissolution, which reflect a capacity for Mn reduction rather than actual rates under ambient conditions, appear to correlate with porewater chemistry and sequential extraction fractions as expected in early sediment diagenesis, and are consistent with previous measurements of in situ reductive dissolution rates. Significant downward advection in this bank filtration setting depletes the Mn and Fe oxides in the sediments and enhances the transport of dissolved Fe and Mn into the infiltrating water.

Hydrologic and biogeochemical controls of river subsurface solutes under agriculturally enhanced ground water flow.

The relative influences of hydrologic processes and biogeochemistry on the transport and retention of minor solutes were compared in the riverbed of the lower Merced River (California, USA). The subsurface of this reach receives ground water discharge and surface water infiltration due to an altered hydraulic setting resulting from agricultural irrigation. Filtered ground water samples were collected from 30 drive point locations in March, June, and October 2004. Hydrologic processes, described previously, were verified by observations of bromine concentrations; manganese was used to indicate redox conditions. The separate responses of the minor solutes strontium, barium, uranium, and phosphorus to these influences were examined. Correlation and principal component analyses indicate that hydrologic processes dominate the distribution of trace elements in the ground water. Redox conditions appear to be independent of hydrologic processes and account for most of the remaining data variability. With some variability, major processes are consistent in two sampling transects separated by 100 m.

Expression dynamics of arsenic respiration and detoxification in Shewanella sp. strain ANA-3.

Because arsenate [As(V)] reduction by bacteria can significantly enhance arsenic mobility in the environment, it is important to be able to predict when this activity will occur. Currently, two bacterial systems are known that specifically reduce As(V), namely, a respiratory system (encoded by the arr genes) and a detoxification system (encoded by the ars genes). Here we analyze the conditions under which these two systems are expressed in Shewanella sp. strain ANA-3. The ars system is expressed under both aerobic and anaerobic conditions, whereas the arr system is only expressed anaerobically and is repressed by oxygen and nitrate. When cells are grown on As(V), the arr system is maximally induced during exponential growth, with peak expression of the ars system occurring at the beginning of stationary phase. Both the arr and ars systems are specifically induced by arsenite [As(III)], but the arr system is activated by a concentration of As(III) that is 1,000 times lower than that required for the arsC system (< or =100 nM versus < or =100 microM, respectively). A double mutant was constructed that does not reduce As(V) under any growth conditions. In this strain background, As(V) is capable of inducing the arr system at low micromolar concentrations, but it does not induce the ars system. Collectively, these results demonstrate that the two As(V) reductase systems in ANA-3 respond to different amounts and types of inorganic arsenic.