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SECTION I. Effects of Benthic Flora on Arsenic Transport in Whitewood Creek, South Dakota
By James S. Kuwabara, Cecily C.Y. Chang, and Sofie P. Pasilis
Field measurements and bioassay experiments were done to investigate the effects of arsenic and phosphorus interactions on sorption of these solutes by the benthic flora (periphyton and submerged macrophytes) in Whitewood Creek, a stream in western South Dakota. Short-term (24-hour) sorption experiments were used to determine arsenic transport characteristics for algae (first-order rate constants for solute sorption, biomass, and accumulation factors) collected in the creek along a transect beginning upstream from a mine discharge point and downgradient through a 57-kilometer reach. Temporal changes in biomass differed significantly between and within sampling sites. Arsenic concentrations in plant tissue increased with distance downstream, but temporal changes in concentrations in tissues differed considerably from site to site. Cultures of Achnanthes minutissima (Bacillariophyceae) and Stichococcus sp. (Chlorophyceae) were isolated from four sites along a longitudinal concentration gradient of dissolved arsenic within the study reach and were maintained at ambient solute concentrations. Arsenic accumulation factors and sorption-rate constants for these isolates were determined as a function of dissolved arsenate and orthophosphate. Cell surfaces of algal isolates exhibited preferential orthophosphate sorption over arsenate. Initial sorption of both arsenate and orthophosphate followed first-order mass transfer for each culturing condition. Although sorption-rate constants increased slightly with increased dissolved-arsenate concentration, algae, isolated from a site with elevated dissolved arsenic in the stream channel, had a significantly slower rate of arsenic sorption compared with the same species isolated from an uncontaminated site upstream.
In diel studies, amplitudes of the pH cycles increased with measured biomass except at a site immediately downstream from water-treatment-plant discharge. Inorganic pentavalent arsenic dominated arsenic speciation at all sites—not a surprising result for the well-oxygenated water column along this reach. Concentration fluctuations in dissolved-arsenic species lagged pH fluctuations by approximately 3 hours at the most downstream site, but no discernible lag was observed at an artificially pooled area with an order of magnitude higher biomass. Furthermore, the amplitudes of diel fluctuations in arsenic species were greater at the pooled area than at the most downstream site. Lack of correspondence between changes in dissolved-orthophosphate concentrations and arsenic species may have resulted from preferential sorption of orthophosphate over arsenate by the biomass. Based on carbon-fixation estimates, the phosphorus demand from photosynthetic activity required water-column concentrations to be supplemented by another source such as phosphate regeneration within the benthic community or desorption of particle-bound phosphate.
TABLE OF CONTENTS
Purpose and Scope
Methods of Study
Results and Discussion
Effects of Benthic Flora on Physico-Chemical Processes
Summary and Conclusions
SECTION II. Evaluation of the Processes Controlling Dissolved Arsenic in Whitewood Creek, South Dakota
By Christopher C. Fuller and James A. Davis
Coupled physical, chemical, and biological processes affect the concentration of dissolved arsenic in Whitewood Creek, South Dakota. In the lower reaches of the stream, dissolved-arsenic concentrations were controlled primarily by adsorption and coprecipitation with iron oxyhydroxides (ferrihydrite) as ground water enriched in arsenic entered the stream. Periphyton photosynthesis induced a diurnal pH fluctuation of 0.25 to 0.5 in surface water that had a pH of 8.1 to 8.7 and a concomitant diurnal cycle in arsenate (30- to 40-percent variation). The fluctuation in arsenate reflects the dynamic equilibrium of adsorption-desorption processes occurring in response to the pH cycle. Kinetics of the sorption processes are slow, which results in a cycle of dissolved arsenic that lags 2 to 4 hours behind the pH cycle. A mass balance for dissolved arsenate indicates that adsorption-desorption of arsenate on ferrihydrite surfaces in and on streambed sediments was the primary control of dissolved arsenic. Uptake of arsenate by algae and input from reducing sediments were of secondary importance to the dissolved-arsenic budget. The results demonstrate the importance of adsorption-desorption processes in controlling trace-element concentrations in aquatic systems and the need to incorporate sorption kinetics into transport models. The effect of diurnal pH cycles on trace-element and nutrient cycling and availability may be of significance in other surface-water systems.
TABLE OF CONTENTS
Purpose and Scope
Description of Study Area
Methods of Sample Collection and Analysis
Statistical Analysis of Diurnal Surface-Water Chemistry Data
Sampling of Ground-Water Inflows
Solid-Phase Characterization and Adsorption Properties
Results and Discussion
Diurnal Fluctuations in Surface-Water Chemistry
Variations in Stream Discharge
Dissolved Arsenic in Synoptic Sampling
Arsenate Adsorption and Isotopic Exchange on Iron Oxyhydroxides
Processes that Contribute to the Diurnal Cycle of Dissolved Arsenic in Surface Water
Ground-Water Sources of Streamflows
Molecular Diffusive Flux from Bed Sediments
Algal Uptake of Arsenate from Surface Water
Potential for Desorption of Arsenic from Suspended Sediments and Bed Sediments
Additional Controlling Processes
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The citation for this report, in USGS format, is as follows:
Kuwabara, J.S., and Fuller, C.C., eds., 2004, Toxic substances in surface waters and sediments—A study to assess the effects of arsenic-contaminated alluvial sediment in Whitewood Creek, South Dakota: U.S. Geological Survey Professional Paper 1681, 48 p.
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Last modified: Monday, January 14 2013, 02:27:48 PM