Environ. Sci. Technol. 1996, 30, 1953-1960
Diel Variation of Trace Metals in the Upper Clark Fork River, Montana CHRISTINE M. BRICK* AND JOHNNIE N. MOORE† Department of Geology, University of Montana, Missoula, Montana 59812
Hourly sampling of an oxic, slightly alkaline river with high concentrations of trace metals stored in bed and flood plain sediments reveals diel cycles in dissolved Mn and Zn and acid-soluble particulate Al, Fe, Mn, Cu, and Zn. Metal concentrations increase 2-3-fold at night as pH and dissolved oxygen decrease. Dissolved Mn and Zn cycles may be the result of redox reactions in river-bed sediments or variations in influx of water from the hyporheic zone as a result of evapotranspiration. Increased particulate metal concentrations result from an increase in total suspended matter at night. Particulate metal concentrations decrease several hours before the decline of dissolved concentrations in the morning. Major elements and ions including Ca, Mg, Na, Si, Cl-, SO42-, and total alkalinity show no evidence of diel variation. The results have implications for monitoring and assessment of reclamation because they show that daytime sampling underestimates the flux of metals in the river.
Introduction Rivers and streams are dynamic ecosystems in which several physical, chemical, and biological processes operate on a diel time scale. These processes occur in riparian and hyporheic zones as well as in stream channels and have the potential to affect the partitioning and mobility of trace metals. Knowledge of the processes that control diel variation in partitioning of metals between solid and solute or substrate and water column is important for assessing the sources, storage, and mobility of trace metals in rivers (1, 2). Photoreduction and pH-dependent adsorption-desorption are two processes known to cause diel cycling of metals in freshwater. Photoreduction of iron transfers ferric iron to soluble Fe(II) in the water column in lakes and streams (1, 3, 4). Manganese oxides have also been shown to photoreduce in seawater (5, 6). Diel variation in arsenate concentrations in streamwater has been linked to photosynthetic control of pH (2). Fuller and Davis (2) demonstrated a relationship between diel pH variation and * Corresponding author e-mail address:
[email protected]; telephone: (406) 243-2341; fax: (406) 243-4028. † E-mail address: gl
[email protected].
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1996 American Chemical Society
adsorption-desorption of arsenate from stream-bed iron oxyhydroxide deposits. They speculate that a similar process could be expected for cationic trace metals at lower pH. Diel changes in in-stream photosynthesis and microbial respiration influence dissolved oxygen concentrations in streamwater and redox equilibrium in stream substrate. Similar cycles in denitrification have been observed in stream bottom periphyton (7) and in stream sediments (8). A diel cycle of methanogenesis has also been documented in lake sediments (9). While these processes have not been related to diel metal concentrations, changing redox conditions in stream substrate could potentially affect trace elements associated with ferromanganese oxides. Other biological processes operate on a diel time scale within the stream and in the riparian zone that cause physical, and potentially chemical, changes within the stream. Evapotranspiration by stream bank vegetation can reduce streamflow and/or groundwater influx during the day. This has the potential to alter transient storage in the hyporheic zone, which is important to the timing of solute fluxes in streams (10). Although several processes can potentially cause daily trace metal variation in rivers, this type of cycling has been documented in very few cases. In rivers with high concentrations of trace metals from anthropogenic sources, diel cycles of these elements could be important sources of variation. In this study, we present results of a 2-day, hourly sampling of an oxic, slightly alkaline river with high concentrations of trace metals stored in the bed and flood plain. We monitored major and trace elements including Ca, Mg, Na, Si, Cl-, NO3-, SO42-, total alkalinity, Al, Fe(total), Fe(II), Mn, Cu, Zn, pH, dissolved oxygen, temperature, and discharge during summer low-flow conditions. These measurements are coupled with data from the hyporheic zone and bed sediments. Our intent is to identify the elements that vary diurnally under these conditions and discuss possible processes causing the variation. A lab experiment was conducted with river sediment to elucidate the potential role of pH. An understanding of the processes that contribute to the dynamic partitioning of metals between substrate and water in rivers is necessary for accurate monitoring, toxicological assessment, and evaluation of reclamation efforts.
Study Site The Clark Fork River in southwestern Montana is a highgradient, cobble-bed river that is enriched in metals as a result of more than 100 years of mining and smelting in its headwaters (11). The river begins at the confluence of the Warm Springs treatment ponds and several tributary streams that drain the mining, milling, and smelting areas of Butte and Anaconda (Figure 1). Mining and smelting wastes were either sluiced directly into streams or washed into them by flooding. Prior to pond construction, flooding in the early 1900s distributed these tailings along the flood plain of the Clark Fork River, resulting in flood plain and river bed sediment significantly enriched in Ag, As, Cd, Cu, Pb, and Zn (12-14). The sulfidic flood plain tailings deposits are a perpetual, non-point source of metal contamination
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TABLE 1
Analytical Methods, Precision of Analyses, Concentrations in Blanks, and Detection Limits
FIGURE 1. Study site location.
to the river, both from erosion of tailings and from leaching of contaminated sediments (11, 15, 16). The water chemistry of the upper Clark Fork River is also influenced by water treatment at Warm Springs Ponds, where lime is added to incoming water to facilitate the precipitation of metals. As a result, the river is slightly alkaline, pH 8-8.7, and the water is saturated with respect to CaCO3. Calcium carbonate encrustations form on stream cobbles during some periods of the year, effectively buffering potential acid contribution from flood plain tailings deposits. The longitudinal chemical gradients observed in many rivers affected by acid mine drainage (17-20) do not occur in this river despite sulfidic flood plain deposits because it is alkaline at its inception. Because of this, water chemistry in the upper river is relatively constant both temporally (within seasonal cycles) and spatially (21). Metal concentrations are correlated with suspended sediment, thus the highest concentrations tend to occur with runoff in the spring (21). Metal concentrations in fine-grained bed sediments have been shown to decrease exponentially away from the source, consistent with the physical mixing of contaminated tailings with clean river sediments (12, 14). The study site is located on the Grant-Kohrs Ranch National Historic Site in Deer Lodge, 37 river km downstream from Warm Springs Ponds. Access to the river is limited in this area, and cattle are fenced out, thus preventing disturbance of the river bed during sampling. The portion of the river sampled was a riffle in midstream where water depth was approximately 30 cm. The average gradient through the study reach is 0.004. The study site is 2 km downstream from a U.S. Geological Survey gaging station in Deer Lodge (no. 12324200) where streamflow and water quality have been measured regularly since 1982. The river bed consists of cobbles, sand, and gravel covered with ferromanganese oxide coatings containing high concentrations of Cu and Zn (ca. 3200 ppm Cu, 15,000 ppm Zn, unpublished data). The cobble surfaces support abundant growth of periphyton, especially in the summer months. High trace metal concentrations (1000-2000 ppm Cu and Zn) also occur in pockets of fine-grained bed sediment in the study reach (12, 14).
Methods Diel Water Sampling. Water samples were collected hourly for 46 h in late July 1994 during a period of warm, clear weather and low discharge in the river. Samples were taken for anions (Cl-, SO42-, and NO3-), major cations (Ca, Mg,
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parameter
method
precisiona (%)
blank (µM)
detection limit (µM)
alkalinity Cl SO4 NO3 Ca Mg Na Si Al acid-soluble Cu dissolved Cu Fe Fe(II) Mn Zn
gran titration ion chromatogr ion chromatogr ion chromatogr ICAPES ICAPES ICAPES ICAPES ICAPES ICAPES GFAAS ICAPES FerroZine ICAPES ICAPES
8 2 2 10 2 2 5 2 10 2 7 2 5 2 2
