Environ. Sci. Technol. 2003, 37, 5127-5134
Biogeochemical Processes and Microbial Characteristics across Groundwater-Surface Water Boundaries of the Hanford Reach of the Columbia River D U A N E P . M O S E R , * ,† JAMES K. FREDRICKSON,† DAVID R. GEIST,† EVAN V. ARNTZEN,† AARON D. PEACOCK,‡ SHU-MEI W. LI,† TINA SPADONI,† AND JAMES P. MCKINLEY† Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, and Center for Environmental Biotechnology, University of Tennessee, Knoxville Tennessee 37932-2575
Biogeochemical processes within riverbed hyporheic zones (HZ) can potentially impact the fate and transport of contaminants. We evaluated a modified freeze core technique for the collection of intact cobble-bed samples from the Columbia River HZ along a stretch of the Hanford Reach in Washington State and investigated microbiological and geochemical parameters of corresponding frozen and unfrozen samples. During three sampling periods (March, May, and November 2000), relatively high numbers of viable aerobic heterotrophic bacteria were recovered from both unfrozen (106-107 cfu/g) and frozen samples (105-106 cfu/g). Relatively large populations of sulfate-, nitrate-, and iron-reducing bacteria were present, and significant concentrations of acid-volatile sulfide were measured in some samples, indicating that anoxic regions exist within this zone. Cr(VI), a priority groundwater pollutant on adjacent U.S. Department of Energy lands, was probably removed from solution in HZ samples by a combination of microbial activity and chemical reduction, presumably via products of anaerobic microbial metabolism. These results suggest that biogeochemical processes in the Columbia River HZ may contribute to the natural attenuation of Cr(VI). Although freezing modestly diminished recovery of viable bacteria, freeze core techniques proved reliable for the collection of intact hyporheic sediments.
Introduction Being physically independent and chemically distinct, groundwater and surface water environments are typically studied separately and in different ways (1-4). Some recent research, however, has focused on the zone where these waters interact beneath the streambed: the hyporheic zone (HZ; 1, 5, 6). Hyporheic flow can represent a significant proportion of basin discharge (7, 8). The flux of solutes through this zone is * Corresponding author phone: (509)372-2098; fax: (509)376-9650; e-mail:
[email protected]. † Pacific Northwest National Laboratory. ‡ University of Tennessee. 10.1021/es034457v CCC: $25.00 Published on Web 10/10/2003
2003 American Chemical Society
spatially and temporally dynamic, governed by physical variables imposed by overlying streamwaters and tempered by hydrogeologic characteristics (5). The microbiological and biogeochemical properties of large river HZ remain fundamentally unaddressed. The Columbia is the largest river west of the North American Continental Divide, occupying a drainage of 671 000 km2 (9) and producing a year-2000 average discharge of 215 600 ft3/s (10), roughly double that of the Nile in Egypt (11). With the bulk of its flow being derived from snowmelt and owing to swift passage through arid lands, the river is low in organic carbon content, and correspondingly, diminished microbial populations might be anticipated. However, trophic relationships within HZ are likely to be controlled by additional factors. Advection- and diffusion-facilitated cycling allow HZ to act as both source and sink of nutrients, potentially regulating biotic productivity (3, 12). Whereas, HZ interstitial waters tend to be oxidizing, zones of microscale anoxia may develop within detrital aggregates and lithotrophic biofilms, even in areas of rapid infiltration (12, 13). Adjacent aerobic and anaerobic microzones may promote the exchange of redox-sensitive metabolites [e.g., Fe(II)/Fe(III)] between physiologically diverse microorganisms, thereby producing an overall stimulatory effect on gross microbial metabolism and community diversity. The River’s “Hanford Reach” in south-central Washington represents the last unimpounded segment within the United States and sole remaining main-stem spawning habitat for fall chinook salmon (Oncorhynchus tshawytscha) (14). The Hanford Reach also bisects the U.S. Department of Energy (DOE), Hanford Site, which receives groundwaters “contaminated with metals and radionuclides”, such as Cr, U, and Tc, resulting from Cold War weapons production. Given the aerobic nature of both the river and the Hanford unconfined aquifer, the expectation has been that transport of these contaminants (mobile in their most oxidized state) to the river would be unimpeded. In contrast to Cr(VI), Cr(III) [as Cr(OH)3] is relatively nontoxic because it is insoluble above pH 5.5 (15, 16) and, once immobilized in this manner, relatively refractory to oxidation by O2 (17). At the Hanford Site, remediation programs employing pump-and-treat as well as in situ redox manipulations to block chromium transport to the river are in progress (18). In the latter, the chromium is immobilized within the aquifer and thus prevented from reaching the river. Alternatively, these elements could be naturally reduced to less bioavailable forms within the HZ by fortuitous biological activity or metabolic byproducts if anaerobic zones were present. However, ephemeral releases from sequestered environmental concentrates remain a possibility against the backdrop of changing redox conditions. The hyporheic zone may thus represent both the last line of defense in the prevention of river contamination and a factor leading to toxic exposures of sensitive species. The objectives of this study were to employ a “freeze core” method to collect intact cobble-bed samples and to obtain information concerning the geochemistry and microbiology of sediments within these zones. This information is required to improve predictions of the fate and transport of groundwater contaminants to surface waters and to minimize their impact on stream and riparian biota. We assess the effectiveness of the freeze core method for sampling HZ material and consider the ecological and site management implications of the results obtained. VOL. 37, NO. 22, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Sampling locations (open circles). Sampling site names are in river km (Rkm), as measured from the mouth of the Columbia. Salmon spawning habitat is denoted by crosshatch pattern, and groundwater nitrate concentration isopleths are denoted as solid lines. Groundwater nitrate data is for the year 2001. Adapted from ref 42.
TABLE 1. Porewater and River Water Chemistry AVSa,b
pH
SO42- b
NO3-
Cl-
DICd
Ca2+
Mg2+
Na+
Fee
U
Cr
Ambient Temperature Cores 1.9 3.4 18.3 19.0 1.7 2.7 24.8 14.7 3.3 1.6 4.6 16.6
21.7 20.1 19.9
6.0 5.2 5.1
4.0 3.8 3.4
82 79 74
0.69 0.41 0.61
4 >4 >4 2
3 >4 >4 5 4 4 3 4 3 2
ndc nd nd nd nd nd 7 7 6 5
FC1d FC2 FC3 FC4 FC5 FC6 FC7 FC8 FC9 FC10
6 6 6 6 6 5 6 5 5 4
3 >4 >4 4 >6 5 >8 5 5 5
2 2 2 1 >3 2 3 4 4 4 3 4 4 2 1
