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Scientific Investigations Report 2007–5239

Scientific Investigations Report 2007–5239

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Previous Near-River Redox Work in Study Area

Hydraulic gradients between ground water and surface water at 32 sites in the study area were measured with a minipiezometer during autumn 2000 (Morgan and others, 2007). One or more measurements of hydraulic gradient were made at each site, at depths of 1.0, 2.0, and (or) 3.0 ft. In conjunction with that work, reconnaissance-level geochemical data were collected at seven sites along the Little Deschutes River (table A2, appendix A). One sample was collected at the center of the river at each site. Additional samples were collected at one of the seven sites: one sample half-way between the center of the river and the edge of the river in each direction. Thus, six sites were represented by single samples, and one site (at river mile 24.9) was represented by three samples.

Samples were analyzed for NO3- and NH4+. O2 was not measured, but the presence or absence of H2S odor was noted (the presence of H2S odor indicates highly reducing water). Samples were analyzed by Oregon Department of Environmental Quality, by methods described in Hinkle and others (2007). Collection and processing protocols were the same as those used in the current study.

Samples collected at each of the six single-sample sites contained H2S, indicating the presence of highly reducing water; these samples were considered to be suboxic. These six samples contained NH4+ at concentrations ranging from 0.34 to 2.6 mg N/L and NO3- at concentrations less than 0.1 mg N/L, reflecting the suboxic conditions. The presence of suboxic ground water is consistent with concepts that describe mechanisms by which ground water may evolve to suboxic conditions in the aquifer prior to reaching the near-river environment, or may become reduced in the near-river environment.

Samples collected at the multi-sample site (Little Deschutes River at river mile 24.9) indicated spatially variable redox conditions. The sample near the left edge of water (defined from a position facing downstream) contained H2S, NH4+ at a concentration of 0.31 mg N/L, and NO3- at a concentration of only 0.011 mg N/L, consistent with reduced conditions. The other two samples contained no detectable H2S, contained NH4+ at concentrations less than the Oregon Department of Environmental Quality MRL of 0.02 mg N/L, and contained NO3- at elevated concentrations (1.29 and 2.73 mg N/L). These data indicate that less reduced (apparently either O2- or NO3--reducing) ground water was present in these two samples. The samples were collected from the center of the river and from a point near the right edge of water. The site was where the Little Deschutes River bends toward the eastern edge of the riparian zone. Less reduced ground water entering the river in the center and right edge where the right edge of the river cuts toward the edge of the riparian zone also is consistent with concepts presented earlier in this report.

Of these seven sites, the site on the Little Deschutes River at river mile 24.9 was particularly valuable because (1) NO3- was detected in some ground water under the river, and (2) the boundary between NO3--bearing (either O2- or NO3-‑reducing) and more reduced ground water was identified. This site was resampled in greater detail as part of this study.

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