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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5239

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Summary

The redox state of ground water exerts a strong control on nitrate (NO₃^-) stability, and therefore on NO₃^- discharge from aquifers to rivers. Oxic ground water was uncommon in sub-river ground-water samples in the La Pine area. Ground-water samples from transects of temporary wells installed near the center of the riparian zone were always suboxic. Where transects were located near edges of riparian zones, ground-water samples from transect centers were always suboxic, as were ground-water samples from points on the inside part of each transect (toward the center of the river/riparian zone system). Oxic ground water (other than hyporheic water), if detected, was detected only near the outside edge of transects at meander bends. However, zones in which oxic ground water discharges to rivers represent locations of potential river vulnerability to NO₃^-.

The redox state of ground water in near-river sediment is controlled by geochemical processes occurring throughout the aquifer and also by geochemical processes occurring near the intersection of riparian zones and ground-water discharge zones. Younger, less geochemically evolved ground water generally enters the near-river environment near the edges of the river corridor, whereas older, more geochemically evolved ground water (water that generally has traversed more of the aquifer) tends to be in more regional flow systems that discharge to locations closer to the center of the river corridor. Riparian zone processes that occur in many near-river sediments become important as ground water nears discharge points in or adjacent to rivers; lateral erosion of river systems away from the center of the flood plain can decrease interactions between ground water and reducing riparian zone sediments. Thus, ground water redox patterns in near-river sediments reflect the position of a river within the river/riparian zone/aquifer continuum. Spatial heterogeneity of redox conditions in the near-river environment reflects the interplay of these (aquifer and riparian zone) processes, and makes it difficult to quantitatively define conditions under which oxic ground water might be expected in near-river sediments. Nevertheless, this general understanding of factors controlling near-river redox state, particularly if coupled to a ground-water-flow-path-based understanding of linkages between upgradient recharge areas and rivers, may be useful for managing river water quality.

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