Sand Hills Stream Water Quality: implications for downstream users
Steven Thomas, Tiffany Messer
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The export of DOC from the Sand Hills may have major implications for downstream water quality. Recent studies have shown an inverse relationship between organic carbon and nitrate across a wide range of environments (Taylor and Townsend, 2010), including streams draining a broad range of terrestrial settings. We hypothesize that water originating in the Sand Hills is relatively low in nitrate due to sufficient DOC supply. While there are potentially several sources of DOC within a watershed, we propose that they can be separated into two broad categories that may can have differing repercussions for DOC-nitrate interactions. First, DOC can be formed in the shallow soil where decomposing terrestrial litter leaches dissolved organic compounds. Second, DOC also forms within stream channels or in adjacent riparian wetlands via riparian litter decomposition and DOC secretion by actively photosynthesizing algae, submerged macrophytes, and wetland vegetation. We hypothesize that the influence of DOC on nitrate flux from the Sand Hills depends on which source dominates DOC delivery to streams (Fork and Heffernan, 2013). If most instream DOC originates in the upland soils and travels with nitrate to the stream via groundwater discharge, we would predict poor quality DOC to dominate and expect reduced influence of this pool on downstream nitrate concentrations. If a significant portion of instream DOC is produced in riparian wetlands or within the stream ecosystem, then we predict the production of higher quality DOC should stimulate microbial activity and reduce nitrate concentrations and downstream flux.
Though these models are not mutually exclusive, understanding which one dominates in Sand Hill rivers has important implications for how exported DOC will impact how downstream reaches process nitrate loading from increased agricultural runoff. Therefore, a third goal of this research is to examine whether DOC transport from sand hills rivers mediates nitrate levels as downstream reaches traverse areas of intense agricultural activity increase.
Identifying mechanisms that remove of large amounts of nitrate from stream water would represent a significant environmental and economic benefit to a broad variety of stakeholders, including municipalities that use surface water or alluvial groundwater for drinking water (U.S. EPA 2015).
The goal of this project is to evaluate the quantity and quality of DOC delivered to and exported from headwater streams in the Sand Hills, and to document longitudinal patterns in the DOC - nitrate relationship within Sand Hill rivers and along downstream reaches that traverse landscapes with intensive agricultural activity. Our specific goals are designed to:
(1) Document longitudinal patterns DOC and nitrate quantity
(2) Assess DOC quality and how it changes as river systems form and traverse the Nebraska landscape (where higher quality DOC is more labile, and available for microbial processes that remove nitrate, e.g. Stelzer et al.2014)
U.S. Environmental Protection Agency (2015) A compilation of cost data associated with the impacts and control of nutrient pollution.
Fork, M. L., and J. B. Heffernan (2013), Direct and Indirect Effects of Dissolved Organic Matter Source and Concentration on Denitrification in Northern Florida Rivers, Ecosystems, 17(1), 14–28, doi:10.1007/s10021-013-9705-9.
Stelzer, R. S., J. T. Scott, L. A. Bartsch, and T. B. Parr (2014), Particulate organic matter quality influences nitrate retention and denitrification in stream sediments: evidence from a carbon burial experiment,
Biogeochemistry, 119(1–3), 387–402, doi:10.1007/s10533-014-9975-0.
Taylor, P. G., and A. R. Townsend (2010), Stoichiometric control of organic carbon-nitrate relationships from soils to the sea, Nature, 464(7292), 1178–1181, doi:10.1038/nature08985.