Stable Isotope and Geochemical Characterization of Nutrient Sources in the Big Creek Watershed of Northwest Arkansas

<p>The establishment of a concentrated animal-feeding operation (CAFO) in Newton County, Arkansas near Big Creek, a tributary of the Buffalo National River, has raised concern over potential degradation of water-quality in the watershed. In this study, isotopic tools were combined with standard geochemical approaches to characterize nutrient sources and dynamics in Big Creek. An isotopic and geochemical reference library of potential nutrient sources in the Big Creek watershed was established by direct sampling of representative potential sources, including septic-system effluent, poultry litter, swine and cattle manure, and CAFO waste lagoons. Representative nutrient sources and Big Creek stream samples were analyzed for δ¹⁵N-NO₃, δ¹⁸O-NO₃, δ¹⁸O-PO₄, and a cation and anion suite. Big Creek stream samples were also analyzed for δ¹⁸O-H₂O and δ²H-H₂O. Nutrient concentrations and isotopic data provide evidence of modification of potential nutrient source signatures by nitrification, atmospheric deposition, evaporation, and denitrification. Chloride-to-bromide ratios of stream samples indicated an anthropogenic influence in Big Creek that could have resulted from any combination of the analyzed sources. Samples taken from the CAFO waste lagoon, a septic system, field and parking-lot runoff, fertilizer, and hog manure exhibited different δ¹⁵N-NO₃ and δ¹⁸O-NO₃ as compared to stream samples. Stream δ¹⁵N-NO₃ and δ¹⁸O-NO₃ cannot be explained by direct input of any one of these potential sources without modification of isotopic composition by mixing or fractionation. Big Creek NO₃ isotope values (-7.59‰ to 9.10‰ δ¹⁵N-NO₃ and -3.41‰ to 6.71‰ δ¹⁸O-NO₃) were similar to NO₃ values expected from nitrification of nitrogen stored in soils sampled in the watershed (3.8‰ to 6.6‰ δ¹⁵N-NO₃ and 3.4‰ to 4.8‰ δ¹⁸O-NO₃). The NO₃ isotope data indicate stream NO₃ is derived in part from NO₃ stored in soils. Discrimination of nutrient source input to Big Creek using δ¹⁸O-PO₄ was complicated by overlap between potential source δ¹⁸O-PO₄ and stream δ¹⁸O-PO₄. Stream equilibrium δ¹⁸O-PO₄ values indicated the influence of both isotopically light and heavy phosphate sources in Big Creek, and the in-stream biological processing of PO₄. The results of this study highlight the importance of effective agricultural, residential, and urban best-management practices in protecting the quality of our waterways. </p>

Name thesis:degree_name

Master of Science in Geology (MS)

Level thesis:degree_level

Thesis

Year dc:date.available

2019

Author dc:creator

• Sokolosky, Kelly Robin

Advisor dc:contributor.advisor

• Hays, Phillip D.

Contributors dc:contributor

• Pollock, Erik D.
• Sharpley, Andrew N.

• Nutrient Tracing
• Stable Isotopes
• Water-Quality
• Environmental Indicators and Impact Assessment
• Environmental Studies
• Geochemistry
• Water Resource Management

Repository record dc:identifier

https://scholarworks.uark.edu/etd/3400

OAI identifier oai:identifier

oai:scholarworks.uark.edu:etd-4950