Date of Graduation

8-2019

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Hays, Phillip D.

Committee Member

Pollock, Erik D.

Second Committee Member

Sharpley, Andrew N.

Keywords

Nutrient Tracing; Stable Isotopes; Water-Quality

Abstract

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.

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