Isotope Investigation of Nitrate in Soils and Agricultural Drain Waters of the Lower Yakima Valley, Washington

Nitrate in the groundwater of the Lower Yakima Valley, Washington has long been known to frequently exceed the EPA maximum contaminant level standard for potable water (10 mg/L), adversely impacting communities with disadvantaged socio-economic status. In this research, nitrogen and oxygen isotopic signatures were determined for nitrate soil leachates and irrigation return flow collected in the Lower Yakima Valley, Washington and compared to previous isotopic studies of nitrate in Central Washington. δ¹⁵N, δ¹⁸O and Δ¹⁷O values are used to constrain sources of to groundwater. Isotope signatures for nitrate from soil leachate largely overlapped with the point cloud of data for nitrate in groundwater in a local EPA study. The groundwater nitrate was largely attributed to a mixture of manure and fertilizer. However, isotope signatures in this study also overlapped with isotopic values attributed to naturally occurring soil nitrate from a study at the nearby Hanford Site, Washington. A mass balance calculation based on Δ¹⁷O data suggests that there is a ~9% atmospheric contribution to nitrate in soil accumulations below caliche layers at several locations. This atmospheric input was consistent across multiple sites. We argue that the consistent atmospheric component implies the nitrate in these soil samples at depth appears to have a largely predominately non-anthropogenic origin, because a significant anthropogenic input would dilute and cause variation in this atmospheric signature. We suggest the flushing of naturally occurring soil nitrate to groundwater during land use conversion to irrigated agriculture may represent a previously overlooked, significant, nitrate input to shallow alluvial aquifers in this region.

Name thesis:degree_name

Master of Science (MS)

Discipline thesis:degree_discipline

Geological Sciences

Year dc:date.available

2017

Author dc:creator

• Jensen, Dallin

Contributors dc:contributor

• Carey Gazis
• Susan Kaspari
• Anne Johanson

• Nitrate
• Groundwater
• Soils
• Isotope Geochemistry
• Hydrogeology
• Yakima Basin
• Earth Sciences
• Geochemistry
• Physical Sciences and Mathematics

Language dc:language

English

Repository record dc:identifier

https://digitalcommons.cwu.edu/etd/672

OAI identifier oai:identifier

oai:digitalcommons.cwu.edu:etd-1682