Title

Tracing Atmospheric Nitrates in the Lower Yakima Valley Basin, Washington

Presenter Information

Gabrielle Cavanaugh

Document Type

Oral Presentation

Location

SURC 140

Start Date

16-5-2013

End Date

16-5-2013

Abstract

A number of past studies have shown that many groundwaters in the lower Yakima Valley are contaminated with high concentrations of nitrate. Recently, the Environmental Protection Agency (EPA) conducted a study to trace the sources of these high nitrate levels and found that some display isotope ratios indicative of an atmospheric source. It is hypothesized that these signatures are a result of leeching from caliche, a calcium carbonate residue in soils. This source was proposed because of the abundance of caliche in the basin and few alternatives that might impart atmospheric isotope signatures on ground water. Other known sources of nitrate include fertilizer, dairy lagoons, combustion, and lightning. This study uses stable isotopes of nitrogen and oxygen to search for these atmospheric sources of nitrates in water and soil samples. Soil, surface water, and groundwater samples were collected from the Lower Yakima Basin, Washington, in the summer of 2012 and analyzed with two methods: major-ion chromatography for nitrate concentrations, and a bacterial denitrification technique for analyzing stable isotopes in nitrate. The results indicate that two of the five caliche samples have atmospheric 15N/14N and 18O/16O signatures while the remaining three have much lower 18O/16O ratios than atmospheric nitrate. Waters leached from the soil samples had isotope ratios indicative of manure and soil nitrogen sources, but not atmospheric. Further testing of the soils should be done to determine the extent of the caliche nitrate and to examine how denitrification or other biological processes affect its isotopic composition.

Faculty Mentor(s)

Audrey Huerta

Additional Mentoring Department

Geological Sciences

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May 16th, 2:40 PM May 16th, 3:00 PM

Tracing Atmospheric Nitrates in the Lower Yakima Valley Basin, Washington

SURC 140

A number of past studies have shown that many groundwaters in the lower Yakima Valley are contaminated with high concentrations of nitrate. Recently, the Environmental Protection Agency (EPA) conducted a study to trace the sources of these high nitrate levels and found that some display isotope ratios indicative of an atmospheric source. It is hypothesized that these signatures are a result of leeching from caliche, a calcium carbonate residue in soils. This source was proposed because of the abundance of caliche in the basin and few alternatives that might impart atmospheric isotope signatures on ground water. Other known sources of nitrate include fertilizer, dairy lagoons, combustion, and lightning. This study uses stable isotopes of nitrogen and oxygen to search for these atmospheric sources of nitrates in water and soil samples. Soil, surface water, and groundwater samples were collected from the Lower Yakima Basin, Washington, in the summer of 2012 and analyzed with two methods: major-ion chromatography for nitrate concentrations, and a bacterial denitrification technique for analyzing stable isotopes in nitrate. The results indicate that two of the five caliche samples have atmospheric 15N/14N and 18O/16O signatures while the remaining three have much lower 18O/16O ratios than atmospheric nitrate. Waters leached from the soil samples had isotope ratios indicative of manure and soil nitrogen sources, but not atmospheric. Further testing of the soils should be done to determine the extent of the caliche nitrate and to examine how denitrification or other biological processes affect its isotopic composition.