Isotope Investigation of Nitrate in Soils and Agricultural Drains of the

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An isotope investigation of nitrate in soils and agricultural drains of the lower Yakima Valley, Washington Dallin P. Jensen, and Carey Gazis ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00086 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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ACS Earth and Space Chemistry

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An isotope investigation of nitrate in soils and agricultural drains of the lower

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Yakima Valley, Washington

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Dallin P. Jensen

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Carey Gazis

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[email protected], [email protected], Department of Geological Sciences, 312 Science

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II, Central Washington University, Ellensburg, WA 98926

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Keywords: Stable isotope, oxygen-17, nitrate in soils, atmospheric nitrate, semi-arid, , caliche

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Abstract

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Nitrate in the groundwater of the lower Yakima Valley, Washington frequently

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exceeds the EPA maximum contaminant level standard for potable water (10 mg/L),

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impacting communities with disadvantaged socio-economic status. Nitrogen and

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oxygen isotopic signatures were determined for nitrate in soil leachates and

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irrigation return flow. Isotope signatures for nitrate from soil leachate had

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significant overlap with both the isotope signatures of naturally occurring soil

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nitrate at the nearby Hanford Site, Washington and of groundwater nitrate

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attributed to manure and fertilizer application in a local EPA study. A mass balance

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calculation using Δ17O data suggests that there is a consistent ~9% atmospheric

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contribution to nitrate in soil accumulations below caliche layers at several

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locations. This agrees with other research on the atmospheric contribution to

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naturally occurring soil nitrates in areas with similar mean annual precipitation

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values. We argue that this consistent ~9% atmospheric component indicates that

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soil nitrate at depth is dominated by naturally occurring, biologically fixed nitrate

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across multiple sites. We suggest the flushing of naturally occurring soil nitrate to

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groundwater during land use conversion to irrigated agriculture may represent a

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previously overlooked significant nitrate input to aquifers in this region.

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Introduction The global nitrogen cycle

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has been greatly perturbed

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by human activities since the

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advent of the industrial

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revolution. Resultant

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environmental impacts have

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resulted in great scientific

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effort in understanding

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nitrogen sources at many

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spatial scales, with stable

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isotopes frequently

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employed as one of the few

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successful approaches for

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understanding nitrate

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source and fate1. The

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aquifers of the lower Yakima Valley, Washington (Figure 1) have been known since

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2002 to contain water with nitrate in excess of the Environmental Protection

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Agency’s (EPA) Maximum Contaminant Level (MCL) for drinking water of 10 mg/L2.

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In 2008, the Yakima Herald Republic highlighted that nitrate-contaminated domestic

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wells are disproportionately impacting a disadvantaged community of Latino


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farmworkers3. This spurred the EPA to investigate and largely attribute

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groundwater contamination to local dairy farms. This attribution was based on

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agricultural chemicals in both dairy manure lagoons and downgradient wells,3 the

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assumption that anthropogenic inputs dominate nitrate sources to groundwater,

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and the use of nitrate stable isotope analysis which has a well-established scientific

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backing4. In addition, the EPA study found nitrate in a few disparately distributed

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wells to be anomalously enriched in 18O suggesting potential atmospheric nitrate

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inputs to some groundwater. They suggested that in this dry environment,

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atmospherically deposited nitrate may accumulate in caliche horizons in soils along

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with carbonates and other minerals3. This study was undertaken to explore this

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possibility, incorporating Δ17ONO3- data for the first time in this region, as it has been

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successfully employed in other landscapes for nitrate source attribution to constrain

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the significance of atmospheric nitrate5,6. A separate stable isotope study at the

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Hanford Site (80 km away) concluded that vadose soil nitrate, naturally occurring at

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depths of greater than 20 meters, was flushed into groundwater after the discharge

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of wastewater onto the ground surface. Our study further investigates the role of

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soils and atmospheric inputs as a potential source of nitrate to groundwater in the

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lower Yakima Valley by analyzing stable isotopes of nitrate from soil leachates and

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irrigation return flow in the region.

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Site Description

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The lower Yakima Valley naturally experiences limited natural groundwater

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recharge (2.5 mg/L and the agricultural drain

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water samples were analyzed at the University of Pittsburgh’s Regional Stable

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Isotope Laboratory to obtain δ17ONO3― , δ18ONO3― , and δ15NNO3― values (±0.1‰) using



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the Sigman – Casciotti bacterial denitrifier method13, and a continuous flow GV

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Instruments IsoPrimeTM stable isotope ratio mass spectrometer. The remaining

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leachates with nitrate concentrations between 1.0 mg/L and 2.5 mg/L were

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analyzed using the same method at the Washington State University Stable Isotope

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Core Laboratory for δ18ONO3― and δ15NNO3― 14 (±0.05‰) using a Gas Bench II and a

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ThermoFinnigan Delta V Ratio Mass Spectrometer. All oxygen isotope data

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presented are calculated with respect to Vienna Standard Mean Ocean Water

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(VSMOW) and reported in per mil. The relationship between δ18ONO3― and δ17ONO3―

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values during mass dependent fractionation was used to calculate Δ17ONO3-15 where:

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Δ17O =δ17O -0.52 x δ18O (1)

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Positive Δ17O values were used to determine the fraction of nitrate from

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atmospheric sources (fatm) which can be estimated based on mass balance

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considerations that assume a mixture of atmospheric nitrate and nitrate along the

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terrestrial fractionation line using the equation16,17:

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𝑓𝑎𝑡𝑚 =

𝛥17𝑂 +23.4‰

(2)

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Equation 2 uses the assumption that Δ17O values will fall under a two end-member

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mixing line where non-atmospheric nitrate will have a value of 0.0‰ and

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atmospheric nitrate will have a value of 23.4‰.

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Results and Discussion


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Nitrate Concentrations. Analytical data for nine soil sites and two irrigation return

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flow sites are presented in Supplementary Information Table S1. For the nine soil

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sampling locations, nitrate in soil leachate was typically detected at levels above 1

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mg/L for at least some portion of the soil profile. Of the nine soils sites sampled,

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three were found to have leachate nitrate concentrations above 5 mg/L and selected

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for detailed analysis. With respect to soil pore water, the nitrate concentrations

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measured in this study have a relative, and not an absolute relationship. It is likely

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that actual nitrate concentrations in soil pore waters are higher because of the 10:1

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mixing ratio between DI water and soil, and the short (10 minute) period of time in

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which leaching occurred. Downward percolating water interacts with the soil

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column in a natural environment for much longer time periods and the ratio of

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water to soil is usually less than 1:1.

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Two of these sampling sites were from road cuts, while the third was

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obtained from a moist silt loam likely wetted by irrigation water. All of these sites

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allowed unusually easy access for sampling throughout the soil profile. In the two

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roadcut soils, the highest nitrate concentrations lie within the caliche horizon. This

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spatial pattern suggests a sampling bias between road cuts and soil cores due to

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difficulty in penetrating nitrate rich caliche layers with a soil corer. IN3 (apple

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orchard) and IN1 (shrub steppe roadcut) had relatively low nitrate concentrations,

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however IN4 (shrub steppe eight meters south of orchard) had nitrate

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concentrations as high as 9.49 mg/L. This may be due to migration of irrigation

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water downgradient from the orchard to IN4. This is likely because IN4 is two



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meters lower elevation than the orchard, implying a significant hydrologic gradient

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for irrigation water.

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Notably, soil leachates from the two orchard locations (IN3 and I1) had the

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lowest concentrations of nitrate out of any produced during this study, including

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natural settings, despite known fertilizer application. It is probable that much of the

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naturally occurring or added nitrate once present in these soils has been used by

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plants or conveyed to groundwater by irrigation.

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δ18O and δ15N nitrate data.δ18O and δ15N nitrate data collected are plotted in

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Figure 2, along with typical source ranges4, data from an EPA groundwater study3,

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and the range of values found in nitrate from soil extractions at the Hanford Site18

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(~80 km east). The EPA groundwater study largely attributed well water nitrate

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contamination to dairy manure with several outliers indicative of a significant

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atmospheric nitrate contribution. The values determined for soil samples in this

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study overlapped the range of values found to occur in groundwater in the EPA

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study3. However, overlap in typical nitrate source values make it difficult to


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distinguish between a natural soil

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FIGURE 2. Chart of δ18O vs δ15N for soil leachate and agricultural drain samples. Typical nitrate isotope source ranges after Kendall et al. (2007), shaded region of natural soil pore water values for a study at the Hanford Site18 (80 km east), groundwater values plotted for an EPA study3, soil leachate from an undergraduate thesis19 and soil leachate values from this study. Two agricultural drain samples from the Marion Drain, and the Sulfur Creek Wasteway are also plotted. Arrows signify typical alteration of isotope signatures from bacterial denitrification from arbitrarily selected δ18O and δ15N values. Drains contain agricultural run-off from a large portion of the study area.

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nitrate versus a mixture of manure and ammonium fertilizer without prior land use

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information or other chemical tracers. Farmers commonly apply a mixture of

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ammonium fertilizer and synthetic nitrate fertilizer20 which will tend to result in

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higher δ18O values and lower δ15N values when mixed with either naturally

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occurring nitrate or manure in soils. During denitrification reactions, the remaining



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nitrate will move along 1:1 and 2:1 trajectories on at δ18O - δ15N plot4 (Figure 2).

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δ15N values for ammonium fertilizer may additionally experience enrichments of up

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to 15‰ during ammonium devolatilization4, this fertilizer may then be converted to

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nitrate.

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Δ17O nitrate values. Δ17O nitrate values of soil leachates and agricultural drains

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ranged from -1.2 to +2.2‰ (Figure 3). It is unknown why Δ17O values may deviate

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negatively from the mass dependent δ 17O versus δ18O line. However, significantly

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negative Δ17O values occurred exclusively in irrigation return flow and irrigation

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influenced soils (IN4). Other negative values have been reported before for biogenic

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soil nitrate in a forested catchment21.

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FIGURE 3. Plot of 𝛅𝟏𝟖𝐎𝐍𝐎𝟑― versus 𝛅𝟏𝟕𝐎𝐍𝐎𝟑― for soils and agricultural drains sampled in the lower Yakima Valley, the trend for caliche containing soils at depth, and the Terrestrial Fractionation Line (TFL) for mass dependent fractionation.


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Figure 4. Trends in nitrate isotope ratios with interpretations (a, d, g), nitrate and carbonate concentrations (b, e, h), and percent of nitrate with atmospheric origin (c, f, i) versus depth for locations RC2, RC1 and IN4. Caliche was observed in the field where carbonate content was found to exceed five percent.

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Interpretation.Soil samples taken from RC2 presented complex trends in δ15NNO3―

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and δ18ONO3― (Figure 4a). The shallowest sample analyzed (15-30 cm) had values

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typical of biologic nitrate, potentially due to biotic processing in the root zone

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(Figure 4c).. The 30-45 cm soil interval showed anomalously high values of δ15NNO3―

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(+12.6‰) and δ18ONO3― (+12.9‰) (Figure 4a). These high values may be explained



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through denitrification fractionation of biologic soil nitrate4. Δ17O values indicated a

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small atmospheric contribution of nitrate to these soil depth intervals. The depth

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intervals 45–60 cm and 60–75 cm yielded δ15NNO3― (+0.2 to 4.6‰) and δ18ONO3―

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(+10.8 to 12.9‰) values suggestive of a mix of biologic and atmospheric nitrate or

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synthetic fertilizer. This depth interval also has relatively low leachate nitrate

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concentrations between 1.5 and 1.0 mg/L. Nitrate concentrations then increase

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through the depth intervals 75-90 cm, 90–105 cm, and 105–120 cm, from 1.2 to 3.0

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mg/L, while δ15NNO3― (+1.0 to 6.4‰) and δ18ONO3― (+5.3 to 6.3‰) more strongly

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reflect biologic nitrate. Δ17O values were near zero from 15–45 cm but were 1.8–

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2.3‰ from 90–120 cm indicating an 8 to 10% atmospheric contribution which

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reached these depths without biologic remediation (Figure 4a). RC1 soil leachate from the shallowest depth interval (0 cm to 15 cm) has δ

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15

NNO3― (8.0‰) and δ18ONO3― (5.7‰) values consistent with biologic nitrate, with a

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nitrate concentrations of 3.2 mg/L (Figures 4c and 4d). The depth interval from 15

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to 30 cm shows δ15NNO3― (1.8 ‰) and δ18ONO3― (7.9‰) values which may be from a

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mixture of biologic nitrate and nitrification of ammonium in precipitation. Leachate

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from 30 to 45 cm yielded slightly elevated δ15NNO3― (9.0‰) and δ18ONO3― (8.2‰)

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values interpreted to be from biologic fixation and then denitrification. Δ17O values

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for 0 to 45 cm indicated a negligible atmospheric contribution (10‰), but δ

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15

NNO3― values (0.2 to 4.6 ‰) were too low to suggest significant denitrification had



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occurred to alter these isotope signatures. The Yakima Valley is known to have high

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concentrations of ammonium nitrate pollution in winter22, which may be deposited

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in the snow pack and subsequently flushed to the subsurface during spring snow

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melt. Over long periods of time this process may have the potential to significantly

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increase nitrate accumulations in the subsurface.

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All samples from soil pit IN4 (Figures 4g-i), except the sample taken from 75–

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90 cm, exhibited δ15NNO3- values higher than the ammonium chemical fertilizer

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range of -10 to +44, and within the natural soil range observed at the Hanford site

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(+3‰ to +8‰)18. It is particularly challenging to uniquely determine nitrate

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sources in an area with abundant natural soil nitrate18 and known usage of both

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chemical ammonia and manure fertilizer. However, the land leaser at site IN stated

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that only chemical ammonia fertilizer has been used on the site since at least 2004.

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Δ17O values were slightly negative, indicating no significant atmospheric nitrate was

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present. Considering that the natural soil nitrate in RC1 and RC2 has an

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atmospheric component of between 7 and 10 percent, the absence of this

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atmospheric component in IN4 is interpreted as largely reflecting agricultural

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inputs. However, higher biological cycling from increased moisture availability via

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throughflow from a nearby irrigated orchard may be an alternate explanation for

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the lack of positive Δ17O values. The δ15NNO3― and δ18ONO3― values for IN4 are similar

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to those for RC1 and RC2, demonstrating the difficulty associated with

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distinguishing natural soil nitrate from anthropogenic inputs in this area using only

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those isotopes.


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The Marion Drain, and the Sulfur Creek Wasteway samples (agricultural

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return flow) yielded δ15NNO3― values of 7.1‰ and 10.1‰ respectively and δ18ONO3―

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values of –4.2‰ and –1.9‰ respectively (Figure 2). These values are consistent

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with a complex mixture of nitrate produced from nitrification of ammonium in

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fertilizer and manure, natural soil nitrate, and nitrate fertilizer. Δ17ONO3- values were

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–0.9‰ and –1.4‰, indicating no atmospheric contribution (Figure 3).

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We conclude that natural soil nitrate represents a potential source of nitrate

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in groundwater upon flushing during irrigation which may lead to elevated nitrate

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concentrations. We interpret our soil leachate data at RC1 and RC2 to reflect

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naturally occurring soil nitrate with a potential contribution of elevated

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atmospheric ammonium nitrate in winter time. IN4 is interpreted to have nitrate

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from commercial fertilizer, biologic fixation and denitrification within the soil.

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Mechanism for nitrate accumulations in high desert soils. A potential

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mechanism for large quantities of nitrate to be transported into soils is millennial-

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scale atmospheric deposition, followed by partial biologic processing. A recent

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study23, has shown that nitrogen cycling in soils depends on mean annual

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precipitation (MAP) with Δ17O values that are increasingly shifted away from the

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atmospheric value with increasing MAP due to increased biological nitrogen cycling.

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Wang et al. (2016) developed an empirical equation to describe this shift:

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MAP ln ( ) 253.8 = Δ17O (3) ―0.12



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Based on the MAP in the lower Yakima Valley (~190 mm/yr), naturally occurring

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soil nitrate would be expected to exhibit a Δ17O value of approximately +2.5‰. This

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is similar to Δ17O values observed at depth at sample sites RC2 and RC1 of +1.8 to

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+2.3‰ (Figures 2 and 4).

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The deposition of atmospheric nitrate and the cycling of nitrogen in soils

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may work in conjunction with the formation of biological soil crusts, symbiotic

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communities of fungi, cyanobacteria, bryophytes, algae and lichens. Studies in the

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cold deserts of the Colorado Plateau, southwest Utah, the Mohave Desert, and the

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Sonora Desert have investigated nitrogen cycling in these communities using micro

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sensors, acetylene reduction assays to measure N2 fixation rates, acetylene

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inhibition assays to measure denitrification rates, and measurements of ammonium

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oxidation rates24–26. The results indicate that biological soil crusts fix an order of

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magnitude more nitrate than is denitrified, due to a lack of denitrification activity,

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leading to a flux of nitrate to the soil below during percolation events24–26. These

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biological soil crusts have been documented to cover between 15 and 20 percent of

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the ground surface of the Yakima Military Training Ground to the north of the study

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area27,28. Therefore, it is possible a similar process has occurred prior to agriculture

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in the lower Yakima Valley.

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Over the long term, accumulated biologic and atmospheric nitrate are likely

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flushed to the subsurface during snow melt which dominate moisture availability in

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this basin. Downward percolating nitrate-rich soil water may be impeded by lower

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permeability caliche deposits until the hot dry summer months when


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evapotranspiration rates,and matric potential increase. Similar to other arid

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environments, as evaporation removes water from the soil column, nitrate and

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other solutes accumulate29 Alternatively, moisture during the brief wet period may

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penetrate the caliche layer and trace nitrate may become redistributed and then

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evapo-concentrated into the caliche horizon during the dry summer months, as has

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been suggest for nitrate accumulations in the Mojave Desert30. This nitrate may not

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be remediated due to a lack of denitrifying bacteria at depth leading to incomplete

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nitrogen cycling, similar to in other high desert environments25. This is supported

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by EPA groundwater data with δ18ONO3― enrichments plotting near a mixing line

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between biological soil nitrate and atmospheric nitrate, with little apparent δ15NNO3―

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, enrichment associated with denitrification (Figure 2).

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More recently, as this area experienced extensive land use conversion,

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enhancing groundwater recharge by over an order of magnitude7, it is likely that soil

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nitrate was flushed to the groundwater. This mechanism is supported by the nitrate

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accumulations documented in this study, δ18ONO3― , enrichment in a subset of area

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groundwaters3, the occurrence of similar processes in other semi-arid areas which

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have experienced land-use conversion to irrigated agriculture29, and the lack of an

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atmospheric Δ17ONO3― observed in irrigated agriculture influenced soils.

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Implications. Studies use elevated δ18ONO3― , characteristic of atmospheric nitrate, in

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groundwater to help assess if natural soil nitrates in caliche may represent a

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significant input to nitrate contaminated water3,4. However, our study has found

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natural nitrate in soil with the potential to impact groundwater lacking this well-



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known elevated δ18ONO3― signature. Studies into nitrate contamination of

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groundwater in this, and other semi-arid regions, should therefore be careful to

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avoid assigning contamination entirely to a mixture of agricultural fertilizer and

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manure based purely on the absence of δ18ONO3― enrichment. Other studies have also

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found nitrate-containing soils can be a significant input in an agricultural setting

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upon land use conversion to irrigated agriculture29.

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As IN4 and the agricultural return drains all exhibited negative Δ17O values of

349

up to –1.4‰ (Figure 2), the use of 0‰ for non-atmospheric nitrate in a two end-

350

member mixing model may underestimate the abundance of atmospheric nitrate in

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soils and water. It is possible atmospheric nitrate was present in these agriculturally

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impacted samples but had its characteristic positive Δ17O anomaly obscured by

353

mixing with nitrate sources with the observed negative values. If our most negative

354

observed value of–1.4‰ is used instead of 0‰ as an end-member in our mixing

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model, atmospheric contributions to soil nitrate in sample RC2 90 to 105 cm are as

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high as 15%, and atmospheric nitrate is present in all soil samples for which Δ17O

357

data was collected.

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Acknowledgements

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Funding for this study was provided by the Geological Society of America under the

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2016 Graduate Student Research Grants program and by a 2016 graduate

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fellowship from the Washington State Chapter of the American Water Resources

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Association. The authors thank Ben Harlow at Washington State University for an

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introduction to the bacterial nitrate method, and Katie Redling of the University of


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Pittsburgh for Δ17ONO3– sample analysis. We thank Anne Johansen and Ashleen

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Reddy for assistance obtaining chemical analysis with the ion chromatograph. We

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also thank the Washington State Department of Natural Resources for granting land

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access.

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Supporting Information

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Table S1. Major anion concentrations, Δ17ONO3― , δ18ONO3― and δ15NNO3― values for soil

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samples, soil leachates, and water samples. Organic content, moisture content, and

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carbonate are given for soil samples.

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δ17O (‰)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 18 17 17δ O versus δ O for 1812 19 10 20 IN4 21 8 RC2 22 RC1 6 Drains 23 24 4 25 2 26 27 0 28 29 -2 30 -4 31 -10 -5 0 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nitrate in lower Yakima Valley Soils e ich Cal

5

th dep t a +2.3‰ ils g so n i TFC tain

con

10

δ O (‰) 18

15

20

25


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Isotope Investigation of Nitrate in Soils and Agricultural Drains of the

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