Sources and fate of nitrate in groundwater at agricultural operations overlying glacial sediments

• Main Authors: S. A. Bourke, M. Iwanyshyn, J. Kohn, M. J. Hendry
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-03-01
• Series: Hydrology and Earth System Sciences
• Online Access: https://www.hydrol-earth-syst-sci.net/23/1355/2019/hess-23-1355-2019.pdf

<p>Leaching of nitrate (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="57a4663cbf0d11bf294d99bb32c9ae29"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00001.svg" width="25pt" height="16pt" src="hess-23-1355-2019-ie00001.png"/></svg:svg></span></span>) from animal waste or fertilisers at agricultural operations can result in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="4c315b3ea451cf26923ad12993612b33"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00002.svg" width="25pt" height="16pt" src="hess-23-1355-2019-ie00002.png"/></svg:svg></span></span> contamination of groundwater, lakes, and streams. Understanding the sources and fate of nitrate in groundwater systems in glacial sediments, which underlie many agricultural operations, is critical for managing impacts of human food production on the environment. Elevated <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a186e28964d6ae507e65dbc91f8b1f71"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00003.svg" width="25pt" height="16pt" src="hess-23-1355-2019-ie00003.png"/></svg:svg></span></span> concentrations in groundwater can be naturally attenuated through mixing or denitrification. Here we use isotopic enrichment of the stable isotope values of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="e16cba38499a6a16cb1a10e488ec56da"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00004.svg" width="25pt" height="16pt" src="hess-23-1355-2019-ie00004.png"/></svg:svg></span></span> to quantify the amount of denitrification in groundwater at two confined feeding operations overlying glacial sediments in Alberta, Canada. Uncertainty in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">15</mn></msup><msub><mi mathvariant="normal">N</mi><mrow><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="42pt" height="18pt" class="svg-formula" dspmath="mathimg" md5hash="f57ee86612cf590febf09c614387be99"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00005.svg" width="42pt" height="18pt" src="hess-23-1355-2019-ie00005.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">18</mn></msup><msub><mi mathvariant="normal">O</mi><mrow><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="43pt" height="18pt" class="svg-formula" dspmath="mathimg" md5hash="2025ff7cbf2aa6a13643b32df03193f0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00006.svg" width="43pt" height="18pt" src="hess-23-1355-2019-ie00006.png"/></svg:svg></span></span> values of the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a02883d0956e7dc256b9fe9fffa70b09"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00007.svg" width="25pt" height="16pt" src="hess-23-1355-2019-ie00007.png"/></svg:svg></span></span> source and denitrification enrichment factors are accounted for using a Monte Carlo approach. When denitrification could be quantified, we used these values to constrain a mixing model based on <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="8a872e45f44a0fc3c08e466e371cfb3a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00008.svg" width="25pt" height="16pt" src="hess-23-1355-2019-ie00008.png"/></svg:svg></span></span> and <span class="inline-formula">Cl⁻</span> concentrations. Using this novel approach we were able to reconstruct the initial <span class="inline-formula">NO₃−N</span> concentration and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn></msub><mo>-</mo><mi mathvariant="normal">N</mi><mo>/</mo><msup><mi mathvariant="normal">Cl</mi><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="62pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="5b87f7e16e6ce23de2042aac34d16bce"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00009.svg" width="62pt" height="15pt" src="hess-23-1355-2019-ie00009.png"/></svg:svg></span></span> ratio at the point of entry to the groundwater system. Manure filtrate had total nitrogen (TN) of up to 1820&thinsp;mg&thinsp;L<span class="inline-formula">⁻¹</span>, which was predominantly organic N and <span class="inline-formula">NH₃</span>. Groundwater had up to 85&thinsp;mg&thinsp;L<span class="inline-formula">⁻¹</span> TN, which was predominantly <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="ecc3e6dd5af0ffb1da8bfbfcb16b8e8b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00010.svg" width="25pt" height="16pt" src="hess-23-1355-2019-ie00010.png"/></svg:svg></span></span>. The addition of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="48a6d5724cc017ced9c974ab9a81c03a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00011.svg" width="25pt" height="16pt" src="hess-23-1355-2019-ie00011.png"/></svg:svg></span></span> to the local groundwater system from temporary manure piles and pens equalled or exceeded <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="822fcc3376206f5298bc14405cca7022"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00012.svg" width="25pt" height="16pt" src="hess-23-1355-2019-ie00012.png"/></svg:svg></span></span> additions from earthen manure storages at these sites. On-farm management of manure waste should therefore increasingly focus on limiting manure piles in direct contact with the soil and encourage storage in lined lagoons. Nitrate attenuation at both sites is attributed to a spatially variable combination of mixing and denitrification, but is dominated by denitrification. Where identified, denitrification reduced agriculturally derived <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="7dd3c683c0655cd2a5c1ed2d08ea01e9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00013.svg" width="25pt" height="16pt" src="hess-23-1355-2019-ie00013.png"/></svg:svg></span></span> concentrations by at least half and, in some wells, completely. Infiltration to groundwater systems in glacial sediments where <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="7248c728767abac31fc80ac33e5f4469"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hess-23-1355-2019-ie00014.svg" width="25pt" height="16pt" src="hess-23-1355-2019-ie00014.png"/></svg:svg></span></span> can be naturally attenuated is likely preferable to off-farm export via runoff or drainage networks, especially if local groundwater is not used for potable water supply.</p>