Transformations And Transport Of Fixed Nitrogen From Septic Tanks In Soil Absorption Systems And Underlying Aquifers

Author : Hannah Waterhouse
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.6/5 (624 download)

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Download or read book Nitrogen Cycling and Transport Under Agricultural Managed Aquifer Recharge written by Hannah Waterhouse and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: In semi-arid regions, including much of California, there is great interest amongst water management and conservation districts to implement agricultural managed aquifer recharge (AgMAR). AgMAR is a concept in which farmlands are leveraged to capture and recharge legally and hydrologically available flood waters to increase regional capacity for recharge to replenish the underlying aquifers and combat overdraft. The potential benefits of AgMAR, in addition to a more reliable future basin-wide water supply, include decreasing downstream flood risks by removing excess water from near flood stage rivers, reducing groundwater pumping costs by increasing groundwater levels, flushing salts from the rooting zone, increasing water storage in the root zone, improving ecosystem health of groundwater dependent ecosystems, and mitigating land subsidence. When flooding farmland for groundwater recharge, of particular concern is the potential for AgMAR to exacerbate nitrate (NO3−) contamination of already at-risk aquifers. Nitrate, when ingested, has been linked to methaemoglobinaemia, or "blue baby syndrome", miscarriages, and non-Hodgkin's lymphoma. Thus, it is necessary to determine if implementing AgMAR increases the risk of transporting residual NO3− below the root zone, as well as legacy NO3− accumulated in the vadose zone, into aquifers used for drinking water. This dissertation focuses on understanding how to mitigate NO3− contamination to groundwater under AgMAR implementation. First, I identified the crops and soils with the lowest potential risk of NO3− loading to groundwater when considering AgMAR. Cores down to 9 m were taken on both permeable and less permeable soils within almond, grape, and tomato cropping systems and stored-NO3− -N within each system were determined. Considerations for historical and current nutrient and water management under AgMAR are discussed as well and discussion is targeted toward stakeholders, including growers, water managers, and policy makers considering AgMAR. Next, I explored the mineralization potential, denitrification capacity, and denitrification potential for subsurface soils and sediments. Denitrification represents a permanent sink to the underlying aquifer and would be a positive outcome of AgMAR implementation. Using the acetylene incubation method, denitrification potential (glucose and NO3− addition) and denitrification capacity (no glucose or NO3− addition) were determined for vadose zone sediment samples to a depth of 9 m. The denitrification potential assays (addition of glucose and NO3−) resulted, on average, in over 108% to 175% of NO3− being reduced to N2O across all layers. Denitrification capacity, the ability of the sediment to denitrify without the addition of glucose or NO3− , resulted in 19-133% of NO3− being reduced to N2O. Across all depths, net immobilization was found on incubations of post-AgMAR soil samples, which represents a delay in NO3− arrival to the underlying aquifer. Finally, chapter 4 examines the transport and transformation of NO3− under varying management of AgMAR and across differing vadose zone heterogeneities using a reactive transport model, TOUGHREACT. Results show that silt loams are important features in the deep subsurface for creating reducing zones where denitrification can occur via both heterotrophic and chemolithoautotrophic pathways. Furthermore, applying floodwaters all at once increased denitrification compared to applying floodwaters incrementally, however, higher concentrations of NO3− moved faster and to further depths when water was applied all at once. Additionally, wetter antecedent moisture conditions while promoting denitrification more readily, increased the depths to which NO3− leached compared to drier antecedent moisture conditions. Thus the geologic heterogeneity, depth to the water table, and antecedent moisture conditions should be considered when applying floodflows for AgMAR. Further work is needed on how varying management practices, such as cover cropping, could retain residual soil NO3− and increase leached dissolved organic carbon to the subsurface to promote denitrification.