Biochar for mitigating stormwater runoff and nitrate load: models, tools, and the role of soil aggregation
Date
2020
Authors
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Publisher
University of Delaware
Abstract
Stormwater runoff is a fast-growing source of nonpoint nitrogen pollution in the United States, causing water quality impairment and eutrophication. Biochar is the product of the thermochemical conversion of biomass in oxygen-limited conditions and may serve as a soil amendment to tackle this problem by increasing infiltration and water retention while simultaneously enhancing denitrification in the retained and percolated water. This dissertation advances scientific and engineering understanding of biochar’s use for this purpose. ☐ Mechanistic models are presented to predict the water retention curve for biochar and biochar-amended soil at low water contents using the biochar and soil physicochemical properties. While the model works accurately for fresh biochar and biochar-amended soil, the models’ applicability for aged systems should be investigated. The underlying causes for naturally occurring biochar segregation in biochar-amended media were investigated, and procedures are proposed to inhibit it. A method is proposed to quantify biochar content in soil with varying organic matter content. The method requires a distinct difference in organic matter composition between soil and biochar and a uniform organic matter composition in soil. Predicting water retention in biochar and biochar-amended soil, creating a homogenous biochar/soil mixture, and quantifying the amount and distribution of biochar in soil are prerequisites to sustainable biochar applications for stormwater management. ☐ The effectiveness of biochar addition to soil for reducing stormwater runoff volume and nitrate loading is a function of interacting physical, chemical, and biological processes and may vary with soil, biochar, and time. The impacts of a wood-based biochar on the structure and hydraulic properties of a sandy soil were assessed under natural field conditions. The dynamic effect of this biochar on another sandy soil’s structural and hydraulic properties and the partitioning of stormwater influent and nitrate load between infiltration and runoff were monitored in a pilot-scale greenhouse experiment. Although biochar amendment decreased surface runoff and nitrate load in the aged greenhouse system, it did not change the total load to the environment for the large storm event studied. In both field and greenhouse studies, a clear link between changes in soil water-stable aggregate size and hydraulic conductivity was observed. Due to the importance of aggregation, further analyses were conducted to understand the mechanisms causing biochar-induced soil aggregation in both studies. In addition to providing insight into changes that might be expected in other biochar-amended soils, these measurements demonstrate the benefits and limitations of biochar application for the biochar and soils evaluated herein. Further studies with additional biochars and soils of varying properties are required to assess biochar application for decreasing runoff and nitrate load for stormwater management.
Description
Keywords
Stormwater runoff, Soil aggregation, Nitrogen pollution, Water quality, Biochar-amended soil, Stormwater management