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Evaluation of Pinyon-Juniper Biochar as a Media Amendment for Stormwater Treatment
AuthorMcCrum, Michael J.
AdvisorHeyvaert, Alan C.
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Stormwater runoff affects the water quality of lakes, rivers and streams around the world, but many stormwater best management practices (BMPs) do not achieve adequate pollutant reductions, especially when receiving waters are very clean. Biochar, a product of controlled combustion under low oxygen conditions, has been used in agriculture for the improvement of soil fertility, water-holding capacity and nutrient retention. Recently, the Nevada Division of Forestry (NDF) has developed a biochar product from forest clearing and thinning of pinyon-juniper (P-J) landscapes. That biochar has been used on a trial basis to improve soil moisture-holding capacity of rain-garden soils in the Tahoe Basin, and its physicochemical characteristics suggests that it may also improve the sequestration of nutrients from stormwater runoff. This study evaluated nutrient adsorption characteristics of NDF biochar using batch equilibrium tests and column experiments with mixtures of Washoe washed sand (WWS) and the P-J biochar. Results showed that biochar adsorption of dissolved inorganic nitrogen and phosphorus was most effective with the smaller size classes (125–250 microns) versus larger size classes (500–4000 µm). Batch equilibrium tests demonstrated significant improvements in phosphorus and nitrate removal with P-J biochar compared to WWS, and modest adsorption of ammonium relative to WWS, but also some leaching of phosphorus from biochar. Subsequent column adsorption-desorption experiments using a 30% biochar mixture in WWS with simulated stormwater showed a net retention of 11% for ammonium-N, 1.5% for orthophosphate-P, and negligible nitrate-N retention. Many studies on adsorptive media simply conduct batch adsorption tests on the media of interest, but we found that column adsorption-desorption experiments improved our interpretation of results and yielded better estimates of net nutrient retention. The 30% P-J biochar with WWS mixture retained 7.6 g m-3 dissolved inorganic phosphorus and 43 g m-3 of dissolved inorganic nitrogen. Subsequent batch equilibrium tests with iron-amended P-J biochar showed a much higher capacity for P adsorption than with P-J biochar, but also some potential ammonium leaching.