This study investigated the direct effect of fly ash used as a soil amendment on anion exclusion in NO3 transport in a variably saturated soil. A sandy clay soil was amended with Class F fly ash (FA) at four levels: 0, 2, 10, and 20% FA. The soils were placed in a small soil column and kept at constant saturation using a steady-state centrifugation unsaturated flow apparatus . Three saturation ranges were investigated, 0.75 to 0.9, 0.4 to 0.45, and 0.27 to 0.29, designated as high, medium, and low saturation, respectively. Ammonium nitrate (0.7 mmol L−1) solution was passed through the soil sample, and the leachate was collected at predetermined intervals up to a total volume equivalent to 12 effective pore volumes. The leachate samples were analyzed for NO3 concentration using ion chromatography, and breakthrough curves were constructed. Experimental and modeling results showed that the amount of fly ash and saturation level had a significant effect on the hydraulic properties of the soil, reducing the hydraulic conductivity and increasing water retention. Furthermore, it was shown that the fly ash treatment slowed the transport of NO3, especially at low saturation levels, when the effect of anion exclusion is thought to be stronger.

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