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TECHNICAL MEMORANDUM <br />To: Bill Lyle <br />Date: October 19, 2007 <br />Page 6 <br />3.2 Fluoride Chemistry <br />The complete soil extract analyses are shown in Table 3-3 for the West Pit ground water <br />soil suspensions and in Table 3-4 for the Salazar Ranch ground water soil suspensions. <br />When using West Pit ground water (initial fluoride = 3.2 mg/L), soluble fluoride remaining <br />after reaction with the soils ranged from 0.40 to 1.1 mg/L. When using Salazar Ranch <br />ground water (initial fluoride = 0.73 mg/L), soluble fluoride remaining after reaction with <br />the soils ranged from <0.10 to 0.82 mg/L. Note also that the final manganese <br />concentrations were generally less than 0.01 mg/L in the batch fluoride attenuation tests <br />(Tables 3-3 and 3-4). <br />A summary of the fluoride attenuation data when using Salazar Ranch ground water (initial <br />fluoride = 0.73 mg/L) shows that the mean fluoride concentration in the soil extracts was <br />0.71 mg/L ± 0.08 (one standard deviation) (Table 3-5). Fluoride attenuation ranged from 0 <br />to > 1.3 mg/kg when using the Salazar Ranch ground water. When using the West Pit <br />ground water (initial fluoride = 3.2 mg/L), fluoride attenuation increased to a mean value <br />of 4.8 mg/kg, while fluoride concentrations in the soil extracts essentially remained the <br />same (0.80 ± 0.24) (Table 3-6). <br />Fluorite saturation index values indicate that the West Pit ground water is initially near- <br />equilibrium with respect to fluorite, while the Salazar Ranch ground water is initially <br />undersaturated with respect to fluorite (Table 3-1). Upon reaction with the soils, however, <br />the soil extractions became consistently undersaturated with respect to fluorite (Tables 3-3 <br />and 3-4). Fluorite undersaturation, coupled with rapid (within 24 hours) fluoride removal <br />and similar fluoride concentrations in the Salazar Ranch and West Pit ground water soil <br />extractions, indicates that fluoride removal results from adsorption rather than <br />precipitation. These results are consistent with a previous study of fluoride retention in <br />calcareous soils, cropped with alfalfa and irrigated with water containing 7 mg/L fluoride <br />(Tracy and others, 1984). The previous study concluded that both adsorption and fluorite <br />P:(Projects1111-Ne mont(SanGris)LSanLuisWF -Shalom RanckSoilsLS k=r Character Eton fiwldoc