Fluorite Precipitation in a Calcareous Soil Irrigated with High Fluoride Water

Abstract

Lysimeters filled with Freedom silt loam soil (fine‐silty, mixed, mesic, Xerollic Calciorthids) were irrigated with geothermal well water from south central Idaho. The water contained 7 mg F/L. Alfalfa (Medicago sativa L.) was grown in the lysimeters and the water was applied at a rate to give either a 0.30 or 0.15 leaching fraction. After irrigating the 1.0 m deep soil samples for 600 d, over 98% of the added F remained in the soil. Soil extract and lysimeter leachate analysis showed that fluorite precipitation was occurring in the upper profile, but only after another fluoride adsorption mechanism was saturated and the fluoride and calcium ion activities exceeded the CaF2 ion activity product. The F concentration in the upper profile extracts exceeded 20 mg F/L and with the additional high F water irrigation, the soluble F would be expected to leach deeper in the soil. Under field conditions the F would be expected to eventually move into the groundwater with continued irrigation with high F water.

Fluoride adsorption by a saline sodic soil irrigated with a high F water

“…In the previous study (Tracy et al 1984), fluoride concentrations in 1 : 10 soil : distilled water extracts varied from less. than 0.05 mMF in the lower half of the lysimeters to over 1.6 mMF in the surface 0.4 m of the profile.…”

“…Bower and Hatcher (1966) found that over the range of 0-1.0 mMF, F adsorption could be described by the linear form of the Langmuir equation in six soils (some had been acid washed to remove the lime) and nine clay minerals. These F concentrations are considerably lower, however, than those in the soil solutions of Tracy et al (1984), since the irrigation water initially contained 0.38 mMF and this solution was concentrated in the soil 3.3 to 6.7 times by evapotranspiration.…”

“…Soil samples from the two lysimeters that had received the 30% leaching fraction irrigations in the previous study (Tracy et al 1984) and samples of the untreated surface soil and subsoil used to fill the lysimeters were used for this study. The 15% leaching fraction data from the previous study are not shown here due to the close similarity to the presented data.…”

“…In a previous study 6 lysimeters filled with 0.7 m of saline sodic subsoil and 0.3 m of sodic surface soil and irrigated at 0.15 and 0.30 leaching fractions, retained over 98% of the applied F when irrigated with a high F geothermal well water (Tracy et al 1984). The water contained 0.38 mMF (7.2 mg F/kg) and had an EC of 2.7 dS/m.…”

Coefficients for estimating SAR from soil pH and EC data and calculating pH from SAR and EC values in salinity models

“…The center of each plot was sampled from the surface to 0.25 m and from 0.25 to 0.50 m. This provided 144 samples for each depth in each of the two parts of the field ( Table 2). The 53 Freedom silt loam (fine-silty, mixed, mesic, Xerollic Calciorthids) samples were collected and analyzed during selection of a site to obtain surface soil (0.0 to 0.25 m) for a previous study (Tracy et al 1984).…”