**7. Missouri rare earth element investigations**

The Alred and Rueter soil profiles were completely characterized, with the rare earth element suite determined with aqua regia digestion. The ochric (A and E horizons) and illuvial (Bt and 2Bt) soil horizons were identified and compared to their rare earth element concentrations. For clarity, eluvial soil horizons occur where strong leaching of clay minerals, oxides, and organic material is observed, whereas illuvial soil horizons contain materials that have been transported downwards either in solution or suspension and subsequently deposited. The illuvial horizons, with their greater clay contents, show greater rare earth element abundances (**Table 4**). Paired t-test comparing ochric and illuvial rare earth element concentrations are significant (0.016 for Alred and 0.020 for Rueter).

The multiple pedons for the Menfro and Kaintuck soil series were characterized for rare earth element concentrations for each soil horizon in their respective soil profiles (**Table 5**). The Menfro soil is an Alfisol with ochric silt loam texture transitioning to a silty clay loam texture in the agrillic horizon. The Kaintuck soil series is an entisol


## *Evaluation of Rare Earth Element Mine Sites for Environmental Impact DOI: http://dx.doi.org/10.5772/intechopen.109161*

#### **Table 4.**

*The rare earth element distribution of the Alred and Rueter series.*


#### **Table 5.**

*The mean rare earth element concentrations and coefficient of variations.*

with sandy loam soil textures througthout the soil profiles. The Menfro soil profiles have greater rare earth element concentrations, a feature attributed to their greater clay contents. The coefficient of variation for the rare earth element concentrations is greater for the Menfro soil pedon, a feature attributed to the discrete clay differences in the ochric and agrillic horisons when compared to the rather uniform texture distribution in the Kaintuck pedons.

The Wilbur soil series (coarse-silty, mixed, superactive, mesic Fluvaquentic Eutrudepts) consists of very deep, moderately well-drained soils formed in silty alluvium. The four pedons are located across a 40 ha production field and were evaluated for morphology, routine physical and chemical characteristics, and rare earth element concentrations across all pedon horizons. Yttrium was included in the rare earth element analysis. As expected, the rare earth element distribution clearly is in accordance with the Oddon-Harkin rule (**Table 6**).

The Wilbur pedons are distant from industry and mining, thus these pedons may be considered pristine. The rare earth element distribution is very uniform within and across pedons, features reflecting the pedon's as belonging to the Inceptisol order and having little soil texture variation. The rare earth elements may be used in the GeoBase for the following pollution indices: (i) geoaccumulation index, (ii) single pollution index, (iii) The enrichment factor, (iv) the contamination factor, (v) the sum of contamination, (vi) the pollution load index, (vii) the average single pollution index, (viii) the vector modulus pollution index, (ix) the degree of contamination, and (x) the hazard quotient. If an estimate of the factor of the rare earth element's toxic reaction becomes available, then (i) the potential ecological risk factor and (ii) the potential ecological hazard indexes can be calculated.


#### **Table 6.**

*Mean and coefficient variation for the Wilbur soil series.*
