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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Salinity and Sodicity of Weathered Minesoils in Northwestern New Mexico and Northeastern Arizona

^a BHP Billiton, New Mexico Coal, 300 West Arrington, Suite 200, Farmington, NM 87401
^b New Mexico Mining and Minerals Division, 1220 S. St. Francis Dr., Santa Fe, NM 87505
^c Inter-Mountain Labs, Inc., 1673 Terra Ave., Sheridan, WY 82801
^d Poudre Valley Environmental Sciences, Inc., 2835 Schooners Ct., Loveland, CO 80538
^e Peabody Western Coal Company, P.O. Box 650, Kayenta, AZ 86033
^f Dep. of Renewable Resources, 1000 E. University Ave., Univ. of Wyoming, Laramie, WY 82071-2000

^* Corresponding author (Brent.Musslewhite{at}bhpbilliton.com).

Received for publication September 13, 2007. Evolving relationships between electrical conductivity (EC) and sodium adsorption ratio (SAR) in reconstructed soils at surface mines have been insufficiently documented in the literature. Some minesoils (i.e., rootzone material) are classified as saline, sodic, or saline-sodic and are considered unsuitable for revegetation. Weatherable minerals such as calcite and gypsum are common in alkaline minesoils and on dissolution tend to mitigate elevated SAR levels by maintaining or increasing electrolytes in the soil and providing sources of exchangeable calcium and magnesium. Topsoils (i.e., coversoils) contribute to mitigation of sodic conditions when soluble cations are translocated from coversoils into the underlying minesoils. This study evaluated the weathering characteristics of minesoils sites from three surface coal mines in northwestern New Mexico and northeastern Arizona. Minesoils were grouped into 11 classes based on EC and SAR. After 6 to 14 yr, differences between upper and lower halves of the coversoils suggest general increases occurred with EC, SAR, chloride (Cl^–), and sulfate (SO₄^2–) with depth. Within the reclaimed minesoils, there were several significant (P < 0.05 or < 0.10) relationships among EC and SAR that related to Minesoil Class. Lower SAR levels with corresponding increases in EC compared to baseline minesoils were more apparent in upper minesoil depths (0–5 and 5–15 cm). Minesoil anion concentrations suggested coversoil leachates and gypsum dissolution influenced EC and SAR chemistry. Over time, chemical changes have increased the apparent stability of the saline and sodic reclaimed minesoils studied thereby reducing risks associated with potential aggregate slaking and clay particle dispersion.

Abbreviations: EC, electrical conductivity • ESP, exchangeable sodium percentage • FV, flocculation value • SAR, sodium adsorption ratio • TEC, total electrolyte concentration