Environmental Implications of Phosphate-Based Amendments in Heavy Metal Contaminated Alluvial Soils.

Releases of heavy metals, primarily lead (Pb), from mining related activities have contaminated sediments and floodplain soils in Missouri.  Past research has demonstrated the immobilization of soluble Pb compounds through in situ application of phosphate-based amendments that induce formation of stable, insoluble pyromorphite [Pb₅(PO₄)₃OH,Cl,F] compounds.  However, water quality threats associated with P-remediation technology continue to be a concern, particularly in riparian areas.  The objective of this study is to investigate the effects of P fertilizer application rates on Pb and P loss in the contaminated alluvial soils of southeast Missouri, and select the rate that maximizes pyromorphite formation while minimizing P loss.  Floodplain soil samples were collected from the 0 – 15 cm depth, air-dried, and passed through a 2-mm sieve.  A suite of standard soil analyses were performed including total elemental analysis, Mehlich 3 nutrient analysis, particle size distribution, CEC, extractable bases, and extractable acidity, among others.  Soils were then treated with triple super phosphate (TSP) at one of three P-treatment levels based on molar ratios of Pb:P (1:4, 1:8, 1:16).  Treated soils were allowed to react with applied TSP for 2 months at approximate field capacity moisture in a greenhouse setting at 21 ¢ªC.  Treated and control soils were then exposed to simulated rainfall at 32 mm hour^-1 for one hour and surface water runoff and soil leachates were collected and analyzed to evaluate P and Pb loss.  Treated and control soils will be analyzed using extended X-ray absorption fine structure spectroscopy (EXAFS) at the Advanced Photon Source in Argonne, IL.  The EXAFS data will be utilized to correlate P-treatment levels to the degree of pyromorphite formation.  Preliminary data investigating differences in soil, solution, and surface runoff concentrations of metals and nutrients as well as EXAFS spectra and interpretation will be presented.