APPLICATION OF SYNTHETIC PRECIPITATION LEACHING PROCEDURE (SPLP) TO WASTE ROCK CHARACTERIZATION, SOUTHEAST IDAHO PHOSPHORITE MINING OPERATIONS

Backfills and dumps from phosphorite mining contain overburden and inter-ore waste that are potential sources of selenium and other contaminants (COPCs) that can be mobilized by meteoric water seepage. Bench-scale, free-draining and/or saturated column leach tests, incorporating material from developmental drill holes, have become an accepted method for estimating waste rock seepage quality. Such water quality estimates are commonly used as source terms for groundwater impact prediction models in EISs prepared for proposed phosphorite mines in southeast Idaho. However, primary reliance on column testing tends to favor testing of small numbers of site-wide mono-lithologic or run-of-mine composites as a way of reducing the number of columns, testing times and costs. Further, the representativeness of such composites and the actual site-wide variability of leachable COPCs have not always been formally assessed, mainly due to the onerous requirements of large column testing programs. A more flexible approach is needed. To this end, overburden geochemical characterization data from five previous EIS submittals was evaluated and showed that SPLP (EPA Method 1312) leachate compositions favorably compare to column leach results for selenium on a cumulative eluted-mass basis for the first five pore volumes. This allows column results to be anticipated inexpensively and quickly. Although more confirmatory testing of this SPLP-column elution curve method is required, it offers promise as a means of economically allowing higher sampling and analysis frequencies, recognizing areal and stratigraphic variations in leachable CPOCs, and allowing analysis of sources of variance in sub-sampling and compositing schemes.