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1 ? <br />® 1 <br />i ? -? <br /> <br />C Potential im ae?s ©f re laced s azl Qn r©uudwater ualit .The operation has disrupted the <br />overburden above the Dakota coals anal has removed the Dakota coals, therefore, these are the only two <br />stratigraphic zones that have been affected. As described in the section on overbuden water quality, the <br />pre-mine quality of the overburden water is poor with TDS generally in the 3??a ppm range and some <br />ions exceeding limits for most water uses. The primary potential for impacts to ground water quality <br />occurs from increased water infiltration causing an accelerated oxidation of pyrite in the spoil, ether salts <br />may also dissolve more readily in the highly permeable spoil. The minor amounts of sulfuric acid <br />produced can cause lower pH, which then results in higher rates of dissolution of other chemical <br />compounds in the spoil, resulting in higher TDS. This water will saturate the spoil at the low-wall and <br />farm a spring at the low point. Also, the spoil water can infiltrate into the low-wall strata of the Dakota <br />Sandstone formation. <br />S Qil water Chemist <br />Concerning impacts from the conversion of overburden to spoil, the available data indicate that a small <br />proportion of the overburden may produce acid through the oxidation of pyrite. Based on laboratory tests <br />on overburden cores, calcite is present thraughaut the overburden. Calcite serves two functions. p'irst, it <br />buffers the pH of the water, which overall tends to slow the oxidation of pyrite, slowing the production <br />of acid. Second, it will neutralize the acid that is produced. The core samples that exhibited low paste <br />pH's are surrounded by non-acid producing, calcite-bearing rocks. The water that contacts the low-paste <br />pH materials will have first reacted with calcite, and therefore developed a pH-buffer capacity of its own. <br />The groundwater monitoring data. indicate that mixed overburden and interburden waters have near <br />neutral pH's. Sample pH's less than ? are associated oniy with the lower Dakota coal. where the coal's <br />permeability is high enough to produce about 5 gpm during sampling, the acid-producing reactions do <br />not appear to be fast enough to maintain a pH of less than ?. Cxidation rates may increase because of <br />the mining process, however, the supply of oxidation is only one of the constraints on the production of <br />acid. ether constraints are influenced by the quantity of calcite present and the reactivity of the pyrite. <br />The paste-pH test, conducted under oxidizing conditions, indicates that a very small proportion of the <br />overburden is likely to produce acid. This overburden is generally located in a thin zone immediately <br />above the coals. No zone near the surface at the New Horizon #1 Mine has ever spawn an acidic nature. <br />The ability of the calcite in the overburden spoil to neutralize any acid produced is dependent upon a <br />number of factors such as: <br />a) the uniform distribution of calcite in the replaced spoil, b) the higher transmissivity of the spoil to <br />allow irrigation water with higher levels of oxygen to move quickly through the spoil, resulting in faster <br />breakdown of the pyrite in the spoil, c} high void channels developing in the spoil at the bottom of the <br />pit which may serve as the primary conduits for flow in the spoil, d) the quantity of calcite available in <br />the areas needed most, e} other chemistry which may influence the neutralization reactions. <br />for these reasons, there is a possibility that water leaching through the spoil may result in a higher level. <br />of TDS for some period of time, until pyrite in the overburden spoil is fully oxidized and removed. This <br />was found to occur at the Seneca II Coal Mine in northwest Colorado and was the subject cif a study by <br />the L?SGS in 1?99?. Sampling data gathered through the last 13 years at the New Horizon 1 Mine suggests <br />that same pyrite is oxidizing but is being neutralized, as described below. <br />The analysis of geochemical controls on groundwater quality at the New Horizon #l Mine spoil suggests <br />that the water chemistry and concentrations of most elements of concern are controlled by mineral <br />reactions that will resist changes in water chemistry. ft appears that any pyrite ?FeS2} oxidation gets <br />neutralized by calcite ?CaCO?} present in the same spoil material. This results in the iron precipitating as <br />iron oxides. The slightly higher than normal pH of the natural water means that there is sufficient