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Colowyo, C1981 -019, PR3 adequacy No. 3 <br />July 22, 2011 <br />Please refer to question 2.05.3(4) above. <br />5. Please use in the calculations of spoil msaturation time a spoil recharge rate, rather than a <br />bedrock recharge rate. A reasonable spoil recharge rate for the Cullom area would be 3.1 <br />inches per year, calculated as 18% X 17 inches per year average precipitation. The 18% <br />factor is based on Williams and Clark's (1994) spoil lysimeter study which found that <br />approximately 18% of annual precipitation recharge spoil at a Routt County mine. The <br />annual recharge values used in the submittal (0.2 to 0.35 inches, and 1.1 inches) are less than <br />7% of annual precipitation in the Cullom area and apply to undisturbed bedrock, whose <br />permeability, and consequently annual recharge, would be a fraction of the value for spoil. <br />'- , , r I 13,IIC use in the calculations ol'spoil saturation time a reasonable <br />surface infiltration rate into Cullum Lite spoil of 3.1 inches pet* year. calculated as 180, X 17 <br />inches per year average precipitation. In order to reflect the 3.1 inches per year infiltration, <br />please change the 0.016 cts value on page 133 to 0.31 efs, and change the 0.75 inches per year <br />recharge rate at the bottom of page 136 to 3.1 inches per year. <br />Our letter on April 22, 2011 explained that the I S "o factor is based on Williams and Clark's <br />(1944) spoil lysinmeter study hi <br />wch found that approximate 189ho of annual precipitation <br />recharge spoil at a Routt County nmiue. The lithologic differences between Dantiath Hills area <br />and the lower Williams Fork Formation that are cited oft page 23 of CCC May 20, 1011 <br />response letter would not cause a significant difference in spoil permeability (and, <br />consequently. spoil saturation rate) because the effective porosity in the spoil is between <br />particles of spoil, rather than inside the particles of spoil. Also, a spoil infiltration study found <br />the Colowyo spoil infiltration rate was not less than infiltration rates at mines developed in the <br />Lower Williams Fork coal sequence (Table 10, item 6 of Exhibit 7 of the Colowyo permit <br />application). <br />The response is adequate. <br />6. To be consistent with the 15% swell factor noted in Table 2.05.6 -5, please use in the spoil <br />resaturation calculations a spoil porosity value of 15 %, rather than 35 %. <br />'I he Division's original comment was a request to use in the Cullum Lite pit backfill saturation <br />calculation that currently is contained on page 137, a value for c%%ctive porosity 350/9 that is <br />shown on that page. Most of the flow and resulting saturation in spoil will occur between the <br />particles of spoil, rather than through particles of spoil; therefore. The porosity between spoil <br />particles is the effective porosity, and effective porosity is the appropriate porosity value for <br />performing a spoil saturation calculation. It seems reasonable in a spoil saturation calculation to <br />add 5 percentage points for ineffective porosity inside spoil particles that may become partially <br />saturated, resulting in 2001, porosity fo' the percentage of spoil backfill that will become <br />