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• drilling programs can decrease OIU characterization cost substantially without jeopardizing the <br />quality of data obtained. <br />The following tasks were accomplished during the two phases of OIU analyses: <br />Phase I: The existing OIU data from the seven overburden baseline locations were <br />reviewed and analyzed, potential chemical and physical problems were identified, <br />and general areas of suitable and unsuitable overburden were outlined. <br />Phase II: Four additional sampling locations were approved for the scraper pit on <br />April 14, 1986 by CMLRD (Peter O'Connor's letter to Vern Pfannenstiel). This <br />program was completed to accurately characterize the extent of suitable and <br />unsuitable material. Constituents which approached or exceeded guideline levels <br />during the Phase I sampling were included on the Phase II analyses list. Modifying <br />the second phase parameter list was desirable to reduce unwarrantable analyses <br />costs and to focus on site specific conditions. <br />H. CROP/SUBCROP DRILLING Twelve exploratory holes will be drilled in order to better define <br />the croplsubcrop boundary. The holes will average 50 feet in depth, and will be 5-1/8" in diameter. <br />• See Exhibit "6-12° for the hole locations and access. No topsoil removal or blasting will be required. <br />Access to the holes will be limited to overland travel with no bladework required. The holes and <br />access will be reclaimed unless they fall within the mine plan which may be revised based upon the <br />drilling results. If reclaimed, the drill holes will be sealed by replacing the drill cuttings, and placing <br />a suitable plug 10 feet below the ground surface to support a cement plug within 3 feet of the ground <br />surface. The top 3 feet will be covered with the drill cuttings or dirt. <br />$~m~q Intensiri. Most OIU sampling and analysis studies indicate that drilling designs should <br />be governed by geological factors, such as the lithologic variability and the depositional environment <br />of the OIU strata (Barth et al„ 1981), Barrett et al. (1980) states that drilling designs and hole <br />spacings must be determined on the basis of the unique set of geologic and geochemical conditions <br />(i.e., lateral variability of strata) existing at each mine site. <br />Recommended drilling intensities range from one hole per 40 acres to one hole per 640 acres. <br />CMLRD, 1982 suggests sampling 1 drill hole/640 acres with a minimum of 3 holes/site. USDI-OSM <br />(1986A) recommends drilling 1 hole/160 acres with a minimum of 3 holes/site. WDEO (1985) <br />stipulates sampling 2 core holes and 6 cuttings holes per 640 acres during Phase I drilling. Phase <br />II sampling is reserved for special handling situations and a density of 1 hole/40 acres is <br />recommended. <br />• <br />(Rev. 9/92) 6-19 <br />