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Sediment Yield <br />Use of Division's independent analysis instead of HGCC's water sample data. The most <br />straiahtfonvard approach to determining ifpost-mining sediment yields exceed pre-minim yields <br />would be to compare pre-mining and post-mining total suspended solids (TSS) field sample data. <br />Such a comparison would not be valid in HGCC's case, however, because the post-mining TSS data <br />in HGCC's bond release submittal are From samples taken from waters that passed through sediment <br />control ponds. Consequently, the Division conducted an independent analysis of sediment yield. <br />The analysis is described in a memorandum dated December 2. 1993 to the current bond release file <br />(SL-03). <br />Methodology used in Division's analysis of sediment yield. The Division compared the bond <br />release parcel's pre-mining and post-mining soil loss rates calculated using the Universal Soil Loss <br />Equation (LISLE). The calculations were adjusted for differing sediment delivery ratios. The <br />reasonableness of the post-mining soil loss rate was checked bycalculating apost-mining rate usinc <br />sediment accumulation data from HGCC's pond sediment surveys. Pre-mining and post-mining data <br />on vegetative cover was used in the analysis. <br />Comparison of pre-mining and post-mining soil loss rate. The calculated pre-mining soil loss <br />rate, adjusted for sediment delivery ratio, was 0.29 ton/acre/year. The calculated post-mininc rate <br />was 0.22 tun/acre/year. Pond sediment survey data supported the calculated post-minim rate. <br />Division's Held observations made during inspections over the years and observations made <br />during the bond release inspection of June 23, 1998. The Division searched for any field evidence <br />of erosion and sediment accumulation during the bond release inspection. Observations made during <br />the bond release field inspection, as well as observations made during previous regulaz inspections <br />and subsequent inspections, indicated that the vegetation on the reclaimed slopes had stabilized the <br />soil. Gullies did not appear to be forming on the reclaimed lands. Rills were found at only a few <br />locations on the reclaimed lands and appeazed to be healing. There was no general erosional <br />instability observed. No significant sediment accumulations, such as deltas and fans, were observed <br />on hillslopes. The Division's field observations appeared to corroborate the conclusion drawn from <br />the calculated soil loss rates that postmining sediment yields are less than premining yields. <br />Division's finding that post-mining sediment yield does not exceed pre-mining yield. Based <br />on the calculated soil loss rates and corroborating field evidence, the Division finds that the lands <br />proposed for Phase II bond release are not contributing suspended solids to streamflow or mnoff <br />outside the permit area in excess of premining levels. <br />Permanent impoundment demonstration. HGCC included in the Phase II bond release submittal <br />a SEDCAD sedimentologic demonstration as proof that ponds HG-3 and 003 are no longer needed <br />for sediment control and for showing their suitability for the postmininc land use of livestock <br />crazing. rangeland, and wildlife habitat. The Division previously approved leaving ponds HG-3 and <br />003 as permanent impoundments based on a similar demonstration submitted by the permittee (see <br />copy of Division's letter of November 2~, 193 in Phase I bond release submittal). The Division also <br />Hayden Gulch Mine Page 10 May 25, 1999 <br />Phase II/III Bond Release <br />