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INTRODUCTION • Applied Hydrology Associates (AHA) was retained by Colorado Yampa Coal Company (CYCC) to evaluate the nature and potential hydrolo- gic consequences of underground water discharges from backfilled area at the Eckman Park operation (Figure 1). A preliminary evaluation based on limited data was submitted to the Colorado Mined Land Reclamation Division (CMLRD) on February 3, 1986. Comments on the report from CMLRD were received by CYCC on March 12, 1986. AHA has updated the original report to incorporate additional data and respond to the CMLRD comments. DATA SOURCES Available hydrogeologic data from CYCC to evaluate the hydrologic characteristics of the backfill material is limited. CYCC completed three monitoring wells on the backfilled area in late 1984. One of the wells, SP-3, has been dry since its construc- tion. The other two wells, SP-1 and SP-2, have monthly water level monitoring data. Two sets of water quality samples have been taken from the SP-1 well, during January and July of 1985. The SP-2 well has been sampled on three occasions, during January, April, and July of 1985. All samples were bailed from the wells. • During the drilling of the SP-1 and SP-2 wells production rates of 20gpm and 60-SOgpm respectively were encountered which would indicate a permeability in the range 10'"to 10"5 ft/sec. A limited amount of spoils permeability data is available from 2 geotechni- cal wells completed in spoils in area 51. The horizontal permeability was measured at 6.5 x 10'Sft/sec and >6.5 x 10'5 ft/sec. The latter requirement is an estimate as the well recovered rapidly for water level measurements to be taken. Backfill well test data from the Seneca II mine indicated a hydraulic conductivity of 10'3 to 10"~ ft/sec. (Slade Dingman, pers. comm.) depending on the analysis method used. Large variability of permeability would be expected in backfilled materials, however, the high values would be more significant with respect to groundwater flows. Consequently, a permeability range of 10'"to lOsft/sec was assumed for this evaluation. Lysimeter studies performed at the Seneca II mine between 1976 and 1981 by the US Geological Survey yielded information on rate and quality of recharge in backfilled areas (Bob Williams, pers. comm.). This study involved five lysimeters constructed in the backfilled areas which allowed the measurement of volumes and quality of water recharging the upper 8 feet of spoil material. The results of measurements indicated a recharge rate of 5.93, 3.86 and 3.21 inches for the first, second and third year of the study, considerably higher than was expected. This constitutes • 15-25 percent of the average annual precipitation of 16 inches.