Hydrologic conditions and water-quality conditions following underground coal mining in the North Fork of the Right Fork of Miller Creek drainage basin, Carbon and Emery Counties, Utah, 2004-2005

Abstract

In 2004 and 2005, the U.S. Geological Survey, in cooperation with the Bureau of Land Management, reassessed the hydrologic system in and around the drainage basin of the North Fork of the Right Fork (NFRF) of Miller Creek, in Carbon and Emery Counties, Utah. The reassessment occurred 13 years after cessation of underground coal mining that was performed beneath private land at shallow depths (30 to 880 feet) beneath the NFRF of Miller Creek. This study is a follow-up to a previous USGS study of the effects of underground coal mining on the hydrologic system in the area from 1988 to 1992. The previous study concluded that mining related subsidence had impacted the hydrologic system through the loss of streamflow over reaches of the perennial portion of the stream, and through a significant increase in dissolved solids in the stream. The previous study also reported that no substantial differences in spring-water quality resulted from longwall mining, and that no clear relationship between mining subsidence and spring discharge existed. During the summers of 2004 and 2005, the USGS measured discharge and collected water-quality samples from springs and surface water at various locations in the NFRF of Miller Creek drainage basin, and maintained a streamflow-gaging station in the NFRF of Miller Creek. This study also utilized data collected by Cyprus–Plateau Mining Corporation from 1992 through 2001. Of thirteen monitored springs, five have discharge levels that have not returned to those observed prior to August 1988, which is when longwall coal mining began beneath the NFRF of Miller Creek. Discharge at two of these five springs appears to fluctuate with wet and dry cycles and is currently low due to a drought that occurred from 1999–2004. Discharge at two other of the five springs did not increase with increased precipitation during the mid-1990s, as was observed at other monitored springs. This suggests that flowpaths to these springs may have been altered by land subsidence caused by underground coal mining. Analysis of possible impacts to the fifth spring were inconclusive due to a lack of data collected during the mid-1990s. Discharge at eight other monitored springs in the study area appears to be controlled mainly by climatic fluctuations and was generally near the value measured prior to 1988. Discharge at one of these eight springs is significantly greater than that measured during the longwall mining period. Concentrations of magnesium, calcium, sulfate, and dissolved solids at one undermined spring were elevated in relation to other springs in the study area. Dissolved solids concentration at this spring ranged from 539–709 milligrams per liter. Dissolved-solids concentration for all other springs in the study area ranged from 163 to 360 milligrams per liter and was near the median value measured prior to longwall mining beneath the NFRF of Miller Creek drainage basin. Baseflow measured at a streamflow-gaging station on the NFRF of Miller Creek located downstream of the mined area during the summer of 2004 was near 5 gallons per minute. Baseflow in 2005 increased to 7–8 gallons per minute, due to increased precipitation. This is slightly greater than the range of baseflow measured near the end of the longwall mining period which was approximately 3–5 gallons per minute. Seepage investigations carried out in the summer of 2004 and 2005 along the NFRF of Miller Creek showed a net loss of surface flow along the studied reach. Specific areas within the study reach had streamflow losses prior to longwall mining, however, the study reach as a whole was observed to gain in discharge when measured in 1986–1988, immediately before longwall mining began. The area where the greatest loss in discharge from the NFRF of Miller Creek occurred corresponds to an area where overburden (material overlying a deposit of useful geological materials or bedrock) is between 700 and 210 feet thick. Overburden thickness at the place where the streambed first dried up was approximately 600 feet thick. In 2004, approximately 1,600 ft of the streambed of the NFRF of Miller Creek was dry. Only 300 feet of the streambed was dry during the wetter year of 2005. Prior to longwall mining, no dry reaches were observed, though seepage loss was documented. Average discharge measured at a tributary to the NFRF of Miller Creek has increased from 1.6 gallons per minute measured during longwall mining to 7.2 gallons per minute measured in 2004–2005. During both years of this study, the lower reach of the stream regained flow from this tributary and from seepage gains. Water quality in the lower reach of the NFRF of Miller Creek downstream of the longwall-mined area, showed significantly higher concentrations of magnesium, calcium, sulfate, and strontium, in relation to water in the upper reach of the NFRF of Miller Creek and to the springs sampled in the area. Dissolved-solids concentration measured in the lower reach of the stream in 2004 and 2005 ranged from 1,880 to 2,220 milligrams per liter, while sulfate concentrations ranged from 1,090 to 1,320 mg/L. The maximum contaminant level for drinking water in the state of Utah for dissolved solids and sulfate is 2,000 and 1,000 mg/L respectively. Concentrations of these ions are slightly greater than those measured during and just following mining beneath the NFRF of Miller Creek drainage basin, but are significantly higher than those measured prior to mining. With the exception of strontium, dissolved metals concentrations in the NFRF of Miller Creek were similar to those measured in area springs. pH in the creek and at all spring sites was near neutral. Qualitative observations of the creek bottom suggest that mining-related activities have had little effect on vegetative growth.