Executive Summary The Need for an Additional Evaluation of Hydrologic Impacts due to Mining an Methane Production
In 1988, the United States Geological Survey (USGS) issued a cumulative hydrologic impact assessment (CHIA) for the eastern Powder River Basin (PRB), which the State of Wyoming used in finding that no material damage was anticipated to the hydrologic balance in the area due to surface coal mining. Wyoming has laws and regulations that address all aspects of surface and groundwater quantity and quality. The state agencies with the authority in this matter are the Department of Environmental Quality/Land Quality Division (DEQ/LQD), Water Quality Division (WQD), and the State Engineer's Office (SEO). The federal Office of Surface Mining (OSM) indicated in 1992 that, given the increase in mining activity in the PRB, the 1988 USGS report had deficiencies and recommended that a new CHIA be initiated. A Cooperative Agreement was entered into in 1993 to accomplish this and to facilitate hydrologic data exchange among the cooperators. The cooperating entities included the DEQ/LQD, the Bureau of Land Management (BLM), the OSM, the SEO, the University of Wyoming (UW), and in 1994, the Wyoming State Geological Survey (WSGS). This report presents results of an analysis of existing and potential surface and groundwater impacts due t coal mining and coal bed methane development on the Little Thunder Creek Drainage located in the south-central portion of the PRB. The report is divided into the following sections:
Table of Contents
Introduction and Background
Surface Water Modeling
The PRB is located in northeastern Wyoming and contains abundant coal reserves that have been undergoing large-scale mining activity. To assess the best method of conducting a CHIA for the entire PRB, the cooperators decided that one drainage basin would be studied in detail (the Pilot Study Area), which consisted of the Little Thunder Creek Drainage and the areas of groundwater impact in the same vicinity. This region was designated as a cumulative impact area (CIA), which is a watershed or region impacted by two or more mines, and was one of four ClAs delineated in the PRB. The Little Thunder Creek Drainage is affected by three surface coal mines. The pilot study was conducted at the Wyoming Initiative Laboratory of UW, with funding and direction from cooperating agencies.
Modeling of groundwater flow in the Little Thunder Creek Drainage was undertaken to quantify the impacts from surface coal mining and coal bed methane (CBM) development in the Pilot Study Area, and to work on a method to assess hydrologic impacts from new or expanded development in the Pilot Study Area or other identified ClAs in the PRB. Groundwater flow impacts were expected to the upper Fort Union Formation aquifers and the Wasatch Formation as a result of mining and CBM development. Surface coal mining in the Pilot Study Area has been ongoing since 1976, with smallscale CBM development beginning in the late 1980's north of Gillette in the Powder River Basin. CBM production has become more significant since 1994. Although commercial CBM production has not reached the pilot study area, it is anticipated in the near future. Mining and CBM development are regulated independently, and they have separate environmental compliance requirements. The cumulative impacts from these two industries had not previously been considered.
The USGS Modular Three Dimensional Finite Difference Groundwater Flow Model (MODFLOW) was used to model the groundwater flow system. This model was chosen because its computer code is verified, widely accepted, easily modified, well documented, and thoroughly tested. Hydraulic data were obtained from DEQ/LQD surface mine permits and were input for each modeled aquifer. The modeled aquifers included: the Wyodak Coal; Clinker; Wasatch; and Backfill. Starting ground water levels were developed from time series data in the DEQ Coal Permit and Reclamation (CPR) database. Information on stresses to the aquifers, due to pit inflows at the mines and pumping of CBM wells, was obtained from the mine permits and SEO records, respectively. The mining sequence was simulated as incremental impacts in one-year stress periods from 1975 to the present, and the predicts simulation of impacts was modeled from 1995 to 2021. Two predictive scenarios were investigated: (1) just surface mining from 1995-2021; and (2) surface mining and CBM production from 1995-2005 followed by just surface mining from 2006-2021.
Calibration of the model was evaluated with respect to three quantitative goals. Minimization of Root Mean Square (RMS) error was used as the primary model goal. Absolute error, or the maximum error observed at a single calibration location, was minimized as a secondary criteria to RMS error. Mean error was checked as an estimator of model bias.
Ground Water Modeling Results
Areas with at least five feet of drawdown in the Wyodak Coal Aquifer cover approximately 250 square miles in 2021, considering surface mining development only. Drawdowns of 100 or more feet occur in a much smaller area of less than five square miles, which is largely within the mine permit boundaries Wasatch Aquifer drawdowns are generally confined to within the mine permit boundaries through 2021.
The 5-foot drawdown contour extends to the west and south model boundaries in 2005, as a result of the 1995 to 2005 CBM pumping. The areal extent of the 5-foot drawdown contour with the added impact of CBM approaches 400 square miles, with a secondary depression of the piezometric surface of greater than 125 feet occurring in the vicinity of CBM production. Recovery from CBM begins almost immediately after the cessation of methane development. CBM impacts are largely undetectable by 2021
Surface mining impacts also recover following the predicted end-of-mining. In mined areas, a pre-mining dual-aquifer system consisting of the Wyodak Coal Aquifer and the Wasatch Aquifers is replaced by a single Backfill Aquifer. Seventy-five percent of the water level recovery occurs within the first 200 years, with nearly complete recovery taking between 500 and 750 years. The increased length of time for recovery from surface mining impacts is due to replacement of generally confined aquifers, characterized by small storage coefficients, with an unconfined aquifer having much larger storage values.
Surface Water Modeling
For surface water modeling, it was necessary to acquire and analyze data pertaining to soils, vegetation, hydrography, mine permit areas, precipitation, and discharge. This information came from a variety of sources, including the UW Water Resource Center Data System; the Geographic Information System Laboratory; the CPR database; and surface mine permits on file with the DEQ/LQD. The modeling was conducted using HEC-1, which is a rainfall/run-off flood prediction model developed by the Army Corps of Engineers. HEC-1 requires that the watershed be divided into catchments, here called hydrologic response units (HRUs), which should respond to a precipitation event in a uniform manner. Primary output from HEC-1 is a set of hydrographs representing the discharge at the base of each individual component of the system. The model-generated hydrograph for the HRU farthest downstream was compared to observed data to determine model accuracy.
The ephemeral nature of stream flow in the Little Thunder Creek Drainage required the acquisition of hourly precipitation and discharge data for the area. Precipitation data were gathered from gages located on mine sites in the Pilot Study Area, and from National Weather Service Stations located in the vicinity. Discharge data came from one USGS station in the Pilot Study Area that had data of sufficient quality and quantity to be used in model calibration. The Pilot Study Area was divided into thirty-three HRUs based on the analysis of clinker abundance, soils, vegetation, mine permit locations, gaging stations, and hydrography. To determine rainfall distribution through time for each HRU, hourly records from precipitation stations were used. The precipitation records analyzed were for four storms selected between 1978 and 1980. The Natural Resource Conservation Service (NRCS) Run-off Curve Number Method was used to estimate run-off from each HRU.
Certain input parameters, which were not particularly variable for an individual storm, were held constant during calibration. Other parameters, which can be highly variable, were more likely to be altered during calibration. Adjustments to the model were required to reflect the impact of mining present in the Little Thunder Creek Drainage at the time of the observed storms. The storms used in modeling were chosen because they represented a variety of antecedent moisture conditions (AMCs), including dry, intermediate, and wet conditions. The AMCs were used to determine run-off curve numbers within each HRU. AMC, in conjunction with the contributing area of each HRU, was also used to simulate reservoir storage in each HRU.
The goal of the calibration process was to generate a model that matched, as closely as possible, the rainfall/run-off relationships within the Little Thunder Creek Drainage. All calibration was done with values that reflected conditions at the time the given storm occurred. Mining has been ongoing since 1976, so the models were calibrated to reflect the state of the watershed, including mine impacts, at the time of the storm. Adjustments were then made to the model to reflect what would have happened had the mines not been in place. The adjusted, or pre-mining models, were used as a baseline for comparison to post-mining models. For post-mining modeling, the pre-mine models were adjusted to represent the changes in the hydrologic regime that would result from mining. NRCS run-off curve numbers were changed to reflect the post-mining environment. The calibrated models generated peak flows and total volumes within 10% of the observed data for all four storms.
Surface Water Modeling Results
Results of the surface water modeling effort include the possibilities of large and small changes in the response of the post-mining landscape. Change with regard to the magnitude and direction in NRCS run-off curve numbers was incorporated into the model and generated changes of varying magnitudes in response to the four storms. The changes in peak flow, resulting from an increase of 1 NRCS run-off curve number, ranged from 0.0 to 9.9%. Changes in total volume of discharge for the same change ranged from 2.0 to 7.1%. Uncertainty associated with the direction and magnitude of change in the post-mining environment led to additional runs to determine a range of possible outcomes. A decrease of 1 NRCS run-off curve number resulted in changes in peak flow that ranged from 0.0 to -12.0%, and changes in total volume that ranged from -0.2 to -6.7%. Additional runs of the models with positive and negative changes of 2, 3, and 4 NRCS run-off curve numbers were also made. Increasing and decreasing the NRCS run-off curve numbers was believed, by the authors, to represent the most extreme changes that would be represented by mining. Increasing the NRCS run-off curve numbers by 4 generated changes in peak flow between 0.0 and 66.8%, and changes in total volume between 11.0 and 29.7%. Decreasing the NRCS run-off curve numbers by 4 generated changes in peak flow between -0.0 and -18.9%, and changes in total volume between -2.4 and -19.1%.
The groundwater model addressed two concerns indicated by the OSM: 1) States CHIA's were not based on the most recent technical information and 2) The USGS 1988 CHIA was a general assessment that was being applied regionally rather than individually with site specific data. This model also presents one of the first efforts to assess the recharge dynamics of the coal/clinker/overburden boundary. Compilation of BLM, CBM, and mine data for the top and bottom elevations of the coal seam (necessary as model input) has provided valuable information for evaluating the subsurface hydraulics. The complexity of the hydrogeologic setting and the lad of widespread data for verification of model results illustrate the difficulties of trying to extend a model of a Cumulative Impact Area, such as the Little Thunder Creek Drainage, to the PRB as a whole. Although the assumptions used to model the smaller area can be extended from one drainage area to the next, 'extension' of the model assumptions to the PRB as a whole is not considered economically feasible at this time.
The representation of the system, including the assumptions made, for each event simulated is considered to be conceptually correct. The consistency between the models with regard to NRCS run-off curve numbers, reservoir storage, and conveyance loss lend credence to this conclusion. Utilization of the models developed for the Little Thunder Creek Drainage will be most efficient il the pre-mining models are not altered. Post-mining effects can be added to the pre-mining models by determining the areas to be impacted, ascertaining post-mining terrain features, and then altering model input parameters. The model is flexible, allowing a large number of scenarios to be tested if future conditions warrant this.
An addendum was prepared by the LQD to list alternatives to the model conceptualization. Selection of some or all of the options could change the predicted drawdown distributions and rainfall/run-off relationships.
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