SIMULATED EFFECTS OF ANTICIPATED COAL MINING ON DISSOLVED SOLIDS IN SELECTED TRIBUTARIES OF THE YAMPA RIVER, NORTHWESTERN COLORADO

x Identifying cumulative effects of mining on dissolved solids downstream from multiple coal-mining operations is particularly important in western basins. The problem of identifying cumulative effects is evident in the Trout Creek drainage, a tributary to the Yampa River in northwestern Colorado, where a number of mines are active and expansions planned. As an evaluation tool, a model was developed and calibrated for the Trout Creek drainage and a reach of the Yampa River main stem. In the model a series of nodes on the stream network is used to sum water quantity and quality through"the network. The model is operated on a monthly time step and was based on data from water years 1976 to 1981. Output is mean monthly discharge, dissolved-solids concentration, and dissolvedsolids load. Observed data are needed to initiate the model and for model calibration. Some data were extrapolated from records of nearby streamflowgaging stations. Some nodes within the stream network were inputs from anticipated mining and were inactive during calibration. After calibration, these nodes were used to input water discharge at a given dissolved-solids concentration to reflect various future mine configurations. INTRODUCTION The demand for energy in this Nation has fostered increased needs for coal. In response to this demand, one area of increased coal-mining activity is the Yampa River basin in northwestern Colorado. Many coal mines are now active in the basin, and many of the mining companies have proposed expansion of active coal mines or have identified sites for new mines. The Yampa River basin is hydr©logically typical of many coal regions in Colorado. Much of the streamflow of the main stem of the Yampa River is derived from high-altitude mountain snowpacks. The geology of these highaltitude areas is very different from the geology of the coal areas in the basin. The basic rock types in the high-altitude areas are igneous and metamorphic and result in streamflow with low dissolved-solids concentration. Coal mines are located in sedimentary-rock areas which may yield higher dissolved solids. The Yampa River main stem serves as a conduit of water through the coal region. The streams within the active coal area can be classified into two groups small tributaries and major tributaries. The small tributaries draining sedimentary-rock basins may contain naturally occurring large concentrations of dissolved solids. In general, most mining activity takes place in these small basins. The major tributaries, which receive water from the small tributaries and carry it to the Yampa River, similarly drain basins underlain by sedimentary rock. Numerous mines are active, proposed, or mine expansions are contemplated in the vicinity of Trout Creek (fig. 1), a major tributary to the Yampa River. In reviewing proposed mine plans, the Mined Land Reclamation Division must evaluate the cumulative impacts to the small tributaries, to the major tributaries, and ultimately to the Yampa River main stem. A cumulative evaluation is difficult because each mine plan is prepared and evaluated independently with no standardized method available to combine the effects of various plans. This difficulty is particularly evident in an evaluation of changes in the dissolved solids of the stream system which can be attributed to mining activity. The changes in dissolved solids may be dramatic both in small tributaries and in major tributaries. In addition, any increase in dissolved solids in the Yampa River main stem could have serious consequences on the continuing problem of dissolved solids in the Colorado River. A modeling study of the area was made by the U.S. Geological Survey for the Mined Land Reclamation Division of the Colorado Department of Natural Resources. This study is an initial attempt to construct a dissolved-solids model of surface water in the Yampa River basin. The model will be used by the Mined Land Reclamation Division to evaluate proposed mine development. To meet planning needs, severe time constraints on model development and evaluation were imposed upon the U.S. Geological Survey. Because of the time limitations, assumptions were made where needed data were not available. Also, the area to be modeled was reduced and focused only on the Trout Creek basin because this area was of immediate concern to the State. Therefore, using this particular model in the Yampa River basin is the first effort at combining mine plans with water-resource information. The overall objective of this report is to determine the cumulative effects of mining on dissolved solids in the Yampa River basin. More specifically the approach is: 1. To develop a model of streamflow and dissolved solids focused on the Trout Creek drainage and a reach of the Yampa River main stem. 2. To calibrate this model with existing data. 3. To insert into the model future mine plans as given by the Mined Land Reclamation Division to determine cumulative effects of mining on dissolved solids in the streamflow. In this paper the model is described, data needs for the model are identified^ methods used to extrapolate missing data needed by the model are shown, model calibration is explained, and anticipated mining activity provided by the Mined Land Reclamation Division is analyzed. These plans are compared with the existing conditions from the calibrated model. The model is to be used by the Division to help in the assessment of cumulative effects of multiple coal mines in a drainage system. Although the model is focused on the Trout Creek drainage, the State also is interested in changes in dissolved solids resulting from mining activity in Dry Creek (fig. 1). Insufficient water-quantity and water-quality data are available from this drainage system for inclusion in the model. Therefore, s'treamflow and dissolved solids were computed manually for Dry Creek following the model algorithm. In this report, an analysis of Dry Creek is given after the description of the model. THE MODEL The model, which routes streamflow and dissolved solids through the stream network, was written by A. W. Burns (U.S. Geological Survey, written commun., 1983) and has been used for other major stream systems in Colorado. The algorithm is an accounting procedure that sums water quantity and quality in monthly time steps from one or more upstream points to a downstream point. The addition of water quantity and quality is done at individual points called nodes. A reach is defined as the stream segment between nodes. In the stream network examined in this report, there are 27 nodes. Data can be entered, modified, or outputted at each node. Although the data are manipulated at these nodes, the changes in quantity and chemical composition of the water are attributed to the reach upstream from any particular node. As an example, a simple stream network with a series of nodes is shown in figure 2. If the concentration in dissolved solids is increased at node 5, this increase is not necessarily due to a point source at node 5 but may be due to diffused sources of increased salinity in the reach bounded by nodes 1 to 5 and 4 to 5. There are three kinds of nodes (fig. 2): input nodes, internal nodes, and output nodes. Input nodes are the upstream nodes (nodes 1, 2, and 3 in fig. 2) in the stream network. Because the summation process of water downstream starts at these nodes, the ideal case is to have streamflow-gaging stations for the input nodes. This is not always possible, and some estimated data must be used. Water and dissolved solids from upstream nodes are accumulated by the model at internal nodes (nodes 4, 5, and 6 in fig. 2). As such, some internal nodes will not be shown in the stream network under analysis in this paper. Internal nodes also are used to input proposed changes in water quantity and quality at individual coal mines (fig. 1). These input changes at a node can be point sources of water from dewatering activities or diffused sources of water such as drainage from a coal spoil pile within the reach upstream from the node. For brevity, there are instances when proposed changes of water quantity and quality for several mines are combined at a single node. Thus, there may not be an internal node for every mine in the watershed.