CHEMICAL MASS BALANCE IMPROVES ESTIMATES OF TOTAL STREAMFLOW GAINS AND LOSSES FOR A STREAM CROSSING UNDERGROUND COAL MINES
Water quality and stream discharge were measured at several locations along West Creek in the fall of 2012 and spring of 2013. Temperature, pH, and specific conductance (SC) were measured in the field. Concentrations of major anions (SO42-, Cl-) in filtered, unpreserved samples were analyzed by ion chromatography.
Streamflow losses along a reach of less than 100 m were indicated by flow measurements upstream and downstream of a visible seep. The stream water upstream of the seep had low pH, SC, and SO42- concentration. The seep water had neutral pH, high SC, and high SO42- concentration. Downstream of the seep, pH, alkalinity, and SO42- concentration in the stream were intermediate of those measured at the upstream and seepage sites because of mixing.
While discharge measurements indicate that losses of 1.91 cfs took place along this stretch of West Creek, mass balance using SC indicates that an addition of 0.13 cfs from the seep would be necessary to produce the specific conductance measured downstream. Visual estimates of the seepage volume were much lower than this estimate. This calculation suggests that the total amount of flow lost through the streambed into the underlying abandoned mine is 2.04 cfs, a 7% increase from direct flow measurements. Similar estimates were obtained using SO42- and Cl- mass balance computations. For systems in which inflow chemistry differs significantly from stream water quality, chemical mass balance can provide more complete estimates of flow gains and losses than would be derived from discharge measurements alone.