CHEMICAL MASS BALANCE IMPROVES ESTIMATES OF TOTAL STREAMFLOW GAINS AND LOSSES FOR A STREAM CROSSING UNDERGROUND COAL MINES

In extensively mined areas, rapid transfers of runoff, recharge, and discharge can drive dynamic variations in surface and groundwater quantity and quality. Because groundwater seepage into a stream can affect ion concentrations and pH along a flow path, chemical mass balance estimates may demonstrate gains from groundwater seeps that may not be apparent solely from flow measurements in a losing stream. Calculating inflow volume from seeps can supplement direct measurements of streamflow to improve estimates of total volume lost or gained. This study investigates the potential for using specific conductance and associated water quality data to refine estimates of flow gains and losses along the West West Branch of the Schuylkill River headwaters.

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 (SO₄^2-, 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 SO₄^2- concentration. The seep water had neutral pH, high SC, and high SO₄^2- concentration. Downstream of the seep, pH, alkalinity, and SO₄^2- 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 SO₄^2- 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.