Chemical Mass Balance Improves Estimates
of Total Streamflow Gains and Losses for a Stream Crossing
Underground Coal Mines
Hillegas, L.,
Cravotta, C., Levine, L., Sherrod, L.,
2014, Chemical Mass Balance Improves Estimates of Total Streamflow
Gains and Losses for a Stream Crossing Underground Coal Mines
[abs.]: Geological Society of America Abstracts with Programs,
Northeastern Section - 49th Annual Meeting Lancaster, PA, (23–25
March), v. 46, no. 2
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 (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.