An application of the plaster dissolution method for quantifying water velocity in the shallow hyporheic zone of an Appalachian stream system

1. A method for quantifying interstitial water velocity based on the dissolution rate of plaster of Paris standards was developed as part of a study of vertical, longitudinal (1-4 order sites) and seasonal variation in the biotic and physical characteristics of the shallow hyporheic zone (0-30 cm) of a headwater stream system in West Virginia, U.S.A. 2. A calibration model was developed using a water velocity simulation tank to relate mass loss of plaster standards to water velocity and temperature. The model was then used to calculate water velocity through artificial substrata embedded in the shallow hyporheic zone of four stream reaches based on in situ mass loss of plaster standards. 3. Water velocity in the hyporheic zone increased with stream order, was highest in early spring and winter during high stream base flows, and decreased with depth into the substratum. There was a strong interaction between depth and season: during periods of high stream discharge, water velocity through the upper level of the shallow hyporheic zone (0-10 cm into the substrate) increased disproportionately more than velocity at greater depths. Mean interstitial velocity in March ranged from 0 cm s(-1) in the lowest level (20-30 cm) to 3.5 cm s(-1) at the upper level (0-10 cm) at the first-order site, and from 2.5 cm s(-1) (20-30 cm) to 9.5 cm s(-1) (0-10 cm s(-1)) at the fourth-order site. Gradients in stream discharge and sediment permeability accounted for treatment effects. 4. Use of calibrated data improved the ability to resolve among-season differences in interstitial water movement over the use of uncalibrated mass loss data. For some applications of the plaster standard method, empirical calibration may not be necessary.