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 d issolution rate of plaster of Paris standards was developed as part of a study of vertical, longitudinal (1-4 order sites) and seasonal vari ation in the biotic and physical characteristics of the shallow hyporh eic 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 simulat ion tank to relate mass loss of plaster standards to water velocity an d temperature. The model was then used to calculate water velocity thr ough artificial substrata embedded in the shallow hyporheic zone of fo ur 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 interacti on 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 vel ocity at greater depths. Mean interstitial velocity in March ranged fr om 0 cm s(-1) in the lowest level (20-30 cm) to 3.5 cm s(-1) at the up per 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. Grad ients in stream discharge and sediment permeability accounted for trea tment effects. 4. Use of calibrated data improved the ability to resol ve among-season differences in interstitial water movement over the us e of uncalibrated mass loss data. For some applications of the plaster standard method, empirical calibration may not be necessary.