flow rates may introduce non-equilibrium conditions, limiting sorption and therefore
affecting the final calculation of Cs.
2.1.3 Flux Determination Under Transient Conditions
Transient conditions more closely represent natural systems and therefore are
important to take into consideration. Under steady-state conditions, the area of the
stream channel is assumed to be constant. Natural channels often are irregularly shaped,
causing stream flows to change in cross-sectional area depending on their stage height.
Variations in both v, and Ach are problematic when trying to estimate water velocity with
a PSFM under transient conditions.
In general, the water velocity Vo can be described by
S= Q(t) (2-10)
Ach (t)
where it can be seen that ifAch(t) is constant, Vo is directly proportional to Q. In cases
where Ach(t) varies with time, there is no direct way to calculate Q without the aid of
additional equipment. Therefore, in this study transient studies were conducted only by
varying solute flux while maintaining a constant water velocity.
When flow remains relatively steady, the water and solute fluxes can be calculated
using the equations developed for steady state conditions, only the solute flux will now
represent an average solute concentration in the stream channel over the duration of
deployment. This average concentration is a direct result of the measurement itself being
time-averaged, therefore no estimates on the concentration range over that time period
may be made.