**6. Nomenclature**

AP, AM1, AM2 – interface areas of the hyporheic zone and the mainstream flow under plug flow, first and second well mixed cells respectively

C(x, t) – Concentration of pollutant in the main stream

CP, CM1, CM2 – Pollutant concentrations at the end of plug flow, first and second well mixed cells respectively

CR – Input pollutant concentration of a hybrid unit

Cs(x, t) – Concentration of Pollutant trapped in the hyporheic zone

D – Depth of effective soil layer/hyproheic zone

DL – Longitudinal dispersion co-efficient

F – Proportionality constant

K(t) – Unit step response of a hybrid unit

k (t) – Unit impulse response of a hybrid unit

Ms – Mass of the pollutant trapped in hyporheic zone

n – Number of cells

Q – Stream discharge

R – Retardation factor

RD – Mass exchange rate constant

t – Time

T1 – Residence time of first well mixed cell

T2 – Residence time of second well mixed cell

u – Stream flow velocity

U ( ) – Unit step function

V0 , V1, V2 – Volumes of mainstream plug flow, first and second well mixed cells

V0\*, V1\*, V2\* - Volumes of hyporheic zones under plug flow, first and second well mixed cells

WP – Wetted perimeter at the interface of hyporheic zone and the main stream

x – Distance

α – Residence time of plug flow cell

δM1, δM2 – Ramp kernel co-efficient of first and second well mixed cells respectively

Δt – Small time interval

Δx – Size of control volumes within plug flow cell.

φ – Porosity
