I am trying to simulate the chlorine decay using spreadsheet. I used the reaction equation (C 0 =C t *EXP(- k* ∆t)) but my results still different from the one in EPANET. I do not know what I am missing.

I have the ∆ t = (300 secounds)

k= (kb * C) ( 1.15741E-05 s^-1 * 5 mg/l)

C t-1 = 5 mg/l

pipe length is 171 m

segment length is shorter than the pipe length (169.77 m)

after one time step, my reading is 4.913943623 mg/l

EPANET reading is 4.98 mg/l

velocity is 0.57 m/s

wall coefficient is neglected

flow rate 10 m^3/s

I see a divergence between my results and EPANET results.

I do not know what parameter I should introduce to have my result similar to EPANET.