Ariver has a heavily-polluted discharge entering it at a low rate of 325 litres per second. the 5-day bod and do of this discharge at 19 degree c are 185 mg/l and 2.5 mg/l respectively. the river flow upstream of the discharge may be taken as 5.0 m^3/sec and have a 5-day bod of 2 mg/l and do of 8.1 mg/l at 16 degree c. the river may be taken as having a width of 11m and a depth of flow of 1.5m in a rectangular channel. the following information is derived from the laboratory results and river field survey: deoxygenation constant (k_1) = 0.35day^-1 at 20 degree c (e-base) reaeration constant (k_2) = 0.63 day^-1 at 20 degree c (e-base) k_1 and k_2 at temperature t can be changed using the following equations: k_1, t = k_20 (1.135)^(t - 20) k_2, t = k_20 (1.024)^(t - 20) a) construct a clearly-labelled graph of do concentration against distance downstream for the first 200 km downstream of the discharge point, making clear the values of all the parameters you have used. b) calculate the distance downstream (x_c) where the minimum dissolved oxygen concentration will occur and show it in the graph of do concentration against distance downstream. assume there are no other discharges downstream. the following streeter- relationship may be assumed: d_o_2, t = k_1 l_u/k_2 - k_1 (e^k_1 t - e^k_2 t) + d_o_2, 0 e^-k_2 t where d_o_2, t is oxygen deficit at time t d_o_2, 0 is initial oxygen deficit at the point of waste discharge at time t = 0. lu is ultimate bod at the point of discharge at time t. k_2 is reaeration constant, d^-1 k_1 is first- order reaction-rate constant, d^-1