Problem $1$

An industry is located adjacent to Spring Creek. The industry uses copper cyanide for plating copper and discharges $0\mathrm{.}08\frac{m^3}{s}(Q_e)$ of copper$-$containing wastewater into Spring Creek. Aqueous copper is toxic to aquatic life at concentrations above $20$ ppb$.$ What is the maximum concentration of copper that the industry can discharge into the creek $(C_e)$ without causing adverse impacts to aquatic life? Upstream of the industry, the flowrate is $0\mathrm{.}25\frac{m^3}{s}(Q_u)$ and the concentration of copper is below minimum detection limits $)=(0.$

$e=$ effluent from the industry; $u=$ upstream of the industry; $d=$ downstream of the industry.