Freshwater flows out of a River and is entrained in the surf zone, being advected north along Beach while also being diluted by water from outside the surf zone (salinity 33.4 ppt).

a) If fecal coliform concentration in the river is 12 x 208 MPN/100mL what is the concentration at 400m, 3km and 6km along the beach, given that salinities of 30.0 ppt, 33.1 ppt, and 33.3 ppt were measured at those locations?

b) If the public health standard is 103 MPN/100mL, how far away from the river mouth should you swim?

Use above info no need to calculate a precise answer.