A sewer construction program has been proposed as a method of controlling eutrophication in a small lake which is surrounded by suburban community. The lake receives practically all of its water from a phreatic groundwater system which surrounds the lake. Evaporation from the lake surface is equal to precipitation.
Suburban development is distributed throughout the groundwater basin.
Domestic waste water is currently discharged into individual septic tanks which drain to the groundwater system. Water supply is also obtained from the ground and consumptive use is small. The discharge of waste water into the aquifer has produced an increase in nitrate concentration in the aquifer and the lake. Biologists have concluded that nitrogen is the limiting nutrient in this lake, and suggest that the concentration of nitrate-nitrogen in the lake will have to be reduced to less than 1 ppm in order to control the excessive eutrophication. For the conditions indicated below, estimate the time required after completion of the sewer system for the eutrophication control to become effective.