3. Consider a lake with a mixing time of a few weeks, and a total volume of 3106m3. The only significant sources of incoming water are a stream supplying 1.5106m3/ year and groundwater supplying 0.5106m3/y. Water exits the lake in stream flow at a rate of 1.5106 m3/ year and by evaporation at a rate of 0.5106m3/ year. The lake and stream water concentrations are in equilibrium with today's atmospheric CO2. a. Until recently, there had been no strong acid inputs to the lake. The alkalinity of the inflowing stream water had been 40 microequivalents per liter and that of the groundwater had been 150 microequivalents per liter. During this historical period, there were no other sources of alkalinity, and alkalinity in the lake was not changing. What was historical alkalinity and pH of the lake? (10,5pts.) b. In recent weeks, acid rain of pH3.8 started falling on the watershed. The stream inflow water pH has dropped to a pH of 4 and stayed at that value, but the groundwater chemistry has remained unchanged and will for many decades because of a huge stock of acid neutralizing capacity in the soil through which the groundwater percolates. What is the new alkalinity of the stream water? (10 pts.) (Yes, alkalinity can be negative.) c. Under the assumptions above, what will the alkalinity and pH of the lake be after a decade or so of this acid deposition? (10, 5 pts.)