Your municipality withdraws 10,000 gallons per day from an
aquifer that originally held 1,000,000 gallons. The recharge
rate to the aquifer is 2,000 gallons per day. How long will it
take before the aquifer is depleted? You have to make an
equation that relates the amount of water (w) in the aquafier to
time (t). Start with the original amount: Now, we know that
every day ten thousand gallons are taken out so we have: And we
also know that every day two thousand gallons are added so we
have: To find when the water runs out, set the equation equal to
0 and solve for t: So, the water will run out after 125 days.