The Ogallala aquifer described in the Closer Look box in Section 18.3, provides 82% of the drinking water for the people who live in the region, although more than 75% of the water that is pumped from it is for irrigation. Irrigation withdrawals are approximately 18 billion gallons per day.

(a) Assuming that 2% of the rainfall that falls on an area of 600,000 km² recharges the aquifer, what average annual rainfall would be required replace the water removed for irrigation?

(b) What process or processes accounts for the presence of arsenic in well water?