The St. Peter-Jordan aquifer in Iowa was studied to understand its hydrologic factors and the effects of pumping. The researchers used a groundwater flow model to simulate the aquifer and found that pumping has led to regional declines in water levels. They discovered that most recharge to the aquifer in northwestern Iowa comes from vertical leakage from overlying rocks, not from outcrop areas as previously thought. Pumping has increased net vertical leakage into the aquifer and reduced discharge across lateral boundaries. Aquifer storage has helped supply water for pumping, but the rate of decline in water levels is expected to decrease by 2020 if pumping rates remain stable.