ABSTRACT
Of the considerable amount of groundwater recharge that occurs on a daily basis, most circulates rapidly to surface water drainage systems such that storm hydrographs respond closely with precipitation. Alluvial aquifers along drainage channels or shallow bedrock aquifers can respond rapidly to recharge and have groundwater with mean residence times on the order of months to a few years (Figure 8.1). Only a small fraction recharges to groundwater systems with much longer circulation times. Deeper and conned aquifers host groundwaters that may circulate over hundreds to thousands of years. Others may retain groundwaters recharged under climate conditions different from current and host fossil groundwaters that are not being recharged today.
