The Northern Guam Lens Aquifer (Lens) provides 80% of water utilized by Guam’s residents and visitors.  The Guam and United States Environmental Protection Agencies have recently questioned whether the vadose zone overlying the Lens provides sufficient filtration to be protective of human health, with regards to Giardia lamblia and Cryptosporidium sp.  The vadose zone is uplifted, eogenitic limestone that has undergone significant secondary alteration due to dissolution and precipitation of calcium carbonate and fracturing.  The vadose zone thickness ranges from 100 to 300 meters.   In order to help characterize the flow processes occurring within the vadose zone a transfer function model was developed.  The transfer function model accounts for vadose zone thickness, precipitation history, and precipitation intensity.  It links with SUTRA to simulate Lens response to storm events of varying intensity and duration.  Simulations illustrate that under intense precipitation the mean travel time to the Lens is less then one day.  When a wet period is followed by less intense precipitation, the mean travel times were between one and two days.  Longer travel times were observed for periods of less intense precipitation and precipitation occurring after extended dry periods.  These results indicate that, under some conditions, flow through the vadose zone may follow more direct pathways, bypassing the filtering processes that occur within the vadose zone.