Shallow alluvial aquifer system and the interaction with Boulder Creek, Boulder, Colorado

Water chemistry in Boulder Creek, Colorado, shows significant variation as the Creek flows through the City of Boulder. This variation may be partially due to ground water inputs, which is not quantitatively understood. The purpose of this study is (1) to understand ground water movement in a shallow alluvial aquifer system and (2) to assess surface and ground water interaction. The study area is located at the Sawhill and Walden Ponds Wild Life Habitat in Boulder, Colorado. This area was reclaimed by the City of Boulder and Boulder County after gravel mining operations ceased in the 1970's. Consequently, ground water has filled in the numerous gravel pits allowing riparian vegetation regrowth and replanting.; A physical hydrology approach is used to examine the shallow ground water and surface water of the study area through field measurements, graphical data analysis, and numerical modeling. Collected field data suggest that lateral heterogeneity exists throughout the unconsolidated alluvial sediment. Measured hydraulic conductivities range from 1 to 24 ft/day. Data analysis suggests that ground water movement parallels surface topography and does not noticeably vary with season. Recharge to ground water via infiltrating precipitation is dependent on evapotranspiration (ET) demands.; An inspection of streamflow records indicates that Boulder Creek loses 0.5 to 3.5 ft3/s as it traverses the study area. Streamflows influence ground water levels for distances of up to 200 feet. At greater distances water levels are mainly controlled by ET and recharge.; Using collected data, a surface and ground water model was developed in Visual MODFLOW. The model was calibrated to reproduce observed stream losses and observed water levels. It was then used to determine the sensitive parameters that are most influential to the ground water system. Stream stage was found to be the primary factor in determining the rate of ground and surface water exchange and ET was another important factor. A two-fold increase in stream stage caused a net increase in stream loss to ground water by 79%. The greatest surface water contributions to the local ground water budget occur from late April to July when both streamflows and ET are high.