FIELD CHARACTERIZATION OF THE SHALLOW HYDROGEOLOGY OF NIWOT RIDGE, FRONT RANGE, COLORADO

Niwot Ridge, approximately 35 km west of Boulder, Colorado, is in the headwater region of the Boulder Creek watershed. At an elevation of around 3400 m, the study area has an annual mean temperature of -3.7 ºC and annual precipitation of around 1000 mm. The objective of this study is to assess the recharge rate to groundwater.

Seven pairs of groundwater wells were installed at the Saddle and Martinelli sites, within the study area, in 2005. Each pair consists of a deep well to a depth of 9.1 m and a shallow well to 2.1 m. Monthly water levels since the installation of the wells were compiled to examine the temporal and spatial characteristics in water level and groundwater flow. Two deep wells at two Saddle sites were instrumented to collect the groundwater level data twice daily since January 2010.

Water level records at the Saddle show a pattern of rapid rise during spring in May-June and gradual decline throughout the remainder of the year. The seasonal fluctuation in water level was 4.7 to 5.4 m at the westernmost Saddle wells and 2.3 m at the easternmost pair. The rising limb of the hydrographs for the three western pairs were steep and occurred one month earlier compared with that of the eastern pair, which indicates the higher rate of recharge at the western wells. Due to infrequent water level measurement during the winter months at the Martinelli site, seasonal trends for the site were not analyzed.

Slug tests and double-ring infiltrometer tests were conducted at all wells in summer of 2010 to assess hydraulic conductivity of the shallow subsurface. These tests indicated that hydraulic conductivity is around 0.30 mm/s at Martinelli and 0.08 mm/s at Saddle.

Groundwater flow is generally from west to east, following the general topography. On the basis of measured hydraulic gradient and hydraulic conductivity, flow rates are estimated to be on the order of 1.3 x 10^-5 to 5.6 x 10^-2 mm/s at the Saddle site and 4.0 x 10^-3 to 1.7 x 10^-2 mm/s at the Martinelli site. Higher rates of groundwater flow are dictated by steeper horizontal hydraulic gradients. Vertical gradients are typically very low at all wells except at the easternmost Saddle pair which had an average vertical gradient of 0.91. These preliminary results suggest a significant spatial variability in groundwater recharge over the study area of approximately 5 square kilometers at the Niwot Ridge.