Characterization of the shallow hydrogeology with estimates of recharge at a high-altitude mountainous site, Niwot Ridge, Front Range, Colorado

摘要

Mountains provide a source of essential water resources, including groundwater, for lower-lying valleys and plains. It is important to understand mountain systems to effectively manage and protect groundwater resources. Yet, basic questions remain regarding mountain recharge. The goal of this study was to improve understanding of spatial and temporal variability of groundwater flow and recharge in a mountainous watershed. It is the first attempt to characterize the hydrogeology of Niwot Ridge, an alpine study site in the Front Range, Colorado. The focus area included two headwater catchments: the Saddle catchment and the Martinelli catchment. Hydrogeology was characterized using results of groundwater level monitoring, infiltration tests, and slug tests. Recharge was estimated using water table fluctuation method and groundwater modeling. The water budget was determined. The water table is at most 8.5 m below ground surface and is a subdued replica of topography. Water levels fluctuated seasonally, implying that one significant recharge period occurred per year. Infiltration capacity ranged 2.87x10-5—6.71x10-4 m/s. Slug test results indicated hydraulic conductivity was 1.92x10-4 m/s for Quaternary-Tertiary diamicton, 8.74x10-4 m/s for Tertiary bedrock, and 8.42x10-6 m/s for Precambrian bedrock. Modeling results indicated hydraulic conductivity was 2.50x10-6 m/s for diamicton and 1.00x10-7 m/s for Tertiary bedrock. Groundwater flow was mostly shallow, flowed from high to low topography, recharged on ridgetops, and discharged to streams. At the Saddle site, recharge averaged 1.25 m/y (3.96x10-8 m/s, 52% of precipitation) and ranged from 0.57 m/y in areas where little snow accumulates to 1.64 m/y in areas where snow depths reach several meters. At the Martinelli site, recharge was at a minimum through winter months, rose quickly to a maximum rate (1.22x10-7 m/s) in June when water table was highest, and gradually declined through the remainder of the year. Assuming 100% water input from precipitation, the annual water budget at Niwot Ridge included 33% evapotranspiration, 15% sublimation, 13% runoff, and 52% recharge (with 13% total error). The results of this thesis contribute to scientific knowledge about the Niwot Ridge hydrologic system as well as to the broader understanding of how groundwater is replenished at its source by mountain recharge.