HYDROCHEMICAL EVOLUTION MODEL AND CHARACTERIZATION OF FLOW IN A FRACTURED-CRYSTALLINE ROCK AQUIFER, TURKEY CREEK BASIN, COLORADO

Recent studies suggest that residential development of the mountainous parts of the Jefferson County, Colorado is one of the primary causes of water quality degradation in the area. This phenomenon is of great concern to planning organizations and residents living in the area, because groundwater is the only source of water for household use. In this study, multivariate statistical techniques and inverse geochemical modeling were used to investigate the sources and factors controlling both groundwater and surface water chemistry in 116.5 square km Turkey Creek Basin (TCB). A total of 180 groundwater and surface water samples collected during spring runoff and baseflow conditions were separated into four homogeneous groups by hierarchical cluster analysis (HCA) using major-ion content and physical properties of the waters. Principal components analysis (PCA) showed that the factors that control the clustering are total dissolved solids (TDS), pH and NO3+NO2. Inverse modeling indicates that a portion of the chemical variation can be attributed to water-rock interaction, but also indicates there is mixing of normal groundwater with an anthropogenic component in some samples. Overall, the systematic spatial variation, with TDS increasing down the topographic flowpath, implies that the fractured-crystalline rock aquifer is acting as an equivalent porous medium on the watershed scale. However, comparison of the baseflow and spring recharge samples reveals areas where the dilution effect is either pronounced or non-existent. This is interpreted to represent areas of high and low infiltration or variations in near-surface permeability. This study demonstrates the value of chemical data in defining the spatial distribution of aquifer properties, which can then be used to better constrain numerical groundwater flow models.