DELINEATION OF GROUNDWATER-SURFACE WATER CONNECTIONS AND PROCESSES USING HYDROCHEMICAL DATA IN A MOUNTAIN WATERSHED, JEFFERSON COUNTY, COLORADO

Almost three hundred samples were taken during spring runoff and baseflow conditions over a two-year period in a populated mountain watershed during a joint project by Jefferson County and the USGS. The samples were classified by hierarchical cluster analysis (HCA) and principal component analysis (PCA) based on physical and chemical properties. The hydrochemical facies formed spatially distinct clusters of samples that included both surface and groundwater. The results were used to delineate the connections and processes between ground water and surface in the watershed, first by examining systematic changes between spring and baseflow samples from the same location, then by analysis of baseflow samples alone.

Analysis of spring versus baseflow samples showed surface water parameters were usually diluted during the spring runoff with the exception of some samples where sulfate, fluoride and total phosphorus increased. Sodium, chloride, nitrate, fluoride and sulfate also increased in the majority of the groundwater samples taken during the same period. Locations where dissolved parameters increased during recharge are assumed to have poor vertical connection between surface and groundwater, or recharge rates that are slower than the time between initial melt and sampling (~1 month).

The baseflow samples were segregated into impacted (high sodium, chloride and nitrate) and non-impacted water samples. In the un-impacted samples, there were distinct chemical changes related to contact with the atmosphere (increased dissolved oxygen, decreased dissolved CO2), precipitation of minerals such as calcite and ferrihydrite and dentrification. While some of these changes were observed in the impacted samples, the changes were often less pronounced.