TWO-WAY CLUSTER ANALYSIS OF GEOCHEMICAL DATA TO CONSTRAIN SPRING SOURCE WATERS

Abstract

The purpose of the study was to use geochemical characteristics and apparent ages of sampled groundwaters to determine which of the two regionally extensive bedrock aquifers, the lower bedrock aquifer or the upper bedrock aquifer, is a more likely source of water discharging to springs in the Nine Springs watershed. The use of summary statistics and our knowledge of the regional hydrostratigraphy resulted in the identification of three groups of monitoring points that are representative of groundwaters with distinct geochemical characteristics. Two-way cluster analysis of the geochemical data supports these groupings and further identifies subtle geochemical characteristics of the groups. One spring, which is representative of smaller springs and seeps found in the watershed, belongs to a group that is characterized by variable nitrate and chloride concentrations. Water discharging from this spring has a groundwater residence time of approximately 8 years based on the tritium/helium 3 dating method. The water discharging to this small spring is thought to have traveled primarily through the unlithified aquifer, as opposed to either of the major bedrock aquifers. Most of the springs in the watershed belong to a group that is characterized by elevated, but consistent, nitrate, sodium, and chloride concentrations. In addition, cluster analysis revealed that potassium and alkalinity concentrations are somewhat low. Apparent groundwater ages for this group range from 10 to 15 years. The water discharging from the majority of the springs in the watershed is thought to have traveled primarily through the unlithified aquifer and the upper bedrock aquifer before discharging into the former glacial lakebed wetland complex. Due to the relatively short groundwater residence times, spring water quality and flow in the Nine Springs watershed are likely to be vulnerable to the rapid urban expansion occurring within the watershed.