VARIATIONS IN ELEMENT RATIOS HELP DISTINGUISH THE EFFECTS OF DILUTION BY MIXING OF INFLUENT GROUNDWATER FROM THE ADDITION OF DEICING CHEMICALS

Bogle Brook, in Weston and Wellesley Massachusetts, has been monitored for specific conductivity and other parameters for several years on a fifteen-minute cycle. We have also accumulated over 140 chemical analyses by ion chromatography for cations and anions dating back to 2012 at numerous places throughout this watershed. The monitoring site at Rivers School has been shown to have stratified flow, with a significant influx of groundwater through the stream bed showing higher conductivity. Sampling between cobbles and at the surface show significantly different ion concentrations, but equal ratios of upper to lower ion concentrations are found during high flow (113 liters/second). Much more varied ratios occur during lower flow conditions (42 liters/second).

Throughout the main stream and tributaries, Ca/Mg ranges smoothly from 2 to 20, with the highest values occurring associated with wetlands, kettle ponds, and a perched pond with chloride concentrations higher than 2000 mg/l and close association with a deicer storage facility. In isolated forest watersheds, the Ca/Mg ratio is typically much closer to constant.

Other ion concentrations such as Na and Cl are very highly correlated with a ratio of 1.688 and correlation R² = 0.996 over a very wide range of concentrations caused by application of road salt deicer. Sodium and chloride also correlate strongly with specific conductance because of their comparatively high concentrations and strong seasonal variation. Analysis of molar concentrations suggests that cation exchange may be affecting sodium contents.