STATISTICAL ANALYSIS OF SURFACE WATER QUALITY DATA OF EASTERN MASSACHUSETTS

Salinization of ground water by winter de-icers is a primary cause of degradation of drinking water quality of water supplies in Eastern Massachusetts as well as in many other areas in northern tier states of the United States and Canada. Our data on surface and ground water quality from a small watershed (approximately 7 mi²) in suburban SE Massachusetts show a continuing steady build up of NaCl in the groundwater system since the early 1990 and a corresponding increase of chloride concentration in the public water distributed to the local community. This data set and other data sets included in the USGS sponsored NAWQA program for the New England region were assembled into a larger dataset of 571 analyzed samples which include all standard water quality parameters. The entire dataset as well as its subsets using a variety of selection criteria are statistically evaluated by a principal component analysis and other statistical procedures to obtain principal components and the dominant independent variables that influence the observed chemical compositional range. In most cases, over 85% of the total variation can be explained by 3 to 4 components. The overwhelming variation is attributed to a large compositional range of Na and Cl seen even if all data are combined into a single dataset. Na vs. Cl correlation coefficients are commonly greater than 0.9. Second components are typically associated with dilutions by overland flows (non winter months) and/or increased concentrations due to evapotranspiration (summer season) or overland flows (winter season) if a snow storm is followed by the application of de-icers on road surfaces. The last two components reflect the relative dominance of the composition of the local bedrock on groundwater chemistry or the effect of nutrient cycling. Comparisons of pre winter and post winter baseflow compositions appear to show no evidence of post winter decline of NaCl concentrations.