Creating a Water Quality Index

Water quality is a global concern — not only for tap water, but oceans, rivers, streams, lakes and ponds. What about our own Choate Pond on the Pace Pleasantville campus? We look at it, walk around it, enjoy turtles sunbathing on the rocks, but what do we know about its quality?

A Water Quality Index (WQI) is a universal language for understanding water quality. Its purpose is to turn complex water quality data into information that can be interpreted and understood by the public. Our plan is to create a WQI for Choate Pond.

Scientists cannot assume that the average person can interpret technical measurements such as mg/L (parts per million) for different parameters. A WQI is a single number, often ranging from 0-100 (0 being poor and 100 being great), that expresses the overall quality of a water body. It is typically based on many water quality parameters that can include but are not limited to: temperature, pH, turbidity, conductivity, biological oxygen demand, dissolved oxygen, total/fecal coliform, total nitrogen and total phosphorus.

The WQI is calculated by averaging the various water parameters and then generating a score. The index is presented and accompanied by a confidence value that expresses the degree of completeness, or how many tests were averaged to generate the water quality index value. For example, if a WQI includes pH, turbidity and BOD, but only pH and BOD were averaged to calculate the index, then the confidence value would be 2/3.

We are fortunate that Choate Pond has the benefit of both the real-time monitoring capacity of Blue CoLab’s Ada platform (see Bulletin #1), and the generous participation of the Westchester County Department of Labs and Research (where I work) which together provide us full analytical capabilities to measure for all the typical parameters. By the end of the semester we will present a proposed model for a WQI for monitoring Choate Pond’s health throughout the seasons, and perhaps extend farther into the Pocantico River watershed.