All About Chlorine

On July 7th & 8th, teams of Mad River Watch volunteers visited their field sites to collect important data and make careful environmental observations for the third time this summer. This article is part of an ongoing series that shares highlights from this fieldwork as well as lessons learned from this year and the past 40 years of Friends of the Mad River’s Mad River Watch program. In the coming weeks, we’ll share more details about the various pollutants we monitor and why we do so, starting with chloride this week.

All About Chlorine
Chlorine, a reactive greenish-yellow gas characterized by a pungent odor, is used in several industrial and commercial applications including water treatment, bleaching, production of plastics and solvents.

Chlorine naturally occurs in our waterways but excess amounts can have severe ramifications for water quality. Regrettably, chlorine can contaminate rivers from multiple sources, including industrial discharges, sewage treatment facilities, water treatment plants, agricultural runoff, and most notably in Vermont, stormwater runoff of de-icing compounds like road salts. High levels of chlorine are known to negatively impact fish populations and can pose risks for consumption, swimming, and fishing. Preliminary results from our last sampling event showed chloride ranges between 2.4 mg/l & 25.3 mg/l. Final results will be verified later and these levels are subject to change. Since this is something new for MRW to monitor, at this time we are just trying to get a sense of what baseflow chloride levels are.

How We Sample
To safeguard the Mad River Watershed and the larger Lake Champlain Basin against the impact of high chlorine content in our waters, Friends of the Mad River works with volunteers to conduct chlorine sampling at five different locations on the main stem of the Mad River through support from the Vermont Department of Environmental Conservation (DEC) La Rosa Partnership Program. Samples are collected every two weeks in the summer and sent off to the DEC laboratory for testing. The data we collect allows us to monitor chlorine levels to ensure water quality remains within acceptable limits. The results may also allow us to pinpoint sources of chlorine pollution, enabling targeted intervention.

Labeled LaRosa Program sample containers ready for volunteer pick-up. Photo Credit: Friends of the Mad River

​While Mad River Watch sampling happens only in the summer months, according to the Watershed Management Division of the Vermont DEC, chlorine sampling can be done at any time of the year and during any flow condition. It is generally highest during the spring snowmelt period with the flush of road and parking lot runoff, however, some streams are showing year-round elevated chloride levels due to groundwater, wastewater, or agricultural discharges. River flow information is also important because it helps to interpret what type of pollutant sources might be present. Generally, chlorine concentration is diluted during higher flows. If we see high concentrations of chlorine during these conditions, it might point us to a direct discharge somewhere upstream.

Luke Foley collects samples from Pony Farm Road while Ira Shadis observes. Photo Credit: Friends of the Mad River

​Our data this season indicates that the chlorine load in the Mad River is relatively low. Our next steps are to continue monitoring chloride at the five sites this year and for the next few years as we establish baseline conditions. We are also looking at expanding our season for conductivity monitoring (which correlates with chloride & sodium) which can help us better understand the flush of salts into the water in the spring. The same probes we use for conductivity could be used to understand the extent to which pollutants like chloride enter the river during storms.

Near bank of Clay Brook. Photo Credit: Mad River Watch Volunteer, Suze Edwards

Observations from the Field - Summer is Heating Up
The weather on both field days was sunny and warm with scattered clouds. Air temperatures ranged from a low of 62F at Brook Road on Sunday to a high of 85F on North Road on Monday. The recent rains also contributed to higher and faster water flows across the watershed. Water temperatures can range greatly throughout the day, increasing with greater exposure to solar radiation – sunshine. Water temperatures ranged from a low of 61.5F at Clay Brook on Monday morning to a high of 75F in the Main Stem near Waitsfield Covered Bridge on Monday evening. The average temperature across both days was 65.8F. As water travels from high up in the mountains to the confluence with the Winooski, it leaves the shady forests for long stretches of open canopy and the possibility of strong sunshine through much of Waitsfield and Moretown.​