Effect of Rainfall and Snowmelt on Specific Conductivity

The Backstory

On 12/16-17/2020 the Candlewood Lake basin received between 12 and 15 inches of snow, and air temperatures remained below freezing for the next several days.  Local roads were treated with brine or salt to melt snow and ice.

Prior to the storm, the lake temerature was about 43°F, and the specific conductivity (a measure of "saltiness") was measured at about 260 µS/cm.

By 12/21 the surface temperature of the lake had cooled to 39°F.  I was unable to get out on the water to measure temperatures at depth (the boat ramp was impassable with snow!) so I made my measurements from a dock, but it's safe to assume that the entire water column had cooled to39°C, the temperature at which water is most dense.

Then, on 12/24-25/2020, the Candlewood Lake Basin was hit with by a powerful storm that brought strong southerly winds, overnight temperatures around 60°F, and several inches of rain. By midday on 12/25 all the snow from the past week had melted, and streams were at or near flood levels.

The Study

I suspected that the meltwater/storm runoff would be carrying significant road salt, and that the salt would raise the specific conductivity of the lake near the mouth of 2 small streams that flow into the lake at the north end of Candlewood Trails. I decided to test that hypothesis by measuring lake water at one site along the shore of the lake far from any streamflow entering the lake (Sample A) and then at the site where the 2 small streams enter the lake (Sample B)

Candlewood Lake (north is to the upper left).  The study area is on the northeast arm of the lake, shaded blue in this image.  The town of New Milford appears just to the northeast of the study area, and the route 7 and 202 corridor runs from the top left to the middle right of the image.

The basin drained by the 2 small streams has an area of about 92 acres. It is traversed by about 0.4 miles of Candlewood Lake Road North (a town road treated with brine prior to the storm), and about 2 miles of private roads in the Candlewood Trails and Candlewood Point communities.  Those roads generally are salted on the hilly sections.

The Small Watershed Basin (92 acres, shaded blue here) drains into Candlewood Lake at "Sample B."  Water was also tested along the shore at "Sample A."

The Data

Discussion and Conclusion

It's been observed that over time the specific conductivity ("saltiness") of Candlewood Lake has been slowly but relentlessly climbing - and that trend has been observed in many lakes in New England.  The causes are varied, but increased development and accompanying road salting is widely considered to be one of the major contributors.  My intent here was to see if I could actually observe changes in lake salinity after the snow melted and ran off into the lake.

Not surprisingly, the water measured near the stream discharge was a little bit warmer than the rest of the  lake - the measurements were taken following 2 days where the air temperature reached daily highs of 60°F.  What was interesting, however, was that the water measured near the stream discharge was actually LESS salty than the lake in general, indicating that the recent runoff of rainfall and snow melt was likely diluting the "saltiness" of the lake.  This does NOT mean that the runoff was not bringing any salt to the lake, just that it was less than I'd expected to see.  Indeed (and not surprisingly), the overall specific conductivity of the lake  in the study area dropped slightly after the recent snow and rain events - from 260µS/cm on 12/16 before the snow to 257µS/cm on 12/27 after the rain.

My experiement here was pretty impromptu, and not done with consistent controls and methods, so it should all be taken with a grain of salt.  But it all points to one important fact: In order to understand the dynamics and ecology of our lake in a way that will help us make informed and effective management recommendations, we need to monitor the lake more thoroughly and more frequently, and include monitoring of tributary streams and runoff as well.