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.  

Housatonic Pump-Up into Candlewood Lake

 As a delegate to the Candlewood Lake Authority I’ve been collecting water quality data in Candlewood Lake since June 2020. Twice a week (more or less) I’ve dropped a probe into the middle of the lake off my waterfront and at 1 meter intervals recorded temperature, dissolved O2, specific conductivity (essentially salinity), pH, and chlorophyll and blue green algae concentrations.

I use a "Manta 40" probe, and off-season I access the sampling sites with a small trailered aluminum runabout.

In the fall, as the lake loses heat to the cooler air above it, the cooled water sinks until it meets even cooler water below.  Eventually, the surface temperature approaches the bottom temperature. The temperature of the lake becomes uniform, and the lake begins to “turn over” as cold bottom waters are replaced by even cooler water from the surface.  At this point, the lake is well mixed and very uniform in temperature and composition.  By 11/03, there was virtually no stratification in the water column, and things remained that way through 11/19 as the water continued to cool.

On 11/25, a new and unexpected thermocline (a boundary layer between warm water above and cooler water below) appeared at a depth of about 11m.  The colder water below the thermocline was “saltier” (as indicated by a higher specific conductivity) and enriched with O2, as if a new body of water had been introduced at the bottom of the lake. The "discovery data" appears below, with the thermocline indicated by a blue line.

I assumed that I had sampled water that the power company that operates the lake had recently pumped up from the Housatonic River. The pumping station is about 2 1/2 miles north of the sample site. I did another sampling the morning of 11/28, and once again saw the thermocline.

To confirm my assumption,  I (with the encouragement and help of an enthusaistically interested friend) hustled down to a bridge over the Housatonic near the intakes at the power company’s pumping station.  Dodging traffic, we dropped the probe over the side of the bridge and made the following measurements:

It seemed like a close match to water I'd observed at the bottom of the lake earlier that day, so my next thought was to map the pumped water as it moved southward and mixed with the lake water.  I chose four sampling sites as shown on the map to the right, and headed out the next day to sample them.

Pumped in at the northern end of the lake, the colder Housatonic water apparently formed a lens of cooler, saltier water that hugged the bottom of the lake as it spread southward. I drew this profile from the data I collected on 11/29/2020 at the four sites. (Data set is here t.ly/1E9g )

As the slug of cool water spilled southward it warmed as it gained heat from the water above it. By the time it reached Brookfield, almost 6 miles to the south, it had largely dissipated into the surrounding waters.  By 12/10/2020 there was no trace of it south of The Narrows, though the specific conductivity of the water there was slightly higher than it was prior to the pump up.

 Hurricane Isaiah on Candlewood Lake

August 4 - 5, 2020

This summer I’ve been doing bi-weekly water quality monitoring in the middle of the lake off my dock on Candlewood Lake in SW Connecticut. As Hurricane Isaiah approached, I thought it would be interesting to measure the lake immediately before and after to see how the storm affected the it.  To that end, I sampled at 8 AM Tuesday as the storm approached, and again at 9 AM on Wednesday after the storm had passed, and compiled the results on this chart.

Not surprisingly, the high, sustained winds mixed up the surface waters.  What was surprising to me was the depth to which that mixing occurred. Surface water temps decreased by almost 2° F, while water deeper down warmed slightly indicating a downward transfer of heat.  Notice also that dissolved oxygen (HDO) shows similar mixing - compare the amount of O2 in the water at 6,7, and 8m before and after the storm - it’s pretty clear that the storm mixing reached a depth of at least 7m (23 feet!!).  Since the storm, BTW, O2 levels in the top 6 meters are back at saturation.
The “bg ppb” column is a measure of blue green algae (cyanobacteria) abundance, and it’s interesting to note how they like to hang out in the colder water this time of year - on 8.14 there were even fewer at the surface, and they were again hanging out in the colder water.  “Sp. cond” stands for specific conductivity, which is a proxy for the salinity of the water, and the numbers we’re recording are at the high end of what we’d like to see. It’s of concern because blue green algae tolerate it better than other plants and animals that normally keep them in check. All fresh water has some salinity, but a likely source of increased salinity in Candlewood are ice melting salts and inorganic fertilizers. Anyway, the influx of fresh rainwater apparently made the lake a tiny bit "fresher".

The Winter Hexagon

(This blog is a rehash of a blog I wrote in December of 2010)

This image is edited from a Stellarium screenshot.  Stellarium is an excellent, free, planetarium program.  Click for a larger view.

As a little boy I was lucky to spend summers under dark skies, and I had parents that helped me learn the constellations and movement of the nighttime skies.  Winter skies, of course, take more effort to observe, but the reward is worth the bundling up.  The "Winter Hexagon" is easy to know, and serves as great guide to the nighttime winter sky.

"Orion's Belt", part of the constellation Orion, is a well known and easily recognized asterism in the northern hemisphere's winter sky (between Betelgeuse and Rigel on the image above).  Six bright stars surround Orion's belt forming the Winter Hexagon, outlined in the image above.  Those stars are easy to find on a dark, clear night - follow the line formed by Orion's Belt down to the left to locate the bright and twinkling star Sirius, drop down perpendicular to the Belt to find blue-white Rigel, follow the line of the belt up to the right to spot Aldebaran (the orange "eye of the bull" in the constellation Taurus).  Look up from Aldebaran to find Capella (in the constellation Auriga), to the left of Capella find Pollux (the brighter of the twins of Gemini), and the sixth star of the hexagon is Procyon, below Pollux on the way back to Sirius. The bright orange star perpendicular to and up from Orion's belt (about as far above the belt as Rigel is below it) is the "red giant" Betelgeuse, the brightest star within the hexagon.

The Moon passes through the Winter Hexagon from right to left (west to east) each month...this month (January, 2019) it will cross the hexagon on the nights of 1/17, 1/18, and 1/19.

June 2013 Solstice and Full Moon

(NOTE: The images below are screenshots from a program called Stellarium, available for free at www.stellarium.org. Click the image for a larger view)

This year (2013), the summer solstice and the June full moon occur within days of each other.  The noon solstice sun (shown in the top image crossing the meridian of my home in western Connecticut with the effects of the atmosphere removed) will have reached its highest point in the sky in 2013.  Note its position on the point of the ecliptic (drawn in red here) that is as far north of the celestial equator (drawn in blue) as can be.  The June 21 sun is in the middle of the ‘winter hexagon’, just above Orion and the star Betelgeuse, and happens to be crossing the plane of the Milky Way galaxy as well.  Venus and Mercury are to the left (east) of the sun, and Jupiter and Mars are just to its right, though none of that will be visible through the sun’s glare.  It’s interesting to note too that the autumnal equinox is just rising due east, and the vernal equinox just setting due west at noon on the summer solstice.

The June full moon, 2 days later, is opposite the sun, occupying the position of the winter solstice at the southernmost point of the ecliptic.  – it will be low in the southern sky all night, following the same path the December sun followed six months ago. And as that full moon crosses your meridian that night, the vernal equinox will be rising in the east as the autumnal equinox sets in the west. The June 23 full moon also happens to be a “perigee full moon” (meaning its slightly eccentric orbit happens to bring it closest to earth on the same day it’s full), and it happens to be the closest perigee of the calendar year, too. While these conditions will make the moon measurably (but not noticeably) bigger, a little brighter, and produce higher than usual high tides, it is nothing extraordinary. You’re likely to see all kinds of “Super Moon” posts in the social media, but keep in mind that perigee full moons occur every 14 months and that this one in particular will not get very high in the sky.

Latest Sunrise of the Year

The latest sunrise of the year will occur on January 5 this year...2 full weeks after the shortest day!  The earliest sunset occurred on December 8 even as the days continued to get shorter and on December 21-22 (the Winter Solstice) we experienced the shortest daylight period of the year. Since then the days have been getting longer, even as the sunrise was getting later.

It might seem as if the latest sunrise and earliest sunset should occur on the shortest day, but both the tilt of the earth's axis and it's slightly elliptical orbit work together to speed and slow the sun relative to our clocks, sometimes pushing the daylight period later into the day (as has been happening in the last month), and other times moving the daylight period into the morning in a predictable pattern we call "the equation of time".

The term solstice means sun stops, or sun stands still.  Of course the sun is always moving east to west across our sky, but from late November through mid January, the sun is nearly as far south as gets (it stops moving further south!) - and that's why we see such uniformity in the length of the day....9 hours 20 minutes on 12/8, 9 hours 13 minutes on the solstice, and 9 hours 20 minutes again on 1/4.  It isn't until late February that you'll really notice rapid lengthening of the day.

Earliest Sunset of the Year

December 8, 2011. For the last few years, I've posted this in early December...12/8 marks the earliest sunset of the year! The daylight period is still getting shorter (people who pay attention to these things know that the shortest day is the Winter Solstice around December 21), but not a lot of people can explain tonight's early sunset.  It turns out that the rate at which the Sun travels across the sky is not constant - the tilt of Earth's axis and its elliptical orbit conspire to push the Sun ahead of our clocks, and then slow it down again, twice every year.  Astronomers call the difference between time told by the Sun (apparent solar time) and clock time (mean solar time) the "equation of time".(If you're interested, you can get the sunrise and sunset times for your location at the US Naval Observatory site.)
The chart on the left above, called the analemma, combines the equation of time with the position of the Sun relative to the equator.  Click it for a larger view, and notice that through most of the fall the Sun has been running ahead of the clock, but in December it began to slow dramatically.
It's the Sun slowing down relative to the clock that's moving the daylight period later into the day even as the days get shorter! The worst of winter is still ahead of us, but at least we'll have a little more evening daylight... (the latest sunrise of the year occurs during the first week of January)
This photo composite was made by Tom Matheson over the course of a year, snapping a picture of the Sun at exactly 8 AM (by the clock) each day.  Here is a labeled image of  Tom's photo.

(This blog is an edited  re-post from December 2009 and 2010)