A Little Dirt Never Hurt...Right?
Updated: Mar 28, 2022
A classic Colorado childhood isn't complete without snacking on snowballs and catching scrumptious snowflakes on the tongue. One of my favorite memories as a dad was watching each of my kids' first reactions to deep, fluffy powder. Each one promptly grabbed a handful of powder and stuffed it in their mouth. Not only does this irresistible white stuff create childhood memories, but it also provides the majority of Colorado's precious water supply. But just how clean is our snow?
Everyone knows to steer clear of yellow snow, but what about pink snow? When I was shoveling fresh snow on 2/25/22, I noticed that the snow had a subtle pink hue. And later that day, I noticed that my windshield wipers had concentrated a fine layer of pink dust on the edge of my windshield. A few days later, I went to a nordic ski race near Purgatory. When a friend of mine from Durango pulled up, I asked him if he’d been off-roading in Moab because his vehicle was covered in a thicker layer of the same pink dust. He said all that dust had accompanied the recent snowstorm.
I'd seen this phenomenon before because the San Juan Mountains are susceptible to these dust-on-snow events. Our dust-on-snow events usually occur when strong southwest winds entrain desert dust as storms pass over the Four Corners area and crash into the San Juan Mountains. These events usually occur in the spring and happen frequently enough that the Colorado Dust on Snow Program (CODOS) was created to monitor and model the impact of these types of events on our water supply.
The surprising and far-reaching consequences of large DOS events are caused by changes in the reflectivity, or albedo, of the snow surface. Clean, fresh snow has an albedo of nearly 1, which means the snow reflects nearly 100 percent of the incoming sunlight. Dust lowers the albedo of the snowpack causing it to melt sooner.
Oftentimes, dust that is deposited at the beginning of a storm is subsequently buried under clean snow that falls at end of the storm. However, these dust layers are easy to find by looking at the full snow profile. As overlying, clean layers melt, dust is exposed and the snowmelt accelerates. If there are multiple dust layers in a snow profile, the upper layer of dust will merge with the lower layers and concentrate the dust on the surface. The reduction in albedo can cause the snow to melt up to 3 to 4 weeks earlier.
Earlier snowmelt is good news if you're ready for summer, but it's bad news for our water supply and perhaps your summer plans. Altering the timing of our snowmelt can shorten the runoff season, which can lead to warmer water temperatures later in summer. Warmer water temperature can stress trout species and impact the fishing industry. Shorter, earlier peak runoff dates can also impact the rafting industry because the peak runoff can happen before the tourist season gets into full swing. The ski industry is also impacted because the dust can create slushy conditions and can ultimately lead to a shorter ski season.
Most importantly, when the snow melts earlier, evaporation and transpiration by plants begin removing water from our soil earlier resulting in a 5% reduction in our water supply as well an earlier onset of the wildfire season.
To answer my initial question about the purity of our snow, I recruited my high school science students. Our hypothesis was that the 2/24/22 storm deposited a disproportionate amount of sediment to our snowpack and would create a measurable impact on the overall water quality of the melted snow.
First, my students read the CODOS storm report and determined that the source region of the dust was Northern Arizona due to strong southwest winds during the storm.
Next, we located a fairly pristine field of snow on the north side of the high school building. Half of my students collected the whole snow profile and the other half collected the upper-most layer of snow from the most recent, sediment-laden storm.
Back in the lab, each group melted their snow sample and measured the snow water equivalent (the depth of the water produced by the melted snow), the conductivity, and pH. Then my students filtered the sediment using the help of a vacuum pump.
The Pagosa Springs High School campus had about 4.8 inches of water in the snowpack and the 2/24/22 storm accounted for 0.8 inches (17%) of that moisture. The pH of the meltwater from the full snow profile was 6.5 and the recent storm was slightly higher with a pH of 6.8. The conductivity of the full snow profile was 18 us/cm and the recent storm was 7 us/cm, which is 40% of the dissolved load. In terms of sediment, the full snow profile contained 0.047g of sediment, and the 2/24/22 storm deposited 0.012g, which is about 25% of the sediment load.
Surprisingly the recent storm didn't contribute as much sediment to the snowpack as we thought. It's possible that the snow collected a substantial amount of dust during the 7-week snowless period from January through early February. Normally, Colorado's topsoil washes into our neighboring states, but if we extrapolate, the wind returned 250 dump truck loads of Arizona topsoil back to the Upper San Juan River's 284 square-mile watershed!
For comparison purposes, we decided to test the pH and conductivity of our tap water. Since Pagosa Springs is the first municipal user of the snowmelt, our tap water is high quality with a conductivity that hovers around 100 us/cm and the pH is usually around 8. As a reference point, pure water is an insulator and has a conductivity of 0 us/cm. On the day we measured, our snowpack was about 9x less conductive than tap water, which means it's cleaner with respect to dissolved minerals. The pH of the snowmelt is lower than our tap water but only slightly lower than a neutral pH of 7.
So should you have second thoughts about eating fresh snow? Based on our short study, if it looks clean, it is probably purer than our tap water. Although we did not test the snow sample for microbes or insoluble contaminants, I’m pretty confident that both constituents are minuscule.
In raising my kids, my mantra has always been, “A little dirt never hurt.” But if you want to avoid dirt, you can always steer clear of the dust layers that are likely found in the snowpack of the San Juan Mountains.
Since I was delayed in writing this blog, the snow is most likely gone from your yard and the dust from the 2/24/22 and 3/4/22 storms caused it to melt a little faster. The Wolf Creek Summit SNOTEL site currently has 4 dust layers and 33.6 inches of water within the 89-inch snowpack. The 2/24/22 dust layer is rated “severe” and once exposed will likely lead to an earlier peak spring runoff event on the San Juan River.
As I was doing research for this article, I came across an interesting study in the Wasatch Range near Salt Lake City. As the Great Salt Lake shrinks, there is an expanding source of salty soil for storms to carry into the surrounding ski country. And then I came across a mind-blowing video about radioactive fallout, which like dust, is entrained and deposited over areas depending upon wind currents. I learned that we all have trace amounts of strontium-90 in our bones and teeth especially if you were alive during the 1950s!