Fire Wax
DB: This is Earth and Sky. The Hayman fire of 2002 was the largest in Colorado’s history. Some 138,000 acres burned.
JB: Fire temperatures in many places exceeded 400 degrees Celsius – that’s 750 degrees Fahrenheit. It got that hot mainly because, after decades of fire suppression, there was an abundance of debris to fuel the fire. Greg Butters is a soil physicist at Colorado State University. He studied the effect of fire on the soil at the Hayman site.
Greg Butters: Here’s the problem. If you have enough organic matter, and a hot enough fire, then you can volatilize organic material out of that debris, the pine needles and the branches and what not, and that will move into the subsoil, will move below the soil’s surface, and condense, and it forms a waxy layer or a waxy coating, that is similar to what you might think about a wax on a car that causes the beading of water.
DB: That waxy layer is very damaging. It can cause severe erosion, floods and water pollution. One way to break up the wax is to apply hydromulch – a manmade material that acts like a spongy cover. Hydromulch prevents runoff by holding rainwater at the surface.
JB: For more about fire damage and about hydromulch, come to earthsky.org. Thanks today to the Bureau of Land Management and to the National Fish and Wildlife Foundation. We’re Block and Byrd for Earth and Sky.
The following person was interviewed for today’s program. Our thanks to:
Greg Butters
Soil Physicist
Associate Professor
Colorado State University
[15]
^^^ I was contacted by the EPA to evaluate what’s going on in the soil beneath the BAER treatments. I don’t actually impose the treatments or decide which treatments are going to be utilized. My job was to find out – if a treatment is effective, why? I was kind of in the science background – looking at water flow and soils beforehand, and that’s an issue that comes up in these burned soils. So that’s how I got involved in it. The person that contacted me, he was actually the EPA guy, he actually was one of the EPA bigwigs, so he might be the one to talk with if you want to know an overview of what they’re doing, and how they decide what treatments and whatnot, he would probably be the best person to talk to [Bob Borbst].
[36]
^^^ What we’re basically trying to do is find out how long it takes for the soil to recover following a burn. Now some soils are not affected, depending on the intensity of the fire. But in the Hayman site, there was some very intense fire that left a residue in the soil, a hydrophobic residue that is actually a condensate of volatilized organic material at the soil surface, when you have a very intense fire. And that hydrophobic material causes a problem with the soil’s wet-ability. So when you get rainfall events following the fire, you know – everyone is familiar with the erosion problem, but one of the reasons you have the erosion problems is that the soil is not readily accepting the water as it would normally do. And, I’m a soil physicist, so what they asked me to do is to find out a little bit on how long does that hydrophobic material persist, and is it’s persistence different under the different BAER treatments. For instance, there’s a hydromulch that they applied by helicopter, and there’s a straw treatment, and there’s a non-treatment or a scarification perhaps. Or just no disturbance at all. And my involvement has been assessing that hydrophobicity under the treatments, and testing if there is a correlation of that hydrophobicity with the soil water content, which I believe there is. I mean that’s not a new thought. One of the things that we’re trying to find out is if the treatments are effective because they help elevate, or maintain higher soil water content. When you cover a soil, that helps intercept the energy of the raindrops, and that in itself reduces the erosion potential. But it may also be that by having a cover on the soil, you maintain higher soil water content, which might help the microbial population in the breakdown of the hydrophobic material, as well as keep the soil a little bit more wet-able, because it already has some water there. So that’s what I’ve been trying to work on, is there a threshold at which, if you can maintain the water content in the soil, are the materials less hydrophobic. And that’s pretty much what I’ve been doing, not a very large part of the study. There are others that you might want to talk to that are looking at monitoring the efficacy of those treatments. How much reduction in sediment is there, when you put this hydromulch on, vs. not doing anything? And they have these really interesting sediment fences that they put across a wash, or a ravine, and they actually go out and weigh the sediment collected in those fences with buckets, it’s very manual. ^^^ That’s the sort of thing that needs to be done if you’re going to look at scales of watersheds. You need to do something beyond just rainfall simulators, and see what really happens at the base of a treatment. How much sediment is that? But other than that, there’s a scientist in our department that’s working on looking at the microbial changes, the changes in microbial population following the burn and in the recovery of those soils, and finding out if the treatments in the straw or hydromulch is affecting that. So that’s pretty much it.
[82]
ES: Could you describe a little bit about what a hydromulch is?
GB: A hydromulch – you’ve seen them, probably on roadsides or and area where they’re doing some landscaping, they spray this green, oftentimes green fibrous material that covers the slope. It’s a composite of different materials – it even has some seed in it to re-seed the system. But it puts on a layer of munched up fiber, goes on wet, and it covers a 1/2 inch thick, and they do it from the air, so it gets the sides of the trees and everything. It’s quite expensive. And the idea there is to protect the soil from the rainfall energy, but it also provides a little bit of a surface material to absorb a little bit of water, but also slow overland flow, to get little dams, perhaps of debris that might form from that material. It’s one of the main treatments, on of the higher end treatments, if you will, one of the more expensive ones.
[107]
GB: ^^^ Here’s the problem. If you have enough organic matter, and a hot enough fire, then you can volatilize enough organic material out of that debris, the pine needles and the branches and what not, and that will move into the subsoil, will move below the soil’s surface, and condense, and it forms a waxy layer, that is similar to what you might think about a wax on a car that causes the beading of water. Well this material prevents the soil from taking the water as readily as it would ordinarily. And so that hydrophobic material is very, very dependent on how hot the fire is, and how much organic material is there to begin with, and the type of organic matter. So, you can have spots in the fire where it’s not a very intense, or a hot burn, and you won’t have this depositing of this volatilizing of organic, and you won’t have the hydrophobicity problem. But in other areas, particularly on a slope with heavy, dense vegetation, perhaps a south facing slope when the fire was really rushing up, you’ll see particularly close to the trees, particularly where the needles have been heaviest dropping, around the drip line of the tree, you’ll get the heaviest accumulation of this hydrophobic residue. But the hydrophobic residue is not permanent. ^^^ That’s one of the issues. It might last up to a year to a couple of years. It depends on the site, and that’s one of the issues that we’re interested in, is to find out just how persistent it is, not just at the soils surface. Sometimes it’s in the subsoil down just a few inches, and sometimes it’s more so at the soil surface. It depends on the exact fire. But typically, you’re talking about a very hot fire. And the Hayman fire was in an area that had not burned for quite a while. So there’s a lot of debris there to burn. So you had the fuel, and the intensity of the fire was hot enough that there was quite a bit of hydrophobicity created in the subsoil. And that’s pretty much the story on that.
[144]
ES: Permit me a dumb question – I can’t promise there won’t be more – but since wildfire is typically a natural process, why are we so concerned about what happens afterwards?
GB: You’re talking about resources close to the fire, water resources and property resources, where you have people who have moved into those areas, and you’re going to have these erosion events that are going to cause problems, particularly with water quality, where you’re get these huge sediment runoff events into these reservoirs. So that’s one of the reasons, to mitigate that problem. Another is – getting back to what you said about fire being a natural process – this issue about how much impact it has on the land depends on the amount of fuel there. So if you have an area that naturally would have burned at some
21:28 what he wants to look at
23:47 “recovery begins fairly rapidly”
25:29
Dr. Greg Butters etc.
27:32 “I was surprised as heck that the ash wasn’t hydrophobic…” “People from different perspectives….water content issue, important point as to whether the BAER treatments work or they don’t work. North vs. South facing slopes. BAERs watching.
29:30 North vs. South facing slopes
31:53 “I was surprised at how variable the impacts [to the soil] are.”
“It’s really going to be water quality that we’re talking about. The water quality is really a major motivator here. “
Additional Teacher Resources
Cyberwest Magazine: Scientists Study Hayman Fire Burn Area
This site reviews a study by Colorado State University on the hydrophobic qualities of soil that occasionally develop after severe forest fires. The article explains how the use of hydromulch may speed up the destruction of the “fire wax”, which disintegrates naturally over time.
