By Alyson K. McDonald and Diana Doan-Crider
The incidence of wildfires in the southwestern United States is often linked to the El Niño Southern Oscillation (ENSO). The desert southwest typically receives above average precipitation during the El Niño episode of ENSO and then experiences drought conditions during the following La Niña episode. Plants flourish during the El Niño periods. Grasses and forbs may accumulate during this time and later dry out, setting up conditions for increased wildfire risk during the La Niña episode.
Although wildfire is a natural process associated with native rangeland ecosystems, the immediate aftermath can be startling. One of the most obvious landscape changes is reduced plant cover. This reduced vegetation can have indirect impacts on surface water bodies such as streams and stock ponds or tanks, affecting water quality, fish populations, livestock, and wildlife (Fig. 1).
Wildfire effects on water quality can be physical, chemical, biological, or some combination of these. However, all are relatively short-term, lasting from weeks to a few years.
Reduced plant cover leaves the soil vulnerable to increased rainfall runoff and soil erosion. If not prevented, the increased erosion can continue to deteriorate the pond or surface water quality
Soil moisture, weather conditions, and the fuel type and quantity influence fire behavior. Plants and litter (organic matter) on the soil surface make up the fuel for a fire. Grasses, trees, and fallen logs all burn differently, destroying some or all of the standing vegetation and the layer of plant litter (Fig. 2).
The first rainfall events after the fire produce the most sediment. Over time, sediment loss can lead to reduced rangeland productivity and turbid or murky water in streams and stock ponds. After the first rainfall, a crust develops on fine-textured soils that have little or no vegetation. While soil crusting reduces rainfall infiltration, it also reduces erosion. This is also true of rain falling on bare soil after a prolonged dry period, even in unburned pastures.
In addition to sediment, ash washes into surface waters with rainfall runoff or settles from the air as dry fallout. A heavy influx of ash and sediment is detrimental to fish and amphibians and can harm terrestrial wildlife and domestic animals that drink the contaminated water.
Ash often contains elevated amounts of ammonium, nitrate, phosphate, and potassium. It may also contain trace levels of antimony, arsenic, copper, lead, mercury, or zinc. (See Water Quality: Its Relationship to Livestock for recommended limits of the concentrations of these chemicals in livestock drinking water.)
Smoke is another source of ammonium and nitrate (Fig. 3), and asphyxiation from smoke inhalation causes many fire-related livestock deaths.
The influx of soil and associated organic nutrients into a pond can cause bacteria to flourish and lower the dissolved oxygen (DO) content of the pond water. Low DO causes fish die-offs as well as odor and foul taste so that livestock may avoid the affected water source.
Wildfire may result in elevated levels of cyanide in the air and water. Various sources can produce cyanide. Burning plant biomass releases cyanide into the atmosphere in the form of hydrocyanic acid. Studies have detected cyanide in ash leachate at levels that are lethal to fish. Some fire retardants contain sodium ferrocyanide, or yellow prussiate of soda.
Sunlight increases the toxicity of cyanide. Lethal concentrations interfere with the ability of red blood cells to absorb oxygen; sublethal doses can impact fish fertility. Depending on the pH of the water, cyanide in streams becomes diluted, biodegraded by bacteria and protozoa, and oxidized or passed off as vapor.
The input of sediment, ash, and nutrients, especially nitrogen, encourages microbial growth. Cyanobacteria, or blue-green algae (BGA), are microorganisms that can produce toxins that cause liver disease or death, which can occur almost immediately or up to several hours after drinking affected water. BGA thrive in warm, still, nutrient-rich waters, especially water with elevated nitrogen. They float on the water surface (Fig. 4) and wind typically causes them to accumulate along the shore.
Because these organisms do not continually produce cyanotoxins, the presence of BGA does not necessarily indicate a danger. About 50 species of cyanobacteria produce toxins; what triggers BGA to produce these toxins is unknown. Fatalities and severe illness of birds, fish, livestock, pets, and wildlife from cyanobacteria blooms occur worldwide, and cases of cyanotoxin poisoning are reported every year in Texas. The Texas Veterinary Medical Diagnostic Laboratory at Texas A&M University can test for some of the more problematic cyanotoxins.
Burn severity maps can be useful to quickly identify critical or severely burned areas where remediation may be necessary. These maps were developed in the western United States to rapidly assess and categorize the impacts of fire on soils and vegetation over expansive and rugged landscapes.
Burn severity is the level of visible impact a fire causes to the soil surface and herbaceous and woody plants. The severity level, measured using remote-sensing techniques developed by the United States Geological Survey and the United States Forest Service, is classified as 1) severe, 2) moderate, 3) low, and 4) none.
Seeding is a common way to establish new plants that filter overland water flow and reduce rainfall runoff. However, seeding efforts may fail if soils are not allowed to recover sufficiently. Because bare soil can be a prime environment for invasive species, seek sound advice and closely monitor affected areas before reseeding.
Small-scale remediation can protect critical areas. Place straw wattles (See Resources on page 4) or other barriers upslope from stock tanks to minimize the impact of runoff and sediment. You can also excavate debris basins upslope from streams or stock tanks to intercept sediment.
Channel treatment involves installing temporary, in-stream stabilizers and check dams to reduce water velocity and minimize channel cutting and erosion of downstream riparian areas.
Adding alum (hydrated potassium aluminum sulfate), Epsom salt (magnesium sulfate), or gypsum (calcium sulfate) can reduce turbidity caused by suspended clay particles in runoff. These products cause clay-sized particles to clump together and settle out to help clarify the water.
The pH of the pond water indicates which amendment is appropriate. Using a do-it-yourself kit, collect a water sample and test the pH level. Adding alum to acidic (low pH) pond water can make the water toxic to fish. Neither gypsum nor Epsom salt will cause an appreciable change in the pH.
A recommended method is to collect four 1-gallon water samples from the pond or tank and add your amendment of choice (alum, gypsum, or Epsom salt) to each gallon at a rate of 1 to 4 tablespoons, respectively, to determine which dose results in acceptable settling and clearing of the sample after 12 hours. Either by wading or from a boat, apply a slurry of the calculated rate of alum mixed with pond water to the surface water across the pond. For each tablespoon of alum required to clear the water sample within 12 hours, apply 30 pounds of alum per acre-foot of water to the pond (See calculation tips above). Each tablespoon of either gypsum or Epsom salt is equivalent to a rate of 80 pounds per acre-foot of water. Use a seed or fertilizer spreader to apply gypsum and Epsom salt directly to the surface water. (See Water Management: Clearing Cloudy and Muddy Water in Ponds and Lakes for more information.)
Many impacts of wildfire on surface water quality are short-term and can be managed by reducing soil erosion, using an alternate water source, or treating with chemicals to remove sediment from the water column (from the surface to the bottom of the body of water).
Texas Veterinary Medical Diagnostic Laboratory
Texas A&M University
Erosion Management Products
L and M Supply Company Texas
126 Texas Ave., Building #2
San Marcos, TX 78666
Phone: (512) 667-6635
Texas Erosion Supply
1327 Century Way
Wylie, TX 75098
Phone: (972) 575-8800
Arizona Bag Company, LLC
2530 West Buckeye Road
Phoenix, AZ 85009-5746
Phone: (602) 272-1333
Toll Free: 1-800-270-2247
Fax: (602) 278-7871
U.S. Fire Statistics
Sink, T. and M. House. Understanding Water Quality Reports for Your Pond. Texas A&M AgriLife Extension Service, EWF-017, http://fisheries.tamu.edu/files/2014/12/UnderstandingWater-Quality-Reports-for-Your-Pond.pdf. Also available at http://www.agrilifebookstore.org/Understanding-Water-Quality-Reports-forYour-Pond-p/ewf-017.htm.
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