By: Donna Scott, City of Boulder, Water Quality
and Environmental Services
The following is an outline of some of the water
quality concerns the city of Boulder has related to the recent Walker
Ranch fire.
Fires and Fish
First and foremost, we must remember that wild land
fires are critical for forest health. However, fires create a
number of management challenges, including erosion, runoff and water
quality.
Forest fires alter the aquatic ecosystem by changing
the deposition of woody debris, sediment suspension, nutrient cycling,
leaf litter input and the types of aquatic organisms present. The effects
of forest fires on streams can be divided into three categories. Immediate
impacts are those directly attributed to the fire itself (e.g. increase
temperature, water chemistry, and food quality) Delayed impacts are
associated with runoff and include physical disturbances such as sediment
deposition and turbidity. The impact of fires varies proportionally
with the intensity and extent of the burned area in the stream basin.
The impact of fire is also regulated by factors influencing runoff and
erosion. Slope, gradient, geology and soil depth play a role in erosion.
The Walker Ranch Fire
The potential impact of a fire on water quality
is dependent on the erosion potential of the burned area and the proximity
of streams. The U.S. Department of Agriculture/ Natural Resource Conservation
Service is currently evaluating the soils and runoff potential of the
Water Ranch Area to determine erosion control strategies.
Potential water quality impacts involve a major
tributary to Boulder Creek and a drinking water supply reservoir. It
is estimated that 500,000 people receive their drinking water from water
resources affected by this fire. These include South Boulder Creek,
which is a drinking water source for the cities of Louisville, Lafayette
and the town of Superior and Gross Reservoir, a water supply for Denver
Water as well as serving Arvada. In addition, several small drainage
ways cross the area. Tom Davis Gulch is an intermittent stream which
runs west to east right through the most heavily burned areas and is
a tributary to South Boulder Creek, just upstream of the city of Lafayette's
and the town of Louisville's diversion structures. Gross Reservoir's
northern corner is within a few hundred feet of the fire area, and South
Boulder Creek forms most of the eastern boundary of the fire.
Short-term Concerns
The Walker Ranch Fire presents a number of short-term
and long-term concerns related to the water quality - for human health
and for aquatic life. The immediate concern is the impact of the fire
retardant used. A September 20 article in the Daily Camera indicated
that the fire slurry used on the Walker Ranch fire might have contained
sodium ferro cyanide, which "when exposed to sunlight, releases pure,
deadly cyanide." However, it is unclear if the fire retardant used in
the Walker Ranch fire contained the cyanide additive. Water samples
taken on September 20, during the first storm event have been analyzed
for ammonia and cyanide, as well as other water quality constituents.
Laboratory results are not yet available.
The Colorado Department of Health and Environment
indicated that if the cyanide additive were present, it would not
be a drinking water concern or human health risk.
Long-term Concerns and Erosion Control and
Forest Rehab
The long-term water quality concern is the impacts
of runoff from the burned area. The sediment load has a potential severe
detrimental effect on water quality and aquatic habitat, particularly
over the next two years.
The difference in the burned areas which had forest
management / prescribed burns and those areas which did not, is remarkable.
The areas that had prescribed burns not only were instrumental in stopping
the fire but in the long term provide the best erosion control measure.
In these areas, the fire did not burn as intensely, therefore grass
roots and patches of forest remain intact.
In contrast, in the areas with no forest management,
the fire burned the hottest and left a stark landscape, void of vegetation.
This will be the area with the most erosion potential and will have
to have the most costly erosion control measures applied. Of particular
concern in this area is channel erosion in Tom Davis Gulch, which may
be responsible for producing and delivering the largest sediment load
into South Boulder Creek.
Fire fighting efforts were in full swing at the
Walker Ranch fire as the State Forest Service started working on revegetation
and erosion control activities. After the fire, Boulder County Open
Space coordinated an erosion control and forest rehabilitation plan
with other agencies including City of Boulder, Colorado Department of
Health, State of Colorado Forest Service, Eldorado State Park, Denver
Water, U.S. Department of Agriculture/ Natural Resource Conservation
Service. A citizen group, the Boulder Creek Watershed Initiative volunteered
to assist as well. Over the next year, Boulder County will be working
with these agencies to implement the plan they develop to minimize the
impact of the Walker Ranch Fire on water quality. The city of Boulder's
Water Quality and Environmental Services will continue to monitor water
quality of South Boulder Creek to help evaluate fire mitigation measures
and assess drinking water concerns.
References
Minshall, G. Wayne, J.T.
Brock, J.D. Varley. 1989. Wildfires and Yellowstone’s Stream Ecosystems.
BioScience 39:10. 707-715.
Berry, Vern P.. December,
1990. An Evaluation of Stabilization and Rehabilitation Procedures at
Black Tiger Gulch Following the 1989 Fire.
