CHLORINE RAIL CAR ACCIDENT, MACDONA TX
AristaTek has seen some feedback saying that analysis of
real-world accidents or chemical releases is of greater interest than
Chlorine Release, 90-Ton Chlorine Rail Car Accident,
Macdona TX, June 2004
from EPA-On-Scene Coordinator Website,
On 28 June 2004, at approximately 5:03 AM, at Macdona TX,
a Union Pacific train collided with a Burlington Northern (BNSF) train
resulting in a derailment of four locomotives and 35 railcars. A filled 90-ton chlorine railcar was
breached releasing 60 tons (120,000 lbs) of chlorine as of three days after
the accident. Also released was about
78,000 gallons of urea fertilizer plus diesel fuel from the four
locomotives. The train conductor was
killed. Two residents in a house
nearby died from chlorine inhalation.
About 43 people were hospitalized from chlorine inhalation, including
6 emergency responders.
The first 911 call was received from a local resident 3
minutes after the accident (a few seconds after 5:03AM). Initial 911 callers mentioned “train
derailment” or “train wreck”, and “smoke” or “difficulty breathing”, but the dispatcher(s)
had difficulty picking up on the emergency calls. At 5:08 AM, one of the BNSF train crew members notified the Union
Pacific train dispatcher that a UP train had come on us “fast” but was now
stopped and that he would access the situation. At 5:12 AM, the Union Pacific Railroad notified the Sheriff’s
Dispatcher of a possible train incident.
Responders from the Southwest Volunteer Fire Department were the first
to arrive on the scene at 5:15 AM, and reported driving into a “yellow cloud”
of an “unknown substance”. The
Southwest Volunteer Fire Department, who
responded to the “medical emergency” based on 911 information were
unaware of the chlorine release, and were unable to enter the area and
requested mutual aid from other agencies.
Some protective clothing and self-contained breathing apparatus became
available, and at 5:40 AM the Fire Department entered the area to look for
survivors. They found the train
engineer at 6:15 AM, and transported him in the command vehicle to the fire
station for decontamination. At 6:10
AM, the Bexar County Office of Emergency Management established the unified
command system. The San Antonio Fire
Department arrived on-scene at 6:15 AM.
The hazardous materials response contractors retained by the railroad
arrived at 6:33 AM and began to assess the chlorine release.
The major concern was rescue of nearby residents. However, the train derailment blocked the
main road to the closest residences, and access to the rear of the residences
was blocked by a flooded river. Access
was also hampered by locked gates and high security fences. The only way to reach the residents was on
foot. Emergency response personnel also
did not have enough Personal Protective Equipment especially supplied air to
enter the stricken area. Immediate
rescue was therefore not feasible. The
nearby residents initially were ordered via the 911 Center to shelter-in-place
while a site assessment was conducted, but there were no instructions to
residents of how to do this. Another
problem was that radio traffic between 911 and responders was not recorded
because of a malfunction. Some of the
residents walked away on foot, but the people in the two closest houses were
trapped inside. Evacuation began for
other residences within a two-mile radius, or about 57,000 houses, based on
preliminary plume modeling (about 7:15 AM), but there were conflicting reports
of who should evacuate and who should shelter in place. The first attempt to enter the area of the
trapped residents occurred at 9:45 AM, but the rescuers became disoriented and
could not locate the houses. A second
team entered the area to come to the aid of one of the entry team
firefighters. A third team entered the
area at 10:12 AM; they reached three residents at 10:55 AM who were in
considerable respiratory distress. The
two residents who had died from chlorine inhalation were reached at 11:55
AM. Six emergency responders and 23
local residents were treated for chlorine inhalation.
At a later time, 45 residents were ordered evacuated for 13
days until the railroad company finished unloading the chlorine car.
from Railroad Accident report showing accident location, the two houses where
residents were trapped inside, and the location of the conductor’s body. Two residents in the house closest to the
accident died from chlorine inhalation.
Chlorine Railcar, photo taken during cleanup operations, from Railroad
At a public hearing, the events just before and right after
the collision were reviewed (26-27 April 2005, Macdona TX, see http:www.ntsb.gov/events/2005/Macdona/iic_opening.htm). At about 5:03 AM, the Union Pacific train
traveling at 45 mph struck the 63rd
car behind the locomotives of
the advancing BNSF train as it entered a siding track. The BNSF crew remained with the train summoning
emergency responders. The Union Pacific
crew, which suffered minor injuries, exited their overturned lead locomotive
and headed west away from the toxic cloud of chlorine gas from the breached
rail car, along a road on the north side of the tracks. During their escape, the train conductor
experienced difficulty breathing and was unable to continue away from the toxic
cloud and collapsed. The train engineer
was unable to carry the conductor, and he continued walking away from the
cloud. The train engineer doubled back,
retracing his route, and was rescued by first responders at 6:15 AM. The train conductor died. The train engineer was hospitalized for
chlorine inhalation. Crew fatigue was
blamed for the train collision
The chlorine gas cloud eventually reached the outskirts of
San Antonio about 10 miles away.
An EPA Region 6 Emergency Response Review on the actions of
all responding organizations to the accident is available at http://www.epa.gov/earth1r6/6sf/pdffiles/union_pacific_macdona_texas_response_review_final.pdf
. The date of the EPA review is August 18,
2004. The Railroad Accident Report is
available at http://www.ntsb.gov/Publictn/2006/RAR0603.pdf
. The Railroad Accident Report also reviewed
actions of responders.
A major criticism on response operations was that although
the responders arrived at the scene promptly, there was a lack of coordination
between the responding agencies, and more could have been done to rescue the
stricken residents sooner. The San
Antonio Fire Department, the Bexar County Office of Emergency Management, and
the Southwest Volunteer Fire Department were involved in what was described as
a certain amount of discordant debate regarding jurisdictional boundaries and
incident command authority. Based on
post accident emergency debriefings and interviews with responders, no
consideration was given to using open farm fields to the south of the accident
site as potential helicopter landing areas or drop sites for firefighters. The firefighters could have used nearby
unpaved roadways from the farm fields, which would allow them to reach the two
houses on foot.
Property damage and environmental cleanup costs exceeded $7
Modeling the Macdona Chlorine Release
There was a mention of an initial plume modeling done about
2 or 2.5 hours after the chlorine release, which was used as a basis of a 2
mile evacuation affecting 57,000 residences, but AristaTek was not able to find
Dr. Steven Hanna (Hanna Consultants) presented a paper at
the Chemical Biological Information Systems conference on 10 January 2007 at
Austin TX sponsored by NDIA. The paper
is available at http://www.dtic.mil/ndia/2007cbis/Wednesday/hannaWed1130.pdf.
Also authoring the paper were Gene Lee, David Belonger,
Peter J. Drivas, Rex Britter, and Olav Hansen. The Macdona chlorine accident was modeled using SCIPUFF. The figure at the left was presented at
the conference, which shows chlorine contour concentrations at ground level 5
minutes after the release. The “pink”
shows concentrations above 2000 ppm, “red” between 40 and 2000 ppm, and
“yellow” between 20 and 400 ppm.
Distances are in kilometers.
The SCIPUFF Model is described as a “second order closure,
integrated puff dispersion model” and is available from The Tritan Corporation,
Princeton NJ. As the name implies, the
word “puff” implies a sudden, transitory release. The Steve Hanna paper, commenting on the choice of models as
applied to releases from selected major railcar accidents, said that the models
SCIPUFF, SLAB, HGSYSTEM, ALOHA, TRACE, and PHAST generally agree within a
factor of two on plume parameters providing all are compared at the same source
The Hanna paper also presented a concentration time plot
as seen by a hypothetical chlorine sensor placed 1 km downwind near or at
ground level. The plot, reproduce at
the left, shows mean concentration in ppm vs time in minutes. The concentration reached 1750 ppm
chlorine at 1 km, 8 minutes into the release.
Unfortunately, the Hanna paper as presented at the
conference did not say how much chlorine was released nor give information on
the meteorology, nor did the paper provide other details essential for
understanding the modeling. Therefore
any comparison between the PEAC tool implies certain assumptions. If time “0” represents the time of the
release, the toxic cloud travel time based on Hanna’s presentation for peak
concentration calculates out to 4.7 mph.
Chlorine is a dense gas which hugs the ground, and its movement would
expect to be somewhat delayed, which means that the wind speed was probably
about 7 mph based on an initial cloud arrival time of 5 minutes.
The Railroad Accident Report said that the San Antonio
Stinson Municipal Airport located 12 miles away measured a southeast wind at 5
knots (= 5.75 mph) at 4:53 AM with a temperature of 75o
F and a dew
point of 73o
F, and that the southeast winds continued for about 7
hours after the accident.
AristaTek was unable to find any record of how much chlorine
was released during the early stages of the release. Remember, the first responders only knew of a train wreck and
could not look into the choking, dense cloud which covered an area about 700
feet in diameter during the initial release.
It was still dark at the time of the accident and when first responders
arrived. Sunrise occurred at 6:37
AM. Their immediate concern was in
saving lives. Hazardous material
responders under contract with the railroad estimated about 60% of the original
chlorine had escaped by the time they plugged the hole with wooden wedges. It took 14 days for the chlorine remaining
in the tank to be removed.
Chlorine is normally a gas which is shipped under ambient
temperature as a liquid at its own vapor pressure. The pressure inside the chlorine railcar depends upon the
temperature. At 75o
example, the pressure inside the railcar would be about 94 psi gage. If the pressure is suddenly released, a
portion of the liquid chlorine would flash.
The fraction of chlorine flashed is treated as a “puff” or sudden
release, and is calculated by
F = Cp
(T - Tb
F = fraction of chlorine flashed
T = initial temperature inside the tank car before the
breach (assume 75o
= boiling point of chlorine (use – 30o
= average heat capacity of liquid chlorine,
H = Heat of vaporization of chlorine, 123.7 Btu/lb
F = 0.1982
The 90-ton chlorine tank was initially full at the time of
the breach according to the Railroad Accident Report. This calculates out to about 35,700 lbs flashed. In addition, there may have been some chlorine
spilled onto the ground that evaporated and some chlorine aerosol entrained in
the chlorine vapor cloud.
The Hanna report only modeled the initial chlorine release,
probably from the liquid flash. It was
evident from the accounts that chlorine continued to be released long after the
hypothetical sensor concentration dropped down to zero. However, concentrations in the air while not
zero should be much less than what occurred after the initial flash.
Using the PEAC Tool
The Macdona TX accident site is located at 29.3264 north
19’35”) and 98.6911 (98o
longitude. If the PEAC user wishes to
obtain an overlay of the chlorine plume cloud on a site map, this information
should be entered under the pull-down screen “options”. We will also select “English units”. The other option is metric.
“chlorine is then selected under “Lookup”.
Only a portion of the display under chlorine is shown. If the user wishes to provide plume modeling
for chlorine, the icon
at the upper right is
The PEAC display will then ask the user questions on
meteorology and type of release.
The PEAC tool then asks the user the level of concern, in
other words, a concentration value. The
distance downwind corresponding to this concentration is calculated, near
ground level at the centerline of the cloud plume. A graph of these concentrations and corresponding distances can
be plotted as follows.
The Hanna paper using the SCIPUFF model predicted a maximum
1750 ppm chlorine concentration at a distance of 1 km downwind. The PEAC tool using a 35,700 lb
instantaneous release predicted a downwind distance of 0.5 miles corresponding
to 1750 ppm chlorine. At 1 km (= 0.621
miles) downwind, the PEAC tool predicts a concentration of about 1400 ppm, so
the PEAC predictions are in the same ballpark as Hanna’s modeling. Also, the Hanna paper may have used a higher
release in their modeling.
The news accounts mentioned several people in an amusement
park about 10 miles downwind from the accident treated for chlorine
inhalation. Fortunately, the chlorine
cloud for much of its distance traveled over relatively unpopulated areas.
Levels of Concern
Perhaps most useful are the acute exposure guideline
levels. The following numbers are as
displayed in the PEAC tool.
Other information can be pulled up in the PEAC tool. For example,
The PEAC user can select a toxic level of concern and
calculate a protective action distance.
For example, if 20 ppm chlorine is selected, a protection action
distance of 4.9 miles is calculated.
This can be displayed on a map of the area.
This display does not mean that locations within the yellow
triangle have concentrations of 20 ppm or greater. The actual plume cloud may be only several hundred or thousand
feet wide. But the cloud can wander or
drift, and winds can shift meaning that the actual path is unpredictable. The wind can shift direction. Because of unpredictability, evacuations may
be done in all directions, not just downwind.
The initial isolation zone distance of 800 feet displayed is based on
recommendations given in the 2004 Emergency Response Guidebook for
transportation accidents involving large spills, but because of the very large
magnitude of the spill, responders should consider using an even greater
All of this information and more can be displayed in the
PEAC tool and can be transferred to a printer.