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SUMMARY. OF SITE HISTORY,
CONTAMINATION, AND SELECTED
REMEDY
Comhusker Aimy .Ammunition Plant is
located near Grand-Island, Nebraska (Figure
1) on a 11,936 acre tract The plant was
constructed in 1942 by the Army for the
production of artillery rounds, bombs,
booster and supplementary charges. The
plant was operated intermittently for 30
years, with operations ending in 1973.
As a result of disposal practices, common at
threat~time in both military and civilian
sectors, the groundwater at CHAAP was
found to contain explosive residue.
Subsequently, the CHAAP was listed on the
National Priorities List (NPL) on July 22,
1987. At this time it was apparent
groundwater containing explosive residue
had migrated beyond the CHAAP boundary,
about 2 miles into the Grand Island Gty
limits.
Investigations to date identified the explosive
residue in groundwater migrated from the
cesspools and leach pits located in the center
of the plant The affected groundwater
encompasses a disjointed area 6 miles long
and one-half mile wide. While the explosive
residue in the groundwater originated from
the CHAAP, the detected nitrates could have
come from several other sources, including
agricultural activities in the area.
As an interim action the Army has
constructed a permanent water supply
system to the affected households by
extending the city water system.
Approximately 800 residences were provided
the opportunity to hook up to the Northwest
Grand Island Water Supply Extension.
During 1987 and 1988, the Army excavated
and incinerated explosive contaminated soils
from the cesspools and leachpits. A second
removal action was conducted in the summer
of 1994 and focused on additional source
areas.
On November 18, 1994, an Interim Record of
Decision was signed for remediation of the
groundwater plume containing explosive
residues, primarily RDX. This Record of
Decision requires the Army to build a
groundwater extraction, treatment and
discharge system that will capture the
contaminated groundwater and treat the
groundwater to remove explosive residues to
levels that are safe for surface water
discharge. The treated groundwater will
then be pumped via underground pipeline to
a surface water discharge.
Compliance monitoring will be performed
with a groundwater monitoring system of the
plume to verify protection of human health
and the environment This system will
prevent further migration of the
contaminants and will reduce the
concentrations of explosive residues in the
'groundwater over time.
DESCRIPTION OF SIGNIFICANT
DIFFERENCES AND THE BASIS FOR
THOSE DIFFERENCES
In June 1996, the VS. Army presented a plan
to discharge treated water from the proposed
groundwater treatment facility at CHAAP to
Silver Creek at a location near the northwest
comer of CHAAP. Previously, the discharge
point was to be the Platte River at a location
approximately 14 pipeline miles from the""*
groundwater treatment plant on CHAAP.
The proposed discharge location was
changed because the design flow rate for the"
discharge was reduced during design
evaluations from approximately 11 cubic feet
per second (5000 gallons per minute (gpm)) to
approximately 5 cfs (2150 gallons per
minute). An evaluation of the.potential for
increased flooding problems associated with
the discharge to Silver Creek was prepared.
The analysis determined if the discharge from
the treatment plant stopped during periods of
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high flow in Silver Creek, there would be
insignificant increases to flooding problems
(RUST, 1996). Rust also estimated the impact
on groundwater elevation near the Creek and
found that during periods of the year. The
discharge to Silver Creek could result in
increased pumping of sumps in basements
located directly adjacent to the creek. The
analysis was presented along with the
proposed change at a June 5, 1996, public
meeting. Public concerns were raised over
discharging water to Silver Creek during the
meeting and during the 45 day comment
period. There was a concern the discharge
would-be burdensome to some residents and
perhaps affect cropland.
The Army, EPA, and the NDEQ identified the
need to eliminate the further migration of
contaminants in the groundwater off-site. In
order to accomplish this the Agencies want to
proceed with a phased approach.
The Army therefore proposes installing the
treatment facility and the extraction wells
planned for the areas of remaining
groundwater contamination on CHAAP.
This action would provide important
environmental improvement and could be
completed at the same time that, an interlocal
agreement which would formalize the
evaluation process of the discharge as
identified in the phased approach. The
agreement will include representatives from
local governments, the EPA, the NDEQ and
the Army. Along with this agreement,
discussions will continue for off-site piping
and further evaluation of contingency
alternatives (for example, the Prairie Bend
Project). The phased approach will also
provide an avenue to evaluate the most
recent off-site data (Dec. 1996 and Feb. 1997)
and the effect of natural attenuation on
improving groundwater quality.
Under the p*hased approach, the Army
proposes to begin on-site extraction and
treatment as soon as possible. The treated
water would be discharged to an existing
drainage canal located on CHAAP. The
water in the canal would discharge to Silver
Creek after traveling through CHAAP for
about two miles. This proposal differs
significantly from the earlier Silver Creek
discharge proposal in two ways:
1. The discharge flow is lower-only 1.7 cfs/|
allons per minute ^The initial period of
lower flow would allow an opportunity to
evaluated the modeled versus actual impacts
from the discharge. This information would
be used to evaluate the need to change
discharge locations in the future. It will allow
real time data to determine the capabilities of
the Creek for any future off-post actions.
2. The revised model demonstrates the
effects of 1.7 cfs discharge into the Eastern
Drainage Canal Under the most likely
conditions, the model anticipates much of the
water would infiltrate through the on-post
canal bottom during transport to Silver
Creek. Therefore the amount of water
reaching Silver Creek would be even less than
the amount of treated water discharged to the
canal.
The design capacity of the system will allow
for capture and treatment of the entire
plume, as identified in the Cornhusker AAP
OU1 Interim Record of Decision, Nov 1994.
The treatment system will be constructed to
treat up to 2150 gpm or 4.8 cu. ft/s.
Under the OU1 Record of Decision the
discharge location is the Platte River. This
would require the effluent to be piped
thirteen and a half mile underground.
Changing the discharge location to the
Drainage Canal would require a maximum of
a quarter mile of effluent pipeline. This
change would save over 13 miles of
underground pipeline, approximately four
months of construction time and five to six
million dollars. This figure does not include
reduction in real estate acquisition
procedures or maintenance costs, which will
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be realized through the change in discharge
location.
Several different scenarios were modeled to
evaluate the potential impact to surrounding
areas under a variety of conditions. The
model estimated during periods of high
groundwater (e.g., groundwater less than'5
feet below the ground surface and a flowing
stream) the discharge would take
approximately 8 to 10 miles to infiltrate.
Model predictions were also made under
drier conditions or when irrigation wells are
pumping. Under these conditions the
groundwater will be 5- 10 feet below the
surface. In this case, the model simulations
show the effluent infiltrating mostly on
Comhusker property (traveling al distance of
2 miles). This scenario assumes the Drainage
Canal will be cleaned and maintained. The
results from the modeling show the effect to
Creek and surrounding area is very
dependent on; 1) the water levels in the
Creek, 2) the groundwater level, 3) the
infiltration rate of the Creek/Canal bed and
4) the area of the Creek/Canal bed.
Studies have been conducted using the flood
insurance modeling, 10,50 and 100 year flood
flows and information from past flooding
occurrences. These reports demonstrate there
will be no additional risk of flooding of Silver
Creek by discharging 1.7 cfs or 4.8 cfs of
treated water. To virtually eliminate the
possibility the discharge from CHAAP could
increase flooding problems along .Silver
Creek, the Army will stop discharging the
treated water during times when the flow in
Silver Creek is more than 250 cfs. This
limitation has been included in other
discharge permits for Silver Creek.
In addition to the modeling effort a
monitoring program has been proposed to
monitor the ^urface water flow in the creek,
take visual observations and monitor
groundwater levels along Silver Creek.
Surface water flow measurements will be
made twice per month for two years. Visual
observations will be recorded to include
documenting distance of observable flow in
the canal and Creek. These observations will
be made during periods of little or no
precipitation (when there is no flow upstream
at the point where the Canal discharges into
the Creek). This will provide data on
infiltration rates through the drainage canal
and Creek bed that can be used to refine the -
model predictions. ' Any ice buildup
conditions in the winter will also be
documented. If ice buildup is excessive the
system will be temporarily shut-down.
According to CERCLA Section 121(e)(l) the
discharge will occur on-site and will not
require a National Pollutant Discharge
Elimination System (NPDES) permit. The
Army will submit a letter request to allow
discharge to Silver Creek. This letter will set
effluent standards equivalent to those set in
the NPDES permit, NE0131725 for
Comhusker Army Ammunition Plant
Groundwater Remediation Project, dated
October 18,1995 (Table 1).
Effluent Parameters
PH
MtretB as Mtrogen
Total Sulfate
Total HMX(1A5>7>
TetraDBodne
Total RDX
2,4,6-Trirftrotoluene
Combined Explosive
Trlcrdoraettylene
incitiorouinuoroetnane
UMts
S.U.
mg/l
mo/I
mo/1
mo/I
mo/1
mo/1
mo/I
mo/I
Doily
Mrirrun
as
Report
Report
02
0.05
0.01
0.1
0.005
as
Table 1 Effluent Concentration Limits as
Proposed in NPDES NE0131725
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The extraction treatment systems will be
constructed and operated as originally
designed. Therefore, the entire phased
remedy will offer the same level of protection
to human health and the environment as was
originally envisioned in the OU1 Record of
Decision. An amendment to the final design,
incorporating the changes to the effluent line
location will be issued for review by the
regulatory agencies.
REGULATORY AGENCY SUPPORT
COMMENTS
EPA and NDEQ support the changes to the
Record-of Decision described in this ESD.
AFFIRMATION OF THE STATUTORY
DETERMINATIONS
Considering the changes that have been made
to the selected remedy' the Army, EPA and
NDEQ believe that the remedy remains
protective of human health and the
environment The selected remedy also
complies with Federal and State requirements
that are applicable or relevant and
appropriate to this remedial action, and is a
cost-effective application of public funds. In
addition, the remedy utilizes permanent
solutions and alternative treatment
technologies to the maximum extent possible
for this site.
Public Participation Activities
A public comment period will be held from
Feb.7,1997 to March 9, T997. The comment
period includes a public availability/
information session during which the Army,
EPA and NDEQ will be available to discuss
new information that supports the change in
discharge locations. This meeting will be
held on Feb. 19,1997 from 10 a.m.-2 p.m. and
5-8 p.m. at the Grand Island City Hall. A
copy of the Response to Comments from the
45 day public comment period and public
meeting discussing the initial ESD that
proposed to discharge 4.8 cfs of treated water
directly into Silver Creek. The responses to
these comments will be available at the
information session and will be placed in the
information repository.
The Army will accept both oral and written
additional comments during the availability
session and through the public comment
period. These comments will be summarized
and responses will be provided. A copy of
these responses will be kept with the
Administrative Record for OU1, which
includes this ESD. It is available for review
and comment at the locations and times
provided on Page 1 of this ESD. To send
written comments or to obtain further
information, please contact:
Tom Jamieson
Comhusker Army Ammunition Plant
102 North 60th Road
Grand Island, NE 68803
(308) 381-0313
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CORNHUSKER 'ARMY AMMUNITION PLANT
GRAND ISLAND, NEBRASKA
PUBLIC MEETING
GRAND ISLAND CITY HALL
19 February 1997
EXPLANATION OF SIGNIFICANT DIFFERENCES (ESD)
OU #1 Groundwater Treatment Plant
Change of Discharge Location
of
Treated Effluent Water
NAME:
ADDRESS:
TELEPHONE:
QUESTION/COMMENTS:
Please turn-in your question/comment or mail to the following:
Cornhusker Army Ammunition Plant
ATTN: Mr. Tom Jamieson
102 North 60th Road
Grand Island, NE 68803
(3
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