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Apache Nitrogen Products Inc. (ANPI) has
manufactured industrial chemicals and explosives
for the copper mining industry, including
nitroglycerin, nitric acid, ammonium nitrate, and
blasting agents in Benson, Arizona since 1922.
Currently, ANP manufactures nitric acid, ammoniu
m nitrate, and nitrogeneous fertilizer solutions.
    
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The Apache facility is located in Cochise County
seven miles southeast of the town of Benson, and
2.5 miles southwest of the town of St. David.
The San Pedro River runs along the eastern side
of the site, running from the southeast corner of
the property towards the northwest.
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The Apache facility is located in Cochise County
seven miles southeast of the town of Benson, and
2.5 miles southwest of the town of St. David.
The San Pedro River runs along the eastern side
of the site, running from the southeast corner of
the property towards the northwest.
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• Wastewaters contain high levels of nitrates
• Prior to 1971, wastewaters were discharged on
  site into dry washes
  
• Flowed directly into the San Pedro River
• After 1971, wastewaters were contained in unlined
  evaporation ponds.

 
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The evaporation ponds allowed seepage of high
nitrate waters into the ground.
This water seeped downward until it encountered
the underlying St. David clay.
The clay trapped the water beneath the ponds.
At one location, the trapped water began to flow
eastward toward the shallow aquifer.
 
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The seepage of high nitrate waters from the ponds
over several years of ANPs operating history
eventually formed a plume within the shallow
aquifer nearly 3 miles long along the west side
of the San Pedro River.
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To eliminate the problem, Apache undertook two
major projects. The first project identified met
hods to eliminate the discharges to unlined
ponds. In 1994 Apache constructed a Brine Concen
trator which is a distillation process.
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• With this distillation process, Apache can
• Reclaimed water is returned to the production
  process
  
• Concentrated impurities become a fertilizer
  ingredient
  
• Achieved zero-discharge
• Reduced groundwater consumption

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By eliminating the practice of discharging
wastewater to the ponds, and allowing them to dry
out, the impact on the shallow aquifer has
significantly decreased. As a result, the area
of nitrate contamination in the shallow aquifer
is shrinking.
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By eliminating the practice of discharging
wastewater to the ponds, and allowing them to dry
out, the impact on the shallow aquifer has
significantly decreased. As a result, the area
of nitrate contamination in the shallow aquifer
is shrinking.
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The second major project dealt with groundwater
remediation at the site. Apache decided to imp
lement a pump-and-treat remediation technology
which utilizes biological treatment methods to
deal with the northern area of groundwater
contamination.
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In November 1997 Apache completed construction of
a treatment wetland designed to reduce the
nitrates in the groundwater extracted from the
shallow aquifer.

• Treat up to 250 gallons per minute of incoming
  water
  
• Containing up to 300 ppm dissolved nitrate
• Uses biological denitrification
• Residence time of 5 days
• Reduces the nitrate concentration to less than 10
  ppm

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• Groundwater is pumped from a 93-foot deep shallow
  aquifer well
  
• Transported via a 5,100 ft long 4-inch pipe-line
  to the wetland
  
• Wetland is comprised of 5 ponds covering
  approximately 4.3 acres

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The groundwater enters the first of a series of
three ponds planted with cattails and flows from
pond to pond via gravity. The ponds planted with
cattails are called the "denitrification ponds".
                                                
                                                  
                                                  
    
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• The purpose of the cattails is to provide a
  source of organic carbon in the pond bottom
  sediments.
  
• The cattails provide this carbon by dying and
  depositing dead plant materials on the bottom of
  the pond.
  
• Water depths in these ponds are relatively
  shallow, approximately 1 to 2 feet, just
  sufficient to provide moisture to the plants and
  bottom sediments.

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• In previous years, cattail caterpillars (Simyra
  Henrici) have decimated the cattails and delayed
  the startup of discharge from the wetland
  project.
• Apache has sprayed an agent that works on the
  alkaline core of the caterpillars. Due to its
  makeup this agent is safe for other insects,
  reptiles and mammals.
• The search continues for predatory insects as a
  more natural means of pest control. Some of the
  possible insects are praying mantis, lacewings,
  and ladybugs.

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Bacteria which feed upon the carbon from the dead
cattails, require oxygen. Since the incoming
water contains 300 ppm nitrates (NO3), which is a
combination of both nitrogen (N2) and oxygen
(O2), the  bacteria steal the required oxygen
from the nitrates. The result is the liberation
of free nitrogen back into the atmosphere, hence,
the removal of nitrate from the water.  
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In the process of removing nitrates from the
water in the denitrification ponds ammonia is
created.  Ammonia is also generated through
decomposition of cattails. Water, with greatly
reduced nitrate concentrations but elevated
ammonia (NH3) concentrations, exits the third
pond and enters a deeper (4 to 6 feet) pond which
is planted with underwater plants. The function
of this pond is to convert any ammonia that was
generated in the denitrification ponds back into
nitrate (NO3) via oxidation.
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The purpose of the underwater plants,
Potamogeton, is to create high levels of oxygen
(O2) in pond 4, to facilitate oxidation. 
The underwater plants accomplish this task by li
berating oxygen via photosynthesis. 
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Upon exiting the fourth pond, the water is routed
through a final denitrification pond, a polishing
cell, to remove residual dissolved nitrate (NO3)
created by oxidation in Pond 4. NO3 is reduced
to a concentration less than 10 ppm.
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At this point the water is fit to be returned to
the environment. The treated water is routed t
hrough a 2,900 foot pipeline back to a dry wash,
where it is discharged onto a limestone riprap.
The limestone helps reduce phosphorous levels in
the effluent stream. The water then may eith
er infiltrate into the wash channel or flow into
the San Pedro River.
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In the southern area the perched zone has
continued to decrease in size.
Isolated pockets of concentrated nitrates
remain. The perched zone remains a potential sour
ce of contaminants to shallow aquifer.
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In the southern area the perched zone has
continued to decrease in size.
Isolated pockets of concentrated nitrates
remain. The perched zone remains a potential sour
ce of contaminants to shallow aquifer.
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Additionally, perchlorates have been identified
in perched zone and the southern area of the
shallow aquifer. Concentrations of both contamina
nts in the shallow aquifer have decreased.
Apache has proposed Monitored Natural Attenuation
with some further action in the perched zone.
In-situ treatment using molasses injection is an
option.
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