Green Remediation Focus

Apache Nitrogen Products, Inc.

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Benson, Arizona

Superfund NPL

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Wetland Design: On a "24/7" basis, water sequentially passes by way of the natural hydraulic gradient through an engineered wetlands system comprising three denitrification ponds, an aerobic nitrification pond, and a final denitrification pond.

Wetland Design

Wetlands Vegetation: Cattails (Typha latifolia) were installed in the first three aneraboic denitrification ponds. The fourth pond contains pondweed (Potamogeton pectinatus) and periphyton algae, which oxygenate the water through photosynthesis.

Wetlands Vegetation

Groundwater Recirculation: A PV array provides energy for a single pump that recirculates water through the multi-pond wetland.

Groundwater Recirculation

Flow Measurements: A micro-scale PV system powers an electrical gauge that measures flow through the multi-pond wetland system; the measurements are stored onsite and periodically retrieved as part of ongoing site monitoring.

Flow Measurements

Windmill Operations: A windmill was initially used to generate mechanical energy for de-watering the perched zone below capped areas.

Windmill Operations

Perched Water Treatment: A single PV panel is now used for pumping contaminated perched water from the subsurface for forced evaporation treatment in a brine concentrator.

Perched Water Treatment

Exterior Building Controls: Additional PV arrays provide electricity for wetlands area lighting, motion detection, security cameras, and gate controls.

Exterior Building Controls

Solar Canopy: The solar canopy offsets approximately 45% of the energy consumed in the facility's administrative buildings while serving as a structure for rainwater harvesting.

Solar Canopy

Cleanup Objectives: Remediate soils and groundwater containing high concentrations of metals, nitrate-nitrogen, and explosives on 1,100 acres of a 9-mile² property (formerly known as Apache Powder Company)

Green Remediation Strategy: Employ a hydraulically driven constructed wetland system, use native clay to cap contaminated soils, and capitalize on sources of onsite renewable energy to power cleanup equipment or offset grid electricity consumption

• Installed a 4.5-acre, tiered engineered wetlands to treat contaminated groundwater in the site's northern half; this remedial approach avoided typical chemical usage, energy consumption, and waste generation associated with a traditional alternative involving a wastewater treatment plant
• Capped ten contaminated evaporation ponds (in the site's southern half) with low-permeability covers made of native clay
• Used onsite solar and wind energy to power auxiliary equipment instead of extending power lines to the wetlands system, which is located in a remote portion of the property; initial equipment included: (1) a 1.4-kW photovoltaic (PV) system equipped with a solar-powered centrifugal pump to re-circulate water between the wetland ponds, and (2) a mechanical windmill to de-water a perched groundwater zone underlying the capped areas
• Maximized reinjection of wetlands-treated water directly into the alluvial aquifer rather than discharging all treated water to nearby surface water

Results:

• Reduced electricity costs for groundwater recirculation and de-watering by approximately 66% over the expected project life, when compared to the expense of installing new power lines and purchasing the electricity
• Operating the groundwater recirculation system only during daylight hours, which matches peak performance of contaminant degradation mechanisms occurring in the wetlands; sufficient PV-generated electricity is generated to re-circulate groundwater in the wetlands system (through 100 feet of 2-inch hosing elevated 10 feet from ground surface) at a rate of 5 gallons per minute
• Replaced the windmill after five years of operation with a small PV system, to accelerate de-watering of perched groundwater below the caps
• Expanded the use of onsite solar resources over recent years through small-scale PV applications that: (1) measure groundwater flow through the wetlands, and (2) operate lighting, motion detection, security camera, and gate control systems around the wetlands area
• Treated over 550 million gallons of groundwater (removing over 563,000 pounds of nitrates) between 1997 wetlands installation and early 2011, at a rate averaging 180 gallons per minute
• Continuing to achieve nitrate-nitrogen concentrations below the 10 µg/L drinking water standard, as compared to pre-remediation concentrations reaching 570 µg/L
• Directly recharging the underlying aquifer with the majority of treated water exiting the wetlands, with the remainder flowing a distance of less than one mile to the San Pedro River
• Providing a wetland habitat for wildlife such as deer, javelina, Gila monsters, bobcats, snakes, lizards, and amphibians, numerous bird species, and insect-consuming bats; wildlife-friendly fencing allows the animals access but denies entry by grazing livestock that can damage sensitive flora
• Installed a 42-kW solar canopy for the site's administrative buildings in late 2010, which is expected to annually save $6,400 in electricity costs and avoid a carbon dioxide (equivalent) emission of nearly 35,000 pounds each year; nearly 100% of the PV capital costs were covered by federal/state tax credits and utility incentives, with system payback expected in six years
• Recovering rainwater from the solar canopy for use in xeriscape landscaping in the site's desert environment; during heavy rain, a network of constructed rock-lined channels capture the runoff for gradual subsurface infiltration or release to the San Pedro River
• Considering future large-scale solar energy development on the capped ponds, if financially viable; a preliminary solar energy assessment suggests that a 2-MW system would meet the peak daily energy demand of the site's current chemical processing operations

Property End Use: Continued industrial operations

Point of Contact: Andria Benner, U.S. EPA Region 9

Update: July 2011

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