2022 – 2024: The Fifth five-year review (FYR) was released in March 2022 for public comment. There were no comments received from the public. 

In 2022, the United States Environmental Protection Agency (EPA) determined that perchlorate did not meet the definition for a chemical to be regulated under the Safe Drinking Water Act, so there is no federal standard. Therefore, the Arizona guidelines are being followed. Currently the site is meeting the remedial action objective of minimizing migration of ground water contamination for Perched Zona A, Perched Zone B, and the Shallow Aquifer in the Southern Area. 

Piezometers in Perched Zone A showed a relatively steady water level in 2023, but the long-term trend continues to display a decline in water level. 

Operations continued through 2024. EPA and ADEQ are working with Apache Nitrogen Products Inc., (ANPI) to update the Operation and Maintenance Plan, and maintain or improve the efficiency of the wetlands. 

2020 – 2021: EPA-approved limited reductions in monitoring for the Southern Area groundwater in 2020 included reduced monitoring frequencies from quarterly to biennial for several wells (effective August of 2021).

SEW-03 started full-scale groundwater extraction in late March of 2021, extracting more than double the nitrate mass of SEW-02.

2019: Cleanup of Legacy Soils was deemed complete in October of 2019; no contamination was left onsite. 

In November of 2019, ANPI decommissioned the P-03 voluntary pilot project infrastructure in the Southern Area. 

Additional drilling was conducted in the Northern Area in 2019 to further define the extent and magnitude of nitrate plume. The highest nitrate concentrations identified were found at borehole PB-5A. Borehole PB-5A was then tested in December of 2019, and renamed SEW-3. 

2018: Test well TW-01 was converted to extraction well SEW-2 and began pumping to the treatment lagoons in July 2018.

ANPI submitted for comments a work plan for the decommissioning and removal of the P-03 treatment system in September 2018.  The system was used for the periodic extraction and evaporation of perched groundwater in the Southern Area Remediation System (SARS) from 2003 to 2017.

2016 – 2017:  ANPI drilled various exploratory borings, installed additional monitoring wells and conducted a pump test to determine the possibility of implementing in-situ remedy enhancements in the Southern Area PZ-B (the former Molinos Creek Sub-Aquifer, otherwise known as the MCA). The results of the studies indicated that due to continued dewatering of this area, the areal extent of water in PZ-B is greatly reduced from its original estimated capacity.  The tests showed there is insufficient yield for this area to be used as a potential potable water supply.  Instead of being hydraulically connected to the shallow aquifer to the east, PZ-B is now defined as a totally isolated perched area, similar to PZ-A.  Consequently, EPA modified the remedy for PZ-B in the 2017 Explanation of Significant Differences (ESD) to eliminate monitored natural attenuation (MNA) as the selected remedy, and just retain long-term monitoring and institutional controls (ICs).

Several EPA legal documents were published during 2017 including:  ESD #4 (July 2017), the Fourth FYR (September 2017) and a Modification to the Consent Decree (November 2017).  During 2017, ANPI conducted various pump tests in the Northern Area. Results of these pump tests indicated significant decreases of nitrate levels in wells in the Northern Area.  The wetlands have performed as designed, and handled the extra volumes of nitrate-impacted groundwater very well. 

2014: Buildings located on the former powder line that had been scheduled for demolition underwent abatement for asbestos and other potentially hazardous materials prior to the scheduled demolition. During activities, it was noted that debris potentially containing nitroglycerin and pentaerythritol tetranitrate (PETN) was discovered in one of the buildings. The EPA designated the removal of discovered debris a Time Critical Removal Action and a work plan was requested. 

2013: The Northern Area treatment wetland continued to operate and nitrate concentrations in the Northern Area continued to decline. ANPI proposed an enhanced remedy in the Southern Area using in-situ bioremediation for treatment of groundwater. Bench-scale and field-scale studies were proposed to determine operating parameters for the full-scale remedy.

2011 – 2012: All required Southern Area Declaration of Environmental Use Restrictions (DEUR) and ICs were in place and the Northern Area remedy was in full operation. The instituted measures were considered protective of human health and the environment for both groundwater and soils. The Northern Area treatment wetland continued to operate and nitrate concentrations in the Northern Area continued to decline. The Third FYR was completed in September 2012.

2009 – 2010: The Northern Area treatment wetland continued to operate; ICs were in place; groundwater monitoring continued; and nitrate concentrations in the Northern Area continued to decline.

2008: In September, EPA issued its preliminary closeout report for the entire site. Because some of the nitrate-N plume in the Northern Area had migrated beyond the capture area of the wetland extraction well (SEW-1) before it was constructed, additional characterization in this area was needed. These investigations were completed during the first half of the year, and they led to the decision by EPA and ADEQ to supplement the Northern Area groundwater remedy with MNA. This decision was formalized in an ESD in July.

The native soil covers for abandoned pond areas in the Southern Area were completed early in the year.

2007: Construction of native soil covers for abandoned pond areas in the Southern Area began in the fall. The Second FYR was completed.

2006: Soil borings installed in summer showed that the shallow aquifer groundwater contamination in the Southern Area was much less extensive than previously thought. These new data indicated that the contamination in the shallow aquifer appeared to be confined to two small areas that are hydraulically isolated from the shallow aquifer in the Northern Area of the site and from the San Pedro River.

2005: The final remedy for the perched aquifer, the Southern Area shallow aquifer, and contaminated soils in the Southern Area was specified in a Record of Decision (ROD) Amendment in September. The selected remedy for the Southern Area groundwater is MNA. The remedy for the Southern Area contaminated soils is a low-permeability native soil cover with ICs including a deed restriction on the southern portion of the ANPI property.

2004: Late in the year, the Northern Area treatment wetland achieved full-scale startup.

1998 – 2003: In November 1998, perchlorate contamination was detected in the Southern Area groundwater at the site ranging up to 670 parts per billion (ppb) in the perched aquifer and 300 ppb in the shallow aquifer. The source of perchlorate contamination is thought to be Chilean sodium nitrate that was used in historic manufacturing processes. Although neither EPA nor ADEQ have a drinking water standard for perchlorate, the Arizona Department of Health Services (ADHS) issued a Health Based Guidance Level for perchlorate in drinking water of 14 ppb in May 2000. 

Between November 1999 and June 2000, an EPA-mandated removal action of 870 tons of trinitrotoluene (TNT)-contaminated soils was completed. The removal action included the excavation, pre-burning, removal, and off-site disposal of these soils. Between December 1999 and June 2000, 262 drums (110 gallons each) containing dinitrotoluene (DNT), in the form of 2,4- DNT and 2,6-DNT, as well as vanadium-pentoxide were removed from the site. In addition, approximately 1,300 tons of soil contaminated with arsenic, DNT, and vanadium-pentoxide were excavated. These materials were transported to a permitted hazardous waste disposal facility where they were treated and disposed. 

Additional characterization of the interaction between the perched zone and shallow aquifer in the Southern Area of the site was conducted in summer and fall 2000. These investigations included construction of additional monitor wells, exploration boreholes, groundwater modeling, and a geophysical survey. 

From 1997 through 2001, the wetland was in its growth phase to develop its aquatic vegetation.  During this time enough biomass was produced to trigger anaerobic denitrification. However, operational problems during the summer of 2002 prevented full-scale startup. Intensified efforts to monitor the wetland and achieve full-scale startup in the summer of 2003 were then developed. The First FYR was completed in 2002.

In 2003, a groundwater characterization report and a supplemental Feasibility Study (FS) for the Southern Area that included analysis of soil and groundwater remedial alternatives, were completed. This work concluded that MNA would be a viable remedial alternative for the groundwater contamination in the southern portion of the site.

1997: In September, per the ROD, construction of a 4.5-acre Northern Area treatment wetland to treat 150 gallons per minute (80 million gallons per year) of nitrate-N contaminated groundwater in the northern portion of the site was completed.

1995: In January, ANPI completed construction of a brine concentrator. All discharges to the evaporation ponds have since ceased. In March, as required by the ROD, eight households that had been supplied bottled water since 1989 were hooked up to deep aquifer replacement wells.

1990 – 1994: The site was formally listed on the National Priorities List on August 30, 1990, and EPA issued a unilateral administrative order to ANPI in December 1994 to conduct remedial design/remedial action activities under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA).

In June 1992, ADEQ and EPA agreed that ADEQ would be responsible for ensuring ANPI’s compliance with state requirements for aquifer protection, air quality and hazardous waste management, and that EPA would be responsible for overseeing ANPI's CERCLA cleanup.  EPA completed a Baseline Public Health Evaluation and Ecological Assessment for the site in September 1992.

The remedial investigation (RI) and FS were completed in June 1994. The RI revealed nitrate-N contamination in perched groundwater at the southern end of the site exceeding 1000 milligrams per liter (mg/l). Nitrate-N contamination in the shallow groundwater of the site was found at concentrations as high as 400 mg/l. East of the site, concentrations of nitrate-N up to 220 mg/l were detected in samples from the San Pedro River. Shallow groundwater sampling along the west bank of the river revealed nitrate-N concentrations up to 570 mg/l. The federal drinking water standard for nitrate-N is 10 mg/l.

In September 1994, EPA signed the ROD for the site. The selected groundwater remedies consisted of pumping and treating the perched groundwater zone by forced evaporation using a brine concentrator, and pumping and treating the shallow aquifer by use of constructed wetlands and then recharging the treated water back into the shallow aquifer. The 1994 ROD also included soil excavation, removal, and treatment at a permitted off-site disposal area, the removal and treatment of drums containing hazardous materials, and capping of any contaminated soils left on site.

1986 – 1989: After additional studies in 1986, ADHS instructed ANPI to obtain a state groundwater protection permit to address source discharges. However, ANPI continued to operate in violation of applicable state water quality regulations from 1988 through 1993. 

The EPA completed a preliminary investigation of the site in June 1988. This investigation confirmed the State's earlier findings of nitrate contamination, as well as heavy metals contamination of site soils.  In 1989, ANPI began supplying bottled water to nearby residents whose drinking water wells were contaminated with nitrate.

1979 – 1980: The ADHS identified potential groundwater contamination problems in 1979. In 1980, EPA found high levels of heavy metals (such as lead, chromium, zinc and strontium) in some of the on-site ponds, and ten shallow aquifer wells downgradient from ANPI were found to contain nitrate at concentrations up to 470 mg/l.

1922 – 1971: The site (now ANPI) began manufacturing dynamite in 1922 for mining and construction projects throughout the southwest. Later, ANPI broadened its product line to include ammonium nitrate, nitrogen-based fertilizer products, blasting agents, and nitric acid. Today, ANPI manufactures various forms of ammonium nitrate and nitric acid. 

Prior to 1971, manufacturing wastewater was discharged on site into dry washes which flow directly into the San Pedro River. After 1971, wastewater was discharged into unlined evaporation ponds resulting in the contamination of a perched groundwater zone. This discharge of wastewaters to the perched groundwater also resulted in contamination of the shallow aquifer and the San Pedro River.