Environmental Protection Agency (EPA) #: AZD008399263

Superfund National Priority List (NPL) Status: Final

Location

Cochise County, Arizona, approximately seven miles southeast of the incorporated town of Benson and two-and-a-half miles southwest of the unincorporated town of St. David. Study area covers approximately nine square-miles and includes 945 acres of land owned by Apache Nitrogen Products Inc., formerly known as the Apache Powder Company. The San Pedro River bounds the eastern side of the site, running from the southeast corner of the property toward the northwest.

Contaminants of Concern

Nitrate in the shallow groundwater aquifer. Perchlorate has been detected in the perched zones (PZ-A and PZ-B) and shallow aquifer of the southern area.

Public Health Impact

Currently there are no known human exposures to groundwater contamination at this site. In 1995, the site replaced a number of shallow aquifer domestic wells that had nitrate contamination with deeper regional aquifer wells and they are now supplying bottled water to one household. Perchlorate has not been detected in any domestic wells.

Site Hydrogeology 

The site is located in the Upper San Pedro River Basin, bounded by several mountain ranges including the Whetstone Mountains to the west, the Dragoon Mountains to the east, the Huachuca Mountains to the southwest and the Mule mountains to the southeast.

The San Pedro River drains approximately 2,500 square miles, of which 700 are in Mexico. The river flows northward from Mexico and joins the Gila River near Winkelman, Arizona. The river is perennial where it intersects the water table, supporting a rich riparian ecosystem. 



At the site, there’s a shallow aquifer and a deep regional aquifer with wells exhibiting artesian conditions. Some wells in the St. David area actually flow at the surface. In addition, there is a small perched zone on Apache's property that resulted from wastewater discharges to unlined washes and ponds. Until recently, the perched zone drained into the shallow aquifer and was a source of nitrate and perchlorate contamination in the shallow aquifer in the southern area of the site. However, this perched zone is now nearly dry and no longer discharges to the shallow aquifer.



In the southern area of the site, the shallow aquifer is further divided into the Molinas Creek Sub-Aquifer and the alluvial aquifer associated with the San Pedro River. Nitrate and perchlorate groundwater contamination only occurs in the Molinas Creek Sub-Aquifer and the perched zone. Perchlorate has never been detected in the shallow alluvial aquifer associated with the San Pedro River or in the river itself. The groundwater flow direction in the shallow aquifer is generally northward (sub-parallel to the river). However, the Molinas Creek Sub-Aquifer appears have almost no flow due to its apparent hydraulic isolation from the rest of the shallow aquifer.

Action Taken

The EPA and ADEQ have completed investigations of the northern area groundwater contamination and chose monitored natural attenuation (MNA) as a final remedy enhancement to the existing remediation system. All remedies for the northern area are now in place and long-term operation and maintenance of these remedies has begun. The design and implementation of the MNA remedy for the southern area groundwater was completed in fall 2007. In addition, institutional controls that are called for in an amended Record of Decision (ROD) were in place by fall 2008. Native soil covers have been constructed for former pond areas.

Status

Additional on-going remedial work continues at ANPI, the site has three major ongoing remedial actions as identified in the 2008 Preliminary Close-Out Report. The current selected remedies are:

Northern Area Groundwater 


Use of a constructed wetland to treat water coming from a nitrite-contaminated shallow aquifer, monitored natural attenuation (MNA) for groundwater at the end of the plume outside the capture zone, and institutional controls (ICs).

Apache Nitrogen Products Inc. (ANPI) continues to work towards accelerating the groundwater cleanup in the Northern Area Remediation System (NARS). Actions taken in 2018 include the following:

Former pilot test extraction well began pumping to the treatment lagoons (wetlands) in July 2018.

A geophysical study (Electrical Resistivity and Induced Polarization (IP)) was conducted in July 2018 to better understand the area geology within the NARS. The information from the study report issued in September 2018 was used to determine the locations for additional drilling and extraction well construction which began in November 2018 with scheduled completion in February 2019.

Southern Area Groundwater
Continuation of long-term monitoring and ICs for the Perched System and shallow aquifer.

Contaminated Soils 
Excavation and off-site disposal or on-site treatment or encapsulation, including ICs for other contaminated soils detected during confirmatory soil sampling in the Legacy Soils Area. ANPI will submit the final Legacy Soils Area Remedial Action Report once all the remaining work is completed (projected completion 2019).

Community Involvement Activities

Public meetings are organized for site issues where there is considerable public interest.

As required by the ROD for this site, Apache Nitrogen Products, Inc. provides semi-annual notices to the surrounding community regarding the extent of groundwater contamination and the danger of using this water for drinking water. Apache also keeps an inventory of private well use in the area, and they monitor/track groundwater drilling in the area.

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