Hargis + Associates Inc

Dynamics of nitrate & perchlorate in groundwater.

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The Apache Powder Superfund Site is located in Cochise County, Arizona, approximately 50 miles southeast of Tucson. The site was listed on the National Priorities List pursuant to CERCLA in 1990. Principal groundwater chemicals of concern (COCs) include nitrate-nitrogen (nitrate-N) and perchlorate. Concentrations of nitrate-N exceeding 500 mg/l and 2,300 mg/l have been detected in the “shallow aquifer” and “perched zone,” respectively. Respective concentrations of perchlorate were approaching 400 µg/l and 800 µg/l. While nitrate-N was released as a direct result of discharges from an 80-year old nitrogen product manufacturing operation, perchlorate was only present as an impurity in a Chile saltpeter feedstock used during only a portion of the manufacturing history. Its serendipitous discovery in 1998 necessitated reconsideration of the overall remedial strategy.

Results of the remedial investigations performed to date indicate a complex contaminant history at the site. Notably, contaminant source areas are associated primarily with former unlined evaporation ponds. These ponds were situated in upland areas, outside the boundary of the shallow aquifer. Because of infiltration of wastewater through these pond bottoms, a perched zone was formed in the sandy-silty sediments overlying an upper clay member of the St. David Formation. Various paleogeomorphic features have been instrumental in controlling the movement of the COCs from the perched zone to the shallow aquifer. Historically, the perched zone drained laterally eastward into the shallow aquifer. Detailed examination of water quality dynamics, coupled with surficial geophysical surveys and exploratory borings, defined paleogeomorphic features on top of the clay. These features play significant roles in routing the perched zone flow and the dynamics.

In particular, a prominent buried paleofeature identified as the Molinos Creek paleodrainage, underlies the perched zone and extends beneath the alluvial plain west of the San Pedro River (Deane, 2000). Fine-grained overbank deposits associated with the Molinos Creek paleodrainage are believed to comprise a laterally confining unit (LCU) within the shallow aquifer. This LCU isolates the western portion of the shallow aquifer hydraulically from the main body of the shallow aquifer. Likewise, the LCU serves to control the flux of COCs. These aquifer divisions defined by the LCU have been distinguished as the Molinos Creek Sub-Aquifer (MCA) and the San Pedro Aquifer (SPA). Field investigations including seismic reflection, exploratory borings, piezometry, hydrogeochemical data, and contaminant spreading, as well as numerical modeling, collectively indicate that this feature apparently isolates contaminant discharges entering the shallow aquifer (MCA) from the perched zone from the rest of the shallow aquifer (SPA), and from the San Pedro River to the east. These hydrogeologic controls, combined with the potential for bioattenuation of both nitrate-N and perchlorate COCs, has permitted continued consideration of the applicability of a monitored natural attenuation remedy for that portion of the Superfund site.

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