Hargis + Associates Inc

Groundwater/surface water interactions along the Curtiss reach of the San Pedro River.

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A reach of the San Pedro River flows east of the Apache Powder Superfund Site and west of the Town of St. David in Cochise County, Arizona. As a result of the remedial investigations associated with the Superfund Site, this reach has been surveyed extensively since the mid-1980’s from approximately the Escalante Crossing to the Pomerene Diversion. In particular, the section downstream from Curtiss Station has been investigated in regard to potential groundwater/surface water interaction owing to the documented presence of groundwater contamination by nitrate-nitrogen (nitrate-N) and perchlorate solutes in the shallow aquifer.

In-stream surveys have comprised quarterly discharge measurements; quarterly sampling and analysis of surface water for nitrate-N and perchlorate; and occasional analysis for major ion suites in samples collected at strategic surface water and wellpoint locations (H+A, 2001). These investigations confirmed the broader conclusions of Usunoff (1984) regarding in-stream transitions from losing to gaining conditions along the reach. In particular, it appears that these reaches may be largely controlled by a combination of hydraulics associated with the width of the aquifer zone west of the river and the degree of river channel meander westward across the northwestward flow path of the shallow aquifer. The width of the aquifer underlying the west bank of the San Pedro River ranges from a few hundred feet to nearly a mile. The river, directed by the upland areas of outcropping St. David sediments, flows generally northwestward, but locally may change from a nearly northeast to due west course. These abrupt directional changes, coupled with incisement of the inner channel, result in discharge from the shallow aquifer to the river.

This regime is most evident during low flow conditions, when one can observe alternating flowing and dry reaches downstream from Curtiss. At these times, a combination of detailed surface water and wellpoint sampling over an approximate three-mile reach of the river revealed an abrupt increase in the concentration of nitrate-N in the river in the vicinity of a suspected discharge area. Concentrations of nitrate-N in excess of 600 mg/l were detected in samples collected from shallow wellpoints installed within the area of the inner channel. Fortunately, perchlorate has not been detected in the San Pedro River to date. Further investigations involving drilling and monitor well construction along the river terraces are planned in an effort to trace this high nitrate discharge back to a specific source area.

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