EPA's Region 6 Office
Serving: Arkansas, Louisiana, New Mexico, Oklahoma, Texas, and 66 Tribal Nations
The Popile site is located in an upland area adjacent to the alluvial valley of the Bayou de Loutre, a tributary of the Ouachita River. Near-surface geology in the area consists of the Cockfield and Cook Mountain formations.
The Cockfield formation is the prevalent surface unit that covers the majority of the site. Within the Removal Action area, the Cockfield formation is beneath the clean backfill. The formation consists of fining upward sequences and is divided into three general stratigraphic units, namely: the upper fine-grained unit (Upper Layer); a perched aquifer consisting of a fine grained sand layer and a carbonaceous sand layer.
The upper layer underlies most of the Popile site and areas east of the site. This unit consists of interbedded clayey or sandy silts, or sandy clays. The unit thickness across the site ranges from approximately 6 to 19 feet. Based on piezometric head rises in wells completed in the underlying sand, this upper layer acts as a confining layer to sands below it. The upper layer appears to grade laterally into a sand facies along the western boundary of the site, consisting of fine- to very fine-grained clean sand with an occasional interbedded silt or clay layer. The fine-grained unit is projected to underlie the Bayou de Loutre.
The shallow aquifer including the sand layer and the lower carbonaceous layer consists of predominantly of a poorly graded, fine to very fine sand. Scattered small pebbles are common, and at some locations, thin beds of gravelly sand or sandy gravel occur. Disseminated carbonaceous matter (up to 3%) occurs throughout the unit. The sand layer averages 15-feet thick and is the aquifer of interest as far as contaminant migration.
The carbonaceous layer is located in the bottom section of the shallow aquifer. A characteristic feature of this unit is that it has layered black carbonaceous matter which acts as a barrier and "filter" to downward migration of contaminants. This carbonaceous layer ranges from 18 to 31 feet thick. The variation in thickness generally reflects the change in elevation of the underlying Cook Mountain surface.
The Cook Mountain formation consists of clays and silty clays and may range up to 160 feet in thickness. It is regionally the lower confining layer for Cockfield formation and also the upper confining layer for the underlying Sparta Sand aquifer, a regional drinking water aquifer.