ABSTRACT

A national solid waste management company has been attempting for several years to open a new sanitary landfill in the St. Martin/Iberia/Lafayette tri-parish area of south Louisiana. The first permit application, for a site near Cade, St. Martin Parish (Cade I) was rejected by the Louisiana Department of Environmental Quality (DEQ) in 1987 in the interests of protecting the Chicot Aquifer, an "important source of drinking water," which is close to the surface in the area.

In 1988, the company submitted a new permit application for a second site (Cade II), less than two miles from the first. Despite geological and technical criticisms of the site voiced by local geologists, by the Louisiana Geological Survey (in its role as consultant to DEQ), and by members of the Solid Waste Division of DEQ, a permit was granted in early 1992.

Geological objections to the site fall into two main categories. Firstly, the site selection process should have given high priority to optimizing geological protection for the aquifer. This consideration was apparently ignored and a location was chosen within the area shown by published studies to have the thinnest clay cover. Cade lies very close to a major recharge area for the Chicot, which is the main source of domestic and municipal water supplies for southwestern Louisiana.

Secondly, site characterization was conducted to engineering standards with no geological interpretations. Fifty-five borings were sampled and five geological units recognized. Lithological logs clearly reveal that the units are highly variable and that gradational contacts exist between #1 and #2 and between #1's 3, 4 and 5, numbered from the surface down. Units #1 and #2 are here interpreted as loess, and units #3 through #5 as representing the upper portions of a late Pleistocene (Prairie Terrace) point bar, a component of the Chicot aquifer system. The clays of unit #3 constitute the main natural protection for the aquifer relied upon in the permit application and were assumed by the engineering consultants to be homogeneous and characterized by measured sample permeabilities of 1x10-7 cm/sec or less throughout. As might be anticipated from numerous point bar studies, the detailed boring descriptions mention very fine-grained to fine-grained sand pockets and laminations which must have higher permeabilities and which are likely to be oblique to the unit boundaries, providing potentially rapid access to the silts and sands of the lower units.

An early and unbiased geological evaluation would have introduced an element of realism to the assessment of the barrier potential of the clay and probably would have condemned the site as a suitable landfill location. This case history provides a good example of the wrong approach to landfill site selection.