USGS - Pontchartrain Geochemistry

Environmental Geochemistry and Sediment Quality in Lake Pontchartrain

Sediment Sampling Surveys (con't)

Boreholes-

Discussion:

Borehole data used in this project is mainly associated with sediment retrieval or cuttings from boreholes drilled into substrata during the engineering phase of infrastructure development, such as the building or expansion of roads, bridges, dams and buildings. A number of engineering factors are derived from the cuttings, including the strength and stability of the material underlying the infrastructure. Lithology and stratigraphy can also be determined from the cuttings. The advantage of borehole data is the extensive dataset developed during the engineering process. Also, borehole depths tend to exceed depths obtained by most intrusive research methods. Interpretations of lithology and stratigraphy from borehole data can sometimes be subjective. This is because borehole data is derived from cuttings, and interpretations are normally done on-site and sometimes lacks the rigorous attention of formal scientific scrutiny.

The borehole data used in this project was acquired by the Army Corps as a part of a large coastal data base of engineering borings throughout the area. In 1994 and 1995 the United States Army Engineer Waterways Experiment Station (US Army Corps of Engineers) produced a series of technical reports (Dunbar, et. al., 1994a; Dunbar, et. al., 1994b; Dunbar, et. al., 1995) that include stratigraphic interpretations and correlation profiles based on the engineering logs of the cuttings. For the reports, the profiles were divided geographically into quadrangles. The quadrangles studied for this project were those surrounding the Lake Pontchartrain and Lake Borgne areas.

The cuttings interpretations were assessed primarily for depth to Pleistocene, to corroborate with the HRSP and vibracore data. Interpretation of the Pleistocene surface in the borehole cuttings were based on engineering criteria (sheer strength), whereas change in lithology identified the Pleistocene surface in the vibracore interpretations. This led to a consistent discrepancy between interpretations. Stratigraphy within the Pleistocene could also be compared between borehole profiles and HRSP, below vibracore depth (Figure 11). For the study, boreholes across the Lake Pontchartrain Causeway, Interstate 10 bridge, Highway 190 and a transect to Alligator point in Lake Borgne were used. The profiles were compared to a model of the Pleistocene surface generated from vibracore data and HRSP, and correlations and discrepancies were identified. The borehole profiles were also used to extend the model of the Pleistocene surface across land, where HRSP and vibracore data were unavailable.

Figure 11:

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