Structure contour maps and isopach maps should be prepared for each water bearing

zone that comprises the uppermost aquifer and for each significant confining layer, especially

the one underlying the uppermost aquifer. A structure contour map depicts the configuration

(i.e., elevations) of the upper or lower surface or boundary of a particular geologic or soil

formation, unit, or zone. Structure contour maps are especially important in understanding

DNAPL movement because DNAPLs may migrate in the direction of the dip of lower

permeability units. Separate structure contour maps should be constructed for the upper and

lower surfaces (or contacts) of each of the zones of interest. Isopach maps should depict

contours that indicate the thickness of each of these zones. These maps are generated from

borings and geologic logs, and from geophysical measurements. In conjunction with the

cross sections, these maps are used to help determine monitoring well locations, depths, and

screen lengths during the design of the detection monitoring system.

A potentiometric surface map or water table map should be prepared for each water 

bearing zone that comprises the uppermost aquifer. Potentiometric surface and water table

maps should show both the direction and rate of ground water flow and the locations of all

piezometers and wells on which they are based. The water level measurements for all

piezometers and wells on which the potentiometric surface map or water table map is based

should be shown on the potentiometric surface or water table map. If seasonal or temporal

variations in ground water flow occur at the site, a sufficient number of potentiometric

surface or water table maps should be prepared to show these variations. Potentiometric

surface and water table maps can be combined with structure contour maps for a particular

formation or unit.

An adequate number of cross sections (at least two for each hazardous waste

management unit at the facility) should be prepared to depict significant stratigraphic and

structural trends and to reflect stratigraphic and structural features in relation to local and

regional ground water flow. On each cross section, the following should be depicted or

reported:

Orientation (aspect);

Horizontal and vertical scale;

Location of match points or intersections with other cross sections or with

geophysical survey lines;

Topography;

Lithology of all stratigraphic units;

Structural features;

November 1992

4 57