Hydrogeologic mapping is a method of gathering and evaluating geological information to create a three-dimensional depiction of the subsurface material in which groundwater occurs. By mapping the spatial distribution of geologic material with distinctive permeability, the hydrogeologist can understand which geologic units will allow movement of groundwater and which units will restrict groundwater movement.
Geologic Controls on Groundwater Flow
Permeability on a regional scale is a function of primary geologic processes such as glaciation , volcanic eruptions, and sedimentation . Understanding the geologic framework of an area and the relation between permeability and geologic processes are fundamental to understanding a groundwater flow system.
The proportion, size, and degree of interconnection of void spaces within a geologic material influence the water-bearing capability or permeability of that material. The proportion of void space and the degree of interconnection of geologic material commonly are due to the character of the rock type, resulting from the processes that formed the material as well as subsequent processes such as faulting and weathering .
Since the subsurface cannot be directly observed, hydrogeologists employ various tools and techniques to make "observations" or inferences at specific sites, and to extrapolate that information to create a continuous three-dimensional picture of the underground material.
Hydrogeologic mapping, like any other scientific endeavor, involves collection, analysis, and interpretation of data. Common data collection techniques include compilation and collection of surface geologic data, surface and subsurface geophysical data, and well-drilling data.
The most abundant data available for an area of interest usually are surface geologic maps and geologic cross-sections. Geologists at state or federal agencies, colleges, and universities have geologically mapped most areas in the United States at some scale. This work is important because the identification of rock units at the surface with distinctive physical or hydrologic properties may be important to understanding the likely permeability of similar units in the subsurface. Also, certain rock units or contacts between rock units may control groundwater flow and be associated with significant surface water features such as large springs.
Geophysical data is another commonly used source of information in hydrogeologic mapping. Geophysical data comes from a number of surface and subsurface techniques that measure physical properties of rocks such as electrical conductance and resistivity, remanent magnetism, and natural radioactivity. Surface techniques are carried out on or above the land surface; subsurface techniques involve lowering instruments into wells. Geophysical data are often used to detect the general character of a rock unit, but some geophysical data can actually be used as an indirect measurement of void spaces and permeability.
The most important data used in hydrogeologic mapping comes from wells. Wells are commonly drilled to depths of tens to hundreds of feet, and measure a few inches to a few feet in diameter. Most wells are constructed to obtain water. However, many small-diameter wells are constructed for foundation engineering and environmental testing purposes.
Many state governments require a well-drilling report to be submitted after completion of a well. A well-drilling report or "well log" often contains valuable information on various factors:
- Well location;
- Depth of well;
- Depth to water in the well;
- A description of the geologic materials encountered during drilling; and
- An estimate of the rate at which water can be pumped from the well.
All of this information is important to hydrogeologic mapping.
Depending on the size of a hydrogeologic area of interest, there may be hundreds to thousands of wells already drilled. Existing wells represent a potentially large source of valuable subsurface hydrogeologic information. In addition, unused wells within an area represent a source of potential wells for subsurface geophysical surveys.
Data on a well report that are of particular interest in hydrogeologic mapping are the geologic material descriptions, the identity of the water-bearing material, and results of pumping tests. Geologic material descriptions from well logs are useful in identifying rock types in the subsurface.
Although most well logs are not completed by a geologist, the descriptions they contain are usually adequate to distinguish significant geologic differences, as in the case of subsurface material classification such as lava versus coarse gravel versus clay. Knowledge of the thickness and spatial distribution of the water-bearing materials, and a rough estimate of the permeability of that material obtained from pumping tests, are paramount in determining the extent of the groundwater resource within an area of interest.
Depicting Hydrogeologic Data
Several techniques are used in hydrogeologic mapping for analyzing and correlating data from individual sites and depicting it as continuous data.
Constructing geologic cross sections is one technique commonly used for visually depicting a hydrogeologic system. A geologic cross section is a two-dimensional, vertical view of the subsurface (see figure). When numerous geologic cross-sections are drawn through an area of interest, they begin to yield a three-dimensional picture of the subsurface.
Geologic cross sections are often used to correlate surface geologic mapping with geologic descriptions obtained from wells. Constructing geologic cross sections allows scientists to visualize those correlations and interpret subsurface features such as boundaries between rock units and geologic faults and folds. Hydrogeologists also use cross sections to understand where water occurs underground and to make inferences about where boundaries to groundwater flow are likely to exist.
Isopach and Structure Contour Mapping.
Other techniques used to create a continuous picture of the subsurface from discrete sampling sites include isopach mapping and structure contour mapping. Isopach mapping, or thickness mapping, is a technique of mapping geologic material based on its apparent thickness. The thickness of a geologic unit (or a water-bearing unit) can be contoured throughout an area by knowing the thickness of the unit in wells or at exposures on the land surface and by interpolating the thickness between the known points. A surface of a particular unit may be contoured in a similar way. By knowing the elevation of a particular unit at discrete points, a surface of the unit can be contoured throughout the area with a technique called structure contouring. A structure contour map is similar in appearance to a topographic map of the land surface.
Kenneth E. Lite Jr.
Fetter, Charles W., Jr. Applied Hydrogeology, 4th ed. New York: Prentice Hall, 2000.
Streltsova, Tatiana. Well Testing in Heterogeneous Formations. New York: John Wiley & Sons, 1988.