Several lines of evidence indicate that the shallow aquifer system functions as an integrated system that is hydraulically and geologically distinct from deeper aquifers and aquifer systems:
- Water levels recorded from different shallow sand and gravel units within a geographically proximate area seldom differ by more than a few feet, even when the units are separated by extensive till sheets. In contrast, water levels may differ by tens of feet between shallow and deep aquifers;
- Three-dimensional mapping of the Wisconsin deposits suggests that interconnection commonly exists between the various sand and gravel units at and above the pre-Wisconsin surface. Such interconnections are most commonly in the form of cut-outs of one sequence by another, and channel-like sand and gravel bodies that pierce till- confining unit s and extend between shallow aquifers at different horizons. Both of these situations are illustrated in Figure 1;
- In contrast, geologic mapping also suggests that a thick, persistent sequence of pre-Wisconsin till units creates a low-permeability confining unit that, in most places, effectively separates the shallow system from deeper aquifers (see, for example, fig. 1);
- In upland areas, water levels in the shallow aquifer system are commonly between ten and several tens of feet higher than those recorded in the deeper aquifers; and
- Water-level patterns associated with most of the shallow units, both individually and collectively, show a strong relationship to the pattern of surface-water drainage and to local surface topography, whereas such a relationship is less pronounced for the deeper systems.
Cross section illustrating the distribution and variety of aquifers occuring beneath Marion County.
Aquifers occur along a variety of horizons and in many configurations beneath Marion County. They include sand and gravel bodies throughout the glacial section as well as limestone and sandstone bedrock . The major confining units consist of sequences of glacial till of late Wisconsin and pre-Wisconsin age, along with shale and siltstone bedrock. The majority of wells tap the shallow aquifer system, which comprises a hydraulically integrated system of un confined , semi-confined, and confined aquifers along and above the pre-Wisconsin surface (heavy red line), including the large outwash complex associated with the White River valley. Several other sizable aquifer complexes and numerous smaller aquifers occur at even greater depth, some of which are concentrated in and above buried bedrock valleys. (Diagram adapted from Brown and Laudick, 2003.)
Brown, S. E., and Laudick, A. J., eds., 2003, Hydrogeologic framework of Marion County, Indiana — a digital atlas illustrating hydrogeologic terrain and sequence: Indiana Geological Survey Open-File Study 00-14, CD-ROM.
Fleming, A. H., Brown, S. E., and Ferguson, V. R., 1993, Hydrogeologic framework of Marion County, Indiana: Indiana Geological Survey Open-File Study 93-05, 67 p.