Southwestern Aquifer Complex
  

by:
Anthony H. Fleming

Location and General Dimensions

The southwestern aquifer complex is a large, generally tabular body composed mostly of sand that extends over nearly 100 sq mi (161 sq km) in the subsurface of south-central and southwestern Marion County (click here and here to view a geologic map of the complex). The complex occurs in two distinct and roughly equal-sized parts that straddle the lower trunk valley of the White River. It acts as the primary groundwater source in those areas, serving hundreds of domestic, commercial, and some industrial wells. Wells developed in the aquifer complex are typically capable of moderate to large yields, with some large-diameter wells reported to produce several hundred gal per min from thicker parts of the complex.

Origin and Geology

The bulk of the southwestern aquifer complex appears to have originated as a single, coherent body of outwash along a well-defined horizon within the Illinoian section. The age of the complex is loosely interpreted to be “middle Illinoian” because it occurs about midway through the thick sequence of Illinoian deposits that flank this part of the White River valley. It is referred to as a “complex” because substantial parts of the body are coalesced with stratigraphically higher (and, in a few places, lower) sand and gravel bodies, producing exceptionally thick, composite sand and gravel sections of widely varying ages and origin. This is particularly true along the margins of the ancestral White River valley, where the aquifer complex is gradually truncated by late Wisconsin outwash fans as one proceeds inward into the valley. The truncation by later sequences is responsible for producing the two disjunct segments of the aquifer complex that lie on either side of the White River Valley. Depending on the degree of truncation and the thickness of the overlying deposits, the top of the aquifer complex lies as little as a few feet to more than 100 ft (30.5 m) below the modern land surface.

The aquifer complex is composed chiefly of coarse sand, which averages 15 to 30 ft (4.6 to 9.1 m) in thickness and is remarkably well sorted. The uppermost several feet of the complex are commonly gravelly, as are the northeastern and northwestern parts of the unit, which also contain lenses and are locally capped by a genetically related till unit. A strong weathering profile, commonly marked by leached, greenish-brown, loamy sand, is preserved at several places atop the aquifer complex; this distinctive paleosol rises up onto the capping till unit towards the margins of the complex, which suggests that the sand and the till represent a genetically related outwash fan sequence deposited during the same Illinoian glacial episode, and were both affected together by a subsequent weathering episode.

Figure 1.
Diagram illustrating inferred conditions in southern Marion County during deposition of the southwestern aquifer complex during middle Illinoian time (Brown and Laudick, 2003).

As shown on the geologic map, the overall shape of the complex also is highly suggestive of an outwash fan: the complex is clearly oriented about the ancestral bedrock valley that underlies the White River trunk valley, and the surfaces of both segments of the complex slope gently but consistently inward toward the trunk valley, which probably acted as a central sluiceway during fan deposition. Sand thicknesses as great as 60 to 80 ft (18.3 to 24.4 m) occur locally along vaguely linear to lobate zones that appear to mark channels or distributaries on the surface of the outwash fan. For all these reasons, the southwestern aquifer complex is best interpreted as a high-energy outwash fan produced in front of Illinoian ice advancing into the ancestral sluiceway from both sides, much like the late Wisconsin ice. The ice front eventually overrode the fan, or parts of it (fig. 1), producing the capping tills preserved on the proximal parts of the complex.

The top of the aquifer complex was undoubtedly eroded and truncated both by later Illinoian glaciers and, especially, by late Wisconsin glaciers and meltwater . The extent of this erosion, however, is difficult to quantify. Along the margins of the ancestral White River, the complex was severely eroded by meltwater, and its central area completely removed by erosion. On the other hand, a relic paleosol is still preserved over sizable parts of the top of the complex away from the central sluiceway, suggesting these areas did not experience severe erosion.

Hydrogeology

The southwestern aquifer complex contains a coherent, well-defined groundwater flow system with distinct recharge and discharge areas and groundwater flow directions. The distinctiveness of this system allowed a map of the hydrogeology of the complex to be developed in a manner analogous to that of the shallow aquifer system . The hydrogeologic map of the aquifer complex shows the direction of groundwater flow in the complex via a series of water-level contours, and identifies several hydrogeologic settings based on the hydraulic behavior of the complex and its relation to superjacent aquifers and the modern landscape at different places.

The water-level contours clearly show that regional groundwater flow in the aquifer complex is inward toward the White River from potentiometric highs located toward the outer margins of the complex. Groundwater recharge is inferred to come from a combination of slow leakage through overlying till confining unit s and via sand- and gravel-filled channels and other hydraulic interconnections that expedite downward flow of groundwater from overlying aquifers. Some of the latter locations are associated with sharp potentiometric highs that strongly suggest that considerable recharge may be focused through such “breaches” in the till-confining units. At places, the aquifer complex directly underlies the floors of several major surface streams, such as lower Eagle Creek and Lick Creek, where a considerable amount of interaction between groundwater and surface water is inferred to take place. Several of these streams act as regional discharge areas for the aquifer complex, suggesting that the complex may contribute a significant amount of the base flow of these streams.

Over most of its extent, the southwestern aquifer complex is confined by moderately thick to very thick sequences of low-permeability glacial till. Hence, it is not thought to be particularly sensitive to contamination from sources and activities at the modern land surface, where most contaminants originate. Sensitivity to contamination is greater, however, where the top of the aquifer complex is intersected by large channels that pierce the confining units, and beneath the floors of the stream valleys where no confining unit is present. As noted above, however, most of the latter places act as regional groundwater discharge areas, and are characterized by upward hydraulic gradient s; this type of setting is likely to limit the impact of any contaminants that might be introduced to the uppermost part of the complex and hinder the migration of contaminants beyond these places.

References

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.



 
 
 
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