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Allen County
  
New Understanding of Lagro Formation Aids Evaluation of Aquifer Sensitivity

by:
M. L. Prentice and S. L. Letsinger

Recent drill- core and seismic-reflection studies indicate that the Wisconsin age Lagro Formation (Wayne, 1963) that surfaces most of Allen County consists of two significantly different clay-rich deposits. Figure 1 is a schematic depiction of these two Lagro deposits overlying the Huntertown drift (Fleming, 1994), which serves as a regional aquifer.

Figure 1. Schematic cross section of Lagro Formation geology

Lagro 1 subglacial till and colluvium overlie Lagro 2 ice-marginal till interbedded with colluvium and glaciofluvial deposits. A weathering zone having vertical fractures has developed. Lagro 1 is composed of subglacial till deposited in a shallow-to-deep subglacial environment. Lagro 1 is characterized by: (1) beds of slightly varying, fine-grained texture having an average thickness of several feet, (2) significant variation in internal bed structure, (3) sharp, subhorizontal bedding contacts, (4) sandy interbeds that are distributed irregularly, and (5) jointing that is distributed irregularly. Lagro 2 is till that formed subglacially but was deposited in an ice-marginal environment along with glaciofluvial sediments and colluvium. Lagro 2 is characterized by: (1) beds of significantly different texture having an average thickness of several feet, (2) bedding inclined at a high angle, (3) gravelly sand interbeds that are distributed irregularly, (4) strong jointing that is distributed irregularly, and (5) variable bed thickness.

Figure 2. Schematic of aquifer sensitivity

The range of Lagro Formation characteristics and their integrated influence on groundwater can be evaluated in terms of aquifer sensitivity to contamination from the surface (fig. 2). In the conceptual model, this influence is organized into four aquifer sensitivity zones that have the following conditions.

Low sensitivity (green): moderate to high thickness, horizontal bedding, slow recharge, preferential flow along bedding planes that does not lead to the water table. In Allen County, this sensitivity class is dominated by Lagro subglacial till.

Moderate sensitivity (yellow): moderate thickness, relatively low density of gravelly sand interbeds, jointing, and weathering fractures, some preferential flow to water table along bedding planes and fractures. Lagro ice-marginal till in Allen County is typified by these characteristics.

Another suite of characteristics can lead to moderate sensitivity to contamination in Allen County. A thin deposit having a relatively high density of gravelly sand interbeds, jointing, and weathering fractures, and some preferential flow to the water table along bedding planes and fractures is often found in Lagro subglacial till over ice-marginal till.

Figure 3. Aquifer sensitivity to contamination (provisional 2012), Allen County, Indiana

Moderate-high sensitivity (orange): moderate thickness, moderate-to-high relative density of gravelly sand interbeds, little jointing, and weathering fractures, recharge to water table through fractures and bedding planes. Lagro ice-marginal till having many fractures and bedding planes hosts this sensitivity classification.

High sensitivity (red): thin, relatively high density of gravelly sand interbeds, jointing, and weathering fractures, rapid recharge, or alluvial sediments with high permeability. Lagro ice-marginal till or alluvial sediments having little or no overlying low-permeability sediments are highly sensitive deposits in Allen County.

Lagro Formation geology strongly influences its hydraulic conductivity and, therefore, the sensitivity of underlying aquifers to contamination. Figure 3 illustrates how the spatial distribution of geologic materials controls water flow through the Lagro Formation into the underlying Huntertown Formation and how this, in turn, influences the sensitivity of the Huntertown aquifer to contaminants from surface sources.



References:

Fleming, A. H., 1994, The hydrogeology of Allen County, Indiana geologic and ground-water atlas: Indiana Geological Survey Special Report 57, 111 p.


Wayne, W. J., 1963, Pleistocene formations in Indiana: Indiana Geological Survey Bulletin 25, 85 p.



 
 
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