1acre impoundment appears to have been created by the railroad bed and primarily receives its water through stormwater runoff and groundwater discharge. Groundwater that discharges to the impoundment may originate from areas near an abandoned landfill (Site20) to the northnortheast of the impoundment (Dames & Moore 1994a).
3.1.1.1 Soil
Soil was mapped and classified by the Jo Daviess County and Carroll County Soil Conservation Service (SCS). The upland soil consists primarily of alluvial sand or loamy sand, which resulted from deposition by the Mississippi River. The upland soil is dry due to a low water holding capacity and high permeability. The dominant upland soil at SVDA is Sparta (Dames& Moore 1994a).
The bottomland soil primarily is located adjacent to and within the floodplains of the Mississippi River and its backwaters. The bottomland soil developed gradually by the deposition of fine sediment (silt and clay) carried onto the bottomlands by floodwaters or from the deposition of soil eroded from hillsides. This bottomland soil is primarily clay loam, loamy sand, and silty clay that is poorly drained. The dominant bottomland soil at SVDA is Birds silt loam (DamesMoore1994a).
3.1.1.2 Geology and Stratigraphy
SVDA can be divided into three general geologic divisions, as shown in Figure34. AreaI, the backwater area, includes the present floodplain of the Mississippi River and encompasses approximately 42percent of the installation land area. Area II, the central and southern upland areas, has sandy soil and gently rolling topography, with elevations typically ranging from 10 to 60feet above the backwater area. Area II covers approximately 48percent of the SVDA land area. Area III, the northern upland area, is a zone of more steeply rolling hills and shallow bedrock along the northern boundary of SVDA that covers approximately 10percent of the SVDA land area. A generalized geologic cross section through the central depot area is shown in Figure 35. This cross section is taken along the line AA’ shown in Figure34. Within SVDA, unconsolidated sediment, ranging in age from Pleistocene to Recent, overlies Ordovician dolomite bedrock of the Galena Group (Dames & Moore 1994a).
Figure 34. General Geologic Divisions
Figure 35. General Depot Area – General East-West Cross Section
The following sections discuss the geology and stratigraphy for each of the three general geologic divisions within the installation.
Geology of the Backwater Area – Area I—The total thickness of the unconsolidated strata in Area I is approximately 145 to 155feet. The unconsolidated stratigraphy near the surface is composed of clay, silt, and sand of the Cahokia alluvium overlying the coarser, alluvial deposits of the Parkland Formation. The Cahokia alluvium was deposited from sediment carried by meandering waters of the Mississippi River, Apple River, and other smaller tributaries. The lithology of the Cahokia ranges from dark gray, medium plastic clay to silt and fine to mediumgrained sand. Soil of the Cahokia alluvium is classified generally as clay with some silt and sand (Unified Soil Classification System [USCS] code CL) or sand with varying amounts of silt and clay (USCS codes SP/SM/SC). These soil classifications are based on data from physical soil tests on samples collected from the backwater area during the 1992 RI. The porosity of alluvial soil samples collected from the backwater area ranged from 32.5 to 49.8percent, with an average porosity of 42.3percent (Dames & Moore 1994a).
A surficial layer of silty clay forms a shallow confining layer in the backwater area near the former old washout lagoons. This silty clay deposit appears to be associated with local deposition during highrunoff events (Dames & Moore 1994a).
The Parkland Formation, underlying the Cahokia alluvium in Area I, is Late Holocene in age and generally alluvial in origin. The formation consists of fine to coarsegrained, moderately wellsorted, brown sand with traces of silt and occasional layers of fine, multicolor, rounded gravel.
The sand and gravel of the Henry Formation lie between the Parkland sand and the bedrock floor of the ancient Mississippi Valley (Willman et al. 1970). During the Wisconsin glacial period, the Mississippi Valley carried an enormous load of glacial outwash, primarily the sand and gravel now classified as the Henry Formation. On the basis of boring logs from monitoring wells 302007 and 302131 from Area I, the Henry Formation is composed of wellgraded glacial outwash material consisting of rounded granitic cobbles, large boulders of dolomite, multicolor gravel, and medium to coarsegrained quartz sand (Dames & Moore 1994a).
Bedrock of the Galena-Platteville Group underlies the Henry Formation. On the basis of boring log data from monitoring well 302121, Ordovician dolomite in Area I grades from slightly weathered, light brownishgray dolomite to fresh, light gray dolomite within the first 5feet encountered. The elevation of the top of the dolomite averages 450feet above msl in Area I (Dames & Moore 1994a).
Geology of the Central and Southern Upland Areas – Area II—The topography of AreaII is gently rolling with surface relief controlled by sandy deposits of the Parkland Formation. Area II is bounded on the east by bluffs carved in the Silurian and Ordovician rock by the ancient Mississippi River. The stratigraphy of the upper Parkland Formation in Area II consists of wellsorted, very fine to finegrained, windblown (i.e., eolian) sand with a trace of silt. The upper portion of the Parkland Formation consists of 5 to 10percent silt by weight. These eolian deposits are interbedded with layers of alluvium consisting of medium to coarsegrained, subangular sand with occasional fine, rounded gravel. Soil of the Parkland Formation is classified generally as sand with trace amounts of silt and clay (USCS codes SP/SW) based on physical soil tests performed on soil samples from RI sites in the upland area (Dames & Moore 1994a). The trace amounts of silt and clay and the amount of sorting within the Parkland Formation probably result from swiftmoving currents within shallow braided channels of the Late Holocene Mississippi River. In the lower section of the Parkland Formation, the lithology is typical of the alluvial deposits. An exception to this is observed in the vicinity of Site 67, where a stratum of very fine to finegrained sand occurs in the lower section of the overburden (Dames & Moore 1994a). Soil samples collected during the 1992 RI determined that the porosity of the sandy soil in the upland area ranged from 39.1 to 48.1percent, with an average porosity of 43.6percent.
The Parkland sand grades to coarsersize deposits representing the Henry Formation at approximately 110feet BLS. The lithology of the Henry Formation within Area II consists predominantly of gravel and medium to coarsegrained sand. The total thickness of the unconsolidated strata in Area II ranges from 132 to 190feet, based on well log data from the SVDA deep water supply wells (Dames & Moore 1994a).
Boring data from monitoring well 306706 indicate that the bedrock is light gray, slightly weathered, and fractured. Sparse subsurface data indicate that the erosional surface dips steeply southwest along the edge of the bluffs in the central portion of the installation (DamesMoore1994a). Well logs from the Illinois State Geological Survey (ISGS) for SVDA deep production wells show that in the southern portion of the installation, the surface elevation of the bedrock averages 450feet above msl and dips 0.7 degrees southsouthwest. These logs indicate the thickness of the Galena Group ranges from 145feet in the central part to 215feet in the southern part (ISGS 1967).
Geology of the Northern Upland Area – Area III—The northern upland area of the installation contains steep rolling hills and dunes of the Parkland sand. The lithology of the Parkland sand is consistent throughout Areas II and III. In Area III, wellsorted, fine sand of wind blown origin typically overlies medium to coarsegrained alluvial sand containing a few fine gravels. The sand of the Parkland Formation thins out in the extreme northern portion of the area and grades to yellowishbrown silt that has the characteristics of residual soil from completely weathered and altered dolomite rock (rock flour).
The Henry Formation is generally thin or absent in Area III. The thickness of the unconsolidated overburden varies with the topography of the bedrock and is normally thicker in valleys and thinner in areas where the bedrock is shallow. The total thickness of the overburden in Area III ranges from 5feet (monitoring well 302206) in the northeast to 100feet (monitoring well 300101) in the southeast.
The elevation of the bedrock surface ranges from 611 to 494feet above msl in Area III, with an average depth to bedrock of 40 feet. The bedrock surface is irregular, indicating a dissected system of hills and valleys. In the vicinity of Site 22, the bedrock surface forms a ridge that dips to the southwest approximately 2.5 degrees. In other parts of Area III, the bedrock surface is relatively flat. The bedrock lithology consists of light gray, finely crystalline to light yellowishbrown moderately weathered dolomite. Bedrock cores retrieved during well installation activities show that bedrock in the northwestern portion of AreaIII exhibits a greater amount of weathering and fracturing than bedrock in the southeastern portion (Dames & Moore 1994a). Mineralized (calcite) solution cavities with traces of pyrite and galena have been observed in rock cores from the installation of monitoring well 301002.
3.1.1.3 Hydrogeology
Shallow groundwater in the central and southern areas of SVDA is first encountered in the sandy deposits of the Parkland sand and Cahokia alluvium. In the northern area of SVDA, groundwater generally is first encountered in the shallow bedrock of the Galena dolomite. These aquifers have the greatest potential to be affected by activities at SVDA. Shallow groundwater is generally unconfined at SVDA, except near the old lagoons and the bomb disassembly plant in the northern bottomland area, where a layer of soft clay overlies the sand and creates a localized confined condition. Although seasonal variations occur, the net direction of groundwater flow in the shallow aquifer is southwest, toward the Mississippi River and bottomland of Crooked Slough in the northern and central portions of SVDA. In the extreme southern area of SVDA, groundwater flows in a radial pattern northeast toward the Apple River and southeast toward the Mississippi River (Dames & Moore 1994a).
The groundwater within the sandy Parkland and Cahokia aquifers is affected by the river stage in the Mississippi River. The direction of groundwater flow during the high river stage of 1992 was northeast, opposite the regional flow direction (Dames & Moore 1994a). In comparison, the direction of groundwater flow in the overburden aquifer during a falling river stage was southeast and toward the river throughout the installation (Dames & Moore 1994a). Because of the greater hydraulic gradient and elevated water table, groundwater flow within the shallow dolomite aquifer of the extreme northeast portion of SVDA during the 1992 RI activities (Dames & Moore 1994a) remained southwest, which is consistent with the regional flow direction.
Slug tests conducted during the 1992 RI (Dames & Moore 1994a) determined the hydraulic conductivities for the overburden and bedrock deposits at SVDA. The hydraulic conductivity values of the overburden range from 1.6 × 104 to 1.1 × 101 cm/sec. The range of values primarily represents variability of grain size and composition found in the sandy overburden deposits—silty sand to clean, wellsorted sand. The average hydraulic conductivity of the overburden was calculated to be 3.8 × 102 cm/sec. In comparison, hydraulic conductivity values of the bedrock range from 9.0 × 105 to 1.3 × 102 cm/sec and averaged 4.6×103 cm/sec.
Dames & Moore (1994a) estimated groundwater flow velocity in both the overburden and within the bedrock from available data. The groundwater velocity calculated for the bedrock should be considered only an estimate. Since the calculation is based on the assumption that the bedrock behaves as a classical porous medium. However, numerous solution cavities and fracture zones were observed in dolomite cores retrieved during well installation activities in the northern area of SVDA. Therefore, groundwater flow through such openings may be much faster than indicated by the gradient and bulk hydraulic conductivity.
Based on the hydrogeologic data, SVDA can be divided into three general areas that are consistent with the geologic subdivisions (Figure34). Each area exhibits unique hydrogeologic characteristics due to the differing environmental settings.
Hydrogeology of the Backwater Area – Area I—Area I includes the backwater area and present floodplain of the Mississippi River. It is unique hydrogeologically because the land area is bisected by Crooked Slough and a number of smaller associated sloughs and isolated catchments. The backwater complex is subject to seasonal flooding by the Mississippi River during the high river stage and the meandering channels provide inlets for surface water infiltration and recharge into the shallow Cahokia aquifer. During low stage heights of the Mississippi River, there is low surface flow in the backwater complex and numerous groundwater discharge seeps occur along land/water interfaces through the predominantly sandy deposits (DamesMoore1994a).
Overall, water levels and flow directions in the backwater area of SVDA are determined completely by the variable low and high stages of the Mississippi River. Rising river stages will hinder, stop, or reverse groundwater movement toward the river. Falling river stages encourage groundwater flow toward the river. Given the short duration of yearly flooding and the southwestern slope of the regional piezometric surface, the net longterm direction of groundwater flow will be toward the river with a downstream component within the backwater area (DamesMoore1994a).
Data collected during the 1992 RI (Dames & Moore 1994a) show that during high stages in the river, groundwater flows generally east toward the installation with a southeast downstream component, which reflects the surge of higher floodwaters upstream. During a falling cycle of the river stage, groundwater elevations in the backwater areas of Crooked Slough were generally 2feet above the elevation of the river stage. During this period, groundwater appeared to flow southwest toward the river (Dames & Moore 1994a).
Hydraulic conductivity values for the overburden in Area I ranged from 2.2 × 103 to 5.4×102cm/sec. The average hydraulic conductivity of the overburden was 1.9 × 102 cm/sec. Using this value, a measured gradient of 0.0005, and the averaged measured porosity of the backwater area (42.3percent), the estimated groundwater flow velocity across the backwater area is 23 ft/yr (Dames & Moore 1994a).