Environmental Setting and Receptors 352

Environmental Setting and Receptors 352

1. Topography and Physiography3521

Topographically, the installation is bounded by the Mississippi River to the south and west, and a line of hills and bluffs to the north and east. The topography of the bottomland in the northwestern portion of SVDA is a result of point‑bar deposition of Mississippi River sediment and the development of intervening, meandering channels and sloughs throughout the areas of bottomland. The northwestern section of SVDA consists of relatively flat bottomland that ranges from 588 to 600 feet above mean sea level (msl), with large areas subject to flooding from the Mississippi River during high water. In May 1992, during peak flow of the Mississippi River, the pool elevation upstream of L&D No. 12, adjacent to SVDA, was 595 feet above msl. The backwater elevation within Pool 13, which is downstream from the dam, was 594 feet above msl, which coincides with the average elevation of the backwater area. During the severe flooding in early July 1993, the stage height of the river in Pool 13 reached a maximum of 21.52 feet (601.2 feet above msl), which is approximately the elevation of West Road in the northern half of SVDA (Dames & Moore 1994a).

The remaining upland areas of the installation are composed of gently rolling hills and flat areas, with elevations ranging from 600 to approximately 660 feet above msl. The topography of the upland area was developed mainly by eolian processes reworking sandy river deposits. In the northern section of the installation, these processes have formed a series of dunes. Elevations of the adjacent bedrock hills and bluffs east of SVDA reach up to 1,000 feet above msl and form the plateaus of the Southern Driftless Area (Dames & Moore 1994a).

2. Climatology3522

The National Oceanic and Atmospheric Administration (NOAA) has prepared a climatological summary for Bellevue, Iowa, for the 30‑year period between 1951 and 1980 (NOAA 1980). The most recent annual climatological summary for Bellevue, Iowa, at L&D No. 12 is for the 1994 calendar year (NOAA 1994). During the winter, SVDA experiences frequent snow, and the temperature drops below 0ºF an average of 18 times each year (NOAA 1980). The soil freezes to a depth of approximately 2 feet BLS and may remain snow‑covered for weeks at a time. In contrast, the area experiences a temperature of at least 90ºF an average of 14 days each year during the summer months (NOAA 1980). Extreme heat and humidity rarely last more than a few days because of cool air masses moving southward from Canada (Dames & Moore 1994a).

 The mean total precipitation in Bellevue, Iowa for the period between 1951 and 1980 was 33.17 inches (NOAA 1980). December, January, and February generally are the driest months, while June, July, and August are the wettest (NOAA 1980). Precipitation during the fall, winter, and spring is generally uniform over large areas. Summer is characterized by short, local showers; however, summer thunderstorms may be severe and are sometimes accompanied by hail or destructive wind. Flooding frequently occurs with the breakup of river ice in late winter and early spring, especially if there has been a thick snow cover that has been removed by rain or unseasonably warm temperatures (Dames & Moore 1994a).

Average annual snowfall for the period 1951 through 1980 was 30.4 inches (NOAA 1980). Heavy snows of 10 to 14 inches occur once or twice per year. Prevailing winds tend to pile the snow into high drifts. Moderate to heavy ice storms occur annually. Damaging winds may develop into tornadoes at any time of year, but they are more likely to occur from March through June (Dames & Moore 1994a).

3. Surface Water Hydrology3523

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  Rivers and streams
  
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  Eastcentral swamp area
  
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  Mississippi River backwater areas.
  

The upper Mississippi River, north of St. Louis, Missouri, has been altered into a series of pools by the construction of 29 locks and dams along the 750‑mile course of the river. U.S. Army Corps of Engineers (USACE) Districts at St. Louis, Missouri; Rock Island, Illinois; and St. Paul, Minnesota constructed the L&D system primarily between 1930 and 1940. The purpose of the system is to maintain a dependable (9‑foot deep) channel to accommodate large diesel‑powered river vessels for inland navigation and interstate commerce (Dames & Moore 1994a).

The 13‑mile section of the Mississippi River that borders SVDA to the west and south is designated Pool 13. Most of SVDA is bordered to the west by the upper portion of Pool 13. The extreme northern portion of SVDA is located north of L&D No. 12 and is bordered to the west by the lower portion of Pool 12. Bellevue, Iowa is located at the western end of L&D No. 12, across the Mississippi River from SVDA. The lock at Bellevue was constructed between February 1934 and November 1935. The dam that presently extends from the lock across the backwater area of SVDA was constructed between September 1936 and July 1938. Following the rise of the water elevation in Pool 12 to achieve a 9‑foot channel depth, L&D No. 12 became operational in May 1939 (Dames & Moore 1994a).

The need for operation of the gates in the dam is dependent on regional precipitation and seasonal river stage fluctuations. During high river stages, the gates of the dam are fully open and the river flows freely through the dam with little resistance. During these high water conditions, the Mississippi River water surface conforms closely to its original natural surface. The gates are adjusted, as necessary, to maintain a minimum water depth of 9 feet in the channel of the river during low water conditions (Dames & Moore 1994a).

Surface water runoff from the elevated bluffs east of SVDA drains onto the installation. At least six intermittent streams and one perennial stream are located in the northern and central sections of SVDA. Beaty Hollow is a perennial stream that eventually discharges into Prairie Lake, which is a component of the Crooked Slough backwater complex. An unnamed tributary located north of K Road is an intermittent stream that flows from the northeast uplands, along the Burlington Northern Railroad, and eventually discharges into Straight Slough, which forms the eastern boundary of the Crooked Slough backwater complex. Five other intermittent streams terminate near the vicinity of the SVDA east central boundary in a swampy groundwater recharge area, which is discussed below (Dames & Moore 1994a).

On most of the installation, no pronounced drainage patterns exist. Generally, the western and central portions of the installation drain either directly or indirectly by sloughs into the Mississippi River. Other drainage from the extreme southeastern portion of the installation flows southeasterly along the eastern boundary and into the Apple River. The area in and around the southern portion of SVDA drains north into a shallow, broad depression, where the water either infiltrates the permeable, sandy surface soil or flows into storm drains that empty into the Apple River (Dames & Moore 1994a).

The Crooked Slough complex, which represents approximately 40 percent of the installation, receives runoff from the upper western half of SVDA. The three distinct surface water features of the Crooked Slough complex are Crooked Slough, Straight Slough, and Prairie Lake. Crooked Slough is the principal slough and generally flows south‑southeast through the center of the backwater area. During above‑normal river stage conditions (greater than 594 feet above msl), surface water flows over a 1,400‑foot‑long spillway along the Mississippi River dam that is immediately north and upstream of the Crooked Slough complex. This dam is referred to as L&D No. 12 and separates Mississippi River Pool 12 (north from the dam) from Pool 13 (south and downstream from the dam). Water from the spillway flows into the Crooked Slough complex. In addition, at times of low flow (i.e., when the spillway is not active), surface water from Pool 13 flows into Crooked Slough at the extreme northwestern corner of the backwater area and at the mouth of Crooked Slough in the central depot area (Dames & Moore 1994a).

Straight Slough is the second largest slough and is located along the interface between the upland and backwater areas. This interface is coincident with and parallel to West Road. Numerous groundwater discharge seeps are located along this interface. Straight Slough is the main channel that connects large areas of the eastern SVDA backwater system to the Mississippi River. During rising river stages, floodwater from the river spills into Straight Slough, reversing the surface water flow direction. This reversed flow occurs for a short period until equilibrium is attained between the water level of the Mississippi River and the eastern backwater system.

Prairie Lake is located in the extreme northern section of the backwater complex, just below the dam, and receives runoff from Beaty Hollow. The approximately 10‑acre lake also is subject to seasonal flooding by the Mississippi River, particularly when the spillway is active (Dames & Moore 1994a).

The smaller backwater complex associated with several SVDA sites is located to the south near the mouth of the Apple River. The principal components of this area include Ordnance School Lake and a smaller, separate, unnamed catchment. Ordnance School Lake is bounded to the north and east by the Burlington Northern Railroad, to the south by the Apple River, and to the west by the facilities area of SVDA. The lake is an oxbow feature of the Apple River channel. The approximately 30‑acre water body receives stormwater runoff from the southeastern portion of the installation. The lake is more topographically isolated than most local backwater areas, although it is connected to the Apple River system by a drainage channel located at the southern end of the lake (Dames & Moore 1994a).

A small, unnamed impoundment is located adjacent to the Burlington Northern Railroad, approximately 400 feet north of the railroad bridge that spans the Apple River. The approximately 1‑acre 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 (Site 20) to the north‑northeast of the impoundment (Dames & Moore 1994a).

4. Soil3524

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 (Dames & Moore 1994a).

5. Geology and Stratigraphy3525

The following sections discuss the geology and stratigraphy for each of the three general geologic divisions within the installation.

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 high‑runoff 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 coarse‑grained, moderately well‑sorted, 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 well‑graded glacial outwash material consisting of rounded granitic cobbles, large boulders of dolomite, multicolor gravel, and medium‑ to coarse‑grained 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 brownish‑gray dolomite to fresh, light gray dolomite within the first 5 feet encountered. The elevation of the top of the dolomite averages 450 feet above msl in Area I (Dames & Moore 1994a).

Geology of the Central and Southern Upland Areas – Area II—The topography of Area II 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 well‑sorted, very fine‑ to fine‑grained, windblown (i.e., eolian) sand with a trace of silt. The upper portion of the Parkland Formation consists of 5 to 10 percent silt by weight. These eolian deposits are interbedded with layers of alluvium consisting of medium‑ to coarse‑grained, 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 swift‑moving 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 fine‑grained 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 48.1 percent, with an average porosity of 43.6 percent.

The Parkland sand grades to coarser‑size deposits representing the Henry Formation at approximately 110 feet BLS. The lithology of the Henry Formation within Area II consists predominantly of gravel and medium‑ to coarse‑grained sand. The total thickness of the unconsolidated strata in Area II ranges from 132 to 190 feet, 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 (Dames & Moore 1994a). 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 450 feet above msl and dips 0.7 degrees south‑southwest. These logs indicate the thickness of the Galena Group ranges from 145 feet in the central part to 215 feet 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, well‑sorted, fine sand of wind blown origin typically overlies medium‑ to coarse‑grained 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 yellowish‑brown 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 5 feet (monitoring well 302206) in the northeast to 100 feet (monitoring well 300101) in the southeast.

The elevation of the bedrock surface ranges from 611 to 494 feet 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 yellowish‑brown moderately weathered dolomite. Bedrock cores retrieved during well installation activities show that bedrock in the northwestern portion of Area III 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.

6. Hydrogeology3526

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 × 10^‑4 to 1.1 × 10^‑1 cm/sec. The range of values primarily represents variability of grain size and composition found in the sandy overburden deposits—silty sand to clean, well‑sorted sand. The average hydraulic conductivity of the overburden was calculated to be 3.8 × 10^‑2 cm/sec. In comparison, hydraulic conductivity values of the bedrock range from 9.0 × 10^‑5 to 1.3 × 10^‑2 cm/sec and averaged 4.6 × 10^‑3 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 (Figure 3‑4). 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 (Dames & Moore 1994a).

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 long‑term direction of groundwater flow will be toward the river with a downstream component within the backwater area (Dames & Moore 1994a).

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 2 feet 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 × 10^‑3 to 5.4 × 10^‑2 cm/sec. The average hydraulic conductivity of the overburden was 1.9 × 10^‑2 cm/sec. Using this value, a measured gradient of 0.0005, and the averaged measured porosity of the backwater area (42.3 percent), the estimated groundwater flow velocity across the backwater area is 23 ft/yr (Dames & Moore 1994a).

The slightly elevated and changing topography of the upland areas and the absence of surface water result in a greater depth to the water table than observed in Area I. Based on data from the upland areas, the depth to groundwater in Area II ranges from approximately 5 to 60 feet BLS. Water level measurements indicate that groundwater flows generally northeast and inland during rising river stages. The slope of the water table (i.e., the horizontal gradient) is greatest along the banks of the river and sloughs bordering Area II and diminishes farther inland. Therefore, the direction of groundwater flow is reversed or altered at most of the sites within Area II during seasonal crests of the Mississippi River because of the proximity of the sites to the rivers and sloughs. The structure of the water table in the southern upland area near Sites 15, 33, and 67 appears to form a trough that dips northwest during a high river stage. The direction of shallow groundwater flow is variable from northeast to north in this area during a high river stage (Dames & Moore 1994a).

Shallow groundwater flow during a falling river stage trends southwest, toward the river, throughout much of the central and southern upland areas. In the extreme southern area, water level data indicate that groundwater flows in a radial pattern from south toward the Mississippi River to northeast toward the Apple River. Groundwater flow during low stages of the river in Area II follows a similar trend observed during falling river stages (Dames & Moore 1994a).

Hydraulic conductivity values for the overburden in Area II ranged from 1.6 × 10^‑4 to 1.1 × 10^‑1 cm/sec. The average hydraulic conductivity of the overburden was 4.2 × 10^‑2 cm/sec, slightly higher than the hydraulic conductivity for the overburden in Area I. Using this value, the measured gradients of 0.0004 and 0.001 for the central and southern areas, respectively, and the averaged measured porosity of the upland soil (43.6 percent), groundwater flow velocity was estimated to range from 40 ft/yr, across the central upland area, to 100 ft/yr, across the southern upland area (Dames & Moore 1994a).

Groundwater flow conditions in the extreme northeastern portion of the northern upland area exhibited negligible influence from variable river stages during 1992 (high stage to falling stage). The direction of groundwater flow tended southwest during these two periods. There is a greater influence of groundwater recharge from the perennial streams Beaty Hollow and the unnamed tributary located north of K Road to the groundwater elevations in the surficial bedrock at this location. In addition, vertical leakage through the adjoining upper bedrock aquifers in the upland plateaus recharges the shallow dolomite aquifer. The horizontal hydraulic gradient across Area III is approximately 0.012 in the extreme northeastern portion, but decreases to 0.003 in the southwestern portion closer to the backwater area (Dames & Moore 1994a).

Hydraulic conductivity values were highly variable in Area III, ranging from 1.2 × 10^‑1 to 9.0 × 10^‑5 cm/sec. Generally, hydraulic conductivity values were greater from wells screened in the Parkland Formation than in wells screened within bedrock. The average hydraulic conductivity of the overburden was 4.2 × 10^‑2 cm/sec. The average hydraulic conductivity of the bedrock was 4.6 × 10^‑3 cm/sec. Using the average hydraulic conductivity of the overburden, a measured gradient of 0.003, and the averaged measured porosity of the Parkland sand (43.6 percent), the groundwater flow velocity in the southeastern section of Area III is estimated to be approximately 300 ft/yr through the overburden. Based on the average hydraulic conductivity of the bedrock, a measured gradient of 0.012, and an estimated porosity of 10 percent for dolomite, the groundwater flow velocity in the northeastern section of Area III is estimated to be approximately 570 ft/yr. This calculation of approximate bedrock velocity 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 Area III. Therefore, groundwater flow through such openings may be much faster than indicated by the gradient and bulk hydraulic conductivity (Dames & Moore 1994a).

Regional Aquifers—At least three general bedrock sources of potable groundwater are located in the vicinity of the installation. The first source of groundwater from bedrock is within the Galena Formation of mid‑Ordovician age, which is approximately 120 to 215 feet thick at SVDA and lies below the sandy deposits of the Mississippi River Valley. The fractured dolomite aquifer of the Galena Formation is unconfined and, to varying degrees, is in hydraulic communication with the overlying sand. Just east of SVDA in the upland plateaus of central Jo Daviess County, the aquifer is locally confined by the overlying Maquoketa shale (Dames & Moore 1994a).

At SVDA, the Galena aquifer is recharged by vertical seepage through the sandy overburden deposits. The Galena aquifer also is recharged directly by rainfall in northern Jo Daviess County, where it is exposed at the surface due to the absence of the Maquoketa shale. The piezometric surface contours for the Galena‑Platteville unit indicate a groundwater ridge in northern Jo Daviess County—an area that topographically is the highest in the state (Viscocky et al. 1985). From the groundwater ridge, groundwater flows to the southwest toward the Mississippi River and to the southeast toward the Apple River Valley (Dames & Moore 1994a). The majority of private water wells within 1 mile of SVDA withdraw from the Galena Formation. These wells yield moderate quantities of water that are sufficient for household use (Dames & Moore 1994a).

The main water supply for SVDA is derived from a deeper aquifer, the Cambrian‑Ordovician aquifer (Russell 1963), which occurs at a depth of approximately 340 to 1,400 feet BLS. The aquifer is composed of various formations of dolomite and sandstone, ranging in geologic age from Cambrian to Early Ordovician. The Cambrian‑Ordovician aquifer is a leaky confined aquifer (Russell 1963). From 1971 to 1980, an average of 103 million gallons per day (mgd) was pumped from the Cambrian‑Ordovician aquifer in northwest Illinois. The volume has increased an average of 15.9 mgd every decade (Viscocky et al. 1985).

The Mount Simon aquifer occurs below the Cambrian‑Ordovician aquifer at approximately 1,200 to 1,300 feet BLS. The Mount Simon aquifer is a sandstone aquifer of Early Cambrian age and is separated from the Cambrian‑Ordovician aquifer by a shale unit. The presence of water with high levels of total dissolved solids (TDS) within the Mount Simon aquifer precludes the use of this deeper aquifer for water supply (Dames & Moore 1994a).

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