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Journal of Environmental Quality 32:490-499 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Ground Water Quality

Surface and Subsurface Geologic Risk Factors to Ground Water Affecting Brownfield Redevelopment Potential

Martin M. Kaufman*,a, Kent S. Murrayb and Daniel T. Rogersc

a Dep. of Earth and Resource Science, Univ. of Michigan-Flint, Flint, MI, 48502
b Dep. of Natural Sciences, Univ. of Michigan-Dearborn, Dearborn, MI, 48128
c Amsted Inc., 205 N. Michigan Ave., Chicago, IL, 60601

* Corresponding author (martyk{at}umflint.edu)

Received for publication May 2, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 STUDY AREA
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A model is created for assessing the redevelopment potential of brownfields. The model is derived from a space and time conceptual framework that identifies and measures the surface and subsurface risk factors present at brownfield sites. The model then combines these factors with a contamination extent multiplier at each site to create an index of redevelopment potential. Results from the application of the model within an urbanized watershed demonstrate clear differences between the redevelopment potential present within five different near-surface geologic units, with those units containing clay being less vulnerable to subsurface contamination. With and without the extent multiplier, the total risk present at the brownfield sites within all the geologic units is also strongly correlated to the actual costs of remediation. Thus, computing the total surface and subsurface risk within a watershed can help guide the remediation efforts at broad geographic scales, and prioritize the locations for redevelopment.

Abbreviations: SIC, Standard Industrial Classification


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
STUDY AREA
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
BROWNFIELDS ARE abandoned, idled, or underutilized industrial or commercial facilities where expansion or redevelopment is complicated by real or perceived environmental contamination. There are approximately 500 000 brownfield sites in the United States alone, which have an estimated remediation cost of several billion dollars (Buehlman et al., 1998). Although the concern over brownfield sites has traditionally been the toxicity level of the contamination present, a second factor—the geologic environment—is equally if not more important, and is generally not taken into account.

The process of brownfield contamination results from a sequence of anthropogenic and natural activities and processes involving a contaminant source, its mobilization through the soil (transport action), and its ultimate site of deposition (sinks). Sources of contamination are the human activities performed primarily on the surface that release toxic substances into the environment. These substances may be transported over time, or stay relatively near their release location. Because contaminants released into the environment are often mixtures of several and occasionally hundreds of chemical compounds (e.g., gasoline), an understanding of the physical chemistry of each of the chemical constituents and the geologic environment to which they are released is necessary to evaluate their fate and transport. After transport, these compounds remain at different locations or sinks for different time periods. Examples of sinks include the near-surface soil and ground water, where contaminants will be transported at rates dependent on the local aquifer characteristics to other sinks, including inland lakes and streams or the ocean.

The contamination at a brownfield site results from the substances released on and just below the surface by human activity and their interactions with the near-surface soils and shallow aquifer (Foster, 1987; Daly and Warren, 1994). From an environmental perspective, the redevelopment potential at a given brownfield site is thus dependent on the extent of the contamination present and the textural characteristics of the near-surface soils.

In a recent study of ground water vulnerability within the Rouge River watershed in southeastern Michigan, Murray and Rogers (1999) evaluated the geologic and contaminant characteristics at more than 400 sites of environmental concern. More than 90% of the sites with ground water contamination derived from anthropogenic sources were located on either glacial outwash, moraines, or a beach deposit, while fewer than 1% of the sites with ground water contamination were located on the clay-rich glacial lacustrine or lodgment till units. Murray and Rogers (1999) also found that it was significantly more expensive to clean up and develop properties with ground water contamination than to redevelop those sites without ground water contamination. The average cost to remediate sites located on the outwash, moraine, or beach deposits was $57 273 per kilogram of contaminant, while the cost to remediate sites located on the less sensitive clay-rich units averaged only $127 per kilogram of contaminant. Fortuitously, the majority of metropolitan Detroit's estimated 10 000 brownfield sites are located on these clay-rich deposits (Michigan Department of Environmental Quality, 2002).

An extensive amount of literature exists on the economic and environmental aspects of brownfield redevelopment (Singer, 2000; Irwin, 2001). However, a systematic evaluation of the existing and potential risks within an environmental geology framework has yet to be performed. Redevelopment potential is conceived here as a function of surface risk, subsurface risk, and the extent of contamination present at the site. This research develops a framework to characterize brownfield redevelopment potential by assessing the known risk from the geologic properties present and the potential risk from the human surface activities that may affect a site. A model developed from this framework is then tested at specific sites, with the efficacy of the model evaluated on its ability to predict the relative costs for parcel remediation.


   STUDY AREA
TOP
 ABSTRACT
 INTRODUCTION
 STUDY AREA
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The study area is the Rouge River watershed in southeastern Michigan (Fig. 1) . This watershed is the most populated in Michigan, with 1.5 million residents and high levels of urbanization. In 2000, 99% of the watershed's population lived within the U.S. Census–defined urbanized areas, and the population density was higher than that of any other watershed in the eastern United States.



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  		Fig. 1. Study area with study sites shown within the geologic units.

 
Water quality in the Rouge River is severely degraded. In recent surveys, the Rouge River was ranked as one of the worst toxic sites in Michigan (Michigan Department of Environmental Quality, 2002). Fecal coliform is one of the most pervasive water quality problems and results from the rural runoff, contaminated ground water, and combined sewer overflows at more than 150 sites throughout the watershed (Murray et al., 2001). Sampling over time has also shown a significant presence of heavy metals in both the sediment and water column of the Rouge River (Michigan Department of Natural Resources, 1988; Murray, 1996; Murray et al., 1999). Within the humid microthermal climate zone of the Midwestern United States the stream system is effluent, meaning ground water is recharging stream channels through baseflow. Thus, any significant contamination of ground water will ultimately be delivered to the surface water drainage network (Murray et al., 2000).

Bedrock Geology
The study area is located on the southeastern edge of the Michigan Basin, a 316 000-km2 area composed of sedimentary rocks, primarily limestones, shales, and sandstones. These sedimentary rocks are Paleozoic in age, and rarely exist as natural outcrops because of the presence of a thick deposit of glacial drift in the region. Beneath the study area, the Paleozoic rocks range from 425 to 730 m thick and gently dip toward the center of the basin to the northwest. The depth to bedrock ranges from more than 110 m in the northwestern portion of the study area to less than 15 m in the southeastern (Rieck, 1981a).

Sediments of Pleistocene age overlie the Paleozoic rocks of the Michigan Basin. This forms an unconformity where deposition had not occurred for approximately 280 million years. This unconformity indicates that following the late Paleozoic, the Michigan Basin was uplifted, exposed, and eroded (Dorr and Eschman, 1988).

Surface Geology and Hydrogeology
Surficial geology in Michigan is dominated by glacial sediments that are typically more than 60 m thick and, at some locations, more than 300 m thick (Rieck, 1981a). These glacial sediments were deposited during the Pleistocene Epoch by the Wisconsinan stage of glaciation, and consist of outwash, moraine, and beach, bar, and lake deposits (Farrand, 1988). Varied and complex lithologies are exhibited within these deposits, which include coarse gravels, fine-grained sands, and clays (Bergquist and MacLachlan, 1951; Mozola, 1969; Rieck, 1981a,b).

Five distinct near-surface geologic units have been identified within the study area (Leverett, 1911; Sherzer, 1916; Farrand, 1982; Rogers, 1996). The units are classified by their composition and include moraine, sandy clay, sand, sandy and silty clay, and clay (Fig. 1). Summaries of the key hydrogeologic features of ground water within each unit are based on the results of investigations at more than 500 contaminated sites, with the following classification adopted from Rogers (1996).

The moraine unit contains interbedded sands, gravels, and clays with occasional glacial erratics. Surface elevation within this unit ranges from 290 m in the extreme northwestern corner of the study area to 250 m along its contact with the sandy clay unit. The thickness of this unit is generally less than 45 m. Ground water encountered within this unit exists under both confined and unconfined conditions. Multiple zones of saturation exist, ranging in thickness from 1 to >10 m. Public and private entities use the ground water within this unit as a potable water source.

Southeast of the moraine unit is a unit of sandy clay. This unit consists of light-brown to gray sandy clay with occasional pebbles. Surface elevation within this unit ranges from 215 to 250 m. In the northern part of the study area this unit is generally less than 2 m thick, but thickens to 6 m in the southern portion. Unconfined and discontinuous ground water exists in this unit, with insufficient water present to be pumped at a sustained rate.

The sand unit is characterized as a moderate yellowish-brown to light olive-gray fine to coarse-grained quartz sand that becomes finer with depth. This textural change with depth is attributed to its deposition during lake regression. The sand is 90% quartz. At many locations, this unit is stratified and has well-developed, cross-bedding ripple marks, and scour and fill features. Localized evidence of reworking and eolian deposition is also present within the upper portion of the sand unit. Surface elevation of this unit ranges from 195 to 220 m, and its thickness varies between 1.5 and 10 m; generally increasing to the east. Ground water in the sand unit is unconfined with saturated thickness varying from 1 to >6 m. Due to its location near the ground surface, recharge of this unit is probably from precipitation. Based on recovery wells installed at contaminated sites, the average yield from the sand unit is 0.02 to 0.04 m3 min-1. Given this adequate rate, the unit has historically been used as a source of potable water.

East of the sand unit is the sandy and silty clay unit. This unit is characterized as a medium to light olive-gray, mottled, sandy to silty clay bearing well-rounded pebbles. Discontinuous fine-grained sand lenses are present within this unit, especially along its western margin with the Rouge River. Well-developed varves have also been observed along the western margin of this unit, where it is interbedded and beneath the sand unit. Individual varves range in thickness from less than 0.25 to nearly 0.65 cm and typically extend vertically for 15 cm. The thickness of the unit ranges from 3 to 6 m, with a higher sand content present along its contact with the sand unit. Surface elevation within this unit ranges from 190 to 195 m. Ground water encountered within this unit is unconfined or semiconfined and, where present, exists within the discontinuous sand lenses, at the contact with the lower clay unit, or in the reworked river deposits along the Rouge River.

The clay unit forms the eastern boundary of the study area. This unit consists of mottled bluish to medium light olive-gray clays, with trace amounts of well-rounded pebbles. Vertical or nearly vertical hairline fractures are present within the clay unit. These fractures are caused by stress changes from wetting and drying cycles and/or freezing and thawing (Freeze and Cherry, 1979; Murray et al., 1997). The fracture frequency decreases with depth. Along the western margin, the clay unit is 3 m thick, and it thickens to more than 10 m along its eastern margin with the Detroit River. Surface elevation within the unit ranges from 175 to 190 m. The characteristics of ground water within this unit are similar to those found within the sandy and silty clay unit.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 STUDY AREA
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
To assess brownfield redevelopment potential, the contamination process is presented within a spatial and temporal framework. This framework facilitates the identification of risk factors and their quantification. The critical risk factors become the model parameters, which are then measured at 32 sites of known contamination.

Model Development
Table 1 presents the conceptual framework for evaluating brownfield redevelopment potential in terms of spatial and temporal characteristics. Cells of the matrix represent the risk factors arising from the interaction of human activities and geologic characteristics present at different spatial scales and time periods. The ensuing discussion is structured by the row entries of the matrix and describes the conversion from the concepts to the model parameters.


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  		Table 1. Conceptual risk assessment framework for brownfield redevelopment.

 
Surface Risk
Surface risk evaluations have traditionally used spatially generalized categories of land use, such as "industrial, commercial, or residential" to represent various levels of risk (Barringer et al., 1990; Eckhardt and Stackelberg, 1995; Secunda et al., 1998). The use of general land use categories is problematic for ground water vulnerability assessment because of their inadequate spatial resolution: spatial resolution is defined as the smallest identifiable element in a sequence (Tobler, 1988). In urban and urbanizing areas, mixed land uses within small areas such as city blocks are common, so the variable risks may be obscured by generalization when the capture zones for wellhead protection are delineated. For example, the "10-yr capture zone" is the subsurface and surface areas from which water (and any contamination it carries) will reach the well over a time period of 10 yr.

To demonstrate this problem of inadequate spatial resolution, we can consider a water well in an area designated as residential land use (Fig. 2) . Within this zone of low risk is a single industrial establishment engaged in metal plating. Metal plating activities exhibit a high incidence rate of ground water contamination, but this specific risk is masked by the generalization of the area within the capture zone as a lower-risk residential category. This is shown as the "mixed effect" (Fig. 2a).



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  		Fig. 2. Effects of different spatial resolutions across multiple land use types on surface risk assessment.

 
In Fig. 2b, two adjacent, but different zones create edge effects. At the edge, the residential zone becomes exposed to the higher risks associated with industrial land, but the sources and amounts of the actual risk from the industrial land near the edge are unknown. In Fig. 2c, the use of the Standard Industrial Classification (SIC) code for each establishment within a general land use category permits an improved parcel-level spatial resolution of the relative risks to ground water. The SIC code is a four digit code: the first two digits identify a major group, such as agriculture, retail trade, and manufacturing; the third digit denotes industry groups within each major group, such as agricultural crop production; and the fourth digit identifies a specific industry code, such as grain storage (United States Office of Management and Budget, 1987).

A comparable measure of risk between different establishment types is achieved through the use of contamination incidence rates (Kaufman, 1997). Incidence rates are obtained by (i) assigning a SIC code to each source of contamination appearing on the current Michigan Environmental Response Act list (Michigan Department of Environmental Quality, 2002); (ii) obtaining the total number of establishments in Michigan for each SIC code from the County Business Patterns (United States Bureau of the Census, 1997); and (iii) dividing the number of contaminated sites with a specific SIC code by the total number of establishments in Michigan with that same SIC code. To scale the scores equivalently to the subsurface risks computed in the next section, these rates are multiplied by 10 and converted to scores between 0 and 10 and summed for a circular area around each water well. This total score represents the relative risk of the surface activities.

Figure 3 shows an example surface risk calculation for a brownfield site located in the sand unit. Here, the computed risk includes only those establishments contained within the sand unit, since the hydraulic conductivity is greatly reduced when crossing into the adjacent sandy clay and sand and silty clay units. The legend box shows the SIC and risk scores for three of the many firms within the radius; for example, SIC 2822 represents a firm producing synthetic rubber with a risk score of 4.00. This particular risk is computed by dividing the two synthetic rubber firms currently on the Michigan Response Act List by the total of five synthetic rubber establishments in Michigan, and then multiplying by 10. The risk scores for the other establishment types shown are computed similarly. The dots in the legend box indicate there are many more scores to add within the radius to obtain the total at the bottom.



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  		Fig. 3. Calculation of surface risk using the risk values computed for specific establishment types within the capture radius around a contaminated brownfield site.

 
Circular areas may be effective for modeling wellhead protection areas under small regional hydraulic gradients and low ground water velocities (Barringer et al., 1990; Camp and Outlaw, 1998). Darcy's Law is used to define the radius:

[1]
where Q = discharge (m3 d-1), K = hydraulic conductivity (m d-1), I = hydraulic gradient (m m-1), and A = area perpendicular to flow (m2).

Dividing each side of Eq. [1] by area (A) yields specific discharge (V) in m d-1:

[2]

Dividing KI by the effective porosity ({eta}) gives the average linear flow velocity, which is the average rate at which ground water moves through the subsurface. For example, the hydraulic conductivity for a sand aquifer may be 0.3 m d-1. Given a hydraulic gradient of 0.1 m m-1 and an effective porosity of 0.1, the water would move 110 m per year, or 1100 m in 10 yr. In this case, the well radius used for the 10-yr capture zone would be 1.1 km. Implicit in this analysis is a pumping rate that induces an average hydraulic gradient of 0.1 m m-1 in all directions toward the pumped well. In turn, this also assumes a relatively level water table or potentiometric surface before pumping begins, and predominately horizontal ground water flow.

The hydrogeologic parameters of each geologic unit within the study area were used, which were the best data available. Table 2 shows the hydrogeologic parameters and the results of the radius calculations. There is a high degree of variation in the hydraulic conductivity (K) and effective porosity ({eta}) for each geologic unit within the study area. Field studies (Rogers, 1993) and literature values (Peyton, 1986; Newell et al., 1990) were used to select the values for K and {eta}. Slope (I) is a natural gradient and was computed for each geologic unit by dividing the elevation difference between the western and eastern margins by the average width. The capture radius selected for this study needed to account for the duration of contamination and remediation efforts often present at brownfield sites. Unlike the wells used for the delineation of potential contamination zones in wellhead protection studies, brownfield sites that are used in this study were already contaminated. In the industrial U.S. Midwest many of these sites have been contaminated since the 1950s, with a large percentage abandoned during the 1970s and 1980s. Under the Resource Conservation and Recovery Act (RCRA) (Pubic Law 94-580; 42 U.S.C. 6901–6992; 90 Stat. 2795), standard practice for monitoring ground water is 30 yr after a site is evaluated as a candidate for closure. This generally applies to treatment, storage, or disposal sites of wastes regulated under RCRA, and the monitoring period is applied only after closure, not while the facility is operating. Thus, a conservative radius of 20 yr was employed. This long duration also accommodates ground water flow that may reach the near surface at brownfield sites unaided by pumping. As shown in Fig. 3, each brownfield site is considered as the center of a circular capture zone.


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  		Table 2. Hydrogeologic parameters and 20-yr capture radius.

 
After the radius was determined, an automated procedure for the surface risk assessment was performed. This was achieved by (i) geocoding a file containing the street addresses of public, commercial, and industrial establishments "firms" coded by their SIC and (ii) using the capabilities of a geographic information system (GIS) to sum the risk scores for each firm within the specified radius on a digital map image (Kaufman, 2000).

Subsurface Risks (Ground Water Vulnerability)
The concept of ground water vulnerability originated in France during the 1960s and was introduced into the scientific literature by Albinet and Margat (1970). Ground water vulnerability has since evolved to include both a distinction between and combination of vulnerability and risk assessment. General vulnerability of an aquifer is seen as a function of the natural properties of the overlying soil, unsaturated zone, and rock column (Foster and Hirata, 1988; Robins et al., 1994).

To measure ground water vulnerability, Murray and Rogers (1999) have developed a numerical rating system using a modified form of DRASTIC (Aller et al., 1987). The model uses weighting coefficients similar to the DRASTIC model for the geologic and hydrologic parameters (Table 3).


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  		Table 3. Vulnerability matrix of subsurface risks.

 
The first parameter shown in Table 3 is the depth to the shallow water table aquifer within the glacial deposits. The second parameter, occurrence of ground water, is based on data collected from sites of environmental concern within the study area (Rogers and Murray, 1997). For example, a geologic unit with a less than 25% occurrence of ground water means there is less than a one in four chance of encountering ground water in a well drilled into this unit to a depth of 6 m. The third and fourth parameters, areas of ground water recharge and discharge, provide a means of understanding the horizontal and vertical components of ground water movement within the study area. Both of these parameters are related to Parameters 5 and 6, the textural characteristics and travel time, which can be used to assess soil and solute transport properties of the near-surface geologic materials. The seventh parameter, source of potable drinking water, refers to whether the geologic unit is, or can be, a source of drinking water.

The weighted values for each geologic unit are summed to determine a final relative index of unit sensitivity (Table 4). These values may represent a composite score based on a unit's heterogeneous materials. For instance, for Parameter 5 (composition), the moraine is classified as silt with a midpoint value of 5 because of its mixture of sand, gravel, and clay. The sand and moraine units are substantially more vulnerable to ground water contamination than the other units in the watershed because they are composed of highly permeable sediments with ground water present and are in areas of the watershed subject to ground water recharge.


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  		Table 4. Sensitivity scoring of subsurface risk Parameters 1 through 7 (from Table 3).

 
On-Site and Off-Site Risks
Four main categories of contaminants have been detected in the study area: (i) volatile organic compounds (VOCs), including dense and light nonaqueous phase liquids (DNAPLs and LNAPLs); (ii) polynuclear aromatic hydrocarbons (PNAs); (iii) polychlorinated biphenyls (PCBs); and (iv) metals (Rogers, 1996). In many cases, the original contaminant release at the site affected ground water. If over time these contaminants are transported away from the site by ground water, then the amount of risk increases because the spatial extent of the contamination increases.

Model Specification and Operationalization
Brownfield redevelopment potential is a composite variable consisting of surface risk, subsurface risk, and the extent of contamination present at the site. The model can be specified as:

[3]

Surface risk scores are derived from the scaled incidence rates of firms that might contribute contamination within a specified well radius. The well radius is adapted to the aquifer's hydraulic conductivity and a stated capture time. An automated procedure is used to compute the risk scores for a well located within each brownfield site. Subsurface risks are computed from a modified version of the DRASTIC model adapted to the specific features of the study area. Surface and subsurface risks are considered additive because of the direct hydraulic and hydrologic connections between the surface infiltration that transports contamination and the receiving ground water areas. In the absence of any studies to suggest otherwise, this is also the most conservative approach. Since the actual extent of contamination at brownfield sites is often unknown, a linear multiplier is used. A value of 1 is assigned if soil alone is contaminated, and a value of 2 used if contamination is present in both ground water and soil. The model will be run with and without the multiplier because it is considered exploratory and is designed to assess its effects on the model's explanatory power.

Data Collection
To capture the inherent variability of contaminants within urban soils, a spatially dispersed sample set was collected over a period of several years. Project files from subsurface investigations conducted at more than 3000 known or suspected sites of environmental contamination were reviewed. A subset of 32 sites was selected, including locations within all the geological units and development patterns within the watershed, but randomness or proportionality to area was not achieved due to the limited availability of remediation costs (Fig. 1).

Soil samples were collected at all sites by using either: (i) a stainless-steel hand trowel, (ii) a 0.6- or 1.2-m steel sampler, which was hydraulically pushed into the ground with a device called a Geoprobe (Geoprobe Systems, Salina, KS), or (iii) a steel split-spoon sampler, which was pounded into the ground at various depths with a truck-mounted drill rig equipped with hollow-stem augers. The analytical results from the soil testing were used to determine the presence and magnitude of contamination through a total priority pollutant scan for organics and inorganics; then if present, to assess the nature and extent of the contamination, or to confirm if remediation was effective. Table 5 describes the analytical test methods used for metals, polynuclear aromatic hydrocarbons (PNAs), volatile organic compounds (VOCs), and polychlorinated biphenyls (PCBs).


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  		Table 5. Test methods (USEPA, 1993).

 
Data Analysis and Model Verification
The means of the redevelopment potential scores of the different geologic units were compared with a one-way analysis of variance (ANOVA). Significant differences between the geologic unit means were tested with the Tukey–Kramer honestly significant difference (HSD). Simple linear regression was used to verify the model by exploring the association between redevelopment potential (with and without the extent multiplier) and the actual costs of remediation. All statistical tests were performed at the 0.05 level of significance.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 STUDY AREA
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
The results of the evaluations performed at the 32 sites are shown in Table 6. Lower surface risk scores are associated with the smaller 20-yr capture radii within the clay, sandy and silty clay, and sandy clay units. Surface risk scores increase substantially within the sand and moraine units possessing much larger 20-yr capture radii. Even before the addition of geologic risk and the extent multiplier, it is clear there are distinctions in risk between these geologic units based on whether clay is present. A one-way ANOVA confirms this observation (F = 825.9, R2 = 0.99, p < 0.001), and is also supported by the Tukey–Kramer HSD test identifying those pairs of means most significantly different (Table 7). Positive values indicate the pairs that have significantly different means. The mean risks of the sand and moraine units are significantly different from all three geologic units containing clay.


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  		Table 6. Site risk and cost evaluation.

 

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  		Table 7. Mean differences using Tukey–Kramer honestly significant difference (HSD).{dagger}

 
The relationship between redevelopment potential and the actual costs of remediation was tested to help evaluate the risk model. Simple linear regression was used with total risk and cleanup cost as the independent and dependent variables, respectively. A logarithmic transformation of both variables was performed due to the small sample size and the non-normal distributions of the variables resulting from their extreme values. The results suggest a strong amount of association between these two variables (F = 41.4, R2 = 0.70, p < 0.001, n = 20), indicating that the cleanup costs at a contaminated site will generally increase with increasing amounts of surface risk and geological risk. The equation for this relationship is:

[4]

When the model was tested with the extent multiplier, there was a slight improvement in its explanatory power (F = 52.2, R2 = 0.74, P < 0.001, n = 20), with this resulting equation:

[5]

The study area is a watershed with an older urban core and newer suburban development on the fringe. Many of the older sites currently contaminated within the clay-related geological units have been contaminated for many years—and often longer than the new suburban sites—yet remain relatively inexpensive to remediate. It would incur less environmental risk to locate new development within those areas with less vulnerability to ground water contamination, and be less costly to redevelop existing brownfield sites within the clay-related layers.

This study has implications for land use planning and regional water resources. Urban sprawl has been an ongoing issue in southeastern Michigan. Figure 4 shows the extent of urban sprawl in the region from 1965 to 1995. Redevelopment of brownfield parcels within the older urban area (shown in the figure as the lighter shaded area) may reduce the amount of sprawl. Many of these parcels have good accessibility to major highways and rail lines. Another issue accompanying urban sprawl in this region has been seasonal water shortages. The Detroit water distribution network serves over 100 communities, and during peak demand periods from 1995 to 2000, pressure problems have created shortages that necessitated rationing. In addition, the expansion of urbanized areas to the headwater regions of the watershed is occurring within those geologic units possessing the highest ground water reserves and known yield. Additional development on the urban fringe and any accompanying contamination may threaten a valuable water resource, which has conjunctive use potential to augment the surface water supply during periods of peak demand.



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  		Fig. 4. Urban growth in southeastern Michigan, 1965–1995. Data from Southeast Michigan Council of Governments (1999).

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 STUDY AREA
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Attempts to redevelop brownfields can use the results from this study to guide remediation efforts at broad geographic scales. Mapping the patterns of geologic sensitivity throughout an urbanized region can prioritize locations for redevelopment. Geographic information system analysis can be extended to include soil types to assess the potential for phytoremediation, which relies on plant root systems to reduce heavy metal concentrations.

Within the Rouge River watershed, industrial development of greenfield sites located on materials ranked as highly vulnerable to ground water contamination is outpacing brownfield redevelopment, despite the potential for the degradation of a potable water resource. These decisions are being made, in part, because of the perception that the development of older brownfield sites within the City of Detroit would be a long and costly process resulting in continued liability. This study and others have demonstrated that older brownfield sites in the City of Detroit are typically located on soils with a much lower vulnerability to ground water contamination and are therefore far less costly to develop. A comprehensive approach to urban redevelopment should include ground water vulnerability studies. These studies can become a basic component of the land use planning process, much as environmental site assessments are to the real estate industry.

Due to the wide variability of aquifer parameters and the uncertainty associated with the extent of the actual contamination, there are opportunities to improve this model. Additional research can characterize the nature and extent of contamination to compute a more robust multiplier, and the inclusion of other parameters, such as those affecting the breakdown of organic contaminants in the vadose zone, may also prove beneficial. It would also be constructive to investigate brownfield locations within other combinations of geology and land use. This will enable a more comprehensive assessment of the relationships between site redevelopment potential and the risk factors at and below the surface.


   ACKNOWLEDGMENTS
 
The authors thank the Michigan Department of Career Development-Employment Service Agency for their permission to use a confidential file of firm addresses and SIC codes. Michigan law mandates that this information does not explicitly, or by inference, permit the identification of any firm.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 STUDY AREA
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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JEQ 2003 32: 377-382. [Full Text]  




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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome