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The Dow Chemical Company Midland, MI 48674

November 15, 2013 Ms. Mary Logan Remediation Project Manager U.S. Environmental Protection Agency, Region 5 77 West Jackson Chicago, IL 60604 Re: Technical Memorandum from Tetra Tech on Geomorphic Surfaces and Proxy

Process for the Tittabawassee River Floodplain Settlement Agreement No. V-W-10-C-942 The Tittabawassee River/Saginaw River & Bay Site Dow Submittal Number: 2013.075 Ms. Logan:

Attached please find a Technical Memorandum from Tetra Tech on Geomorphic Surfaces and Proxy Process for the Tittabawassee River Floodplain (Tech Memo). The Tech Memo was developed as part of the Administrative Settlement Agreement and Order on Content (AOC) AOC CERCLA Docket No. V-W-10-C-942 and Appendix A, Section 8 of Statement of Work (SOW) of the Settlement Agreement. Please let me know if you have any questions. Sincerely,

Todd Konechne The Dow Chemical Company Project Coordinator CC: Al Taylor, MDEQ

Diane Russell, U.S. EPA Joseph Haas, U.S. Fish and Wildlife Steve Lucas, Dow Peter Wright, Dow Mary Draves, Dow Kip Cosan, Dow

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November 15, 2013

To: Todd Konechne, The Dow Chemical Company From: Dave Richardson, Tetra Tech Pat McGuire, Tetra Tech Jen Holmstadt, Tetra Tech  SUBJECT:    Geomorphic Surfaces and Proxy Process

Tittabawassee River Floodplain Midland, Michigan

Introduction River systems are complex and dynamic because many physical variables influence flow, erosion, and deposition. Fluvial geomorphology is a discipline that studies river formation, evolution, and function (energy distribution). Geomorphic analysis considers those physical parameters, including surface features, erosion, and deposition that influence the distribution of river sediments. The discipline is suited for identifying, delineating, and understanding river system contamination subject to fluvial processes (e.g. contaminants adsorbed to sediment). Fluvial geomorphology applications also provide a basis for spatially extrapolating contaminant concentration from an area of known concentration to an adjacent area of unknown concentration. Hereafter, this process is defined as a proxy process (the function to substitute for another). The proxy process may be used where geomorphic mapping and a soil sampling strategy cannot be conducted due to sample location selection or limited property access. The proxy process considers site geomorphology, soil profile characteristics, and existing analytical data. The information from an area with analytical lithologic soil data is used to extrapolate contaminant concentrations to areas where access for sampling has been limited or denied. This memorandum describes the process used along the Tittabawassee River floodplain to map geomorphic surfaces, determine geomorphic polygons, and apply toxicity equivalent quotient (TEQ) values to floodplain areas that require proxy values. The validity of the proxy process was tested by conducting a site-specific blind proxy evaluation and was determined to be a conservative approach for assigning TEQ values to geomorphic surfaces not sampled in the floodplain.

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Delineation of Geomorphic Surfaces Background Data Review A floodplain geomorphic surface is an area formed by similar physical factors related to morphology and time (e.g., elevation, floodplain configuration, and deposition/erosion environment). Fluvial geomorphology provides a basis for supporting the development of depositional or erosional environments, and therefore contaminated sediment distribution, using multiple lines of evidence. Each line of evidence is evaluated independently to develop an understanding of its effect on deposition or erosion. For the Tittabawassee River where the contaminant of concern is associated with graphitic particles, the multiple lines of evidence can be merged to support geomorphic interpretation of river system contaminant distribution. This method also provides a means to identify inconsistencies and data gaps that may require additional review or data collection. The following are the lines of evidence used to determine the depositional pattern and geomorphic surfaces:

Aerial photographs (recent and historic) One-foot contour interval maps Channel longitudinal profile (gradient) Surface aspect Geomorphic setting Soil profile characterization Water velocity Water depth Channel width Valley width Land use Anthropogenic impacts (e.g., channel armoring, dams, bridges, dredging)

The first step in mapping the geomorphology of the Tittabawassee River floodplain was to delineate geomorphic surfaces based on elevation change. Aerial photographs and one-foot topographic contours were used to support the development of preliminary geomorphic surface boundaries. Several topographic factors were considered when delineating geomorphic surfaces using contours. Steep elevation changes were identified by tight groupings of contours and were used to indicate two different geomorphic surface boundaries. The best example for Tittabawassee River elevation change is the steep slope associated with the valley wall between the geomorphic surfaces in the floodplain and the upland area outside of the floodplain. Subtle changes in topography were also used in the delineation. For example, broad surfaces of relatively uniform elevation were delineated as the same geomorphic surface. Figure 1 shows an example of an upland scarp and the subtle contour changes for a low surface. Meandering contours not parallel to the river usually indicated a tributary stream. Anthropogenic impacts were sometimes apparent on the topographic map. It was possible to identify areas of urban development based on unnatural right angle topographic contours. A good

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example was on the southwest bank in Reach O where a broad upland area (identified by the tight contours) was urbanized. Buildings and roads were also apparent in the contours. Figure 1 provides an example of urban development. Information related to geologic history and past anthropogenic activities were reviewed to understand past influences on the fluvial process. Historical information was obtained from public records and published literature sources. The information reviewed in the preliminary geomorphic analysis, including the historical aerial photographs, was used to support the historical analysis. Industrial and urban development, structures (e.g. dams, channel alignment, rip rap), and watershed condition, including land use change from agricultural or logging activities, were incorporated in the geomorphic surface mapping. Field Confirmation The desktop review and creation of geomorphic surface maps required field confirmation to improve the accuracy of the geomorphic surface boundaries. Contours mapped from aerial photographs were less reliable when obtained from forested areas. Since neither ground elevation nor tree height can be determined using aerial photographs, the contours were interpreted. The field confirmation focused on the terrace or geomorphic surface scarps by evaluating elevation change on the geomorphic surface and vegetation change. Anthropogenic features were identified or confirmed in the field. These features are important because of the effect they have on the deposition/erosion patterns. Figure 2 shows the geomorphic surface mapping in Reach O. Soil Sample Location Selection Geomorphic soil sample locations were based on the understanding that the deposition, erosion, and contaminant distribution are the result of known physical processes. The geomorphic sampling approach focused sampling to areas with high potential for contamination and/or high spatial contaminant concentration variability. This approach is more efficient and provides more informative data than a uniform sample location grid. Transect and sample locations are based on geomorphic surface types, which are representative of the depositional/erosional environment. Small isolated surfaces were not included in a sampling transect. Figure 2 shows the soil sampling transects in Reach O. Soil Sampling – Soil Profiles The last step to define the geomorphic surface boundary was the soil profile description. Geomorphic surfaces with similar origins, flood history, and weathering characteristics develop similar soil profiles. A separate unique geomorphic surface was mapped with change in the soil profile, soil development factors, and deposition pattern. Soil profile characterization, including color, texture, structure, plasticity, organic content, mottling, and other special features were used to differentiate horizons and classify geologic origin (e.g., till, lacustrine). These descriptions and available analytic data were used to correlate soil horizons with contaminant concentration ranges. These descriptions provided a means to establish depth of contamination or a basis for defining degree of contamination associated with

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distinct soil profile horizons. For example, Pleistocene glacial deposits (i.e., till) often functioned as the natural lower boundary of possible contamination because they pre-date contaminant release. The soil profile descriptions were used to finalize the geomorphic surface area boundaries. The fluvial processes responsible for forming geomorphic surface areas also influence the floodplain soil profile. Therefore, a geomorphic surface area resulting from deposition should also have a soil profile that is consistent with deposition. Geomorphic Surfaces and Contaminant Distribution The mapping of geomorphic surfaces to define depositional patterns in the Tittabawassee River floodplain is a science-based approach that uses multiple lines of evidence. Understanding the depositional pattern provides confidence in floodplain contaminant distribution assessment. This confidence was used to develop the sampling plan for the floodplain and interpret the soil and contaminant data. Geomorphic Polygons A surface that is topographically uniform may not be geomorphically uniform. Geomorphic polygons are developed by evaluating soil profiles and the spatial contaminant concentration data from a previously established geomorphic surface area thus defining polygons of similar deposition and contaminant concentrations. A geomorphic polygon represents one geomorphic surface, but a geomorphic surface could be segregated into more than one geomorphic polygon. An example of a low geomorphic surface with segregated polygons due to differences in soil profile and TEQ concentration is provided in Figure 3. The channel width in Figure 3 is constricted near the bottom of the figure. The channel constriction results in diminished flow capacity, ponding, and inundation of the low surface area during flood stage. The ponding during flood stage promotes sediment deposition on the low surface. The spatial distribution of sediment and associated contaminants throughout the low surface from flooding is influenced by proximity to the channel constriction, the tributary, and the main channel. Therefore, the topographically uniform low surface may be represented by two or more polygons based on differences in soil profile and TEQ concentration that were influenced by the depositional pattern. For each geomorphic polygon, a surface average concentration (SAC) value was calculated based on the surface TEQ samples collected within the polygon. If no samples were collected from a geomorphic surface, the SAC geomorphic polygon was assigned a proxy TEQ, which was based on the TEQ SAC from a similar geomorphic polygon in the vicinity. Proxy Process The objective of collecting soil samples along a transect was to define the floodplain depositional pattern. This purpose was achieved without sampling every geomorphic surface. The soil sample transects were perpendicular to river flow and spatially positioned based on the geomorphic surface and river configuration (e.g. channel width, valley width, river sinuosity, or

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anthropogenic features) and not a specified distance between transects. Figure 2 shows three sampling transects. The transects do not intersect every geomorphic surface, so every surface was not sampled. The proxy process was designed to extrapolate the TEQ data from one geomorphic polygon to a similar geomorphic polygon where TEQ data does not exist. Thus by applying the proxy process, all geomorphic polygons are populated with a TEQ value either by analytical data or the proxy process. A geomorphic polygon may not have TEQ data and require a proxy value due to limited property access or if the geomorphic surface did not require sampling to assess the depositional pattern. The majority of the floodplain geomorphic polygons that required a proxy value were due to limited property access. The selection of the geomorphology polygon to obtain a proxy TEQ value was based on the following factors:

Particle size (sand, silt, or clay soil matrix) Similar geomorphic surface (e.g., floodplain, low surface, natural levee) Similar side of the river Proximity to the river Similar river reach Geomorphic setting

- Sinuosity - Anthropogenics - Valley and channel width

If more than one geomorphic polygon met the criteria, the polygon with the highest TEQ concentration was selected. This provided a conservative value for the proxy polygon. Blind Proxy Analysis The blind proxy analysis was used to evaluate the effectiveness of the proxy process in estimating the surface TEQ concentrations where samples were not collected and/or not analyzed. The blind proxy analysis was developed to address the validity of applying proxy values to un-sampled areas. The analysis began with selecting a target project site area in a reach of the Tittabawassee River that could be blinded to apply the proxy process. Reference areas on either side of the target area were identified to provide a method for comparison after the blind proxy target area was complete. The target area selected was Reach O (Figure 4). The reference areas were Reaches N and P (Figure 4). The soil information and TEQ data for Reach O were not available to the evaluator during this blind study. The reference area data was also not provided to the evaluator. The target area data was limited to the topographic map with contours and boundaries of the river reach. The geomorphologist (evaluator) used this information to develop geomorphic surfaces and TEQ concentration bins for each geomorphic polygon. Field confirmation of the

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geomorphic surfaces was not conducted and soil descriptions or TEQ data were not provided for the target area. The blind proxy process started with geomorphic surface mapping using topographic contours (see Figure 5). The topographic maps were constructed from one-foot contours, which were used to support development of geomorphic surface boundaries. Several topographic factors were considered when delineating geomorphic surfaces using surface elevation contours. Slope was the primary indicator used to delineate surfaces. After slope, subtle changes in topography were identified. For example, broad surfaces of relatively uniform elevation were delineated as the same surface. Depressions (identified by hash marks) were identified as low surfaces or wetlands.

Geomorphic surface types were based on their spatial and topographic relationships including topography, proximity to the river, and the elevation differences between the surfaces (Figure 5). The lowest surfaces in the overbank were categorized as geomorphic wetlands based on topography. The other surfaces directly related to topography were low surfaces, intermediate surfaces, high surfaces, and uplands. These surfaces were denoted by increasing elevations relative to each other. Tributaries and levees were categorized primarily by their geomorphology. Tributaries were easily identifiable streams or channels that intersect the Tittabawassee River. They have distinct topographic signatures such as meandering, tightly grouped contours. Levees were usually rounded at the crest, and often occur in multiples, which have a distinct ridge and swale topographic signature. Red arrows in Figure 6 show where inconsistencies occurred between the blind proxy mapping and the reference areas.  

The blind study required the evaluator to select the appropriate TEQ bin for the geomorphic surfaces in the target area based on predefined TEQ parts per trillion (ppt) bin values (e.g., <100 ppt TEQ, 101-1000 ppt TEQ), shown in Figure 7. Bin values for proxied polygons were selected based on the evaluator’s knowledge of how contaminants are distributed across surfaces in the floodplain. For example, generally high surfaces away from the channel have low TEQ concentrations. Therefore, the “<100 bin” was selected for polygons in the target area that fit this description. This completed the blind proxy process. Table 1 and Figures 8 and 9 provide the results of the blind proxy test. The results are provided on a polygon basis. The blind proxy results indicate a good comparison between the blind and the actual data with 77% of the surface area (acres) correctly identified. When the blind proxy was incorrect the TEQ value was typically overstated, which is conservative compared to the actual values. The evaluator understated the TEQ bins in only three polygons. This understatement applies to seven of the 79 acres. The three understated polygons were only one TEQ bin lower than the actual value. The TEQ SWAC analysis for the entire Reach O target area indicates the blind proxy was conservatively higher than the actual TEQ values by 115 ppt (Table 1). The blind proxy process was based on the development of geomorphic surfaces using multiple lines of evidence. Delineation of two or more polygons within a geomorphic surface was based on available TEQ concentration data and soil profile descriptions. Geomorphic interpretation was used to support the process and apply proxy values to un-sampled geomorphic surfaces.

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Table 1 Blind Proxy Analysis Results

Polygon Reconciliation

Area (acres) Area (%) Comments

Agreement 25 61 77

Overstated 17 11 14 (12 off by 1 bin) (5 off by 2 bins)

Understated 3 7 9 (3 off by 1 bin)

Total 45 79 100

Proxy Analysis*

Actual Data*

Reach O Surface TEQ SWAC 762 ppt 647 ppt

*Calculated based on median bin values

Summary and Conclusions

The proxy process started with the use of multiple lines of evidence and the geomorphic surface mapping. The geomorphic surfaces were confirmed during the field confirmation task. The soil sample transects were selected based on changes in the geomorphic surfaces and other river characteristics. The geomorphic surface boundaries were finalized using the soil profile data. The geomorphic surfaces were used to interpret the floodplain depositional pattern. Available analytical data and soil profile descriptions were used to define geomorphology polygons on the geomorphic surfaces. The proxy polygons were identified during the geomorphology polygon process. A geomorphic polygon may require a proxy value because property access was not granted for sampling or because geomorphic surfaces were not included within the sampling transect to determine the depositional pattern. The majority of the geomorphic polygons in this study that required a proxy were due to limited property access. The blind proxy analysis was used to validate the proxy process. The results of the blind analysis demonstrated that the proxy process was conservative. When more than one geomorphology polygon could be applied to the proxy polygon, the polygon with the highest TEQ concentration was selected for the proxy value.

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The proxy approach was appropriate and conservative to address the TEQ values for geomorphic surfaces that were not sampled. As a result, the nature and extent of the floodplain has been adequately characterized for the purposes of the Tittabawassee River Floodplain Response Proposal.

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