PDF p. 38 (lines 972-984, 1021-1031)
The geology has been investigated in detail nearby in Cottage Grove.
972 Detailed studies of surficial glacial deposits and underlying basement have been conducted in the
973 Village of Cottage Grove near Madison, Wisconsin (Meyer 2016; Harvey et al. 2019). The
974 Cottage Grove site is analogous to Truax Field due to the proximity of the two sites (less than
975 10 miles apart) and their common setting with respect to the Milton Moraine (both roughly 10-15
976 miles northeast of the moraine). As discussed in Exhibit 2, the characteristics of the facies
977 architectural elements found at the Cottage Grove site (Harvey et al. 2019) are likely pertinent to
978 understanding the hydrostratigraphic controls (preferential flow pathways and barriers to flow)
979 for PFAS fate and transport at Truax Field. Hydraulic properties of each architectural element
980 will vary and can depend on the parent material deposited by glaciers and fractures. The
981 proximity of the Cottage Grove site to Truax Field suggests that many of these features
982 (architectural elements that make up the subsurface of ice marginal land systems) may exist in
983 the study area and play a role in determining the fate and transport of any PFAS plume in the
984 area.

at line 1139 the workplan shows output from a model constructed for the Cottage Grove site,
Exhibit 4 Hydraulically Constrained Geologic CSM at the Cottage Grove Site
Previous investigations at Truax itself also indicate a complex geology
1021 Geologic cross sections and maps included with previous investigations for Truax Field indicate
1022 that the bedrock surface topography and geologic formations that subcrop in the vicinity of the
1023 Base are highly variable, suggesting the pre-glacial landscape was dominated by incised
1024 dendritic drainage. The Mt. Simon Formation is believed to underlie Truax Field with a thickness
1025 of approximately 350 ft and other formations likely subcrop below downgradient areas
1026 (Figure 10-5). Previous investigations (Advanced Sciences, Inc. 1994) have suggested that high
1027 angle basement faults and associated fracture zones (Exhibit 3) may have been active in the area
1028 after deposition of the Paleozoic sequence. The existence and possible significance of these
1029 structural elements in the location of preglacial dendritic drainage patterns, the variable thickness
1030 of glacial deposits, or hydrogeology of the area is unclear and will be further evaluated as part of
1031 the RI.

Truax geologic cross
        section
A 1950 State Journal article provided by Maria Powell documents the initial failure to drill well #8 because it was located in the ancient gorge where the upper bedrock layers had been eroded and replaced with glacial drift.

This complexity leads to large variations in the ability of soils to transmit water
1053 The three unlithified hydrogeologic units show high variability of hydraulic
1054 conductivity. The study noted that k-values range over approximately 4 orders of magnitude for
1055 the sand and gravel deposits, approximately 3.5 orders of magnitude for the silt and clay
deposits, and approximately 5 orders of magnitude for the sandy diamicton (Bradbury et al. 1999).
1102 Based on information collected during previous investigation activities, MWs within the water
1103 table zone at the Base indicate shallow groundwater flow at the Base has changed over time as
1104 discussed below. The water table is generally encountered at depths of 5-10 ft bgs; groundwater
1105 flow velocities have been estimated at less than 1 ft per day. Comparison of groundwater
1106 elevation contour maps developed for previous investigations at the Base are illustrated in Figure
1107 10-7. [see PDF file page 79] In January 1993, the groundwater flow direction was to the southeast based on a large set
1108 of data from wells distributed across the Base (Advanced Sciences, Inc. 1994). In June 2010, the
1109 groundwater flow direction was to the northwest and in October 2010, flow was bifurcated along
1110 an apparent groundwater divide to the northwest and southwest based on a subset of the original
1111 well network located in the central portion of the Base (MWH Americas, Inc. 2011).
1112 Environmental reports associated with studies conducted at the Truax Landfill and Former Burke
1113 Wastewater Treatment Plant (WWTP) south of the Base indicate that groundwater flow
1114 directions within the unlithified aquifer in the surrounding area vary based on localized
1115 conditions. For example, preferential flow paths at the former Burke WWTP have been
1116 attributed to subsurface stormwater infrastructure (Section 10.9.3.2) and radial flow in the
1117 vicinity of the Truax Landfill has been observed due to a mounding effect from the Landfill
1118 (Section 10.9.3.3).
Geologists often refer to this using the term heterogeneous. In such cases, it is common for the vast majority of the contaminants to be moving through a very small percentage of the plume cross section.

Large variations over short distances call for methods generally referred to as High Resolution Site Characterization (HRSC) or the Triad approach. It requires sophisticated logging tools and many samples over short vertical intervals.

***SIWP2 line 2305 describes HPT/EC/groundwater borings
Collection of discrete grab groundwater samples for PFAS analysis at various depth
2319 intervals to provide nature and extent data for PFAS in the aquifer (laterally and
2320 vertically). Preference will be given to collecting samples in zones that are indicated to be
2321 of higher hydraulic conductivity.

At the ground surface, the sample points are usually in an arc or straight line perpendicular to the groundwater flow, called a transect. In the Truax map below they are represented by violet parallel lines labeled with capital letters:

Fig 17-1 Sampling
      Locations

As samples are analyzed, the consultant will look at the samples at the ends of the line. If the contaminant levels are above a specified screening level, they will 'step out' by extending the transect so as to capture all of the plume.

HRSC also calls for sophisticated modeling tools. One example which is suitable for the glacial till under Truax is Environmental Sequence Stratigraphy (ESS), which was developed at great expense for the oil industry.

PDF p. 64 (line 1883)
10.10.1.2 Truax Field-Specific Fate and Transport Considerations
Several observations were made in addition to the ones I already listed in the parent document.
1893 • Groundwater flow direction from PRLs relative to other potential off-Base sources has
1894 implications for source attribution and potentially co-mingled plumes.
1895
1896 • Groundwater delineation is most important to the southeast, southwest, and northwest of
1897 the Truax Field, based on observed changes to regional groundwater flow over time and
1898 the potential for groundwater-surface water interaction in these directions.
1899
1900 • Well 15 to the southeast (within 1 mile), other municipal water supply wells, and shallow
1901 potable water supply wells could have historically (or actively be) influencing
1902 groundwater flow direction and/or downward migration.
1903
1904 • The 2016 Groundwater Flow Model for Dane county, Wisconsin (Parsen et al. 2016)
1905 illustrates that groundwater drawdown occurs in both the overburden and bedrock
1906 aquifers in the vicinity of Truax Field, which could have implications for plume
1907 migration downward from surface water and/or the unlithified aquifer to the bedrock
1908 aquifer.
1909
1910 • Review of local well drillers’ logs indicate that clay-rich glacial till is common in the
1911 vicinity of the Truax Field; however, thick intervals of sand and gravels are frequently
1912 noted and may present preferential pathways for vertical or lateral migration where
1913 present.
Bottom line: there are a lot of variables potentially affecting groundwater flow within and downstream (as the groundwater flows) of Truax.