Comments on Geophysical Investigation at the Meral Farm ... · Comments on Meral Farm ERI Investigation 24 Oct 2018 . 1 . Comments on “Geophysical Investigation at the Meral Farm

Comments on Meral Farm ERI Investigation 24 Oct 2018

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Comments on

“Geophysical Investigation at the Meral Farm located in Mable, Minnesota” prepared by 3D Geophysics, 9675 Summit Place, Chaska, MN 55318

dated 11 September 2018

Comments by

E. Calvin Alexander, Jr.Morse-Alumni Professor Emeritus

Earth Sciences Department University of Minnesota Minneapolis, MN 55455

24 October 2018

Introduction:

The 11 September, 2018, Electrical Resistivity Imaging (ERI) survey Letter Report (Report) contains information from three sub parallel ERI lines at the site of the proposed Catalpa Concentrated Animal Feeding Operation (CAFO) barns. The Report describes an ERI survey using only one array geometry (dipole-dipole) instead of the two or three different arrays that are the professional standard in this type of survey. The results are presented in the form of three 2D cross-sections. No attempt is reported to combine the three 2D cross-sections into a 3D interpretation of the site. The Conclusions section of the Report makes no serious attempt to note or explain any of the significant features of their three cross sections. The Report identifies only one feature in the cross sections -- the heavy black line separating the surficial materials above from the “weathered bedrock” below. The bedrock formations involved are not named or discussed. There is no reference to, apparent knowledge of, or interest in the extensive literature on the karst processes that have affected the “weathered bedrock” beneath the site.

The three 2D cross-sections contain abundant evidence that this site is an active karst area. That evidence directly contradicts the Report’s final conclusion, “Based on the 2D resistivity data and a physical examination within the study area we do not interpret an active Karst hazard that should preclude construction activities at the site.” (Report, p. 4, Conclusions, 1st paragraph).

Background:

Martin Larsen, Olmsted County SWCD, has already commented on the Report. His excellent, perceptive comments are copied below, with his permission, along with his figure as Fig. 1. I agree with and support all of Martin’s comments and his interpretation in Fig. 1. Fig. 1 is a composite of the Report’s Fig. 3 through 6 with Martin’s annotations.

1. Usually, we do more than one “line” geometry. With that said, we determine jointtrends in the area and lay out the line spacing and direction accordingly. The

p-ear2-144p


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lines should be “hash-tagged” over the site. This gives a better image of the site. ERI images are rendered “slices” under the line electrodes.

2. Only one array was run – Dipole – dipole. I like to see multiple arrays run per line. There are two commonly used. Dipole – dipole, and Wenner-Schlumberger. The different arrays can give different images.

3. Only one electrode spacing was used. I like to see multiple configurations. Close electrodes (5ft) give more detail closer to the surface but can’t interpret as deep. Further electrodes see deeper with a little less resolution.

4. It is clear a weathered joint was discovered by this ERI. I have attached a crude sketch with the joint identified. A weathered joint IS a karst feature! Is it a hazard to limit construction of the barn? It is unclear. Infiltrating 5 acres of storm water near a weathered bedrock joint CERTAINLY CAN induce an active sinkhole. If this site is to proceed with construction, I do not support a storm-water infiltration basin.

Figure 1. Martin Larsen’s interpretation of the ERI data set. (Martin Larsen, e-mail, 26 Sep 2018).

Martin’s first three points emphasize that this minimal ERI survey is significantly less rigorous than the standard of many ERI studies of CAFO sites in SE Minnesota. His fourth point highlights the complete absence of any attempt to bring the third dimension into the Report’s interpretation, identifies a very


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significant feature, and points out the specific high risk associated with the proposed storm water infiltration point. I will return to the last part of Martin’s fourth point later in this critique.

The Minnesota Geological Survey staff estimate (written communication, 2018) that the St. Peter Sandstone/Shakopee Formation contact at an elevation of about 335 m (1100 ft) at the Catalpa barns proposed location and the Shakopee/Oneota Formation contact is at about 314 m (1030 ft). The “weathered bedrock” in the ERI cross sections in the Report site consists of the bottom part of the St. Peter Sandstone, all of the Shakopee Formation and the top part of the Oneota Formation. Both the St. Peter/Shakopee and the Shakopee/Oneonta contacts are major unconformities in the geological column. The tops of the Oneota and the Shakopee were subjected to long periods of karst weathering after they were deposited and before the overlying formations were deposited. Both contacts are significant paleo karsts with significant secondary porosity and permeability. The current surface and ground water processes are reactivating and enlarging that karst porosity and permeability.

Fig. 2, 3 and 4 below are annotated versions of the three ERI cross-sections from the Report. Only the colors, the heavy black wavy lines and the three black “weathered bedrock” labels were in the original cross sections. Note that the “Resistivity” scale bars are log arithmetic -- they span three orders of magnitude from 1 to 1000 ohm-m. The vertical blue arrows, the light blue lines and the other labels were added.

Figure 2. Annotated version Fig. 4 of 3D Geophysics Meral Farm Letter Report.


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Figure 3. Annotated version Fig. 5 of 3D Geophysics Meral Farm Letter Report.

Figure 4. Annotated version of Fig. 6 of 3D Geophysics Meral Farm Letter Report.

Inaccurate, Incomplete and Clearly Wrong Statements in the Report:

(A) The color coded resistivity values are defined as: “the near surface sediments are conductive clays/silts and are shaded green while the weathered bedrock is the most resistivity material and is shaded yellow, orange and red.”(Report p. 4, sect. 4. Results, 2nd para.)

The resistivity of soils above the top-of-bedrock line range from red orange discontinuous patches of high resistivity surface soils through yellow sub soil through discontinuous green deeper sediments and then back through yellow colors at the interpreted sediment/bed rock interface. The surface soils are relatively high resistivity and patchy. The lower resistivity subsoils are discontinuous

The colors below the top-of-bedrock lines range from high resistivity reds through oranges and yellows up to the light green area of weathered bedrock near the south end of middle cross section.


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The resistivity range both above and below the top of bedrock lines overlap and is almost the same -- from green to red orange

(B) “The resistivity results suggest the weathered bedrock surface is relatively flat and resides at a depth 4-6 meters (13-20 ft) below the surface of the site.” (Report p. 4, sect. 4. Results, 3nd para.)

The operative words here are “relatively flat”. There actually are many two to four meter depressions in the top-of-bedrock line as drawn and those lines could easily have been drawn with even more, deeper depressions in the line. In Fig. 2, 3 & 4 the blue arrows point to some of the many locations where the depressions in the top-of-bedrock lines correlate with color changes suggesting the downward movement of less resistive materials. The evidence of downward movement indicates that the depressions in the “relatively flat” top-of-bedrock lines are recording filled sinkholes or solution enlarged vertical joints (such as the one identified in Martin Larsen’s comments above) which can easily be reactivated by the construction and subsequent operations of a CAFO at this location.

(C) “The low resistivity values of the near surface soils suggest a high clay content …” (Report p. 4, sect. 4. Results, 3nd para.)

The surface soils in the three ERI lines are patchy red orange. They are high resistivity, not low resistivity materials. The statement in the Report is incorrect.

“suggest the surface layer is a clayey material, which probably has limited infiltration of surface water.” (Report, p. 4, Conclusions, 1st paragraph)

The surface soils at this site are well drained. The recent backhoe excavations the site revealed well developed macropores that effectively drain the soils.

“The site also has a significant thickness of weathered bedrock which would also limit surface water infiltration.” (Report, p. 4, Conclusions, 1st paragraph)

I agree that the site is underlain by a significant thickness of weathered bedrock. However, that weathered bedrock does not and will not “limit surface water infiltration”. That phrase is completely wrong and reveals a total lack of understanding on how karst landscapes operate. The weathering of the St. Peter Sandstone and Shakopee and Oneota Carbonates dramatically increases the porosity and permeability of those bedrock formations and is the reason the Prairie du Chien is an important regional aquifer. The clearly evident weathering has enhanced and facilitated surface water infiltration -- which in turn significantly increases the probability of creating new karst features and or reactivating paleo karst features -- in a powerful positive feedback loop.

(D) “No evidence of active surface sinkhole development is interpreted in the resistivity data recorded during this investigation.” (Report, p. 4, Conclusions, 1st paragraph)

This statement is correct -- this report indeed does not interpret any evidence of active surface sinkhole development at the site. However, the Report’s interpretation only reflects that the Report does not adequately interpret the data it contains. The Report ignores abundant evidence that karst processes are ongoing at the site, and that building and operating a CAFO at this site has a high risk of sinkhole formation. Their unjustified interpretation also does not mean that the operation of a CAFO at this site will present no risk of groundwater contamination with or without a new sinkhole forming.


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“800 Pound Gorillas” apparently invisible to the Report’s authors:

OPDC High Transmissivity Zone (OPDC HTZ): One of the fundamental parts of the karst hydrogeology of the Ordovician Prairie du Chien in all of southeastern Minnesota is the ubiquitous regional network of anastomosing karst conduits of the strata -- the OPDC HTZ. The OPDC HTZ is a sub horizontal network of solution cavities (usually partially filled with soft sediments) that rapidly transports water through the aquifer. All three of the Report’s ERI cross sections image various parts of the OPDC HTZ beneath the proposed Catalpa Site but these features are not mentioned in the Report. These features are the yellow to orange areas outlined with blue lines in Fig. 2-4. These features connect the paleo karst at the St. Peter/Shakopee through the Oneota as deep as the ERI survey measured.

Fig. 5 below is a photograph of the OPDC HTZ as exposed in quarry near Plainview, Minnesota. The scale of the photo is comparable but a little larger than the scale of ERI sections in Fig. 2-4. The photo illustrates what the “weathered bedrock” visible in the ERI sections actually looks like. The resemblance between Fig. 5 and Fig. 3 is particularly striking. Note also the “relatively flat top-of-bedrock” exposed by the quarrying operation in Fig. 5. A “relatively flat top-of-bedrock” is fairly typical and does not indicate or imply the absence of karst cavities in the underlying weathered bedrock.

Figure 5. Photo of cross section of the Prairie du Chien High Transmissivity Zone. The photo, complements of the Minnesota Geological Survey, was taken in a quarry near Plainview Minnesota. This feature is a mappable unit throughout the Prairie du Chien formation in Minnesota. There is a person for scale in the left side of the photo.

Numerous waste water treatment lagoons, water retention dams and other water impoundments built on top of this type of karst weathered bedrock in Southeastern Minnesota have catastrophically failed. The solution enlarged joint that Martin Larsen identified above serves as the vertical connection between the surface sediments and the horizontal OPDC HTZ in the bedrock. Note that in Fig. 2 and 4, the yellow OPDC HTZ material’s extent all the way through the bedrock imaged. These connections provide rapid pathways for contaminants to move quickly, deep into the bedrock aquifers.

The Large Green Low Resistivity Cavity at the south end of Line 2: The Report makes no mention of the large light green low resistivity area near the south end of line 2. This feature is a large pocket of low resistivity material in the weathered bedrock underlying the general area of the proposed


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storm water infiltration pond at the Catalpa Site. (This feature is relevant to Martin Larsen’s 4th point above.) Building a storm water infiltration pond on top of weathered karst bedrock is a high risk proposition. Proposing to build a storm water infiltration pond on top of a large mapped cavity in weathered karst bedrock is unconscionable, and probably an actionable geotechnical error.

There are three well documented ways that human activities can induce catastrophic sinkhole development:

1) Land surface by construction activities in karst landscapes: The land surfaces in karst regions are delicate, dynamic equilibriums between depositional and erosional processes. Moving surface soils, creating fills, excavating cuts, creating impervious surfaces, and adding roofs alter those equilibriums and have a long history of inducing sinkhole collapse.

2) Impounding water, particularly in water bodies with permeable bottoms in karst landscapes: Lakes, lagoons, ponds or the simple discharge of water onto the land surface also have a long, well documented history of inducing sinkhole collapse.

3) Pumping water out of aquifers under karst landscapes: High capacity wells routinely induce sinkhole collapse by lowering the buoyant support of the overlying karst landscapes.

The construction of a storm water infiltration pond adjacent to the barns at the proposed Catalpa Barns (with excavated pits beneath them) and near a new high capacity water supply well will be a triple threat for catastrophic sinkhole collapse.

A better approach in karst areas would be to collect precipitation run off from barn roofs via gutters into large cisterns and use that collected precipitation water on site to avoid a storm water retention pond and reduce the amount of groundwater pumping required for a CAFO.

The numerous vertical columns of reduced Resistivity Penetrating the Top-of-Bedrock: The blue vertical arrows in Fig. 2-4 show vertical columns of reduced resistivity sediments and bedrock (relative to the materials on both sides of the columns). These columns represent rapid vertical pathways for enhanced downward infiltration from the surface, through the sediments and down to the OPDC HTZ and below. These columns correlate with depressions in the top-of-bedrock lines in the Report’s cross sections and with the intersections of the ERI lines with Martin Larsen’s inferred solutionally enlarged joint.

There are no obvious surface sinkholes in the area covered by the three ERI lines. But the ERI cross sections contain clear evidence of a dense array of subsurface karst features which will provide rapid, direct infiltration connections between the land surface and the bedrock aquifers. There is no reason to assume that the proposed Catalpa site is any different from the proposed manure spreading fields. Manure spread on those fields will rapidly infiltrate to the groundwater under those fields. There is no reason to assume that moving the locations of the barns short distances in any direction will significantly improve the situation. The entire Catalpa Project Area is not an appropriate place for a large CAFO.

Conclusions: 1. The 3D Geophysics ERI Report contains clear evidence that the extensive karst weathering of the

St. Peter, Shakopee and Oneonta bedrock beneath the Catalpa/Meral Farm site renders the site unsuitable for a CAFO. By extension, the proposed spreading fields also present a high risk for


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groundwater pollution. This project presents an unacceptable risk to the environment and should not be permitted.

2. If the proposal is not rejected or subject to an Environmental Impact Statement, the most

dangerous component is the proposed storm water infiltration pond near the south end of the barns. Under no circumstances should an infiltration pond be built anywhere near the barns.

3. If the proposal is not rejected it should be subject to an EIS which should require a much more

extensive geotechnical site investigation. That extensive investigation needs to be focused on the weathered karst bedrock beneath the site. The next step should be deep drilling to investigate the karst weathered bedrock. The unconsolidated surficial sediments are not the problem. The first target should be the large low resistivity anomaly near the south end of the middle ERI line. Soil borings to “refusal” (the first hard object hit by the soil borer) are not adequate to identify voids in the underlying bedrock. The problems are the karst weathering voids and sediment or water filled cavities in the bedrock. The drilling will need to extend to depth of 35 to 40 meters into the bedrock and should be Rotosonic or some other technique of drilling capable of recovering core from both solid bed rock and unconsolidated cavity fillings.

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Comments on Geophysical Investigation at the Meral Farm ... · Comments on Meral Farm ERI Investigation 24 Oct 2018 . 1 . Comments on “Geophysical Investigation at the Meral Farm