Attachment Q MODFLOW Modeling to Simulate a Pipe and Trench in a Wetland - Ray Woulo’S Rebuttal Testimony (Schedule B)

Attachment Q MODFLOW Modeling to Simulate a Pipe and Trench in a Wetland - Ray Woulo’s Rebuttal Testimony (Schedule B)

Ex. ENB-10, Wuolo Rebuttal, Schedule B

Technical Memorandum

To: Bobby Hahn, Enbridge Energy Limited Partnership From: Evan Christianson P.G. and Ray Wuolo P.E., P.G. Subject: MODFLOW Modeling to Simulate a Pipe and Trench in a Wetland Date: August 6th, 2020 Project: 23/69-1530

To evaluate the hydrologic effect of a pipe buried in wetland sediments a groundwater flow model was developed using the industry standard modeling code MODFLOW. MODFLOW simulates three- dimensional, steady-state and transient groundwater flow using finite-difference approximations of the partial differential equation of groundwater flow:

where: Kxx, Kyy, and Kzz: are the three principal directions of the hydraulic conductivity tensor W: sources and sinks Ss: specific storage h: hydraulic head t: time MODFLOW was developed by the U.S. Geological Survey and is in the public domain. It is widely used and accepted. The version of MODFLOW used for this model is MODFLOW-2005 (Harbaugh, 2005).

The model was set up to represent a typical pipe and trench configuration within a typical wetland hydrogeologic setting. It represents a three-foot diameter pipe at the base of a nine-foot deep backfilled trench together with the wetland and underlying hydrostratigraphic units. To simulate this, the model domain was subdivided into five different hydraulic conductivity zones allowing for different scenarios to be simulated with varying hydraulic conductivity values for wetland sediments and underlying hydrostratigraphic units. The general model setup and hydraulic conductivity zonation is shown on Figure 1.

The model represents a cross section that is 2000 feet long and 30 feet deep. All finite-difference cells, with the exception of those in model layer 1, are 0.5 feet x 0.5 feet in size. Cells within model layer 1 are 0.5 feet wide and 1.5 feet thick allowing for the water table to fluctuate within layer one without drying out cells which could cause issues with model stability and solver convergence. In total, the model is 4000 columns by 1 row by 57 layers. A gradient was imposed across the model domain using constant head boundary conditions on both the left and right side of the cross section. Two different gradients were

Barr Engineering Co. 4300 MarketPointe Drive, Suite 200, Minneapolis, MN 55435 952.832.2600 www.barr.com

Ex. ENB-10, Wuolo Rebuttal, Schedule B To: Bobby Hahn, Enbridge Energy Limited Partnership From: Evan Christianson P.G. and Ray Wuolo P.E., P.G. Subject: MODFLOW Modeling to Simulate a Pipe and Trench in a Wetland Date: August 6th, 2020 Page: 2 imposed; 0.0001 (approximately 6 inches per mile) and 0.00025 (approximately 16 inches per mile). These gradients are typical of wetland settings in Minnesota.

Thirty scenarios were simulated using the two gradients and a range of potential hydraulic conductivity values for wetland sediments, trench backfill, and underlying geology. Hydraulic conductivity values were chosen based on a range of literature values and slug tests conducted at LaSalle Creek Crossing (Barr 2020). The hydraulic conductivity values and gradients for each of the thirty scenarios is shown in Table 1.

Table 1 Modeled Hydraulic Conductivity and Gradient Values

Scenario Horizontal Hydraulic Conductivity (ft/day) Vertical Anisotropy (Kx/Kz) Horizontal Zone1 Zone2 Zone3 Zone4 Zone5 Zone1 Zone2 Zone3 Zone4 Zone5 Gradient 1 0.05 0.05 - 0.05 20.0 10.0 1.0 10.0 10.0 10.0 0.0001 2 0.05 0.05 - 0.05 20.0 10.0 1.0 10.0 10.0 10.0 0.0001 3 0.0087 0.0087 - 0.009 20.0 1.0 1.0 1.0 1.0 1.0 0.0001 4 0.0087 0.0087 - 0.009 20.0 0.1 0.1 1.0 0.1 0.1 0.0001 5 0.36 0.36 - 0.36 20.0 1.0 1.0 1.0 1.0 1.0 0.0001 6 0.36 0.36 - 0.36 20.0 0.1 0.1 1.0 0.1 0.1 0.0001 7 0.0087 0.0087 - 0.36 20.0 1.0 1.0 1.0 1.0 1.0 0.0001 8 0.0087 0.0087 - 0.36 20.0 0.1 0.1 1.0 0.1 0.1 0.0001 9 0.0087 0.0087 - 0.009 1.0 1.0 1.0 1.0 1.0 1.0 0.0001 10 0.0087 0.0087 - 0.009 1.0 0.1 0.1 1.0 0.1 0.1 0.0001 11 0.36 0.36 - 0.36 1.0 1.0 1.0 1.0 1.0 1.0 0.0001 12 0.36 0.36 - 0.36 1.0 0.1 0.1 1.0 0.1 0.1 0.0001 13 0.0087 0.0087 - 0.0087 0.0087 0.1 0.1 1 0.1 0.1 0.0001 14 5 5 - 0.0087 0.0087 0.1 0.1 1 0.1 0.1 0.0001 15 5 5 - 5 0.0087 0.1 0.1 1 0.1 0.1 0.0001 16 0.05 0.05 - 0.05 20.0 10.0 1.0 10.0 10.0 10.0 0.00025 17 0.05 0.05 - 0.05 20.0 10.0 1.0 10.0 10.0 10.0 0.00025 18 0.0087 0.0087 - 0.009 20.0 1.0 1.0 1.0 1.0 1.0 0.00025 19 0.0087 0.0087 - 0.009 20.0 0.1 0.1 1.0 0.1 0.1 0.00025 20 0.36 0.36 - 0.36 20.0 1.0 1.0 1.0 1.0 1.0 0.00025 21 0.36 0.36 - 0.36 20.0 0.1 0.1 1.0 0.1 0.1 0.00025 22 0.0087 0.0087 - 0.36 20.0 1.0 1.0 1.0 1.0 1.0 0.00025 23 0.0087 0.0087 - 0.36 20.0 0.1 0.1 1.0 0.1 0.1 0.00025 24 0.0087 0.0087 - 0.009 1.0 1.0 1.0 1.0 1.0 1.0 0.00025 25 0.0087 0.0087 - 0.009 1.0 0.1 0.1 1.0 0.1 0.1 0.00025 26 0.36 0.36 - 0.36 1.0 1.0 1.0 1.0 1.0 1.0 0.00025 27 0.36 0.36 - 0.36 1.0 0.1 0.1 1.0 0.1 0.1 0.00025 28 0.0087 0.0087 - 0.0087 0.0087 0.1 0.1 1 0.1 0.1 0.00025 29 5 5 - 0.0087 0.0087 0.1 0.1 1 0.1 0.1 0.00025 30 5 5 - 5 0.0087 0.1 0.1 1 0.1 0.1 0.00025 Note: Zone 3 is inactive (no-flow boundary condition; impermeable) for simulations with the pipe present. For simulations where the pipe is not present Zone 2 and Zone 3 are set equal to Zone 1.

P:\Mpls\23 MN\69\23691530 L3 Replacement & Deactivation\WorkFiles\_15_L3 Replacement\100_General Env Support\Contest Case\Wetland_Trench_Groundwater\Wetland_Cross_Section_GW_Model_Memo_d2.docx Ex. ENB-10, Wuolo Rebuttal, Schedule B To: Bobby Hahn, Enbridge Energy Limited Partnership From: Evan Christianson P.G. and Ray Wuolo P.E., P.G. Subject: MODFLOW Modeling to Simulate a Pipe and Trench in a Wetland Date: August 6th, 2020 Page: 3

For each scenario, two steady-state simulations were conducted; one without the pipe and trench present and one with the pipe and trench present. The difference in simulated hydraulic head between these two simulations was calculated to determine the change in hydraulic head caused by the pipe and trench backfill. The hydraulic head in layer 1 is the phreatic surface (i.e. the water table). For simulations that represent those without the pipe and trench, Zone 2 and Zone 3 hydraulic conductivity values were set equal to Zone 1.

Figure 2 to Figure 31 show the results of the thirty scenarios. Table 2 summarizes the change in the water table immediately up gradient and immediately down gradient of the pipe and trench.

As shown in Table 2, model results indicate that depending on the hydraulic conductivity of the wetland deposits and the underlying hydrostratigraphic units, the pipe and trench are estimated to increase the water table by 7.0E-7 feet to 2.6E-4 feet on the upgradient side of the pipe and trench and decrease it by the same range of magnitude on the downgradient side of the pipe and trench. The maximum change is estimated to occur for Scenario 29 which represents a flow condition with a high hydraulic gradient, high permeability shallow wetland sediments and trench materials, and low permeability materials at depth. In this scenario, the shallow sediments in which the pipe is placed act as a high flow layer, so the pipe has the largest effect on the water table. The minimum change is estimated to occur for Scenario 8 which represents a flow condition with a low hydraulic gradient, low permeability shallow wetland sediments and trench materials, and high permeability materials at depth. In this scenario, the shallow sediments in which the pipe is placed act as a low flow layer, so the pipe has minimal effect on the water table.

Table 2 Simulated Water Table Changes due to the Pipe and Trench

Scenario Change in Water Table Immediately Up Change in Water Table Immediately Down Gradient of the Pipe and Trench (ft) Gradient of the Pipe and Trench (ft) 1 +5.7E-05 -5.7E-05 2 +5.7E-05 -5.7E-05 3 +3.2E-05 -3.2E-05 4 +7.2E-06 -7.1E-06 5 +3.3E-05 -3.3E-05 6 +7.8E-06 -7.8E-06 7 +1.0E-05 -9.6E-06 8 +7.0E-07 -6.9E-07 9 +3.2E-05 -3.2E-05 10 +7.5E-06 -7.5E-06 11 +3.8E-05 -3.8E-05 12 +1.6E-05 -1.6E-05 13 +2.3E-05 -2.2E-05 14 +9.7E-05 -9.6E-05 15 +3.1E-05 -3.1E-05 16 +1.5E-04 -1.5E-04 17 +1.5E-04 -1.5E-04 18 +8.6E-05 -8.6E-05 19 +1.9E-05 -1.9E-05 20 +8.7E-05 -8.7E-05

Scenario Change in Water Table Immediately Up Change in Water Table Immediately Down Gradient of the Pipe and Trench (ft) Gradient of the Pipe and Trench (ft) 21 +2.1E-05 -2.1E-05 22 +2.7E-05 -2.7E-05 23 +1.9E-06 -1.8E-06 24 +8.6E-05 -8.6E-05 25 +2.0E-05 -2.0E-05 26 +9.9E-05 -9.9E-05 27 +4.3E-05 -4.3E-05 28 +6.0E-05 -5.6E-05 29 +2.6E-04 -2.5E-04 30 +8.2E-05 -8.1E-05

References

Barr Engineering. 2020. Geotechnical Data Report. Line 3 Replacement, LaSalle Creek Crossing, Clearwater and Hubbard County, Minnesota. Prepared for Enbridge Energy, Limited Partnership, May, 2020.

Harbaugh, A.W., 2005, MODFLOW-2005, The U.S. Geological Survey modular ground-water model—the Ground-Water Flow Process: U.S. Geological Survey Techniques and Methods 6- A16.

Zone 2 Zone 1 9’

Zone 3

7’

Zone 4 11’

Zone 5 10’

2000’ Full width not shown HYDRAULIC CONDUCTIVITY ZONES

Note: Not to scale FIGURE 1 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 1

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 2 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 2

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 3 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 3

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 4 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 4

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 5 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 5

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 6 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 6

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 7 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 7

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 8 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 8

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 9 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 9

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 10 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 10

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 11 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 11

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 12 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 12

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 13 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 13

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 14 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 14

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 15 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 15

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 16 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 16

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 17 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 17

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 18 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 18

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 19 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 19

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 20 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 20

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 21 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 21

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 22 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 22

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 23 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 23

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 24 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 24

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 25 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 25

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 26 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 26

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 27 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 27

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 28 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 28

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 29 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 29

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 30 Ex. ENB-10, Wuolo Rebuttal, Schedule B

SCENARIO 30

Note: Only model output near the pipe is shown. Further from the pipe simulated head change diminishes FIGURE 31