Modelling Approach Demonstration

To demonstrate the complete modelling approach taken in this study, this section illustrates the internal workings of the model for time-lapse ERT monitoring of two monitoring steps of a DNAPL remediation scenario. Figure 4-2 presents a schematic to summarize the modelling approach and corresponding images. The simulated scenario involves the release of the chlorinated solvent chlorobenzene (CB) in a two-dimensional (2D) domain (32 m wide x 6 m deep) characterized by a heterogeneous distribution of intrinsic permeability. 1.2 m3 of CB was released over 5 days into an initially water- saturated domain, allowed to redistribute, and the final distribution of DNAPL corresponds to Time 1 (i.e., when the first ERT scan was obtained, T1). CB was then subject to enhanced dissolution in groundwater flowing from left to right across the domain and a second ERT scan was conducted when the initial DNAPL mass had depleted by 20% (i.e., Time 2, T2).

As illustrated in Box 1 (Figure 4-2), DNAPL3D-MT provides the evolving distribution of DNAPL. Although not part of the modelling approach, to illustrate the difference in the interval between T1 and T2, the time-lapse saturation change (i.e., the “true” DNAPL removed) is also shown in Box 1. The DNAPL-ERT linkage methodology described in Section 2.1 converts the hydrogeological domains at T1 and T2 into their corresponding bulk electrical resistivity domains, as illustrated in Box 2. This represents the “true” distribution of resistivity in the subsurface. The differential resistivity image between the two electrical domains is shown on the right of Box 2. In this ratio image, areas with a ratio value of 1.0 (yellow) indicate no resistivity changes between T1 and T2, areas with a ratio value less than 1.0 (green-blue) indicate decreasing resistivity changes, while areas with a ratio value greater than 1.0 (orange-red) indicate increasing resistivity changes. As expected, the only changes occurring between T1 and T2 are decreasing resistivity changes (green-blue), and correspond to the changes in DNAPL saturation (Box 1).

Figure 4-2: The time-lapse modelling approach adopted in this study to monitor DNAPL source zone remediation. Each domain is 32 m (marked at 5 m intervals) x 6 m (marked at 2 m intervals). The colour scale bars represent different parameters at each step, identified below each figure. The right hand figure in each case represents the change between the left two images, with some presented as a difference and some as a ratio, following common practice for each. A full description of the steps and subfigures is provided in the text.

Time 1 (T1) Time 2 (T2) T1 – T2

(3) Periodic ERT Surveys: Apparent resistivity with noise added (1) Actual DNAPL Volume Remediated

DNAPL-ERT linkage

(2) Bulk Electrical Resistivity Domains

- =

- =

(4a) Time-lapse Inversion Step 1: Pre-estimation

- =

(4b) Time-lapse Inversion Step 2: 4D-ATC Inversion

- =

(5) Estimated DNAPL Volume Remediated 0.05 0.20 0.40 0.60

Sn k

1E-09 1E-10 1E-11 1E-12 1E-13 1E-14 0 10 20 30 40 50 60 70ΔS (%) n

30 40 50 60 70 80 30 40 50 60 70 80 (m ) 2 ρ (ohm-m) ρ (ohm-m) 0.8 0.9 1.0 1.1 1.2 0.02 0.04 0.06 0.08 0.10 A 0.05 0.20 0.40 0.60 Sn 0.05 0.20 0.40 0.60 Sn ρ (ratio) T2 / T1 T2 / T1 T2 / T1 T1 – T2 ERT ERT 30 40 50 60 70 80 30 40 50 60 70 80 ρ (ohm-m) ρ (ohm-m) 0.8 0.9 1.0 1.1 1.2 ρ (ratio) 30 40 50 60 70 80 30 40 50 60 70 80 ρ (ohm-m) ρ (ohm-m) 0.8 0.9 1.0 1.1 1.2 ρ (ratio) - = 0 10 20 30 40 50 60 70ΔS (%) n

The ERT forward model then simulated a surface ERT survey at each of the two times (Box 3). In this case, the survey used the dipole-dipole electrode array with an inline electrode spacing of 1 m. The apparent resistivity data generated for T1 and T2 are not visualized as inversion is required for its interpretation. To render modelling results more realistic, the recorded synthetic data was contaminated by levels of Gaussian noise. As discussed, the 4D-ATC inversion is conducted in two steps: pre-estimation and 4D inversion. The pre-estimation of expected subsurface changes (dotted box and arrows) is obtained from differential imaging between independently inverted images of each time step (Box 4a). This ratio image, shown in the right side of Box 4a, illustrates the changes between T1 and T2. It is evident from comparison to the “true” resistivity changes in Box 2 that additional areas of resistivity changes (both decreasing and increasing changes) occur in the ratio image in Box 4a due to the artifacts (i.e., false anomalies) typically introduced by independent inversion. The ratio image is then used to formulate the outputted temporal Lagrangian matrix A (image between boxes 4a and 4b), where low values (green-blue) correspond to areas with large expected change and large values (orange-red) correspond to areas with little expected change.

The second step of the inversion process (Box 4b) then revisits the original apparent resistivity data and performs simultaneous time-lapse inversion on all monitoring steps; note that all 11 monitoring steps, tracking the complete removal of the DNAPL, were simultaneously inverted although only two are shown. The differential image on the right hand side of Box 4b provides the 4D ERT measurement of the DNAPL region remediated. Comparison of the difference images for the 4D-ATC (Box 4b, right) and independent inversions (Box 4a, right) illustrates that the 4D-ATC scheme eliminates the majority of artifacts evident in the independently inverted image.

This is important for remediation monitoring of DNAPL, where artifacts can be falsely interpreted as regions that are either (a) invaded with remobilized DNAPL or (b) cleaned up. Moreover, note the difficulty in deducing the static DNAPL distribution at T1 (Box 1, left) from either ERT interpretation (Box 4a or Box 4b, left). A number of tests (not shown) similarly highlighted the difficulty of static detection of complex targets such as DNAPL source zones. All of the examples reveal (like Box 4b, right) the greater

potential of time-lapse ERT for monitoring DNAPL removal. Estimation of time-lapse DNAPL saturation changes from the 4D-ATC inverted resistivity fields were obtained using Archie’s law (Box 5). The differential image on the right hand side of Box 5 is the ERT-measured estimate of the true DNAPL change (Box 1, right). How well this result compares to the actual changes is explored in Section 5 using a variety of 3D scenarios. First, confidence in the geophysical method and the coupled model is developed with an analog laboratory experiment described in the following section.

4.3

4D ERT NAPL Experiment

In order to demonstrate this new technique for mapping changes in NAPL distributions with time in a physical system, a 3D controlled laboratory experiment was conducted. The experiment was also independently simulated with the DNAPL-ERT model to develop confidence in the model for simulating these systems, particularly the model’s ability to reconstruct NAPL changes with time from simulated ERT surveys.