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4D interpretation of time-lapse electrical resistivity monitoring data to identify preferential flow path in a landfill, South Korea

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Abstract

Monitoring the leakage of leachate from a landfill is critical in preventing possible contamination in the surrounding area. Time-lapse (TL) electrical resistivity tomography (ERT) has been performed along eleven survey lines at four different time points in a landfill in Korea. The TL data sets were interpreted using an in-house 4D inversion algorithm. Changes in 4D inversion results were analyzed in order to interpret a leachate-contaminant region. Since the rainy season started during obtaining TL ERT data sets, the effects of precipitation on TL ERT data are also analyzed. Changes in electrical resistivity (ER) showed that precipitation increases ER of contaminant zones. As hydrogeochemical data offer contamination information in some areas where boreholes are located, these are helpful to interpret and compare with ERT inversion results to evaluate the extent of the contaminated plume. We also classified soil textures from particle size analysis on soil samples and analyzed electrical conductivity (EC) and dissolved oxygen (DO) using groundwater samples obtained from observation wells in the survey site. The information on soil structure as well as the results of 4D inversion provided insight into the location of a preferential flow path.

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Funding

This research was supported by the Institute for Korea Spent Nuclear Fuel (iKSNF) and Korea Foundation of Nuclear Safety (KOFONS) grant funded by the Korean government (Nuclear Safety and Security Commission, NSSC) (No. 2109092–0121-WT112) and the Basic Research Project [GP2020-007] of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Science and ICT (MSIT).

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The first author is Seo Young Song, and the corresponding one is Myung Jin Nam. The second is Bitnarae Kim; the third is Juyeon Jeong, and the fourth Samgyu Park. Seo Young Song and Myung Jin Nam wrote the main manuscript text. Seo Young Song and Bitnarae Kim conducted the ER inversions. Bitnarae Kim and Juyeon Jeong prepared Figs. 4, 9, and 10. Samgyu Park conducted the TL survey and collected data sets. All authors reviewed the manuscript.

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Correspondence to Myung Jin Nam.

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The authors declare no competing interests.

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Highlights

• 4D inversion was applied to time-lapse ERT data obtained in a leachate-leaked site.

• The utility of the time-lapse ERT data was realized when considering the effects of precipitation.

• Time-lapse 2D inversion cannot unravel the effects of precipitation on the ERT data.

• 4D inversion showed precipitation increases the electrical resistivities of contaminated zones.

• 4D inversion properly detects leachate-contaminated zones despite precipitation.

Appendix

Appendix

Characteristics of changes in ER due to the rainfall and the leachate

The ER is affected by the degree of saturation of groundwater in the soil and the magnitude of electrical conductivity of groundwater according to Archie’s law (1942). In the ER monitoring survey in the contaminant region, ER changes are caused not only by changes in groundwater ion concentration caused by leachate leakage but also by increased water saturation and groundwater level fluctuation due to rainfall. Understanding the characteristics of changes in ER is significant to interpret the contaminant area with the leachate.

Changes in ER due to the rainfall

Rainfall increases the water saturation of the underground, which causes decreases in the ER (Suzuki & Higashi, 2001; Zhang et al., 2016). ER is affected by the degree of water saturation, which is explained by the following equation (Archie, 1942):

$${\rho }_{b}=a{\varnothing }^{-m}{S}_{w}^{-n}{\rho }_{f}$$
(A1)

where \({\rho }_{b}\) is the bulk resistivity of the media, \(a\) is the proportionality constant, \(\varnothing\) is the porosity, m is the cementation factor, Sw is the water saturation, n is the saturation exponent (usually assumed to equal 2), and \({\rho }_{f}\) is the ER of the pore fluid. In addition to saturation, ER of the pore fluid has a significant influence on bulk ER changes.

The decrease in ER due to rainfall appears in the upper region of bedrock. The ER reduction area appears near the surface immediately after rainfall and appears in a deeper region over time (Travelletti et al., 2012). The degree of ER decreases varies depending on the geological characteristics of the site. The average reduction in ER caused by rainfall in the Vence landslide was about 7% (Lebourg et al., 2010), and ER reduction area of up to 8% was shown in the ER inversion result after 4 days from the maximum rainfall (over 400 mm/day) in the mountainous area in the central Kyushu region of Japan (Suzuki & Higashi, 2001). On the weathered clay-shale slope in France, ER decreased by about 15% as a result of ERT (Travelletti et al., 2012).

In addition to the degree of ER reduction, different types of underground medium affect not only the rate of rainfall penetration but also the residual degree of the low ER anomaly (Kim et al., 2012). Kim et al. (2012) analyzed the changes in the ER due to the rainfall according to the various geological conditions (granite weathered soil, sandstone weathered soil, unconsolidated mudstone, sandstone) in Korea. Infiltration and diffusion of rainfall appeared relatively quickly in areas where unconsolidated sedimentary rocks are distributed. In addition, rainfall infiltrates through the fractures or permeable zone in small-scale rainfall, whereas it penetrates the overall survey area in the case of heavy rain, which affects ERT results. The low ER region caused by rainfall decreases over time and the degree of decrease in ER varies depending on the amount of rainfall (Suzuki & Higashi, 2001).

Changes in ER due to leakage of leachate

In a contaminant plume, the concentrations of ions increase the EC of groundwater (Naudet et al., 2004). The ER in the vicinity of landfills where leachate leaks or leachate injection generally decreases with time (Audebert et al., 2014; Grellier et al., 2008; Tsourlos et al., 2003). Rainfall affects the dilution and flow of leachate, so rainfall is an important factor to consider when conducting ER monitoring in contaminated areas. Aduojo et al. (2018) conducted ERTs in leachate-contaminated areas in wet and dry seasons and analyzed changes in ER. Low ER anomaly appeared wider in the wet seasons because leachate moves well in the wet season compared to the dry season and saturation increases. In the dry season, the area of the low ER anomaly that appeared in the rainy season was reduced, and the low ER anomaly appeared in the more intensive region.

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Song, S.Y., Kim, B., Jeong, J. et al. 4D interpretation of time-lapse electrical resistivity monitoring data to identify preferential flow path in a landfill, South Korea. Environ Monit Assess 195, 625 (2023). https://doi.org/10.1007/s10661-023-11149-1

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