Mapping groundwater contamination in a landfill site using frequency Electromagnetic method.

Shaohong Zhang

College of Resource and Environment Engineering, Liaoning Technical University, Liaoning, Fuxin 123000, China

ABSTRACT

The Electromagnetic Method (EM) is one of the most promising geophysical techniques for remediation-process and subsurface environmental monitoring. The method is sensitive to fluid content, saturation, and composition, as well as subsurface temperature. The EM has a demonstrated ability to measure very small changes in subsurface properties over time. For these reasons the technique has seen increasing use for monitoring of aggressive, environmental remediation processes. In the study the EM measurement is carried out to map the extent of contamination at a landfill site, to detect the spread of groundwater contamination, to locate possible pathways of leachate plumes and to locate the buried groundwater pipeline. Based on the analysis of electrical conductivity of the survey area, the geophysical anomaly zones are delineated. Because the electrical conductivity of contaminated formations tends to be higher than that of uncontaminated formations, as the dissolved salts due to pollutants diminish the electrical resistivity of the formations containing them, the EM method is sensitive to the conductive layers and demonstrates the ability to measure small changes in subsurface properties involving ground water.The measurement shows the behavior of groundwater contamination and the position of groundwater pipelines, and it is beneficial to help the waste management processes and to determine possible locations of monitoring wells in the survey area in the future. The study shows that the EM technique has high accuracy, and it is quite suitable for providing a rapid, reliable and time-saving means for the environmental monitoring.

KEY WORDS: Electromagnetic Method, contamination, ground water, pipeline, anisotropy

INTRODUCTION

Groundwater resources are very important for public water supply. Many of the environmental problems are directly or indirectly related to the location of groundwater and its protection from contamination sources of various kinds. But due to the poor urban planning, overpopulation and rapid industrial development, there are many open dump sites and landfill sites, etc., which cause surface and groundwater contamination.

It is well known that groundwater contamination can easily occur and carry on for a long time, because recovery is slow and difficult. The problems associated with municipal, abandoned or non-controlled landfills are of general concern, especially because of the interactions between the hazardous contents of the leachates derived from them and the groundwater. The fast and inexpensive methods to investigate the shallow subsurface are becoming increasingly important.

The physical properties of many rocks are significantly altered by the presence of groundwater in pores and fissures. For example, the electrical conductivity of contaminated formations tends to be higher than that of uncontaminated formations, as the dissolved salts due to pollutants diminish the electrical resistivity of the formations containing them. This makes it possible to investigate the quality of water using appropriate geophysical techniques (Benson et al. 1997; Sauck 2000; Atekwana et al. 2000; Baba et al. 2004; Baker et al. 2004; Porsani et al. 2004; Martinho et al. 2006).

The Electromagnetic Method (EM) has many unique qualities that make it amenable to environmental monitoring, and has seen increasing use for detecting of buried objects such as subsurface pipes, fuel oil tanks and utilities, etc, leak detecting of groundwater pipeline, monitoring of aggressive and environmental remediation processes (Karlik et al. 2001; Kaya et al. 2007).

In the landfill site of the survey area the groundwater contamination is manifested because the groundwater is located above permeable and porous formations, which aid in contamination scattering. The objective of the survey is to obtain geophysical information from ground surface to approximately 5m below surface using The EM to investigate the groundwater contamination.

METHODOLOGY OF EM

The EM method transmits primary magnetic field using magnetic dipole coil and measures the induced secondary electromagnetic field during the break-up of the primary field (see Figure 1). The secondary field is a electromagnetic field produced by subsurface conductive body and is the comprehensive response of underground geologic body. The EM is an indirect method to investigate the underground engineering and hydrologic geology based on the physical difference of subsurface body.

Proceedings of the 9th SEGJ International Symposium –Imaging and Interpretation- , Sapporo, Japan 12-14 October 2009

Normally in an EM field survey, the quadrature-phase (vertical component) of magnetic field is measured, which is sensitive to metallic objects and hence very useful in looking for buried metal materials (e. g. pipelines). The quadrature-phase component gives the ground conductivity, usually in millisiemens per meter (mS/m); and the apparent conductivity is given by the following formula:

sHHa )/()/(4 2012 ωµσ =

Where σa is the apparent conductivity, H2 is secondary magnetic field at the receiver coil, H1 is primary magnetic field at the receiver coil, ω=2πf is the circle frequency and f is the frequency of the electromagnetic wave(Hz), μ0 is permeability of free space, s is distance between the transmitter and receiver coils (m).

The EM meter normally consists of two coils separated by a set distance. The transmitting coil induces currents in the ground which generate a magnetic field detected by the receiving coil (Figure 1).

SURVEY SITE AND FIELD DATA ACQUISITION

The survey area is at one abandoned hydrocarbon site (see Figure 2), approximately 130m × 120m. The target area is on the north-east side of the road. The fact that the landfill is located in the vicinity of groundwater used for domestic consumption, urges the investigation of possible groundwater contamination. There past service was done by drilling to monitor the ground water (6 monitor wells done). However, additional information is often required to map the subsurface structure, the lateral and vertical extent of the groundwater contamination and to locate the groundwater pipelines.

In the survey area the local geology consists mainly of Quaternary sedimentary units composed of alluvium,

sand dune and mud formations of Pleistocene. Alluvium unit is the major aquifer of the study area. The static groundwater level is approximately 2m in the site. The Quaternary formations contain a free Holocene aquifer on top. The Pleistocene mud layers of semi-confined aquifer constitute a heterogeneous aquitard, which does not isolate the lower aquifer and therefore contamination can reach the lower sediments. The aquifers are recharged by direct infiltration of surface water and by leakage of groundwater pipelines.

The EM survey was carried out with a Geonics EM31-MK2 terrain EM meter in 2008. The intercoil spacing is 3.66 m, operating frequency is 9.8 kHz, the line spacing is 5m and sample rate is 0.5s. The quadrature-phase of vertical electromagnetic field is measured and it gives the ground conductivity, in millisiemens per meter (mS/m). Both the processing and interpretation of acquisition data are shown on Figure 3 and Figure 4.

DISCUSSION OF RESULTS

From the general characteristics of these maps and the previous geologic information, the maps can be interpreted as follows.

On the north-east side of the road the survey area can be divided into three zones: ZONE 1 - original area or untouched area which are closest to the road on the south end, ZONE 2 - transitional zone area in the middle of the survey area, and ZONE 3 - contaminated area on the north and east boundaries. These 3 zones are marked by thin dotted lines on Figure 3 and Figure 4. ZONE 3 shows high conductive values compared with ZONE 1 and ZONE 2. This fact can be related to a more likely salt intrusion in ZONE 3, owing to its shorter distance to the mud layers - a heterogeneous aquitard - compared with ZONE 1 and ZONE 2. These maps also show a strong geoelectrical property contrast among three zones.

Proceedings of the 9th SEGJ International Symposium –Imaging and Interpretation- , Sapporo, Japan 12-14 October 2009

Figure 2. Location and setting of the survey area.

Figure 1. Schematic map of EM method.Both primary and secondary magnetic fields are shown.

The result also reveals a north-eastward evolution of contamination plume.

Figure 3. Color contour map of conductivity.Three anomaly zones are delineated by thin dot lines.

Figure 4. Interpretation of conductivity data. One pipeline is located by a thick hidden line. Three anomaly zones are delineated by thin dot lines.

The apparent conductivity map (Figure 3 and Figure 4) shows a great variation in the apparent conductivity values of the subsurface soil layer. These values range from less than 30 mS/m to more than 75 mS/m. This wide conductivity range reflects a comparable wide variation in the soil (or rock) constituents over the area. The lower conductivity contours can be attributed to dry and compact soils on the north-east, while the higher contours are attributed to soil logging, seepage or to wet soils on the south-west. The EM data shows that high conductivity is associated with contaminated areas, and the occurrence of higher geoelectrical values can be due to 2D or 3D geometrical effects related with contaminant plume. The conductivity values in the contaminated zone (ZONE 3) are from 50 to 80 mS/m, transitional zone (ZONE 2) between 45 -55 mS/m, and untouched zone (ZONE 1) with low values of less than 45 mS/m.

The conductivity data confirm that contamination extends from the free aquifer to the lower semi-confined aquifer, which can be due to discontinuous muddy levels that allow the hydraulics connection between the two aquifers or rupture of that level. These discontinuities are likely to have been induced by mechanic means when the landfill was deepened.

One pipeline position and shape can be identified on the boundary between ZONE 3 and ZONE 2 in the information of conductivity, which is typically featured by lower values with high values on both sides (see Figure 3 and Figure 4). The groundwater pipeline takes a continuous path, marked by thick hidden lines on the interpretation map (see Figure 4).

CONCLUSIONS

In the survey the contamination behavoirs can be defined on the maps of conductivity. The EM data shows that higher conductivities are associated with contaminated areas, and the occurrence of high geoelectrical values can be due to contaminant plume. According to the contamination evaluation of grounder water, the studied area can be divided into three parts: original area or untouched area, transitional zone and contaminated area. The survey also shows that the contamination involving area carries on and evolves northeastward. Thus the study shows that EM is sensitive to the detection of groundwater and can be used to dynamically monitor the groundwater contamination.

One subsurface pipeline is located and the anomaly is obvious on the apparent conductivity map. The groundwater pipeline basically has characteristic of low conductivities with high values on both sides. The study demonstrates that EM method has strong ability to locate the groundwater pipeline.

Proceedings of the 9th SEGJ International Symposium –Imaging and Interpretation- , Sapporo, Japan 12-14 October 2009

ACKNOWLEDGEMENTS

These research results are carried out in 2008 and financially supported by the National “973”Project Foundation of China (No. 2007CB209600). The author thanks Ms. Meng Qingping for computer drawing for the paper.

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Proceedings of the 9th SEGJ International Symposium –Imaging and Interpretation- , Sapporo, Japan 12-14 October 2009