APPLICATION OF INTEGRATED GEOPHYSICAL TECHNIQUES IN

Ozean Journal of Applied Sciences 4(1), 2011

Ozean Journal of Applied Sciences 4(1), 2011 ISSN 1943-2429 © 2011 Ozean Publication

APPLICATION OF INTEGRATED GEOPHYSICAL TECHNIQUES IN THE INVESTIGATION OF GROUNDWATER CONTAMINATION: A CASE STUDY OF MUNICIPAL SOLID WASTE LEACHATE

ABDULLAHI, N.K*, OSAZUWA, I.B** and SULE, P.O** *

Department of Applied Science, College of Science and Technology, Kaduna Polytechnic, Kaduna, Nigeria

**

Department of Physics, Faculty of Science, Ahmadu Bello University, Zaria, Nigeria *E-mail addresses of Corresponding author: [email protected]

_________________________________________________________________________________________ Abstract: An integrated geophysical study involving 2D electrical resistivity /induced polarization imaging, Very Low Frequency-EM, and Seismic refraction tomography was conducted at Unguwan Dosa municipal solid waste–disposal site in Kaduna metropolis, North Western Nigeria. Six 2D resistivity profiles both around the perimeter and inside the dump were investigated with maximum length of 200 m. Results of the resistivity imaging delineated the leachate plume as low resistivity zones (6 - 33 ohm - m), while the corresponding induced polarization models have assisted in the distinctive identification of the saline zones (with maximum chargeability value of 4.05 msec from the clay-rich zones which exhibit a relatively high chargeability (8 to 13 msec). Quantitative interpretation of the VLF-EM data correlates excellently with the results of the 2D resistivity imaging delineating fractures as subsurface contaminant pathways. The seismic refraction tomography data measured on top of the dump suffers severe attenuation in the unconsolidated top soil material, as a result, distorted energy penetration into the underlying layers. This prevented imaging the subsurface targets (fractures). Results of the physio-chemical analysis of water samples from existing hand dug wells reported elevation in concentration of the measured parameters indicating contamination of the groundwater as a result of solid waste leachate accumulation, consequently, complimenting the geophysical data.

Key words: geophysical techniques, seismic refraction, groundwater _________________________________________________________________________________________

INTRODUCTION

Fresh water is a renewable resource; however, the world's supply of clean fresh water is steadily decreasing as a result of disposing of urban waste materials, mainly domestic garbage, in land surface, shallow excavation, rivers and stream channels. These activities pose a serious threat of polluting both groundwater and downstream surface water. Pollution of groundwater happens mostly due to percolation of fluvial water and the infiltration of contaminants through the soil under waste disposal sites. Kaduna municipal with an average daily production of 180 metric tons of municipal waste and refuse (Ministry of Environment and Natural resources, Kaduna state,

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personal communication, 2008) is not an exception to this environmental problem. The entire (324) municipal waste disposal sites are the uncontrolled open dump, thus creating serious threats to local environmental quality and public health. Consequently, Kaduna metropolis will face critical problem pertaining to its groundwater resources with the coming years if this problem of waste disposal sites which indiscriminately litters the city is not adequately addressed. One of the most common demands in metropolitan areas includes detecting the location and extent of contamination patches in areas as small as landfills. In such circumstance, the integrated use of geophysical methods provides an important tool in the evaluation and characterization of contaminants generated by urban residues (domestic and/or industrial). Geo-electric, electromagnetic, magnetic and ground penetrating radar geophysical techniques can be applied in landfill investigations because dissolved plume (leachate) can influence resistivity /conductivity, dielectric constant, chargeability, and magnetic susceptibility. The use of Geo-electrical and electromagnetic methods applied to landfills studies are well documented (Porsani et al., 2004; Karlik et al., 2000; Benson et al, 1997; Mukhtar et al, 2000; Fatta et al, 2000). This work is aimed to investigate the effects of Unguwan Dosa dump site located in Kaduna North, North-West Nigeria on groundwater by employing an integrated use of 2D electrical resistivity and induced polarization imaging in order to delineate the leachate plumes. The inclusion of seismic refraction tomography is to determine depth to water table as well as structures responsible for migration of the leachate plumes while VLF-EM technique is to compliment the resistivity/seismic data in detecting the contaminants pathways. Site Description. 0

'

0

'

The survey area (Fig 1) is located on Abdullahi Bello Road with co-ordinates 10 34 N and 007 27 E in Unguwan Dosa, Kaduna North Local Government Area of Kaduna State, Nigeria. The maximum difference in elevation between the top of the solid waste and the surrounding area is 5m. Two hand dug wells, one located 2 m from the dumpsite (Named well1) and the other at 16 m from the dumpsite near a mosque (Named well 2) located at the northern margin of the dump was the motivating factor for the geophysical survey in order to educate the residents about the health hazards associated with the use of the well as a source of water. The study area has a typical Savannah climate with distinct wet and dry seasons. The rainy season extends from April to October and the dry season is between November and March. Average annual rainfall for Kaduna is 1270mm (Eduvie, 2003) and rainfall usually reaches a peak in August. Temperatures vary between less than 15 0C in December/January and 320C in March/April. Geologically, the survey area lies entirely within the Basement Complex of Northern Nigeria. The rocks consist of series of granites, gneisses, migmatite, low-grade schist, quartzite and amphibolites that have been grouped by the British authors as ‘Basement Complex’ of the Precambrian age (Olugboye, 1975). Groundwater occurs under water table conditions in the clay sand/sand aquifer and under semi-confined to confined conditions in the weathered /fractured zone.

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Fig.1: Map of Nigeria showing the study area.

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METHODOLOGY

The geophysical surveys of the municipal waste dump have been based on the use of electrical resistivity imaging (ERI), induced polarization (IP), VLF-EM and seismic refraction tomography (SRT). Four ERI/IP measurements around the perimeter of the dump were performed during the rainy season (August) and these measurements were repeated in the dry season (March). This is done to determine the influence climatic changes would have on the generation and migration of leachate and the effects that changes in hydrogeological conditions (i.e. seasonal changes) will have on the response of each method used. Two additional ERI were measured inside the dump in order to detect subsurface structures that may influence migration of the leachate outside the dump. The geo-electrical surveys (ERI and IP) were acquired using the ABEM 4000Terrameter equipped with a multi-electrode switch system with 42 channels. The apparent resistivity and chargeability measurements were inverted using the 2-D computer software (RES2DINV). The used inversion approach consists of applying the 2D inversion routine by Loke and Barker (1996), which is a Gauss– Newton least squares method based on the finite-difference model of the subsurface. To define better the boundaries of the leachate, a robust inversion approach has been applied. This option attempts to minimize the absolute changes in the resistivity values, producing models with sharp interfaces. Two seismic refraction tomography data were acquired (Fig.2) during the dry season in order to measure the permanent water table and subsurface structures that could provide contaminant pathways with ABEM MK6 Terralock. Seismic energy was sent into the ground with the aid of hammer blow on a base plate placed at every geophone (separated by 5 m) and in between geophones (2.5m) on the profile. The seismic refraction data was subjected to different stages of processing using the REFLEXW package to enhance the signal-to-noise ratio. Two VLF-EM measurements were also conducted to compliment the ERI/SRT results in detecting subsurface structures (Fractures) in the survey area. The VLF-EM data were collected along the resistivity/seismic profiles and measurements were made with a station separation of 10 m using the Scintrex Envi meter in the VLF-EM mode (i.e. measuring the ratio of the polarized magnetic field).The lengths of the profiles were 140 and 130m respectively. The in phase and quadrature data were presented as single profiles. To locate the anomalies, measured data were processed using Fraser Filtering. Quantitative interpretation of the single frequency (23.4 kHz) VLF-EM data was achieved using the inversion code of Monteiro et al (2006) to yield subsurface resistivity distributions of the fractures. In order to assess the level of groundwater contamination by the solid waste leachate, water quality analysis was conducted on water samples from two hand dug wells both in the rainy and dry seasons. The water samples were analysed for physical and chemical parameters. The results of the physio-chemical analysis are presented in table 1.

RESULTS AND DISCUSSION

Resistivity and IP surveys around the dump perimeter The aim of the geophysical survey is to detect and map possible migration of the leachate outside the dump. Both ERI and IP data were collected. Line BB'1: Both Resistivity and Chargeability measurements were taken at this profile which defines the northern margin of the dumpsite. About six (6) electrodes were located where the refuse has just been removed and leveled. Well 1 at the time the Resistivity/Chargeability data were taken was 2 m from the northern margins of the dump. The electrode spacing adopted was 2.5 m which gave a total length of 100 m, thus allowing depth of investigation down to 16 m. This depth covered the depth beyond the water table.

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BB'1

BB'3

DD'1

BB'4

BB'2

DD'2

Explanation Solid waste Residential/mosque buildings Hand dug well ERI/IP Profiles ERI/VLF/SRT Profiles

Fig. 2: Map of the study area showing the positions of the profiles.

11

0

300m

N


Resistivity model: Examining Fig 3(a) from west to east, we find a trend of decreasing near surface resistivities at depth of 0.6 m down to 5.4 m. The substantial decrease in resistivity(19.8 – 54.8 ohm-m) obtained from the 2-D data at these depths is believed to be due to ground water contamination as a result of accumulation of leachate, a conclusion supported by the water analysis for the well 1 which showed elevations in concentration of organic/inorganic parameters exceeding the permissible limits (Table 1). Fig 3(b) is the resistivity model collected in the dry season. Between 60 m and 65 m on the horizontal scale, to a depth of 10m below the surface, low resistivity zone is evident. Comparing the two models, there is apparent lateral and vertical migration of the contaminant along this profile.

Fig 3a: Wet Resistivity model along Line BB'1

Fig 3b: Dry Resistivity model along Line BB'1 Chargeability model: Fig 3c shows the corresponding chargeability model of wet season. Increased near surface chargeabilities anomalies correspond with low near surface resistivities at depth of 0.6 m down to 5.4 m. According to (Barker, 1990) chargeability will increase as salinity of the groundwater increases up to 500 mg/l. Thus, it appears that there is a correlation between the increased chargeability, towards the well and increased ion concentration as a result of leachate contamination at these depths. The high chargeability anomaly (> 21 msec) is believed to be due to disseminated organic waste and not clay which according to (Aristedemou et al, 2001) has chargeability value of < 10 msec. There is no agreement between the wet model and the dry chargeability model, Fig 3d except the disseminated organic waste anomaly. This could be attributed to the steel electrodes used for the potential on dry and hard surface.

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Fig 3c.Wet Chargeability model along Line BB'1 Line BB'2: This profile was taken at eastern margin of the dumpsite. On this line, the central electrodes (21& 22) and electrodes at take-outs 18, 19, 23 and 24 were located on the evacuated portion of the dumpsite. The electrode spacing was 2.5 m which gives a total length of 100 m.

Fig 3d. Dry Chargeability model along Line BB'1 Resistivity model: Figure 4(a) is the resistivity model of the profile taken at the eastern perimeter of the dumpsite in the wet season. At positions 32.5-65 m and 77.5-92.5 m there are indications of saturated zones represented by low resistivity(15.3-40.5 ohm-m), starting at the ground surface down to 5 m depth. This low resistivity reflects the positions of the central electrodes and those at take-outs 18, 19, 23, and 24 and is believed to be due to accumulation of leachate. The colour scaling changing from deep blue to light blue reflects the changes in the concentration of the leachate as it seeps down due to filtration by the sediments. It is evident that the low resistivity anomalies shown in the wet model is reflected in the dry model, Fig 4b. As observed in line BB'1, there is evident of lateral and vertical migration of the leachate plume.

Fig. 4a: Wet Resistivity model along Line BB'2

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Fig. 4b: Dry Resistivity model along Line BB'2 Chargeability model: The wet chargeability model, Fig 4c, do not show IP anomalies at the profile positions 32.5 – 65 m which correspond to the low resistivity zones in the resistivity model because, saline water does not produce appreciable chargeability anomaly in the absence of clay (Osazuwa and Abdullahi, 2008). The inverse model shows high chargeabilities (16-40 msec) from the ground surface down to 5 m depth at profile positions 35-37.5 m. There was not such anomaly in the resistivity model as clearly as in the chargeability model. One possible explanation of this inconsistency is that chargeability assists in distinguishing IP effects due to predominantly electrolytic controls from effects due to structural (primarily clay control) variation better than resistivity measurement. The high chargeability >16 msec is due to the presence of disseminated organic waste and not clay. No correlation between the wet and dry season model shown in fig 4d could be established due to the noisy nature of the dry IP data.

Fig. 4c: Wet Chargeability model along Line BB'2

Fig.4d: Dry Chargeability model along Line BB'2

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Line BB'3: This profile was taken at the southern perimeter margin of the dumpsite. The electrode spacing used was 5 m which gives a total length of 200 m. ERI/IP data were collected in the wet season and a repeat of the ERI measurement was conducted during the dry season only. Resistivity model: Fig 5(a) shows the Resistivity model along line BB'3. The topsoil with 30 ohm-m and 209 ohm-m values are in sharp contrast. This is interpreted as sand with various levels of compaction and water contents. The model with rms error of 2.7% showed the bedrock with varying degree of weathering from 10 m down to a depth of 30 m. The corresponding dry season model with rms error of 2.2% is shown in Fig 5(b). There is good agreement between the two models (wet and dry) except in the range of resistivity values of the topsoil and the bedrock which are slightly higher in the dry resistivity model. This is expected as rising vapours in the dry season will drive out soil moisture with attendant increase in bulk resistivity. No contamination plume (leachate) is detected along this profile.

Fig. 5(a): Wet resistivity model along Line BB'3

Fig 5a: Dry resistivity model along Line BB'3

Chargeability model: The wet chargeability model of Line BB'3 shown in Fig 5c has a value of 27% of rms fit between the observed and calculated data. This high discrepancy is attributed to background noise as a result of low voltage difference measured by the potential electrode. This is a common problem for battery based systems due to low currents (Loke, personal communication, 2009). A few zones of mainly high chargeability are visible at 21.7m depth which is indicative of the weathered bedrock.

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Fig 5b: Wet Chargeability model along Line BB'3

Fig 5d: Dry Chargeability model along Line BB'3.

Line BB'4: This profile was measured at the western perimeter margin of the dump. 2.5 m was used as electrode spacing which gives a total length of 100 m. Resistivity Line: Fig.6(a) is the wet resistivity model which shows low resistivity values ranging from 47 – 71 ohm-m recorded between x= 0.0 m and 20 m marks. The lower end (47 ohm-m) is where about five electrodes were located on the evacuated portion of the dump and therefore are attributed to leachate bubbles within the refuse itself (Abu-Zeid et al., 2003). The zone with range of resistivity values of 357 ohm-m to 798 ohm-m which dominates the profile with varying depths is interpreted as the fresh basement rock with varying degree of weathering and water content. Dry season model (Fig.6b), shows decreased resistivity values (36 ohm-m) in the zone interpreted as the leachate bubble in the wet model. The decreased in the resistivity value is attributed to the broken down of much of the biodegradable mass with time (Furqhan, 1980).

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Fig 6(a) Wet resistivity model along line BB'4

Fig .6a: Dry resistivity model along line BB'4 Chargeability model: Fig 6c is the wet chargeability model along line BB'4. Strong correlation exists between the wet chargeability model and the resistivity model (Fig.6a). Appreciable chargeability values (2- 8 msec) corresponds to leachate bubbles (low resistivity zone) and high chargeability (> 8 msec) correspond to in-situ weathered clay in- the fresh basement rock. The dry chargeability model with fitting error of 2.0 % is shown in Fig 6d. Leachate bubbles show increased in chargeability (2-7 msec) which corresponds to decreased in resistivity in the dry resistivity model. The zone of high chargeability ( > 14 msec) visible at 10 m depth is interpreted as IP-effects resulting from mineralization in the contact metamorphism zone between the weathered and the fresh basement rock (Dahlin et al, 2002).

Fig. 6c: Wet chargeability model along line BB'4

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Fig.6d: Dry chargeability model along line BB'4

Resistivity, Seismic and VLF-EM surveys on top of the dump. In order to investigate the subsurface structures that may provide pathways for groundwater and contaminant transport, 2D Electrical resistivity, Very-low- frequency (VLF-EM) and Seismic refraction tomography data were collected on top of the dump after evacuation and leveling. The integrated data were collected with a view of identifying and detecting loose ground, faults or fractures that may be responsible for the migration of the contaminant plumes outside the waste disposal sites as well as studying the effectiveness and response of each of the methods used on top of the dump that represents unconsolidated materials. Two profiles in the N-S azimuth separated by 5 m were investigated (Fig 1). Results of the investigated profiles are presented below. Line DD'1: Fig 7 is the resistivity model measured with 3m electrode spacing along profile DD' 1. Low resistivity zone (6-102 ohm-m) is visible starting from the ground surface down to 3m which is indicative of contaminated top soil. The model also shows the presence of vertical contacts at profile positions 63, 77 and 105 m between the surface material and the country rocks. These features are interpreted as vertical and subvertical fractures overlain by 6 m thick overburden. The vertical feature at 63 m mark deeps steeply. Fig 8 is the VLF profile data (in-phase and quadrature) and Fraser-filtered response of the in-phase component. The VLF-data make sign crossover above profile coordinates 63 m, 77 m and 105m which correspond with the vertical contacts observed in the resistivity model. The Quadrature response shows a positive inflection at 63 m, and may indicate a relatively weak conductor in non-conductive ground (Jeng et al., 2004). However, a good conductor in weekly conductive ground could cause a negative inflection shown at 77 m. The Fraser-filtered data (in-phase) have high positive peak response at position 63 m and is almost symmetrical; this is an indication of anomalous body of large dimension and steeply dipping source. This conclusion is supported by the resistivity data. The smaller amplitude of the in-phase response of the cross-over at 105m indicates a relative good conductor in conductive ground while the broader shape of the VLF-anomaly indicate greater depth. This anomaly is absent in the resistivity model due to constrained impose on the profile length. Fig 9 is the 2D resistivity model generated using the 2DINVLF code (version 1.0) developed by Monteiro Santos (2006) for quantitative interpretation of the VLF data. Weak conductors have resistivity values ranging from 260 – 420 ohm-m while the thin conductive subsurface material located at 77 m is reflected by resistivity values below 260 ohm-m. The low resistivity end, 40 ohm-m may indicate fracture filled with contamination plume. Figure 10 is the result of the Seismic refraction tomography along profile DD'1.The velocity zone (798 – 1514 ms-1) represents the soft overburden comprising of sand and lateritic soil. The seismic data could only be modeled with two layers as a result of the attenuation of the signal by the top unconsolidated materials on the dump. The signal attenuation coupled with noise contamination introduced by traffic (human and vehicle) severely affected detailed subsurface structures imaging, hence this may have prevented the identification and delineation of the vertical contacts reported in both the resistivity and the VLF-EM data.

Fig.7: Resistivity inverse model along line DD'1

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250 200 150

Hs\Hp

100 50 0 -50 0

20

40

60

80

100

120

140

-100 -150 -200

Profile distance (m) Observed data(In-phase)

Observed data (Quadrature)

Fraser Filtered(In-phase)

Fig.8: In-phase, Quadrature and Fraser Filtered (In-phase) along line DD'1

Fig.9: 2Dresistivity model obtained from inversion of VLF data along line DD'1

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Fig.10: Seismic refraction model along profile DD'1 Line DD'2: Figure 11 shows the model sections of the resistivity survey of profile D 2 measured with 5m electrode spacing. At this line also, low resistivity at the surface reflects the presence of thin layer of surface contamination with values ranging from 11 – 102 ohm-m. The prominent feature in the model section is the presence of subvertical contact overlain by 10 m thick overburden located at profile position 60 m. The VLF profile plot of the in-phase and quadrature responses as well as the response of in-phase Fraser filtered profile is shown in Figure 12. The in-phase component decayed to zero at a distance of 50 m while the quadrature component make sign crossover at 57.5 m. The Fraser-filtered response shows a high positive peak at crossover between the in-phase and quadrature. The high positive Fraser peak at the crossover point is a favourable location for fracture (Sundararajan et al., 2007). The 2D resistivity model generated from the VLF data (Fig.13) correlates quite well with the Fraser filtered responses of the VLF data. The 2D resistivity model shows resistivity value of 270 ohm-m at the crossover point which is indicative of fracture in the crystalline basement rock. The low resistivity zone (70 ohm-m) at profile position x = 100 m may indicate fracture filled with contamination plume. The result of the seismic survey is shown in (Fig.14). Despite the fact that the seismic data suffered the same noise contamination as reported in the interpretation of profile DD' 1, the model shows week zone at profile position x = 100 m which corresponds to the zone identified as fracture loaded with contamination plume in the 2D (VLF) resistivity model. Seismic velocity ranging from 812 to 1622 ms-1 is representative of sand and lateritic soil.

Fig.11: Resistivity inverse model along line DD'2

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500 400 300

Hs\Hp(%)

200 100 0 -100 0

20

40

60

80

100

120

-200 -300 -400

Observed data(In-phase)

Distance(m) ObservedProfile data(Quadrature)

Fraser Filtered(In-phase)

Fig.12: In-phase, Quadrature and Fraser Filtered (In-phase) along line DD'2

Fig.13: 2Dresistivity model obtained from inversion of VLF data along line DD'2

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Fig.14: Seismic refraction model along profile DD'2. Physio- Chemical Analysis Of Hand Dug Wells. Table 1 is the result of water analysis of Well 1 and well 2. All measured parameters showed concentrations exceeding the permissible limits (WHO, 1992) which signified groundwater contamination. Comparing the wet and dry parameters, there is decrease in concentration of BOD, COD, CHLORIDE and TDS in the dry season study. This is attributed to the breaking down of much of the biodegradable mass with time (Furqhan, 1980) while the decomposition of the organic components of the waste by the action of microorganisms increases the level of organic matter for the period of the wet season .The Fe concentration also is seen to decease in the dry season. This according to Drever(1997) is expected since the solubility of iron minerals in oxygenated waters decrease for increasing pH values, so at low pH values, ion concentration should be higher as shown in table. Also the high value of pH in the dry season is an indication that the landfill is in its methanogenic phase (Ibe and Njoku, 1999). The high concentrations of detrimental substances (Lead, Chloride and Chromium) observed in both wells call for urgent concern.

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Table 1. Physio-chemical analysis of Hand dug wells Parameter

Conductivity

Unit

µs

pH

Wet Season

Dry Season

WHO

Mosque

Dump

Mosque

Dump

(1992)

1024

283

455

1044

100

6.72

6.63

6.73

7.13

6.5-8.5

TDS

mg/l

400

1000

195

528

COD

mg/l

742

902

360

550

80.0

BOD

mg/l

395

480

3.1

2.3