Bacterial indicators of faecal pollution and physiochemical assessment of tributaries of Ganges River in Garhwal Himalayas, India Bakterijski indikatorji fekalnega onesnaženja in fiziološko-kemijska ocena pritokov reke Ganges v Garhwalski Himalaji v Indiji Archna Sati1, Anchal Sood1, Shivesh Sharma2, *, Sandeep Bisht1, Vivek Kumar3 department of Microbiology, SBS Post Graduate Institute of Bio-Medical Sciences and Research Balawala, Dehradun, Uttarakhand, India 2Department of Applied Mechanics (Biotechnology), Motilal Nehru National Institute of Technology, Allahabad, Uttar Pradesh, India 3Microbiology Section, Department of Soil and Water Research, Public Authority of Agricultural Affairs & Fish Resources, PO Box 21422, Safat-13075, Kuwait Corresponding author. E-mail: dr.shiveshsharma@gmail.com Received: February 22, 2011 Accepted: March 27, 2011 Abstract: A study was undertaken to investigate the water quality of Alaknanda and Bhagirathi rivers (tributaries of River Ganges) in Garhwal Himalayan region during the periods of monsoon, summer and winter seasons. Both the rivers are sacred and are important source of water for drinking and irrigation. Water samples were analyzed for various bacteriological parameters including total viable count (TVC), total coliform (TC), faecal coliform (FC) and faecal streptococci (FS). Also, physicochemical attributes viz. dissolved oxygen (DO), biological oxygen demand (BOD) and chemical oxygen demand (COD) was assessed. Total viable count exceeded the maximum permissible limits in all the samples irrespective to different seasons. The high most probable number (MPN) values and presence of faecal coliforms and streptococci in the water samples suggests the potential presence of pathogenic microorganisms which might cause water borne diseases. A direct effect of season and human activities on the pollution status was observed at all the water sampling sites. The over all objective of this work was to investigate the incidence of these indicator organisms, coliform, faecal coliform, faecal streptococci and physi-ochemical parameters during different seasons in two main tributaries of Ganges River. Izvleček: Namen študije je bil raziskati kakovost vode rek Alaknanda in Bhagirathi (pritokov Gangesa) na območju Garhwalske Himalaje v monsunskem, poletnem in zimskem obdobju. Obe reki veljata za sveti in sta hkrati pomemben vir pitne in namakalne vode. V vzorcih vode so določali različne bakteriološke parametre, kot tudi celotno število za življenje sposobnih organizmov (TVC), celotne koliformne organizme (TC), fekalne koliformne organizme (FC) in fekalne streptokoke (FS). Določali so tudi fiziološko-kemijske lastnosti, kot so raztopljeni kisik (DO), biološka potreba po kisiku (BOD) in kemijska potreba po kisiku (COD). Celotno število za življenje sposobnih organizmov presega najvišje dopustne meje v vseh vzorcih, ne glede na čas vzorčenja. Visoke vrednosti najverjetnejšega števila (MPN) in navzočnost fekalnih koliformov ter streptokokov v vzorcih nakazuje možno navzočnost patogenih mikroorganizmov, ki utegnejo povzročati obolenja, ki se širijo z vodo. Na vseh vzorčnih mestih je mogoče opazovati vpliv letnega odobja in človekovih dejavnosti na stanje onesnaženosti. Poglavitni namen dela je bil raziskati pogostnost indikatorskih organizmov, koliformov, fekalnih koliformov, fekalnih streptokokov in fiziološko-kemijskih parametrov v različnih obdobjih leta v dveh glavnih pritokih reke Ganges. Key words:, coliforms, bacteriological, physicochemical, Ganges, river Ključne besede: koliformi, bakteriološki, fiziološko-kemijski, reka Ganges Introduction The Ganges or Ganga rises in the Northern Himalayas on the Indian side of the Tibet border. Its five headstreams i.e. the Bhagirathi, Alaknanda, Manda-kini, Dhauliganga and Pindar rise in Uttarakhand region. Of these, the two main headstreams are the Alaknanda (Latitude: 30°7'60'' N, Longitude: 78°35'60'' E) about 4 402 meter above sea level (the longer of the two), which rises about 30 miles north of the Himalayan Peak of Nanda Devi and the Bhagirathi (Latitude: 30°7'60'' N, Longitude: 78°34'60'' E) about 3 050 meters above sea level in an ice cave at the foot of the Himalayan glacier known as Gangotri, merges at Dev Prayag to form river Ganges, flows through the northern Indian planes, providing drainage and water for around 400 million people. In the recent past, expanding human population, industrialization, intensive agricultural practices and discharges of massive amount of wastewater into the river have resulted in deterioration of water quality. The impact of these anthropogenic activities has been so extensive that the water bodies have lost their self-purification capacity to a large extent. Therefore, there is a grown recognition and need that aquatic water bodies or ecosystem like Ganga must be sustained so that they may support human life. This has resulted in scarcity of potable water supply and loss of biodiversity in aquatic system. The health and well being of the human race is closely tied up with the quality of water used (Sharma et al., 2005). Most of the people in the Himalayan region use surface water for drinking which is most vulnerable to pollution due to the surface run off. Almost all major rivers have been tapped at source for drinking water supplies, but there is no monitoring of water quantity or quality on regular bases. During bathing the river water is also used for drinking (Aachman), irrespective of its water quality. But, it is evident from a course of studies carried out by different (Srivastava et al., 1996; Kulshres-tha & Sharma, 2006) that Ganges water is highly contaminated with coli-forms. Microorganisms are widely distributed in nature, and their abundance and diversity may be used as an indicator for the suitability of water (Okpokwasili & Akujobi, 1996). The use of bacteria as water quality indicators can be viewed in two ways, first, the presence of such bacteria can be taken as an indication of faecal contamination of the water and thus as a signal to determine why such contamination is present, how serious it is and what steps can be taken to eliminate it; second, their presence can be taken as an indication of the potential danger of health risks that faecal contamination posses. The higher the level of indicator bacteria, the higher the level of faecal contamination and the greater the risk of waterborne diseases (Pipes, 1981). A wide range of pathogenic microorganisms can be transmitted to humans via water contaminated with faecal material. These include enteropathogenic agents such as salmonellas, shigellas, entero-viruses, and multicellular parasites as well as opportunistic pathogens like Pseudomonas aeroginosa, Klebsiella, Vibrio parahaemolyticus and Aero-monas hydrophila (Hodegkiss, 1988). It is not practicable to test water for all these organisms, because the isolation and identification of many of these is seldom quantitative and extremely complicated (Cairneross et al., 1980; World Health Organization (WHO), 1983). An indirect approach is based on assumption that the estimation of groups of normal enteric organisms will indicate the level of faecal contamination of the water supply (WHO, 1983). The most widely used indicators are the coliform bacteria, which may be the total coliform that got narrowed down to the faecal coliforms and the faecal streptococci (Harwood et al., 2001; Pathak & Gopal, 2001; Kistemann et al., 2002). Concurrently, contamination of water by enteric pathogens has increased worldwide (Craun, 1986; Islam et al., 2001). However, to the best of our knowledge, no report is available on the bacterial as well as physiochemical parameters analysis of two main tributaries of Ganges River in Garhwal Himalayan region. The overall objective of this work was to investigate the incidence of these indicator organisms, coliforms, faecal coliforms and faecal streptococci in relation with physiochemical parameters of Alaknanda and Bhagirathi rivers in different seasons in Garhwal Himalayas, India. Materials and methods Collection of water samples Intensive survey of the study area was done to select different sites from Gangetic river system of Garhwal region. The Ganges River in Garhwal Figure 1. Map of the study area of Bhagirathi and Alaknanda river system of Garhwal Himalayas. Table 1. Sample collection sites of Alaknanda and Bhagirathi rivers. Alaknada Bhagirathi A1 Vasundhara B1 Bhojwasa A2 Mana B2 Chirwasa A3 Badrinath (Gandhi ghat) B3 Gangotri A4 Badrinath (Rishi ghat) B4 Harsil A5 Gobind ghat B5 Jhala A6 Hanuman Chatti B6 Bhaironghati B7 Gaumukh Himalayas comprises of two tributaries Bhagirathi and Alaknanda, so sampling was done from both the rivers (Figure 1). The total stretch covered in this study was about 250 km, out of which Alaknanda comprised a stretch of 135 km and Bhagirathi about 115 km. Samples were collected during the monsoon, summer and winter seasons. The samples were carefully collected in triplicate from 13 different places (Table 1) in sterile containers, and were transported in ice boxes at 3° C and brought to the laboratory for analysis (Sharma et al 2010). The results presented in the table are average of triplicate samples of a particular site. Bacterial analysis The bacterial population (total viable count, TVC) in different samples was estimated by inoculating nutrient agar plates with 0.1 mL of suitable dilutions. The results were expressed as colony forming units (cfu) per unit volume, enumerated after 48 h of incubation. The water quality was determined by the standard most probable number (MPN) method. Coliforms were de- tected by inoculation of samples into tubes of MacConkey broth and incubation at 37 ± 1 °C for 48 h. The positive tubes were sub cultured into brilliant green bile broth (BGBB) and were incubated at 44.5 ± 1 °C. Gas production in BGBB at 44.5 ± 1 °C was used for the detection of faecal coliform after 48 h incubation. Faecal streptococci were detected by inoculation of water samples into Azide Dextrose broth and incubation at 37.5 ± 1 °C for 24-48 h (APHA et al, 1999). All the culture media were obtained from Hi-Media Pvt. Ltd., Mumbai, India. Physiochemical analysis Physicochemical parameters including total dissolved solids (TDS), conductivity and pH were analyzed on site at the time of sample collection by water analysis kit (Model LT-61, Labtron-ics, Guelph, Ontario, Canada) as per manufacturer instruction. Other parameters i.e. dissolved oxygen (DO), biological oxygen demand (BOD) and chemical oxygen demand (COD) were performed in laboratory by standard ti-trimetric method (APHA et al, 1999). The data were analyzed statistically by using analysis of variance (ANOVA) to find out significance at 5 % levels. In figures, error bars indicate standard error of the mean, where error bars are not visible; they are smaller than the marker. Results The TVC value showed a regular trend (Figure 2). The values increased in monsoon season, thus generally highest counts were observed, intermediate in summer season and least in winter season for each sampling site. The highest TVC was noted in Badrinath ghat of Alaknanda river and Gangotri of Bhagirathi river, where the values were as high as 22.2 x 103 and 19.8 x 103, respectively. The lowest value 10.2 x 103 were recorded in Gobind ghat of Alaknanda and 10.2 x 103 in Bhairong-hati of Bhagirathi river, respectively. The total coliform count was high in all water samples (Figure 3), values ranged from 24/100 mL to 310/100 mL. The highest MPN (310/100 mL) was recorded during monsoon at Vasundha-ra of Alaknanda, the least count MPN (24/100 mL) was obtained in summer and winter season from Bharionghati of Bhagirathi. Even the water samples during less human activities in winter season were not found suitable for drinking as per the Bureau of Indian Standards (BIS), (1991). Results for FC and FS counts have also shown a similar trend to TVC and TC, i.e. higher in monsoon season, intermediate in summer season and least during winter season (Figure 4 and 5). Highest FC count was observed in Alaknanda at Badrinath (160.4, 122.3, 101.2)/100 mL and lowest count was at Mana (15.3, 10.2, 9.8)/100 mL during monsoon, summer and winter season, respectively. In Bhagirathi the Chirwasa and Gangotri sites showed almost similar trend of highest count (45.7, 35.4, 29.8) and 45.3, 36.9, 32.1)/100 mL during monsoon, summer and winter seasons, while the least was observed in Bhaironghati (5.9, 4.5, 3.9)/100 mL during monsoon, summer and winter seasons. Similar trend was also observed in FS, the higher count in Alaknanda was at Badrinath (25, 20, 18)/100 mL, lowest at Mana (8, 7, 7)/100 mL, while in Bhagirathi, highest at Harsil (12, 11, 10)/100 mL and least at Chirwasa (6, 3, 3)/100L. The DO value in Alaknanda ranged from 14.2-18.9 mg/L in monsoon samples and 16.9-23.1 mg/L in winter samples. In Bhagirathi DO values ranged from 10.2-15.4 mg/L in monsoon and 13.2-19.8 mg/L in winter season (Figure 6). Badrinath and Chir-wasa showed a remarkable increase in DO in winter season. Though, in general the DO content of all the river water samples show a uniform trend with varying seasons i.e. least during 25 20 15 x 1 " 10 A6 B1 B2 B3 B4 Sampling sites SI Monsoon [U Summer El Winter B6 B7 Figure 2. Total viable count (TVC) from Alaknanda and Bhagirathi rivers 'il A2 A3 A4 A5 A6 0 Monsoon Q] Summer El Winter B1 B2 B3 B4 B5 B6 B7 Sampling sites Figure 3. Total coliforms Alaknanda and Bhagirathi rivers Ei Monsoon [Q Summer 11 Winter A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 B7 Sampling sites Figure 4. Feacal coliforms count in Alaknanda and Bhagirathi rivers 30 25 20 6 o o 15 10 E3 Monsoon [D Summer El Winter A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 B7 Sampling sites Figure 5. Feacal streptococci from Alaknanda and Bhagirathi rivers monsoon, highest during winter and intermediate in summer season. However, all the samples were found to be saturated with oxygen and were fit for bathing, wild life and irrigation with respect to the amount of dissolved oxygen. The BOD values for most of the water samples were above the permissible limit (Figure 7), samples in monsoon season have high BOD value, and thus the water was not fit for drinking. Considerably higher COD values were recorded in the monsoon season in all the sites of study area, the COD ranged from 4.5 mg/L to 31 mg/L in all water samples (Figure 8). The effect of season was observed in the pH of water samples throughout this study. The pH was slightly alkaline in winter, but al- most neutral in summer and monsoon seasons. Conductivity and TDS in all the sites were found to be well within the minimum prescribed limits (APHA et al, 1999) (data not given). Discussion In present study, all sites were found to have high TVC. In fact, the water of Ganga is used for drinking (Aachman) as part of rituals in this region. Although the higher TVC values suggest that this practice should be avoided. Earlier Baghel et al, (2005) and Sood et al., (2008) have also observed high TVC values in the entire stretch of river Ganga in Uttarakhand region. Baghel AI A2 A3 A4 A5 A6 0 Monsoon CD Summer H Winter B1 B2 B3 B4 B5 B6 B7 Sampling sites Figure 6. Dissolved oxygen in Alaknanda and Bhagirathi rivers 25 20 15 I 10 Al A2 A3 A4 A5 A6 Sampling sites E3 Monsoon □ Summer El Winter B1 B2 B3 B4 B5 B6 B7 Figure 7. Biological oxygen demand in Alaknanda and Bhagirathi rivers 35 30 25 20 15 10 5 0 i Al A2 A3 A4 A5 A6 Sampling sites 0 Monsoon □ Summer 0 Winter B1 B2 B3 B4 B5 B6 B7 Figure 8. Chemical oxygen demand in Alaknanda and Bhagirathi rivers et al, (2005) concluded that large number of animals used by natives and pilgrims in upper stretch of Gangetic river system increase FS load. As a matter of fact, the banks of Alaknanda are more densely populated and face heavy anthropological activity as compared to Bhagirathi. Earlier, Fokmare & Musaddiq (2001) have correlated high content of MPN in surface and ground water of Akola, Maharashtra (India) with the population density. Also the fact that the number of sub-tributaries falling in Alaknanda is more than Bhagirathi may be responsible for the higher col-iform count. The less number of FC and FS in most of the sites of study area may be attributed to the fever anthropological activities. All the sites included in this study were found suitable for bathing purpose with respect to the maximum permissible limits of FC and FS counts as per the standards laid by National River Conservation Directorate (NRCD), India. Earlier, Sood et al, (2008) have also studied water quality of Ganga in Utt-arakhand Himalayas, India and have reported a high level of BOD due to introduction of organic matter into the system as a result of anthropogenic activities. Also these values showed a proportional relation with human activities i.e. the fewer the human activities (in winter), the better the water with respect to physicochemi-cal parameters. Higher BOD values in most of the water samples suggest that either these rivers are rich in organic matter or organic matter is being introduced in the rivers by anthropogenic activities (Tijani et al, 2005), since, BOD provides a direct measurement of state of pollution. Relationship between BOD, COD and microbial count was found inversely proportional, implying that at high organic loading rates, the ecosystem retards the growth of aerobic microorganisms and favors the growth of anaerobes; our findings draws support from Mtui & Nakamurs (2006). The use of coliform bacteria as a measure of the faecal contamination of streams and lakes has been in practice for many years. Our study gives an indication of the extent of relation of microbial pollution and physiochemi-cal parameters; any further addition of wastes may deteriorate the existing hygienic quality in the area. These results suggest that increase of population of coliforms in a river environment are directly proportional to the degree of sewage and human waste pollution, which is reflected by BOD and COD levels. Sah et al. (2000) have stressed on the point that the pollution in rivers and water bodies from industries may adversely affect aquatic life of water bodies' as well human health in the vicinity of rivers/lakes. In a broad view, the river site with higher catchments area, soil cover and land use are more polluted, owing to more anthropogenic activities. McLellan et al, (2001) stated that faecal pollution indicator organisms can be used to a number of conditions related to the health of aquatic ecosystems and to the potential for health effects among individuals using aquatic environments. The presence of such indicator organisms may provide indication of water-borne problems and is a direct threat to human and animal health. Our studies on microbial ecology and physiochemical analysis in the upper Gangetic tributaries in relation to pollution have clearly revealed that there is significant presence of bacterial indicators of faecal pollution; the situation is serious and alarming. Presence of bacterial indicators of faecal contamination in river water at origin clearly revealed the bacteriological status of the water at that site. For this reason, monitoring of microbial contamination in river should be an essential component of the protection strategy in that area. The base line data generated on bacteriological water quality of rivers may serve as biomonitoring standard and comparisons for other rivers and may be useful for all scientists, decision makers and resource managers working with environmental planning and management of such areas. Conclusions The rationale of this study was to evaluate the impact of season and human activities on the pollution status of main upper Gangetic tributaries. This study revealed that tributaries at origin are threatened by high influx of pollutants and enteric pathogenic contamination and it can be concluded that In Alaknanda River, Badrinath is most polluted and Mana is the least, while in Bhagirathi River Gangotri is most polluted and Gaumukh is least. The constant surveillance of these water bodies with respect to the bacterial indicators and physicochemi-cal parameters provides us with the opportunity of true microbiological monitoring of the area as well as proper management actions could be applied in order to improve the quality of these holy rivers and consequently reduce public health risk. Acknowledgments Authors are grateful to the Management of SBS Post Graduate Institute of Bio-Medical Sciences and Research Balawala, Dehradun, (UK), India for providing research facilities required to carry out this work. References APHA, AWWA, WEF (1998): Standards for Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington DC USA. Baghel, V. S., Gopal, K., Diwedi, S. & Tripathi, R. D. (2005): Bacterial indicators of faecal contamination of the Gangetic river system right at its source. Ecol. Indicators, Vol. 5, pp 49-56. BIS (1991): Indian standard specification for drinking water. 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Integrated geophysical and geotechnical investigation of the failed portion of a road in basement complex Terrain, Southwest Nigeria Povezane geofizikalne in geotehnične preiskave poškodovanega dela ceste na ozemlju metamorfne podlage v Jugozahodni Nigeriji Osinowo, O. Olawale1, *, Akanji, A. Olusoji1, Akinmosin Adewale2 University of Ibadan, Department of Geology, Ibadan, Nigeria 2University of Lagos, Department of Earth Sciences, Lagos, Nigeria *Corresponding author. E-mail: wale.osinowo@mail.ui.edu.ng Received: March 23, 2011 Accepted: May 3, 2011 Abstract: Several efforts by the local authority to fix the bad portions of Ijebu-Ode-Erunwon road, southwest Nigeria have yielded no meaningful result, as the road often get deteriorated shortly after repairs. Geophysical investigation integrated with geo-technical studies were undertaken to determine causes of the consistent failure of the highway. Very Low Frequency Electromagnetic (VLF-EM) and Electrical Resistivity (ER) methods were employed to map sections of the road with anomalous electrical responses and interpreted in-terms of structures, li-thology and water saturation. VLF-EM plots identified positive peaks of filtered real amplitudes greater than 30 % which correspond to major and minor linear fractures within the basement rocks. High current density >30 and low resistivity <10 Q m delineated rock units underlying the failed pavement to be water saturated. Liquid limit, linear shrinkage and plastic limit index results; 24.0-48.5 %, 2.1-12.9 %, 7.5-27.4 % respectively, indicate excellent to good engineering index properties. However, soaked and unsoaked CBR results; 70.3-83.9 %, and 12.9-31.6 % respectively, indicate percentage reduction in strength with wetness up to 80 %. This study implies that integrated geophysical and geotechni-cal investigation offers very useful approach for characterizing near surface earth which could be helpful in site preparation prior to construction. Izvleček: Vrsta poizkusov krajevnih oblasti, da bi popravili slabe odseke ceste Ijebu-Ode-Erunwon v jugozahodni Nigeriji ni bila uspešna, ker se je navadno stanje ceste poslabšalo kmalu nato, ko so jo popravili. Da bi ugotovili vzroke za ponavljajoče se propadanje ceste, so opravili geofizikalne raziskave v povezavi z geotehničnimi študijami. Z zelo nizkofrekvenčno elektromagnetno metodo (VLF-EM) in metodo specifične električne upornosti (ER) so preiskali odseke ceste z anomalnimi električnimi lastnostmi in jih interpretirali z ozirom na zgradbo, litolo-gijo in nasičenost z vodo. Na diagramih VLF - EM so ugotovili pozitivne vrhove filtri-ranih realnih amplitud večjih od 30 %, ki ustrezajo večjim in manjšim linearnim razpokam v kamninah podlage. Visoka gostota toka >30 in nizka specifična upornost <10 Q m sta značilni za zemljine, nasičene z vodo, ki leže pod poškodovanim cestnim površjem. Vrednosti meje tečenja 24.0-48.5 %, meje krčenja 2.1-12.9 % in indeksa meje plastičnosti 7.5-27.4 % nakazujejo od odlične do dobre inženirske indeksne lastnosti. Toda rezultati preskusa CBR v nasičenem in nenasičenem stanju, 70.3-83.9 % in 12.9-31.6 %, kažejo, da se zmanjša nosilnost pri vlagi do 80 %. Raziskava priča o tem, kako uporabno je povezati geofizikalne in geotehnične preiskave za karakteri-zacijo pripovršinskih tal, kar utegne biti smotrno pri preiskavi terena pred samo gradnjo. Keywords: electromagnetic, resistivity, geotechnical, basement complex, Ijebu-Ode Ključne besede: elektromagnetna metoda, specifična upornost, geotehnične metode, kamnine podlage, cesta Ijebu-Ode, Nigerija Introduction Flexible highway aids easy and smooth vehicular movement, and has been very useful for transportation of people, goods and services from one point to another, especially in developing countries where other means of transportation such as rail, underground tube, air and water transport system have remained largely undeveloped. However, bad portions of road, many of which result from poor construction or being founded on incompetent sub-grade and sub-base materials had been found to do more harm than good. They have been responsible for many fatal accidents, wearing down of vehicles and waste of valuable time during traffic jams. The various types of road failure identified in the study area include failure of the black top surfacing, especially along wheel cracks, pitting or minor dent, shear or massive failure (pot-holes) extending through the pavement occasionally to the subgrade. (Plate 1) The integrity of near surface geophysical investigation methods to complement geotechnical studies in some foundation engineering problems cannot be overemphasized. This research therefore integrates Electromagnetic, Electrical Resistivity and geotechnical techniques to study the causes of consistent failure of Luba-Erunwon axis of Ijebu-Ode-Erunwon road. It involves lateral and vertical probe of the failed, fairly stable, fairly failed and stable portion of the road in order to characterize the near surface geologic materials that constitute the sub-grade, sub-base and the foundation upon which the pavement was founded. The study area is situated within the southwestern part of Nigeria, it lies between longitudes 6049' N and 6°52' N, latitude 3056' E and 3058' E and the studied portion of the pavement is about 2 Plate 1. Failed section of Luba-Erunwon axis of Ijebu-Ode-Erunwon road km in length. The road was initially constructed in 1983 and have since suffered major failures, especially in the northeastern end, towards Erunwon axis of the road. The road has been repaired severally, the repairs usually include minor repair of the road element and resurfacing. However the northeastern part of the road always starts to deteriorate barely six months after reconstruction. Geological setting Ijebu-Ode and environ lies within the transitional zone between the Precam- brian Basement Complex rocks of the southwestern Nigeria and the Cretaceous sediments of Abeokuta Group in eastern part of Dahomey Basin. The basement rocks occur predominantly in the north, northwest and northeastern parts of the field and it is predominantly a Migmatite Gneiss Complex of biotite granite gneiss, biotite-horn-blende gneiss with varying degrees of fracturing (Olayinka and Osinowo, 2009). The southern part of the field is overlain by Ise Member of Abeokuta group that unconformably overlies the basement rocks. Litho-stratigraph-ically, Abeokuta Group comprise of Figure 1. Geological map of Ijebu-Ode and its environ with geological map of Nigeria inserted grits, arkosic sandstones, siltstones and clay with occasional conglomerate of predominantly arenaceous materials (Omatsola and Adegoke, 1981). Figure 1 shows the geology of Ijebu-Ode and its environ. Materials and methods Electromagnetic method is one of the geophysical methods commonly used in foundation investigation and environmental studies (Olorunfemi & Mesida, 1987; Sharma, 1997). The principle is based on induction of a secondary magnetic field Hs in the subsurface conductor of conductivity o due to effect of an artificially generated primary field Hp. Electromagnetic measurements are usually presented as the mutual impedance ratio Z/Z0 or relative charge in the impedance over a conductor which has ability to provide clear information about the subsurface conductivity and structure. Z/Z0 - 1 = HHrn. o. h. s) HpZ (s) " ABEM WADI was used for VLF-EM measurements, it uses military transmitters as the source of primary electromagnetic waves Hp which is located several kilometers away at the high powered military communication transmission stations. The transmitter's antenna transmits signals continuously at low radio frequency range of 15-30 kHz. The signals generated can travel long distance and able to penetrate the subsurface to induce eddy current in buried conductors. The technique measures the components of Very Low Frequency EM field which are related to the geoelectric structure of the subsurface. (Chouteau et al, 1996). Five VLF-EM profile stations were occupied with the profile length ranging from 250 m to 850 m. Readings were taking at station interval of 3 m and 6 m. Measurements such as raw real, raw imaginary, station's latitude and longitude and the signal strength were recorded against station interval. Electrical resistivity investigation of the subsurface involved determination of the distribution of ground resistivity based on its response to the flow of electric current injected during surface measurement. True ground resistivity of the subsurface can be estimated and can further be employed to interpret the subsurface qualitatively and quantitatively ((Loke, 2001). Georesistiv-ity survey involved measurement of potential difference generated by the current electrodes adapted to Wenner and Schlumberger electrode configurations. p = AV/I • K 'a K is the geometric factor. Two measurement methods were adopted; 1-D Vertical Electrical Sound- ing (VES) and 2-D resistivity measurement using Electrical Resistivity Traversing (ERT) technique. The 1D VES measurements aimed at determining the variation in the geoelectric parameters with depth at the probed stations while 2D method mapped resistivity continuity useful to delineate structurally weak zones that could be responsible for continuous failure of the road. Ge-opulse Tigre resistivity meter was used to measure ground resistance. Current electrodes for 1D measurement were spread from AB/2 of 1 m to 133 m for VES measurement. Two dimensional measurements was made by increasing the electrode spacing along the levels. Ten levels along profiles were covered with electrode spacing range from 3 m to 30 m at incremental step of 3 across 100 m long profile. Geotechnical studies to determine some engineering index properties of sub-grade and sub-base materials employed to corroborate the geophysical measurements involved collection of twelve disturbed bulk samples from four pits each drilled to depth of 1 m and at sampling depths of 0-0.3 m, 0.3-0.6 m and 0.6-1.0 m from each pit. Sample recovering pits were constructed at the failed, fairly stable, fairly failed and stable parts of the road at 80 m, 247 m, 300 m and 470 m on the road. Mechanical sieving helped determined particle size distribution of gravel and sand proportions of dried coarse frac- tion. Consistency Limit Tests generally known as the Atterberg limits gave the plasticity characteristics of the cohesive fraction of the sieved samples. The consistency limit test includes; liquid limit, plastic limit and linear shrinkage test. The difference between the liquid and plastic limits gave the plasticity index, which is the range of moisture contents over which the soil remains plastic. California Bearing Ratio (CBR) test, widely used to characterize and select sub-grade materials for use in road construction was carried out. The test was devised by the California Highway Association and it is simply the ratio of the load that cause a penetration of 2.5 mm or 5.0 mm material to a standard load that causes similar penetration on a standard California sample, notably 13.24 kN and 19.96 kN respectively. C B R = Load that caused a penetration of 2.5/5.0 mm x 100 % 13.24/19.96 (kN) Both soaked and unsoaked CBR tests were carried out and swelling of samples was carefully monitored during the 96 h of soaking period to assess the likely effect of water ingress on the swelling of base material. The samples were compacted at the modified AASHTO level as described under procedure for compaction test in a standard CBR mold. Data processing The obtained raw real (in-phase) and raw imaginary (quadrature) components contain valuable diagnostic information of the subsurface but in a complex pattern that cannot directly and easily be related to the causative body. They contain noise, the raw real/ imaginary data are also often wrongly located on the source along the profile. To correct the above effects and obtain profiles or pseudo-section/images that are easy to interpret, two different data processing techniques were applied. Fraser (1969) and Karous & Hjelt (1977, 1983) filtering operators. Fraser filter is a linear high-frequency bandpass filter that yields semi-quantitative interpretation of data. It transforms the in-phase components into contourable data with noise reduced to the best possible minimum. VLFPROS MATLAB code for processing VLF-EM data developed by Sundararajan et al. (2006) was employed to carry out both the Fraser and the Karous and Hjelt filtering operations. Electrical resistivity data processing involved cleaning the data to remove spurious readings. Resultant VES data were plotted on bi-log paper and partial curve matched using standard two layer curves and auxiliary curves; Cagniard graph (Koefoed, 1979), to obtain some geoelectrical parameters such as layer depth/thickness and layer resistivity values (Orellana & Mooney, 1966). The obtained geoelectrical parameters from partial curve matching were used as initial model parameters to interpret the geoelectrical sounding curves using inversion model software RESIST (vander velpen, 1988) and WinG-Link. The inversion algorithm involves the calculation of curves for observed data by convolving the resistivity transform with appropriate filter coefficient, (Ghosh, 1971 and o'Neill, 1975). The inversion algorithm filters spurious data, enhance signal as well as correct depth matched for obtained geoelectric layers. Data Quality Check (QC) was carried out on the obtained ERT data for spurious data. The resultant data were inverted using the DIPRO inversion software based on the inversion principle presented by Yi & Kim, (1988). The software is a 2 2 dimensional inversion subroutine designed based on the Least Square inversion algorithm and uses two different modeling and smoothening approaches. The FDM Inversion performs smoothness constrained least square inversions based on the finite difference modeling assuming flat topography, while the FEM performs smoothness constrained least square inversion based on finite element modeling. The software automatically determines a two dimensional resistivity model of the subsurface for the obtained data. A forward modeling 150 osinqwq, o. o., Akanji, o. A., Akinmqsin A. subroutine is applied to calculate theoretical apparent resistivity values and a non-linear least squares optimisation techniques was used for the inversion subroutine, (DeGrqqt-Hedlin & Constable, 1990 and Sasaki, 1989). Results Palacky et al. (1981), De Rqqy et al. (1986), Hazell et al. (1988) and other authors have shown the relevance of EM method to be in overburden thickness estimation and basement fracture delineation. Figures 2 (a-f) present the VLF-EM plot of raw real and filtered real components against the profile distance in meters. Two basic anomaly types were identified using characteristic feature curves of coincident inflections on real component anomaly curves as well as the amplitude of the filtered real anomaly. The sign 'F' indicates point with positive peak filtered real anomaly with amplitude ranging between 30-60 %. It characterizes regions or points along the profile with major linear displacement at depth <5 m which may represent a fractured or sheared zone. The sign 'f' indicates positive filtered real anomaly of amplitude <30 % and characterizes zones or points with loose materials at depth <5 m. Five major linear features F1-F5 were delineated at 87 m, 178 m, 298 m, 657 m and 810 m of the profile. Features F1, F2 and F5 were identified at the failed portion while features F4 and F3 were found at the fairly failed part of the road respectively. This shows that 60 % of the identified major features underlie the intensely failed portion while 40 % underlies fairly failed portion. Also, features fj-fg were detected around locations 203 m, 3330 m, 410 m, 482 m, 553 m, 578 m, 848 m, 1040 m of the profile length. Figure 3 is the current density plot along profile 1 which traversed the failed, fairly stable and stable portions of the road from NE-SW. The profile indicates relatively high conductive zone as evident by high current density (up to 30) close to the surface in the north-eastern end and central part of the profile. These zones coincide with the failed portion of the road, it also coincides with the highly fractured part. The high conductivity is likely due to high water filled fractures in the basement rock. Similarly, low resistivity section at the north-eastern end of the road (high conductivity) was identified on the 2-D inverted section obtained from the ERT profile (Figure 4) and resistivity section (Figure 5) constructed from VES data around the study area. The stable portion has a relatively thick and dry sandy unit upon which the pavement rests directly. Vertical Electrical Soundings (VES) identified three to four layered earth interpreted as top soil, loose saturated clayey sand unit and highly saturated fractured basement at the failed section of the road. Figure 6 (a and b) present the representative around the stable and failed portion of curves and interpreted log of VES data the road. t id idp 11» ilo CI 1u 134 up Hi Figure 5. Inverted Electrical Resistivity section from VES data around the study area. VES 31 Layers Resistivity (Q m) 311 77.1 808 2813 Depth (m) 0.45 1.1 18.9 Interpretation Top soil Sandy clay Unsaturated sand Fresh basement rock Figure 6a. Representative VES curve and interpreted log around stable part of the Ijebu-Ode-Erunwon road VES 25 Layers 2 Resistivity (Q m) 516 283 4144 Depth (m) 1.57 16.1 Interpretation Top soil Fractured basement rock Fresh basement rock Figure 6b. Representative VES curve and interpreted log around failed part of the Ijebu-Ode-Erunwon road 1 2 3 4 1 3 4 5 Results of Geotechnical analysis Geotechnical investigation of the highway is aimed at ascertaining geotechnical bases for the road failure. Table 1 presents a description of the recovered samples. Grain size distribution Table 2 presents the summary of the grain size distribution. Samples 2C, 3B, 3C, 4B and 4C have higher fines constituent than other samples. The percentage of fines ranges from 22-26 %, 23-45 %, 21-37 % and 24-46 % for samples from failed, fairly stable, fairly failed and stable parts of the road respectively. The result indicates that the soil units below the stable parts of the road exhibit better engineering properties than those of the failed, fairly failed and fairly stable portions. Table 1. Soil profiles in the study area Depth Range (m) Road condition Pit No Sample code Colour Name 0-03 1 1A Brown Clayey sand 0.3-0.6 Failed 1 1B Brown Clayey sand 0.6-1.0 1 1C Very brown Clayey sand 0.0-3 2 2A Brown Clayey sand 0.3-0.6 Fairly stable 2 2B Brown Sandy clay 0.6-1 2 2C Reddish brown Sandy clay 0-0.3 3 3A Brown Sandy clay 0.3-0.6 Fairly failed 3 3B Reddish brown Silty clay 0.6-1 3 3C Reddish brown Silty clay 0-0.3 4 4B Reddish brown Silty clay 0.3-0.6 Stable 4 4B Reddish brown Silty clay 0.6-1 4 4C Reddish brown Silty clay Sample Depth (m) Medium Gravel/% Fine Gravel % Coarse Sand/% Medium Sand/% Fine Sand % Fines (Clay and Silt)/% 1A 0-0.3 7.0 7.0 19.0 27.0 14.0 26.0 1B 0.3-0.6 2.0 4.0 20.0 37.0 14.0 23.0 1C 0.6-1.0 1.0 1.5 17.5 38.0 20.0 22.0 2A 0-0.3 0.1 0.1 16.9 42.0 17.0 23.9 2B 0.3-0.6 0.1 0.1 20.8 45.0 13.0 21.0 2C 0.6-1.0 0 0 12.5 40.0 10.5 37.0 3A 0-0.3 3.0 2.0 19.5 39.5 13.0 23.0 3B 0.3-0.6 0.1 1.9 16.0 33.5 10.5 37.0 3C 0.6-1.0 0 0.1 15.9 30.5 8.5 45.0 4A 0-0.3 2.0 1.5 17.5 41.5 13.5 24.0 4B 0.3-0.6 0 0 21.0 27.0 14.5 37.5 4C 0.6-1.0 0 0 15.0 29.5 9.5 46.0 Table 2. Grain size distribution of the studied soil samples Table 3. Atterberg Limit result of the clay fraction of the recovered samples Sample Depth (m) Road Condition Liquid Limit (%) Plastic limit (%) Plasticity Index Linear Shrinkage 1A 0-0.3 Failed 27.0 15.6 11.4 6.4 1B 0.3-0.6 26.8 15.0 11.3 7.8 1C 0.6-1.0 24.0 14.7 9.3 6.4 2A 0-0.3 Fairly Stable 24.0 16.1 7.9 2.1 2B 0.3-0.6 26.0 18.5 7.5 5.7 2C 0.6-1.0 50.0 29.4 20.6 12.9 3A 0-0.3 Fairly Failed 24.0 13.7 10.3 2.1 3B 0.3-0.6 48.5 21.1 27.4 9.3 3C 0.6-1.0 56.0 26.2 29.8 12.1 4A 0-0.3 Stable 26.0 13.1 12.9 2.1 4B 0.3-0.6 36.0 17.7 18.3 8.6 4C 0.6-1.0 51.0 25.3 25.7 10.4 According to the American Association of State Highway and Transportation Official (AASHTO) classification system, samples from failed portions, samples 1A,1B and 1C fell into A-2-6, A-2-6 and A-2-4 groups which are excellent to good materials for sub-grade soil in rating, while samples 3A, 3B and 3C of the fairly failed portions fell into A-2-4, A-7-6, and A-7-6 groups which corresponds to excellent to good and fair to poor soils respectively for sub-grade material. Also, samples 4A, 4B and 4C of the stable portion of the road fell into A-2-6, A-6 and A-4 groups which corresponds to excellent to good and fair to poor soils respectively. Samples 2A, 2B and 2C fell into A-2-4, A-2-4 and A-7-6 groups and also correspond to excellent to good and fair to good soils respectively for subgrade materials. Table 3 presents the plastic limit test results, the range and mean values range from 14.7-15.6 % and 15.1 %; 13.726.2 % and 20.3 %; 16.1-29.4 % and 21.3 % and 13.1-25.3 % and 18.7 % at the failed, fairly failed, fairly stable and stable portions respectively. The respective plasticity indexes which is the difference between the liquid limits and plastic limits range and mean of 9.3-11.4 % and 10.7 % respectively for failed portion, 10.3-29.8 % and 22.5 % respectively for fairly failed portion, 7.5-20.6 % and 12.0 % respectively for fairly stable portion and 12.9-25.7 % and 19.0 % respectively for stable portion. Linear Shrinkage The linear shrinkage at the failed, fairly failed, fairly stable and stable portions have range and mean of 6.4-7.8 % and 6.9 %; 2.1-12.1 % and 7.8 %; 2.1-12.9 % and 6.9 %, and 2.1-10.7 % and 7.1 % respectively. Madedor (1983) and Adeyemi (1992) gave the maximum value of 8 % as linear Shrinkage for highway subbase materials and maximum of 10 % was specified for sub-grade materials. It can therefore be concluded that at the failed portions the liquid limits, plasticity index and linear shrinkage for samples 1A, 1B and 1C satisfied all the required standards for highway sub-base and while at the fairly failed portion sample 3A satisfied all the require standards for sub-base materials, but samples 3B and BC failed the requirements for sub-grade material. At the stable portions, samples 4A and 4B satisfied the required standard by FMWH (2000) for sub-base and subgrade soils respectively while samples 4C has slightly higher liquid and plasticity index. At the fairly stable portion, samples 2A and 2B also satisfied the requirements for sub-base and subgrade materials for highways but samples 2C has slightly higher liquid and linear shrinkage values. From the Casagrande chart (Figure 7), the classification of the soil samples 1A, 1B, 1C, 3A, 2A, 2B and 4A plotted within the region of low plasticity while samples 2C, 3B, and 4B, 4C plotted within Figure 7. Casagrande Chart Classification of studied samples region of medium plasticity. Sample 3C plotted completely within region of high plasticity. The result indicates that samples 1A, 1B andlC (of failed portion) 2A, 2B (of fairly stable portion), 3A (of fairly failed portion) 4A, 4B and 4C (of the stable portion) are good sub-base and sub-grade materials while samples 3C of fairly failed portion is unsuitable for sub-grade material. California Bearing Ratio Strength The dry density of the soils and optimum moisture content of the soils compacted at the modified AASHTO is shown in the Table 4. It shows percentage reduction in strength as a result of soaking of the compacted samples and it ranges between 55-83 %. Figures 8 (a-e) present the unsoaked and soaked CBR curves of the studied samples. Most of the analyzed samples have the required 80 % unsoaked CBR value recommended for highway sub-base and sub-grade soils by the FMWH (2000) but only sample 1B satisfied the required unsoaked CBR value of 30 %. Table 4. Maximum dry density and optimum moisture content of the samples compacted at the modified AASHTO level. Sample Depth (m) Road condition Maximum dry density (kg/m ) Optimum moisture content (%) 1B 0.3-0.6 Failed 1970 10.95 1C 0.6-1.0 1910 10.6 3B 0.3-0.6 Fairly failed 1730 12.4 4B 0.3-0.6 Stable 1830 12.0 4C 0.6-1.0 1780 14.0 Table 5. CBR result Sample unsoaked CBR (%) Soaked CBR (%) Percentage Reduction in strength (%) 1B 71.3 31.6 55.68022 1C 83.9 15.3 81.764 3B 76.7 12.9 83.18123 4B 70.3 19 72.97297 4C 77.6 17 78.09278 158 osinqwq, o. o., Akanji, o. A., Akinmqsin A. Figure 8. California bearing ratio of the studied soils compacted at the optimum moisture content of the modified AASHTO Discussion The geophysical investigation results gave horizontal variation in conductivity of the subsurface materials underlying the flexible highway pavement at different portions in view of ascertaining geophysical bases if any for the causes of the highway failure. The interpretation of the plots of real and filtered real curves of the geophysical survey showed that majority of the identified major and minor linear features which represent zones of anoma- lously high conductivity were detected at the intensely failed portions of the road. These features are suspected to be numerous fractures in the underlying basement rocks and hence, conductive discontinuity in the shallow overburden. The geotechnical analysis of the twelve bulk soil samples collected at different portions of the highway helped determined the variation in engineering index properties of the subbase and sub-grade soils. Geotechnical results revealed that the sub-base and sub-grade soils in the depth range of 0-1.0m of the failed portions satisfied all the requirements of liquid limits, plasticity index, linear shrinkage, grain size distribution characteristics specified by the Federal Ministry of Works and Housing. These soils also plot within the region of low plasticity on Casagrande chart classification and under AASHTO classification into groups A-2-6, A-2-6 and A-2-4 which indicate that the soils are excellent to good materials for subgrade soils. In terms of shear strength most of the analyzed samples have adequate values of unsoaked CBR. However, they suffered very high reduction in shear strength of up to 80 % in soaked condition. The numerous major and minor linear fractures mapped in the shallow basement as delineated by VLF - EM methods have tendency to increased the permeability of the portion and thus cause the it to be highly saturated. This is evident in high current density and low resistivity values associated with the failed region. This has particularly made the excellent to good sub-base and sub-grade materials to perpetually remain in soaked condition. The geotechnical implication of this is that the highway pavement is resting on sub-grade, sub-base and foundation materials that lost as high as 80 % of shear strength as a result of ingress of water. Conclusion Both geophysical and geotechnical investigation methods have been used to characterize different portions of the flexible highway in the study area. The highway was reported to always fail barely six months after reconstruction. Geophysical investigation delineated numerous anomalously high current density and low resistivity zones as well as identified some major and minor linear fractures in the shallow underlying basement rocks. Also, the geotechnical investigations although showed that the samples collected generally displayed excellent to good engineering index properties as sub-base and sub-grade materials, but suffers up to 80 % reduction in strength with wetness. The interaction of the sub-grade and sub-base soils with water from numerous fractures in the basement rocks has greatly reduced shear strengths and therefore incessant failure of the overlying pavement. Removal and replacement of the subgrade and sub-base materials upon which the flexible pavement rests (up to 1 m) or soil stabilization / treatment that would improve materials shear strength under wet condition is suggested as well as construction of proper drainage system to drain the whole length of the road. 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