Geophysical Investigation of the Impact of Solid Waste Dump on Subsurface Soil and Groundwater in Eneka, Rivers State Nigeria

GeoScience Enginee ing Vol. 66 (2020), No. 3 h p://gse. sb.cz pp. 121–135, ISSN 1802-5420 DOI 10.35180/gse-2020-0037 GEOPHYSICAL INVESTIGATION OF THE IMPACT OF SOLID WASTE DUMP ON SUBSURFACE SOIL AND GROUNDWATER IN ENEKA, RIVERS STATE NIGERIA Cy il N. NWANKWO 1, Ma k OGORO 2 1 Uni e si y o Po Ha cou , Depa men o Physics, Nige ia 2 Uni e si y o Po Ha cou , Depa men o Geog aphy and En i onmen al Managemen , Nige ia E-mail: cy il.nwankwo@unipo .edu.ng ABSTRACT The geophysical elec ical me hod was used o in es iga e how he e use dump in Eneka (Ri e s S a e, Nige ia) a ec s he subsu ace soil and g oundwa e wi hin i s en i ons. The esea ch was ca ied ou applying Wenne and Schlumbe ge a ay con igu a ions o elec ical esis i i y echniques o image he subsu ace esis i i y wi hin he a ea using ABEM SAS 300 Te ame e . A o al o en loca ions we e in es iga ed and he measu ed da a ac oss he p o iles we e p ocessed using RES2DINV and A cGIS 10.4 compu e i e a i e so wa e. The esul ing in e se esis i i y model isola ed h ee esis i i y zones (anomalously low, in e media e and high esis i i y). The anomalously low esis i i y zone was in e p e ed as con aminan leacha e plumes and land ill gases om he dump a ea was obse ed o ha e a elled o dep hs o 14 m below he su ace and o e 30 m dis ance eas wa ds om he dump si e, showing he endency o a m land and c ops pollu ion. Howe e , he aqui e laye in he a ea es ima ed a he dep h o 40 m may no ha e been con amina ed. The spa ial end o almos all he esis i i y alues measu ed a equidis ance e eals low alue o lines 3 and 4 (which a e a he away om he dump si e) a he beginning and high alue a he end and ice e sa o lines 1 and 2. A he a e a which he leacha e has in il a ed he subsu ace he aqui e wi hin he a ea is likely o be con amina ed in he u u e i adequa e measu es a e no aken. Keywo ds: Dump si e; Elec ical esis i i y; Eneka; G oundwa e ; Leacha e plume. 1 INTRODUCTION Ac i i ies o man on Ea h gi e ise o esidual ma e ials ha may no be o immedia e use. Such ma e ials may be ecycled, eclaimed, o e-used; o he wise hey cons i u e was e (pollu an s) ha will ul ima ely be eleased o he en i onmen in mobile o m o in si u [1]. These pollu an s pose common en i onmen al p oblems ha ha e c ea ed he need o ind sui able me hods o moni o ing he ex en o such en i onmen al damage [2]. A eas nea land ills o dump si es a e p one o g oundwa e and soil con amina ion because o he p esence o leacha e o igina ing om he sou ce o he na u al en i onmen . The umes/odou emana ing om solid was e can also lead o skin and blood in ec ions, eye and espi a o y in ec ions, as well as ch onic diseases such as cance s [3]. Economic ac i i ies a e also a ec ed as people will no like o loca e hei companies, businesses and houses in oxic a eas/en i onmen . Land and wa e bo ne diseases and in ec ions may also occu nea such dump si es. 122 GeoScience Enginee ing Vol. 66 (2020), No. 3 h p://gse. sb.cz pp. 121–135, ISSN 1802-5420 DOI 10.35180/gse-2020-0037 The use o geophysics o enginee ing and g oundwa e con amina ion s udies has inc eased in ecen ime wi h apid ad ances in compu e so wa e and associa ed nume ical modeling solu ions. The geoelec ical imaging me hod has been widely used in en i onmen al and geo echnical in es iga ions o de ec he su ace e ec s p oduced by he low o elec ic cu en inside he ea h [4]. The e ical elec ical sounding (VES) echnique has been used in a wide ange o geophysical in es iga ions such as mine al explo a ion, a cheological in es iga ion, enginee ing s udies, geo he mal explo a ion, pe ma os mapping and geological mapping [5–8]. Soil which is an essen ial esou ce o he exis ence o plan li e, ood p oduc ion and indus ial ma e ials can easily be a ec ed by con amina ion om su ace pollu ion, was e disposal o oxic indus ial was es [9]. In his s udy, e ical elec ical sounding (VES) and wo-dimensional (2-D) geoelec ic me hods we e used o delinea e he aqui e dep h and le el o con amina ion on he subsu ace soil and g oundwa e om land ill mixed wi h bo h haza dous and non-haza dous ma e ials om indus ies, homes and o ices. This was achie ed by measu ing he elec ical esis ance o he subsu ace, e alua e he dep h and di ec ion o low o he leacha e esul ing om he was e ma e ials and hence delinea e he possibili y o con amina ion o nea su ace o ma ion and aqui e , based on in e p e ed esis i i y models. Se e al wo ks ha e been documen ed on he use o a ious geophysical elec ical echniques in p obing he e ec o leacha e plume a land ill si es on soil and g oundwa e [10–13]. Roseq is in [14], ca ied ou 2-D esis i i y imaging aimed a mapping possible leacha e plumes a wo land ill si es in Sou h A ica. The in e se model sec ion e eals he ex en o he leacha e plumes in he land ills, wi h esis i i y alue o less han 10Ω-m. Resea ch in [15] mapped con amina ed plumes a municipal solid was e disposal si es in Malaysia using geoelec ic imaging echnique. The esul o he in e p e a ion de ined he con amina ed leacha e plumes as elec ically conduc i e anomalies o ela i ely low esis i i y alues less han 10Ω-m. Also, Ehi im e al. in [16] ca ied ou 2-D esis i i y imaging o de e mine he impac o municipal solid was e land ill in Po Ha cou Municipali y. Two dis inc i e zones we e mapped, he zones o anomalously low and high esis i e s uc u es. The low esis i e s uc u es we e in e p e ed as ock ma e ials con amina ed wi h leacha e plumes ha a e p edominan ly me hane, ammonium, hyd ogen sulphide and ca bon dioxide. 2 GEOLOGY OF THE STUDY AREA Eneka own is loca ed wi hin he No h-Cen al axis o Ri e s S a e, Nige ia be ween la i udes 4°53’N h ough 4°54’N, and longi udes 7°0’E h ough 7°2’E (Fig. 1). The egion is cha ac e ized by low lying plain wi h mean ele a ion 13 me e s below mean sea le el. Eneka lies wi hin he opical bel , cha ac e ized by abundan ain all almos all yea ound, wi h he excep ion o he mon hs o Decembe h ough Feb ua y. An annual ain all o abou 240 cm, ela i e humidi y o o e 90 % and mean annual empe a u e o 27°C is common in he a ea [8]. The ainy/we season occu s be ween Ap il and Oc obe wi h hea y ain all and ela i ely cons an humidi y. Thick mang o e o es , a ia palms and ligh ain o es a e he majo ypes o ege a ion. The s udy a ea is a ypical Nige Del a sedimen a y en i onmen wi h h ee s a ig aphic uni s, namely, he Benin, Agbada and Aka a Fo ma ions in o de o inc easing age [17]. The o ma ions consis o pe meable sands, al e na ions o shale, sands ones and sil s one; and hick shale sequence a he base. The a ea is known o high aqui e po en ial, wi h he g oundwa e lowing in he NW-SE di ec ion, in line wi h he Nige Del a end. 123 GeoScience Enginee ing Vol. 66 (2020), No. 3 h p://gse. sb.cz pp. 121–135, ISSN 1802-5420 DOI 10.35180/gse-2020-0037 Figu e 1. Map o he s udy a ea 3 MATERIALS AND METHODS The geophysical in es iga ion in ol ed wo elec ical esis i i y echniques: Ve ical Elec ical Sounding (VES) using he Schlumbe ge con igu a ion and 2-D elec ical imaging using Wenne a ay. The VES echnique uses he p inciple o s a ic elec ical a ay ha is pulled ac oss he g ound su ace o con inuous co e age o he esis ance o he subsu ace, u ilizing esis i i y me e , ABEM Te ame e SAS 300. A o al o six s a ions we e occupied along he a e se wi h a maximum cu en sepa a ion o 300 m and po en ial elec ode sepa a ion o 15 m. The eels o cables we e laid along he a e se wi h he cu en and po en ial elec odes dis ances measu ed wi h measu ing apes. The po en ial elec odes MN we e placed e y close o he sounding poin such ha MN is abou i e imes less han he cu en elec odes, AB. Po en ial elec ode sepa a ion (MN/2) is measu ed a 0.5 m in e al and a each ixed poin , cu en was injec ed in o he g ound and he esul ing po en ial d ops be ween he elec odes we e measu ed by he me e , which au oma ically calcula es he esis ance ha was displayed on he sc een. The cu en elec odes we e hen mo ed o nex ma ked poin o ano he shoo ing while he po en ial elec odes emained ixed a he poin , bu mo ed only when he eading becomes e y small. The acqui ed da a we e p ocessed using esis i i y in e sion (RES2DINV) compu e i e a i e so wa e and p esen ed as sounding cu es, which a e plo s o appa en esis i i y alues agains cu en elec ode sepa a ion (AB/2). Quan i a i e in e p e a ion o he sounding cu es ga e in e p e ed esul s as geoelec ic pa ame e s ( ha is, laye esis i i y and laye hickness). In he 2-D esis i i y imaging, ou ho izon al p o ile s a ions we e occupied, u ilizing a mul i-elec ode sys em wi h equal minimum spacing ‘a’ be ween successi e elec odes (Fig. 2). The mul i co e cable was laid on he g ound in a s aigh line along he a e ses wi h in e al spacing anges o 5 m o 30 m a each sounding s a ion. The elec odes we e connec ed o a Cen al swi ching sys em (Te ame e ) and cu en s we e injec ed in o he g ound ia he cu en elec odes (C1 and C2) loca ed a he ex e io o he po en ial elec odes (P1 and P2). The po en ial di e ence be ween he po en ial elec odes we e measu ed and he esis ance o he g ound was calcula ed 124 GeoScience Enginee ing Vol. 66 (2020), No. 3 h p://gse. sb.cz pp. 121–135, ISSN 1802-5420 DOI 10.35180/gse-2020-0037 C C 1 P P a au oma ically by he me e . A e aking he i s eading a s a ion 1, he cables and elec odes we e mo ed in a leap- og manne o he nex posi ion o he second eading. This p ocess con inued un il all measu emen poin s along he a e se we e co e ed. Two hund ed and nine y- wo da a poin s we e acqui ed and subsequen ly p ocessed using he RES2DINV and geog aphic in o ma ion sys em (A cGIS 10.4) so wa e. Figu e 2. Wenne a ay con igu a ion 4 RESULTS The RES2DINV p og am displayed he esul o 2D imaging ield da a as in e se pseudo-sec ion ha gi es he esis i i y o he subsu ace laye s as a unc ion o e ical dep h (Figu es 3 o 6). Th ee dis inc zones we e iden i ied om he model sec ion as zones o low, in e media e and high esis i i y. Figu e 3. 2-D esis i i y pseudo sec ion o P o ile 1 Figu e 4. 2-D esis i i y pseudo sec ion o P o ile 2 a a 125 GeoScience Enginee ing Vol. 66 (2020), No. 3 h p://gse. sb.cz pp. 121–135, ISSN 1802-5420 DOI 10.35180/gse-2020-0037 Figu e 5. 2-D esis i i y pseudo sec ion o P o ile 3 Figu e 6. 2-D esis i i y pseudo sec ion o P o ile 4 P o ile 1 The in e se model Pseudo sec ion has esis i i y alues lowe han 4.89 Ω-m and g ea e han 3308 Ω-m. The con amina ed low esis i i y zone (deep o ligh blue) wi h esis i i y alue ha is lowe han 4.89 Ω-m was isola ed a he lowe sec ion o he p o ile a su ace poin s 35.5 m o 52.0 m and dep hs ange o 12.5 m o 14.8 m. This zone is o e lain by mo e esis i e laye s wi h alues ange o 30.6 Ω-m o 500.5 Ωm (ligh g een o yellow). The high esis i i y zone (yellow o pu ple) alues a e om 514 Ω-m o o e 3380 Ω-m a h ee di e en shallow su ace poin s. The highes esis i e sec ion (pu ple) was isola ed wi hin he dep hs o 2.52 m and 3.58 m, a 4.26 m o 7.52 m su ace poin s. P o ile 2 The esis i i y o he uncon amina ed zone anges be ween 441Ω-m and 251Ω-m (pu ple o o ange), spanning h ough mos op sec ion o he p o ile. The highes esis i i y is loca ed a he wes e n pa o he p o ile a a dep hs ange o 0.938 m o 10.0 m and su ace poin s o 5.02 m o 35.0 m. The zones o low esis i i y (deep o ligh blue) we e isola ed a he uppe and lowe pa s o he p o ile sec ion wi h alues ange o less han 61.6 o 81.6Ω-m. The dep h o he uppe pa anges om 0.938 m o 2.21 m, a su ace poin s o 45.2 m o 70.0 m. The lowe pa o his sec ion was isola ed a dep hs ange o 13.0 m o 14.8 m and su ace poin s o 36.5 m o 50.2 m. The in e media e esis i i y zone (ligh g een o yellow) has esis i i y alues anging be ween 108 and 251Ω-m and sp ead om he su ace o a dep h o abou 2.81 m, and a su ace poin ange o 30 m o o e 90 m. This in e al was also isola ed a he cen e o he p o ile a su ace poin 30 m o 60 m and dep hs ange o 6.95 m o abou 14.8 m. 126 GeoScience Enginee ing Vol. 66 (2020), No. 3 h p://gse. sb.cz pp. 121–135, ISSN 1802-5420 DOI 10.35180/gse-2020-0037 P o ile 3 The high esis i i y zone wi h alues ange om 1206Ω-m o 814Ω-m (pu ple o o ange) spanned h ough he op pe meable soil o ma ion o he bo om o he in es iga ed dep h (14.8m), be ween 28.0 m and 90 m su ace poin s. I co e s mos pa s o he p o ile. This zone o e lies he less esis i e zone (ligh g een o yellow) a su ace poin s 6.0 m o 60.0 m and dep hs ange o 0.938 m o 14.0 m. The esis i i y ange o his zone is 814 o 452Ω-m. The low esis i i y zone (deep o ligh blue) wi h alue ange o 306Ω-m o 452Ω-m we e isola ed a dep hs ange o 0.938 m o 4.78 m, a su ace poin s ange o 6.0 m o 9.0 m; and also a a dep h ange o 13.0 m o 14.8 m and su ace poin s o 35.0 m o 50.0 m. P o ile 4 This p o ile exhibi s almos he same cha ac e is ics as P o ile 3. The con amina ed low esis i i y zone (deep o ligh blue) has a esis i i y ange o 386 o 461Ω-m, a su ace poin s 5.0 m o 8.0 m and dep hs ange o 13.0 m o 14.8 m. The zone o e lies a highe esis i i y zone (ligh blue o yellow) ha ing alue o 560 o 785Ω-m and spanning om 0.938 o 14.8 m a su ace poin s ange o 6.0 o 58 m. The anomalous esis i i y zone (pu ple o o ange) has esis i i y in excess o 1336 o 937Ω-m a dep hs ange o 0.938 o 14.8 m and su ace poin s 40 o 90 m. Aqui e Dep h De e mina ion The geoelec ic cu e o each sounding poin we e p ocessed using RES2DINV au oma ic analysis so wa e. The ep esen a i e cu e esul is p esen ed in Figu e 7 and he compu e modeled analysis is shown in Table 1. The compu e -modelled esul s a e summa ized in Table 2. Figu e 7. Model cu e o VES 1 127 GeoScience Enginee ing Vol. 66 (2020), No. 3 h p://gse. sb.cz pp. 121–135, ISSN 1802-5420 DOI 10.35180/gse-2020-0037 Table 1. In e p e ed compu e model o VES 1 App Resis i i y Thickness (m) Dep h (m) Ohms-m (m) (m) 92.5 1.5 1.5 160 3.35 4.85 273 9.19 14.04 768 37.5 51.54 329 The i s laye has a esis i i y o 92.5Ω-m and is desc ibed as he opsoil wi h a hickness o 1.5 m. This laye consis s o so , b ownish, la e i ic sandy clay and sil clay. Laye wo has a esis i i y o 160Ω-m and appea s a a dep h o 4.85 m. I is desc ibed as medium o dense, poo ly g aded la e i ic sand. The unde lying laye wi h a esis i i y o 273Ω-m has a o al hickness o 9.19 m is a medium o dense ligh b own, poo ly-g aded sands. The ou h laye is a medium o coa se well-so ed sands (aqui e laye ) wi h esis i i y o 768Ω-m and hickness o 37.5m. The dep h o he i h laye wi h esis i i y o 329Ω-m was no de e mined. Table 2. Summa y o he esul s o compu e modelling o all he sounding s a ion 5 DISCUSSION The impac o municipal solid was e land ill on g oundwa e and soil was in es iga ed using 2-D esis i i y imaging echnique. The esul o he model sec ion e eals ha he su ounding soil and g oundwa e a ound he land ill may no ha e been con amina ed o dep h beyond 14 m, which is below he p oduc i e aqui e dep h o abou 40 m and abo e in he s udy a ea. The 2-D esis i i y imaging mapped h ee dis inc i e zones o anomalously low, in e media e and high esis i i y. The anomalously low esis i i y (deep o ligh blue) in he p o iles we e in e p e ed as high conduc i e leacha e con aminan plumes (as a esul o decomposing land ill was e) con aining o ganic and ino ganic subs ances, pa hogens and Dissol ed solids. The leacha e con aminan plume is obse ed o ha e seeped om su ace o dep hs exceeding 14 m in he in e se model sec ion. This obse ed seepage is enhanced by he na u e o he pe meable sandy laye cha ac e is ic o he a ea. The zones o inc easing esis i i y (ligh g een o yellow) wi h esis i i y anging om 79.9 Ω-m o 937Ω-m in he en i e a ea we e also iden i ied as po ous and pe meable sandy laye s o a ying g ain sizes and mois u e con en . The zones o anomalously high esis i i y (pink o pu ple) wi h esis i i y g ea e han 3308 Ω-m in he en i e p o iles we e in e p e ed as uncon amina ed wa e - illed sands. The esul s show ha he modelled subsu ace is mo e esis i e (uncon amina ed) as we mo e away om he land ill as seen in P o iles 3 and 4 which we e aken abou 100 m away om he si e (Fig. 8), wi h he esis i i y inc easing eas wa ds. This was co obo a ed by he spa ial end o he equidis ance measu ed esis i i y (Figu es 9–14) ac oss di e en lines analyzed wi h GIS so wa e. Figu e 9 shows esis i i y p o ile a 5 me e s equidis ance ac oss di e en lines a ound he dumpsi e. I e eals high esis i i y nea he sou ce o lines 1 and 2, while in line 3, high esis i i y was obse ed in he middle and a he end o lines 3 and 4 espec i ely. Low esis i i y uns VES s a ion no. Laye Resis i i y (Ohm-m) Laye Thickness (m) Dep h (m) P1 P2 P3 P4 P5 1 2 3 4 5 h1 h2 h3 h4 h5 1 162 303 539 1029 785 1.9 4.01 8.56 17.6 - 1.9 5.91 14.5 32.1 - 2 736 589 1859 3428 2612 1.84 1.79 4.85 23.4 - 1.84 3.63 8.48 31.9 - 3 92.5 160 273 768 329 1.5 3.35 9.19 37.5 - 1.5 4.86 14 51.5 - 4 90.76 947.6 924.20 2452.00 490 0.60 1.83 2.43 37.40 - 0.60 2.43 486 42.26 - 128 GeoScience Enginee ing Vol. 66 (2020), No. 3 h p://gse. sb.cz pp. 121–135, ISSN 1802-5420 DOI 10.35180/gse-2020-0037 ac oss he middle o he end o lines 1 and 2 and also in he beginning o bo h lines 3 and 4. Figu e 10 shows p o iles a 10 me e s equidis ance ac oss di e en lines in he s udy a ea. I e ealed high esis i i y a he sou ce o lines 1 and 2, while in line 3, high esis i i y is obse ed in he middle and a he end o he p o ile. Line 4 eco ded high esis i i y a he end o he p o ile only. Low esis i i y uns ac oss he middle and end o lines 1 and 2, while he e is obse ed low esis i i y a he beginning o lines 3 and 4. Figu e 11 displays esis i i y dis ibu ion a 15 me e s equidis ance ac oss di e en lines a ound he dumpsi e. I e ealed high esis i i y a he sou ce end o lines 1 and 2, while in lines 3 and 4, high esis i i y was obse ed a he end. Low esis i i y uns ac oss he middle o he end o lines 1 and 2 espec i ely, while lines 3 and 4 obse ed low esis i i y a he beginning. Figu e 12 shows p o ile a 20 me e s equidis ance ac oss di e en lines in he s udy a ea. I shows high esis i i y a he sou ce o lines 1 and 2, while in lines 3 and 4, high esis i i y was obse ed a he middle and he end espec i ely. Low esis i i y uns ac oss he middle and end o lines 1 and 2, while lines 3 and 4 obse ed low esis i i y a he beginning o he p o ile. Figu e 13 displays he p o iles a 25 me e s equidis ance ac oss di e en lines nea he dumpsi e. I shows high esis i i y a he sou ce poin o line 1 and a he middle o line 2, while in lines 3 and 4, high esis i i y was obse ed a he end. Low esis i i y is dominan obse ed nea -middle o line 1 and nea -end o line 2, while lines 3 and 4 eco ded low esis i i y a he beginning o he p o ile. Figu e 14 shows p o ile eco dings a 30- me e equidis ance ac oss di e en lines in he a ea. This e ealed low esis i i y a he beginning o all he p o iles, high esis i i y a he midpoin o lines 1, 3 and 4 and low esis i i y was also obse ed a he middle and end o line 2 and 3, espec i ely. Figu e 8. Loca ion o Da a poin s in he S udy A ea The VES esul s e eal ha he main aqui e zones occu wi hin he 4 h geo-elec ic laye s wi h a esis i i y ange o 768 o 2452 Ω-m and dep h a ying om 31.9 o 51.5 m and laye hickness o e 37.5 m. This inding was alida ed wi h he aqui e dep h de e mined om wo ks o o he au ho s done e y close o he s udy a ea [18]. 129 GeoScience Enginee ing Vol. 66 (2020), No. 3 h p://gse. sb.cz pp. 121–135, ISSN 1802-5420 DOI 10.35180/gse-2020-0037 Thus, he aqui e laye wi hin he a ea may no ha e been con amina ed. Howe e , he loca ion o his dump si e in he a ea will se iously a ec he ag icul u al p oduce due o he p esence o he leacha e om he land ill. ¨ Figu e 9. The spa ial end in 5m equidis ance measu ed esis i i y be ween and wi hin obse ed lines