1 Introduction

In the face of a growing human population, ensuring access to high-quality water is essential to guarantee food security, boast industrial productivity, reduce economic losses, and serve other goals [1,2,3,4] and must be prioritized at all costs. Countries worldwide have not been able to cope with the increasing population and the rate of waste generation. Urbanization and industrialization have brought dumpsites closer to people and also within the radius of influence of drinking water sources. In Nigeria, most of the waste generated is not treated except for a few cases where the government compels industries to treat their waste before disposing of it; even at this, industries still find ways to subvert such guidelines. Despite the treatment, these wastes are still dangerous. In Nigeria, an open dumpsite (an old traditional manner of collecting and disposing of solid waste) is the usual practice with few landfills. Both practices come at a price of disadvantage, gradually resulting in the deterioration of the environmental system and conditions. The fact that it is an open dumpsite suggests that it is unplanned and uncared for; hence, the poor management practices. Ajah et al. [5] noted that there are two major concerns regarding waste disposal on land. They are: (i) contamination of surface and groundwater by leachates; and (ii) bioaccumulation of toxic heavy metals in animals and plants; uptake of these heavy metals by plants; and the progressive buildup of these metals in the food chain. Many times, the composition and volume of leachate produced are a function of the age of the dumpsite, the size of the dumpsite, the quantity of constituents and concentrations of waste materials on the dumpsite, as well as the level of rainfall or fluid precipitation [6,7,8]. The migration of leachates from dumpsites or landfills and the release of pollutants from sediments (under certain conditions) pose a high risk to groundwater resources if not adequately managed [9]. On the other hand, the ease of transportation of this leachate within the soil is a function of the soil porosity, permeability and grain size. The effect of dumpsite leachates is not as massive as landfill leachate on groundwater because of the distance of travel from the surface to groundwater. Soil pollution alters the ecosystem and affects agricultural activities [10, 11]. Yamusa et al. [12] noted that in the selection of a landfill site, the geotechnical properties of the soil should be a major factor to consider.

Researchers like Sunil et al. [13], Nayak et al. [14], Ramakrishnegowda et al. [15], Amadi et al. [16], Alhassan [17] and Alimohammadi‐Jelodar and Karimpour‐Fard [18] have tested the effect of leachate, organic solvents, alkali, and common salt on the engineering properties of subsoil in Nigeria and India and found out that both index and strength properties of subsoil were altered, which resulted in either an increase or decrease of measured parameters at a significant level. To ensure the sustainability of future projects involving the dumpsites, it is important to take steps to improve the geotechnical properties of the subsoil through chemical stabilization and the use of predictive methods such as Bayesian and Kriging interpolation [19,20,21,22].

The northern part of Ondo State has a peculiar complex geological terrain with shallow overburden and inadequate protective capacity to protect soils and aquifers from contamination [23]. Open-to-surface fractures, though localized and varied with locations, serving as groundwater conduits are also another major factor capable of enhancing contamination migration into aquifer units in this complex terrain [24]. Given the above, this study is aimed at determining the effects of leachate on the physical and engineering properties of dumpsite subsoil at different depth layers when compared to the control samples of the same parent rock type. This will provide adequate results on the suitability of the subsoil for the design and construction of a landfill, for other construction purposes and containment measures to make aquifer units safe for long-term groundwater abstraction in and around the investigated areas.

2 Study area

The study area is located in the tropical environment of southwestern Nigeria, where there are alternating wet and dry seasons. The study area lies between latitudes N7° 13′ 00″ and N7° 33′ 00″ and longitudes E005° 36′ 00″ and E005° 47′ 00″ of the Greenwich Meridian, with an average elevation of 375 m (Fig. 1). The three investigated  dumpsites are in the towns of Owo and Ikare-Akoko, which are surrounded by Arigidi-Akoko, Ugbe-Akoko, Ogbagi-Akoko, Ifon, Isuada, Iyere, and Emure-ile towns. The average annual rainfall ranges from 1400 to 1600 mm, and the average annual temperature ranges from 21 to 30 °C [25]. The increased population in the study area is due to the presence of a regional market, two universities, and a polytechnic. These dumpsites are poorly designed, without clayey liners or polyethylene geomembrane liners beneath the surface soil to limit leachate flow or prevent it from reaching the subsurface aquifer units. The study area has fair to good geotechnical ratings of the subsoil [26,27,28,29,30,31]. The study area lies within the Basement Complex of southwestern Nigeria, which is east of the West African Craton. This Basement complex dates back to the Late Precambrian to Early Paleozoic Pan-African Orogenesis [32]. The dominant rock types in the environments of Owo Dumpsite, Ikare-Akoko Dumpsite 1 and Ikare-Akoko Dumpsite 2 are schist, porphyritic granite gneiss and granite gneiss respectively  (Fig. 2). The drainage pattern in the study area is dendritic (Fig. 2). A typical modern landfill cross section is shown in Fig. 3.

Fig. 1
figure 1

Map of Nigeria showing the location and acquisition map of Owo town and Ikare-Akoko town; (a) Map of Nigeria, (b)map of Ondo State, (c) map of Owo–Ikare area of Ondo State, (d) Ikare dumpsite 1, (e) Ikare dumpsite 2, (f) Owo dumpsite

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Fig. 2
figure 2

Geological map of Owo-Ikare highway, Southwestern Nigeria [33]

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Fig. 3
figure 3

Typical landfill cross section

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3 Materials and methods

Thirty-six disturbed soil samples were obtained from twelve trial pit profiles at a vertical depth of 0.5 m (top layer), 1.0 m (middle layer), and 1.5 m (third layer). Two trial pit profiles each were located on the three dumpsites in Owo and Ikare-Akoko, and their control soil profiles (two each) were located between 50 and 65 m away from the dumpsites but within the radius of the same parent rock type. The location of trial pits on the dumpsite was based on the interpretation of a dipole–dipole geophysical investigation. The soil samples were put in a well-labeled polythene bag using the description of TP for the trial pit, v1 for the top layer, v2 for the middle layer, and v3 for the third layer. The soil samples were air-dried for fourteen days, after which they were subjected to geotechnical tests of the Atterberg limit, linear shrinkage, grain size analysis, specific gravity, permeability, compaction, and California Bearing Ratio (CBR) using the British Standard Code of Practice [34] guidelines. For the index tests of the Atterberg limit and linear shrinkage, the soil samples were passed through sieve No. 40 (425 µm). The American Society for Testing and Materials (ASTM) standards of [35, 36] were followed in carrying out liquid limit, plastic limit, and linear shrinkage tests. The plasticity index is a calculated difference between the liquid limit and the plastic limit. The specific gravity using the water pycnometer method and grain size analysis were carried out in accordance with ASTM [37] and ASTM [38] standards. The falling head permeability test was used to measure the permeability according to the ASTM D [39] specification. Only soil samples taken at a depth of 1.0 and 1.5 m were subjected to a permeability test. The compaction test was carried out according to the specification of ASTM D [40] using the Proctor standard compaction test method, while the CBR test was carried out using the ASTM [41] specification. A soil sample weighing 20 g (each from the total of 36 samples) was mixed well with 40 mL of distilled water and the mixture was allowed to settle for an hour. The chemical properties of pH and TDS and the physical properties of EC were determined using a digital LCD pH meter and a portal digital EC/TDS meter (Ketotek).

3.1 Analysis of Variance (ANOVA) and Pearson’s Correlation Coefficient (PCC)

The one-way non-parametric Analysis of Variance (ANOVA) was used to compare the mean values of the physical and geotechnical properties of the subsoil samples around the three investigated dumpsites in the northern part of Ondo State. The three dumpsites serve as the independent variables, while the chemical, physical and geotechnical parameters of pH, total dissolved solids (TDS), electrical conductivity (EC), liquid limit (LL), plastic limit (PL), plasticity index (PI), fines, specific gravity (SG), permeability, optimum moisture content (OMC), maximum dry density (MDD) and CBR are the dependent variables. The degree of freedom (df) of the ANOVA for the different physical and geotechnical data is thirty-five (35) and has an alpha level of 0.05. There are two possible outcomes of either the null hypothesis or the alternative hypothesis in every analysis. The null hypothesis states that there is no difference in the mean values of the parameters obtained from the trial pits on the three dumpsites in the northern part of Ondo State (H0:\(\mathbf{\varphi }\)1 = \(\mathbf{\varphi }\)2 = \(\mathbf{\varphi }\)3). The alternative hypothesis states that at least one of the three major dumpsites in the northern part of Ondo State has different mean values for the measured parameters (HAlt:\(\mathbf{\varphi }\)1 ≠ \(\mathbf{\varphi }\)2 ≠ \(\mathbf{\varphi }\)3). Pearson’s correlation coefficient was employed to establish the relationship that exists between physical parameters and the strength properties of pH, TDS, EC, fine, OMC, MDD, CBR and permeability for each of the three dumpsite soil samples and their control soil samples. It also provides information about the strength and direction of the relationship (either positively or negatively). The obtained values are usually between − 1 ≤ PCC ≤  + 1. The Surfer and Excel software packages were used for statistical analysis.

4 Result and discussion

4.1 Chemical and physical properties of trial pit soil samples

The values of soil pH obtained from trial pits on and around Ikare dumpsite 1, Ikare dumpsite 2, and Owo dumpsite range from 5.52 to 7.03 (coefficient of variation (CV) = 8.6), 6.83 to 7.87 (CV = 4.0), and 4.86 to 7.10 (CV = 12.9) respectively (Table 1). The values for EC in trial pit soils in and around Ikare dumpsite 1, Ikare dumpsite 2 and Owo dumpsite range from 92 to 1644 µs/cm (CV = 105.3), 162 to 826 µs/cm (CV = 54.2) and 12 to 380 µs/cm (CV = 83.6) respectively (Table 1). The values for TDS in trial pit soils in and around Ikare dumpsite 1, Ikare dumpsite 2 and Owo dumpsite range from 46 to 822 ppm (CV = 104.8), 79 to 415 ppm (CV = 54.2) and 9 to 172 ppm (CV = 82.5) respectively (Table 1). The World Health Organization’s [42] recommendation of between 6.5 and 8.5, 1000 ppm and 1000 µS/cm as the limit for pH, TDS, and EC in drinking water should also form the yardstick for pH, TDS, and EC ratings in soil. The soil in the study area is classified as acidic soil, whose content is reduced in the dumpsite soil. Increased TDS concentrations in the dumpsite soil could affect the ability of the soils to support plant life. The top layer of the profiles had the best pH values in most trial pits, while the third layer recorded the lowest TDS and EC values for most trial pits. All the soil samples have TDS and EC values below 1000 ppm and 1000 µS/cm except for soil samples from trial pit 1 (Table 1). On average, trial pit soil samples in Owo dumpsite and its environs have the lowest pH values; the lowest total dissolved solids and are the least conductive of the three dumpsites.

Table 1 Chemical and physical properties of dumpsite and control soils
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4.2 Geotechnical properties

4.2.1 Index test

4.2.1.1 Atterberg limit

The liquid limit (LL) values of trial pit soils in and around Ikare dumpsite 1, Ikare dumpsite 2, and Owo dumpsite range from 35.6% to 48.1% (CV = 11.6), 36.1% to 54.3% (CV = 12.4) and 43.2% to 50.2% (CV = 5.1) respectively (Tables 2 and 3). The soil samples from the trial pits in Owo dumpsite and Ikare dumpsite 2 recorded lower values when compared to the control soil samples of the same parent rock type (Fig. 4a). This, by implication, means that the leachate has helped to lower the LL values. The PL values for the trial pit soils in and around Ikare dumpsite 1, Ikare dumpsite 2 and Owo dumpsite range from 19.2% to 25.8% (CV = 11.7), 19.4% to 32.6% (CV = 18) and 20.8% to 26.9% (CV = 8.9) respectively (Tables 2 and 3, Fig. 4b). The PI values of trial pit soils in and around Ikare dumpsite 1, Ikare dumpsite 2, and Owo dumpsite are in the range 10.6% to 25.0% (CV = 25.1), 16.7% to 24.8% (CV = 11.9) and 21.5% to 25.2% (CV = 4.6) respectively (Tables 2 and 3, Fig. 4c). The coefficient of variation between the Atterberg limit values of dumpsite and control soils is highest at around Ikare dumpsite 1 followed by Ikare dumpsite 2. The third layer recorded the least set of values for PI, followed by the first layer. According to Oeltzschner [43], Daniel [44] and Rowe et al. [45], subsoil material for landfills and dumpsites should have a LL value not higher than 30% and a PI value not higher than 10%. None of the soil samples from the three dumpsites and their control trial pits satisfied the basic requirements as barrier materials in landfills or dumpsites. Kabir and Taha [46] reported that soils with a high liquid limit generally have low hydraulic conductivity. This study's results are consistent with the findings of previous studies, such as Ige [47], Ezema et al. [48] and Mohammed et al. [49], which found a reduction in Atterberg limit values for leachate-contaminated soils (Fig. 4a-c). This result is contrary to Goswami and Choudhury's [50] assessment of a significant value increase when lateritic soil samples were mixed with leachate.

Table 2 Geotechnical Properties of Dumpsites and Control Soils
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Table 3 Statistical values of geotechnical properties of soil
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Fig. 4
figure 4

Average values of dumpsite soil against control soil; a liquid limit test, b plastic limit test, c plasticity index test, d linear shrinkage test

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4.2.1.2 Linear shrinkage (LS)

The LS values of trial pit soils in and around Ikare dumpsite 1, Ikare dumpsite 2, and Owo dumpsite range from 7.1% to 10.0% (CV = 12.1), 7.9% to 14.3% (CV = 21.9) and 10.0% to 12.1% (CV = 7.3) respectively (Tables 2 and 3, Fig. 4d). These values are fairly consistent with depth. Brink et al. [51] stated that a LS value below 8% is an indication that the soil is good as a subgrade material, while Gidigasu [52] put the subgrade specification value below 10% as a good material. Trial pits 1, 2, 3, 4, 5, and 6 have their values within the approved specification.

4.2.1.3 Particle size analysis (PSA)

The values of fine contents for trial pit soils on and around Ikare dumpsite 1, Ikare dumpsite 2 and Owo dumpsite range from 10.5% to 44.8% (CV = 31.3), 21.7% to 49.8% (CV = 20.7) and 18.0% to 46.0% (CV = 26.6) respectively as presented in Tables 2 and 3. Daniel [44] and Rowe et al. [45] recommended that subsoil material for landfills and dumpsites must have a value greater than 30% for gravel, while the fine content must be less than or equal to 30%. The following samples of TP2v2, TP2v3, TP4v1, TP4v2, TP4v3, TP5v2, TP9v3, TP10v2, and TP10v3 met this criterion, which is a representation of 25%, and are basically found in the middle or last layer. The coefficient of variation in particle size distribution between dumpsite and control soil is highest around Ikare dumpsite 1. The control trial pit samples of Ikare dumpsite 2 and Owo dumpsite recorded fine content values higher than the dumpsite trial pits, which imply that the dumpsite soils for these two areas are better off. All the soil samples have relatively higher percentages of coarse fraction than fine fraction and are well graded because of the wide range of particle sizes on the grain size distribution curve (Figs. 5, 6, 7) and can be compacted [53]. The soil around Ikare dumpsite 1 has the best average ratings (Fig. 8a).

Fig. 5
figure 5

Grain size analysis curves of trial pit soils in Ikare dumpsite 1 and its environs

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Fig. 6
figure 6

Grain size analysis curves of trial pit soils in Ikare dumpsite 2 and its environs

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Fig. 7
figure 7

Grain size analysis curves of trial pit soils in Owo dumpsite and its environs

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Fig. 8
figure 8

Average values of dumpsites soil against their control soil; a percentage of fine, b specific gravity results

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4.2.1.4 Specific gravity (SG)

The SG values of trial pit soils on and around Ikare dumpsite 1, Ikare dumpsite 2, and Owo dumpsite range from 2.60 to 2.67 (CV = 0.7), 2.59 to 2.72 (CV = 1.6) and 2.66 to 2.74 (CV = 0.9) respectively (Tables 2 and 3). Soil profiles around Ikare dumpsite 1 have the lowest SG ratings, while soil profiles around Owo dumpsite have the highest SG ratings (Fig. 8b), which imply that soil samples in Owo have undergone more weathering. The general values range of 2.59 to 2.74 for this study fall within the range of 2.5 to 3.6 given by Maignien [54] for the lateritic soils found in equatorial Africa with a moderate degree of laterization and dehydration. In agreement with Sunil et al. [13], these soil profiles have the ability to filter out pollutants and also prevent them from getting into the groundwater.

4.2.1.5 Casagrande plasticity chart

The modified Cassagrande plasticity chart (ASTMD 2487) [55] and activity chart perform the functions of both activity and Cassagrande plasticity charts by revealing the percentage of clay and slit and establishing the link between clay fraction and plasticity index in percentage. Most of the soil samples from the twelve trial pits are above the A-line except for TP4v1 (with silt medium content), TP7v2, TP7v3, TP8v1, and TP8v2 (with silt high content) (Fig. 9). The control trial pits were located near hand-dug wells, which could have influenced the soil properties due to the washing away of clay into the wells. All the samples above the A-line have medium compressibility (CI), which is highly recommended for a dumpsite or landfill (Fig. 9). The soil activity values for samples on and around Ikare dumpsite 1, Ikare dumpsite 2, and Owo dumpsite range from 0.73 to 1.4, 0.74 to 1.03% and 0.58 to 1.37 respectively (Table 2 and Fig. 9). In the classification of clay, all the samples are classified as normal with values between 0.75 and 1.25 except for TP1v3, TP3v3, TP10v1, TP10v2, TP12v1 and TP12v2 soil samples with either active or inactive ratings [56].The primary clay minerals in all of the soil samples are either kaolinite or illite (with low to moderate expansion potentials); hence, they are suitable dumpsite/landfill barrier materials (Fig. 9).

Fig. 9
figure 9

Plots of the 36 subsoil samples obtained in the three dumpsites soil and their environs on modified Cassagrande Plasticity chart (ASTMD 2487) and activity chart

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4.2.1.6 AASHTO and USCS classification system

The American Association of State Highway and Transportation Officials [57] for soil classifications is specifically designed to assess soil suitability for support of road pavement but has been extended to so many other projects, even the construction of a landfill. It has a group classification that ranges from A-1 (best soils) to A-8 (worst soils). The third layer of all twelve trial pits has the best ratings of granular materials, with good and fair general ratings of either A-2-6, A-2-7, or A-6 as subgrade soil, except for trial pit 3 (Table 2). Trial pits 2, 4, 5, 9, and 10 have good ratings of either A-2-6 or A-2-7 for their middle and third layers. Trial pits 4 and 9 have completely good soil profiles. Nine out of the twelve top layers of soil have poor AASHTO ratings of either A-7-5 or A-7-6 (Table 2). The Unified Soil Classification System (USCS) shows that 80.5% of the tested soils are classified as clayey sand, while the remaining 19.5% are classified as either sandy silt or gravelly sand.

4.2.2 Strength tests

4.2.2.1 Permeability

The permeability values obtained for soil samples on and around Ikare dumpsite 1, Ikare dumpsite 2, and Owo dumpsite range from 2.78 × 10–5 to 7.00 × 10–5, 2.07 × 10–6 to 1.49 × 10–4 and 3.01 × 10–6 to 8.45 × 10–5 respectively (Table 2). The permeability values obtained in this study were compared with the US Bureau of Reclamation criterion, which states that soils are considered impermeable if their permeability is less than 10–6 cm/s, semi-permeable if their permeability is between 10–6 and 10–4 cm/s, and permeable if their permeability is greater than 10–4 cm/s [58]. All the soil samples from the dumpsites and control pits were semi-permeable with low permeability and low to medium infiltration rates. This also implies that flow velocity, seepage pressure, and internal erosion potentials will be low; as such, they are good materials to slow down leachate movement. The trial pit soils at the dumpsites show greater permeability than the control soil except for Ikare dumpsite 2, basically because its operation is less than 5 years. The increase in permeability values observed in this study, as a result of leachate infiltration is consistent with the findings of Evangelin et al. [59] and Mohammed et al. [49] in India and Iraq. These variations may have altered the clay mineralogy and led to particle flocculation [13, 60, 61] and increased pore sizes [15].

4.2.2.2 Compaction

The MDD values for the trial pit soils on and around Ikare dumpsite 1, Ikare dumpsite 2 and Owo dumpsite range from 1718 to 1933 kg/m3 (CV = 4.5), 1604 to 2032 kg/m3 (CV = 8.7) and 1630 to 1744 kg/m3 (CV = 2.2) respectively (Tables 2 and 3, Fig. 10a). The corresponding OMC values of the trial pit soils on and around Ikare dumpsite 1, Ikare dumpsite 2, and Owo dumpsite range from 14.1% to 16.3% (CV = 5.1), 13.8% to 17.2% (CV = 7.4) and 15.9% to 16.7% (CV = 1.3) respectively (Tables 2 and 3). According to Taha and Kabir [62], subsoil material must have a minimum MDD rating of 1740 kg/m3 to be suitable as a landfill or dumpsite barrier. Some of the MDD values are below this standard and are mostly from the control trial pits, except for trial pit 10 (Table 2). Variation in dry density and optimum moisture content was highest around Ikare dumpsite 2. Ikare dumpsite 1 has lower MDD values when compared to their control soils while Owo dumpsite soil samples showed no difference in values. This suggests that leachates reduced the soil quality by flocculating the clay minerals in the soil [60, 63]. The reduction in compaction values between dumpsite and control soils for Ikare dumpsite 1 is a reflection of the analysis carried out Ouria and Farsijani [64], Harun et al. [61] and Ezema et al. [48]. On average, the middle layers have the highest MDD ratings, while the third layers have the lowest OMC ratings. This may be due to leachate characteristics. Early-formed leachates flocculate the soil to create space for the passage of other leachate forms. On the other hand, if the laboratory compaction of 90% is achieved in the field, these soil samples can provide long-term protection for groundwater.

Fig. 10
figure 10

Average values of dumpsites soil against their control soil; a Compaction results, b CBR results

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4.2.2.3 Unsoaked California Bearing Ratio (CBR)

The maximum load that a dumpsite or landfill can carry (in terms of tonnes of waste and the pressure generated by heavy-duty trucks depositing these wastes) is a function of the bearing capacity of the subsurface soil. The unsoaked CBR values for trial pit soils on and around Ikare dumpsite 1, Ikare dumpsite 2, and Owo dumpsite are in the range 21% to 28% (CV = 10.1), 14% to 26% (CV = 20.9), 14% to 22% (CV = 13.4) respectively (Tables 1 and 2, Fig. 10b). There is no significant difference in the properties of dumpsite and control soils at the time of investigation. These values are similar to those obtained in the central region of Cameroon by Kamtchueng et al. [65]. The FMWH [66] recommended not less than 5% values for unsoaked CBR for subgrade soils. All the soil samples have values that are far higher than the specification and are capable of successfully carrying the generated waste. The third layer of trial pits 2, 4, 5, 8, 9, 10 and 11 have the highest values in the soil profiles. Figure 11 is a graph showing the average values of the Atterberg limit tests, percentage of coarse content, CBR and maximum dry density of the three dumpsite soils with the average values of their control soils.

Fig. 11
figure 11

Average value plot of dumpsite soil and their control soil for the geotechnical properties

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4.3 ANOVA and Pearson’s Correlation Coefficient

The analysis of variance has a constant Fcritical value of 3.28492 for all the parameters. Fstatistical values (test hypothesis) and P-values for the different parameters range from 0.001012 (CBR) to 17.5 (LS) and 0.001012 (CBR) to 0.199036 (pH) respectively (Table 4). The P values of the following parameters; pH (0.004859), TDS (0.031869), EC (0.031983), CBR (0.000044), OMC (0.006723), MDD (0.034297), SG (0.00045), LS (0.000049), PI (0.000912) and LL (0.002746) are significantly important because they are lower than the alpha level of 0.05. This implies that we reject the null hypothesis in favor of the alternative hypothesis. 21 pairwise correlation data values were generated separately for each of the three dumpsites and their controls. The results were divided into four groups based on the strength of the correlations: strong negative correlation (> − 0.5), weak negative correlation (< − 0.5), weak positive correlation (< + 0.5), and strong positive correlation (> + 0.5) [67]. Ikare dumpsite 1 soil against its controls produced 11 (52.4%)/9 (42.9%) strong positive correlation values, 1 (4.8%)/0 (0%) weak positive correlation value, 7 (33.3%)/11 (52.4%) pairwise weak negative correlation values and 2 (9.5%)/1 (4.8%) strong negative correlation values (Tables 5 and 6). Ikare dumpsite 2 soil against its controls produced 10 (47.6%)/3 (14.3%) strong positive correlation values, 3 (14.3%)/8 (38.1%) weak positive correlation values, 5 (23.8%)/3 (14.3%) weak negative correlation values and 3 (14.3%)/7 (33.3%) strong negative correlation values (Tables 7 and 8) while Owo dumpsite soil against its controls recorded 5 (23.8%)/6 (28.6%) values as strongly positively correlated, 3 (14.3%)/8 (38.1%) values as weakly positively correlated, 4 (19.0%)/2 (9.5%) values as weakly negatively correlated and 9 (42.9%)/6 (28.6%) values as strongly negatively correlated (Tables 9 and 10). Ikare dumpsite 1, Ikare dumpsite 2 and Owo dumpsite have combined strong positive and negative correlation values of 17, 15 and 9 respectively. 14 positive and negative pairwise correlation values are common to Ikare dumpsite 1 and its control samples. They are pH-TDS, pH-EC, pH-MDD, pH-OMC, pH-CBR, TDS-fine, TDS-MDD, TDS-OMC, TDS-CBR, EC-fine, EC-MDD, EC-OMC, EC-CBR and OMC-CBR (Tables 5 and 6). Three similarities of strong positive and negative pairwise correlation of fine-CBR, MDD-CBR and OMC-CBR exist between soil samples in the Ikare dumpsite 2 and its control from granite gneiss origin (Tables 7 and 8). No pairwise parameters of positive or negative correlation exist between Owo dumpsite soil and the control soil of granite gneiss origin (Tables 9 and 10). Some correlations are common to all three dumpsites. These include the pairwise correlation of pH-MDD, TDS-CBR, and EC-CBR, apart from the well-known established relationship of OMC-MDD and TDS-EC (Tables 5, 7 and 9). This new relationship might be a result of the leachates generated.

Table 4 ANOVA for the geotechnical and physical parameters of trial pit soils in the three dumpsites
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Table 5 Pairwise correlation coefficient (Pearson's) of soil parameters in Ikare dumpsite 1
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Table 6 Pairwise correlation coefficient (Pearson's) of soil parameters in Ikare dumpsite 1 environs
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Table 7 Pairwise correlation coefficient (Pearson's) of soil parameters in Ikare dumpsite 2
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Table 8 Pairwise correlation coefficient (Pearson's) of soil parameters in Ikare dumpsite 2 environs
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Table 9 Pairwise correlation coefficient (Pearson's) of soil parameters in Owo dumpsite
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Table 10 Pairwise correlation coefficient (Pearson's) of soil parameters in Owo dumpsite environs
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5 Conclusion

This study investigated the potential harm posed by leachate to the engineering qualities of subsoil collected from dumpsite trial pits and their environs. The following are the study's key findings:

  • The leachate effect lowers the values of the Atterberg limits of dumpsite soils when compared to their controls;

  • The results of grain size analysis, linear shrinkage and CBR revealed that there was no significant difference in the properties of dumpsite and control soils at the time of investigation;

  • All the soil samples have low permeability that ranges from 2.07 × 10–6 to 1.49 × 10–4;

  • The trial pit soils at the dumpsites show greater permeability and lower MDD values than the control soil except for Ikare dumpsite 2, basically because its operation is less than 5 years;

  • Increased TDS concentrations in the dumpsite soil could affect the ability of the soils to support plant life;

  • The dumpsite trial pit soils have good CBR and compaction properties and can be used for construction purposes after prior treatment with lime or cement;

  • The following pairwise parameters: pH-MDD, TDS-CBR, EC-CBR, OMC-MDD and TDS-EC recorded strong positive correlation values for the three dumpsites. Similarly, ten parameters were significant at a 0.05 level of significance;

  • To mitigate the environmental risks associated with leachate contamination, geosynthetic clay liners should be added to the soil in order to lower the permeability; and

  • It is recommended that the three dumpsites be converted to modern sanitary-engineered landfill system with regular monitoring.