MATERIALS AND METHODS

Soil sampling: The study area is located within Roundhill landfill vicinity located at latitude 32°53’13.66" S and longitude 27°37’26.20" E in South Africa’s Berlin town. The area is sloppy, has hot climate and previously was used as a natural grassland. Soils were collected from 8 sites and a reference site 2 km away from the landfill. Of the eight sampling sites, 5 were collected at 0 (L0), 50 (L50), 100 (L100), 250 (L250) and 500 (L500) meters away from the landfill site, one on the east (East 1), another on the west (West 1) and one on the south west side (West 2) of the facility. At each sampling site, soil was taken from 30 and 100 cm depths to represent the topsoil and subsoil, respectively using a soil auger. Soils were emptied in plastic bags for further analysis at Eureka laboratories of the University of South Africa, Florida campus. The research was conducted between October, 2018-February, 2019.

Physical analysis: Physical characteristics including analysis of soil particle sizes, bulk density, Water Holding Capacity (WHC), particle density and moisture content were determined. The clod method was used in bulk density determination⁸, while Piper⁹ method was used to measure the WHC of soils. Particle density was determined using the submersion method¹⁰ while particle sizes were determined by sedimentation method¹¹. Gravimetric method was used in determining soil moisture content¹².

Chemical analysis: Soil pH was determined by potentiometry while Electric Conductivity (EC) and Total Dissolved Solids (TDS) were determined using their respective meters¹³. Total Organic Carbon (TOC) was determined using the Loss On Ignition (LOI) technique where a known amount of soil was subjected to 700°C heat for 4 h to destroy its organic matter¹⁴. The soil was cooled, reweighed and differences of initial and final soil weight were used to calculate percentage TOC.

Mineralogical analysis: To identify the mineral phases in the soil samples, X-ray Diffraction (XRD) analysis was conducted using a prescribed method¹⁵. The soil samples were oven-dried at 105°C, crushed, milled and homogenised to powders of below 50 μm particle size. About 2g of each sample was placed on XRD’s acrylic holder in readiness for analysis. Using XRD equipment (Siemens D500) at CuKα radiation, 30 mA, 40 kV and 2θ reflections, soil samples were analyzed at 0-80 degrees every 2 sec count time and 0.02° step size. Mineral identification was done by matching observed spectra with the International Centre for Diffraction Database (ICDD) of 2004 and results were presented as spectrum.

Statistical analysis: Descriptive statistics including graphs, arithmetic mean, maximum, standard deviation and minimum values were used to describe the physicochemical parameters of sampled soils. One-way analysis of variance (ANOVA) was used to assess significant differences in the means of various parameters at α = 0.05 significance level. Obtained data was processed using XLSTAT statistical tool at 95% significance level.

RESULTS

Physical characteristics: Physical characteristics of sampled soils are shown in Fig. 1 and Table 1. Soils of the study area were mainly fines with a higher percentage of clay compared to silt and sand particles (Fig. 1). The distribution of soil particles was uniform in top and subsoils of all sampling sites.

Results of moisture content, WHC, bulk and particle density in soils were as shown in Table 1. Moisture content ranged from 12.7-18.7% for top-soils and 15.2-30.6% for subsoils. Bulk density values of top-soils ranged between 1.15 and 1.27 g cm^–3 while subsoils were between 1.4 and 1.89 g cm^–3. Subsoils at East 1 and L500 sampling sites had higher bulk density values of 1.79 and 1.89 g cm^–3, respectively. Particle density values ranged from 2.55- 2.78 and 2.58-2.8 g cm^–3 for top and subsoils, respectively. WHC levels were between 41-49 and 42-50% for top and subsoils, respectively. Standard deviation, Coefficient of Variation (CV) and skewness values of each parameter were low and depicted their minimal variability in relation to the mean. ANOVA reported a p-value of 1 and confirmed no significant differences between the means of various sampling sites at α = 0.05 significance level as was shown in Table 2.

Chemical characteristics: Results of pH, EC, TDS and OC in top and subsoils soils were as shown in Table 3. Soil pH levels were slightly alkaline and ranged between 7.14-7.9 and 7.4-7.9 for top and subsoils, respectively. Mean EC levels of top and subsoils were 250.8 and 220.3 μm cm^–3 while TDS values were 168 mg L^–3 and 147.6 mg L^–3 in respective order. TOC levels ranged between 1.6-8.9 and 0.8-5.3% for top and subsoils, respectively. The standard deviation values of EC and TDS were higher compared to other parameters and depicted high variability in the sampling sites although ANOVA reported a p-value of 0.97 that showed no significant differences in the means of various sampling points (Table 2, 3).

Table 1:	Physical characteristics of soils in the study area

Fig. 1:	Percentage sand, silt and clay in the soils

Table 2:	ANOVA results of physical and chemical characteristics of soils at 95% significance level

Table 3:	Results of assayed chemical parameters of soils
EC: Electric conductivity, TDS: Total dissolved solids, TOC: Total organic carbon

Table 4:	Minor phases of minerals in top and subsoils of various sampling sites

Fig. 2(a-b):	Major mineral phases found at sampling site L0, (a) Topsoils and (b) Subsoils
	All 9 sampling sites had quartz and halloysite minerals in both top and subsoils, although in different percentages

Mineralogical analysis: Mineralogical composition in top and subsoils of various sampling points determined by XRD were as shown in Fig. 2 and Table 4. From Fig. 2, it is observed that quartz and halloysite were the major mineral phases identified in all soils irrespective of depth.

Other minor mineral phases identified at various sampling sites are shown in Table 4. There was great variation in the composition of minerals identified and they were enriched with elements such as Na, Mg, Ca and K. Albite occurred in 5 of the 9 top soils (L0, L50, L100, East 1 and West 1) but only in 1 subsoil (West 1).

DISCUSSION

Current findings showed that area soils had high clay content, high moisture content, slightly alkaline pH, low TOC and were dominated with silica and aluminium in their mineral phases. Soils of the study area had a silty clayey nature irrespective of depths, which corresponded to their low drainage, permeability and high pollutant retention capacity. The findings agreed with a previous research that characterised area soils as red dolerites containing more than 55% clay content¹⁶. Soils had uniform particulate distribution, possibly because of their natural occurrence rather than by alluviation as observed in a related study that attributed non-uniformity of particles to inter-horizon translocation of soil particles⁶. High moisture content compared to 0-10% range of semi-arid soils could be due to the landfill’s ability to store water that runs-off as leachate, a trend that was reported in Gaborone landfill of Botswana^6,17. High moisture content could be attributed to the clayey nature of area soils since such soils are de-aerated and have displaced air and oxygen that promote water retention¹⁸. Bulk density values of soils were within the prescribed range of 1.1-1.6 g cm^–3 for fines¹⁹ except two sampling sites whose values corresponded to rocky, compacted and strongly indurated soils²⁰. Particle density values were within the normal ranges of 2.65-2.85 g cm^–3 for soils where quartz is the main mineral while WHC values were in the normal range of 45-55% for clayey soils²¹ and depicted the soil particles as fines with great surface area. From the physical characteristics results, area soils were protective barriers of leachate migration to groundwater just like in Abakaliki landfills where soils had low implication on groundwater due to a clayey nature, strong induration and high WHC⁴.

Assayed pH values of soils agreed with findings of previous studies in dumpsites where soils were slightly alkaline^6,18,22. The EC and TDS results corresponded to the presence of soluble salts in soils possibly from their enhancement by landfill leachate as reported in Nigerian soils near landfills¹⁸. Metal scraps that were part of landfilled waste were possible causes of high EC levels as reported in Enugu-Port Harcourt landfills of Nigeria²². Evaporation in top-soils compared to subsoils leading to salt concentration possibly explained the high EC and TDS levels of the former, as was the case in Wardha region of India²³. TOC values depicted sampled soils as having low fertility, which is characteristic of soils from semi-arid areas due to high temperatures that enhance rapid decomposition^22,24. Leachate concentration in topsoils compared to subsoils explain why the latter had lower TOC levels comparatively. An analysis of TOC in Gohagoda landfill vicinity soils suggested that high levels in top-soils resulted from leachate’s organic content²⁵.

Dominance of quartz and aluminosilicate (halloysite) minerals was explained by their role in soil formation, dissolution of other elements and resistance to weathering. Similar results were reported in a mineralogical assay of landfill soils in Botswana⁶. Feldspar containing halloysite dominance was associated to the presence of mafic igneous rocks in the study area, as established in a mineral assay of Brazilian soils where such minerals were common phases²⁶. Enrichment of these minerals with alkali and alkaline earth metals could have been prompted by anthropogenic enhancement from leachate. These metals occur naturally in their oxide forms unless in the presence of human enrichment²⁷. Occurrence of minor phases of gismondine and clinochrysotile minerals suggested alteration of feldspar probably by landfill leachate in addition to natural occurrence. Leachate contamination transforms feldspar to kaolinite and other Ca, K, Mg, Na and Mn containing minerals⁶. Another study in Lessebo region of Sweden associated the presence of these minerals to leachate contamination²⁸. Occurrence of albite, especially in topsoils showed high inorganic and organic pollutant retention in the soils as was established in a study at a sanitary landfill of western Greece²⁹. Overall, the dominance of aluminosilicates in the area explained their role in attracting pollutants through sorption, which could delay groundwater contamination³⁰. Ilmenite, which results from cation exchanges of kaolin minerals (identified at East 1 top soil) confirmed the effects of landfill leachate on area soils as suggested in an Australian study where structural collapses of clay minerals in the presence of leachate resulted to its formation³¹. Possibilities of soil structure alteration and contaminant infiltration reported in the current study should be validated through periodic follow-up studies in the area.

CONCLUSION

This study assessed physicochemical characteristics of soils in Roundhill landfill vicinity and their role in influencing groundwater contamination. Soils were found to be clayey, which correlated to high pollutant retention capacity. High moisture content was associated to mobilisation of landfill leachate while high levels of bulk density were due to induration that can deter groundwater pollution. The presence of clay minerals mostly aluminosilicates in area soils corresponded to their potential to adsorb leachate contaminants. Assessment of soil characteristics in this study showed susceptibility to leachate infiltration but with low groundwater pollution potential.

SIGNIFICANCE STATEMENT

This study discovered a link between physicochemical parameters of soils and groundwater pollution potential at Roundhill landfill vicinity. The results can help to devise measures towards pollution buffering and contaminant attenuation in land and water resources. The research can be replicated in other areas for comparative analysis.

ACKNOWLEDGMENT

The authors are grateful to the staff of Global Consulting Laboratories, Eastern Cape and Eureka laboratories for their support during fieldwork and in conducting experiments.