The term “trace element” is loosely used in scientific literature to refer to a number of elements that occur in natural systems in small concentrations  . Other terms such as “trace metals”, “heavy metals” etc. have been considered synonyms to the term “trace elements”. The term “heavy metals”, is the most commonly used and widely recognised term for a large group of elements with density greater than 5.0 g/cm3. The trace elements are defined as those elements having less than 0.1% average abundance in the earth’s crust  . Using this definition, the elements Al, Ca, Fe, Mg, K, and Na with average abundances over 1.0%, are considered “major elements” in this work.
Trace elements are ubiquitous in the earth crust. Their natural levels in soil vary widely, as a function of the geology (nature of parent materials from which soil form) and soil-forming processes     . These natural levels in soils have, in many areas, been affected by anthropogenic activities such as mineral exploration, mining and smelting, agriculture, manufacturing, waste disposal and transportation    . Industrial effects are relatively well-documented and are usually largely concentrated around the mine site or form dispersion trains along drainage basins. This explains why whenever there are environmental problems related to high trace elements levels in soils or groundwater, there is always a tendency for the public to blame the most visible industry first without proper technical assessment of other possible unnatural or natural causes  .
Girei District is situated within the Yola Arm of the Upper Benue Trough (Figure 1). In recent years, MAUTECH (Modibbo Adama University of Technology) Campus, (a relatively small area within the district), has experienced and is still experiencing rapid infrastructural expansion/development including construction of students’ hostels, faculty complexes, laboratories, etc. Similarly, the renewed interest in food production has also led to increased agricultural activities on the university land. All these activities have the potentials to affect the natural trace elements levels in soil. Such effects can only be properly determined if there exist a reference data of background trace elements distribution in the area. However, the general lack of background data on natural trace elements distribution patterns in soils makes the determination, monitoring and management of such anthropogenic influences very difficult if not impossible.
This work provides the first comprehensive, reliable scientific database on background levels of trace elements in soils of MAUTECH Campus. Such reliable reference data are essential to any systematic monitoring and accurate assessment of trace elements effects when environmental issues related to elevated or reduced trace and major elements levels in MAUTECH soils are being considered.
2. Geological Setting
MAUTECH Campus is situated within the northern part of the Benue Trough
Figure 1. Regional geologic map of Northeastern Nigeria  .
(Figure 1). The Trough is a NE - SW trending rift depression filled with continental and marine sediments. Different models have been proposed for the evolution of this megastructure.  presented the structure as a basin which has experienced deformation (aulacogen).  and  interpreted the Benue Trough as a set of juxtaposed pull-apart basins initiated in the Early Cretaceaous, and formed by sinistral movement along a NE - SW transcurrent fault inherited from the Atlantic oceanic crest.  and  suggested that the Benue Trough is genetically related to the opening of the equatorial domain of the South Atlantic. All the models imply an intraplate rifting for the genesis of Benue Trough.
The northern part of the Benue Trough is subdivided into three sub-basins: the N-E trending domain to the south, the N-S Gongola Arm to the north and the E-W trending Yola Arm to the east (Figure 1). MAUTECH Campus lies within this Yola Arm. The arm is bounded to the northeast by the basement rocks of the Hawal Massif and to the south, by the Adamawa Massif.
In the Yola Arm, the Precambrian basement is unconformably overlain by the Aptian-Albian Bima sandstone which is the oldest and most extensively outcropping formation in the sub-basin   . The Bima sandstone is overlain by transitional Yolde Formation (interbeds of shale, siltstone and calcareous mudstone) and followed upward by the Dukul Formation (mainly of gray shales and thin silty beds), and the Jessu and Numanha Formations. These sequences are overlain and capped by poorly to moderately sorted sandstone of Lamja Formation.
3. Materials and Methods
3.1. Sample Collection
Sixteen (16) representative soil samples were analysed for this work. The samples were collected over a period of four months from July to October 2016. The sampling sites were mostly from agricultural fields distant from known areas of contamination on campus. Here, the fear of trace elements input from agro- ecosystem (fertilisers, pesticides etc.) may arise. However, such input is balanced by output represented by losses of trace elements through plant tissue removal for food, erosion etc  . Therefore the background concentrations of trace elements in soils are probably not significantly altered by short-term agricultural use. Harmason and de  calculated that it would take three centuries of phosphate fertiliser at 100 kg P2O5 per hectare per year to enrich the top 20 cm of soil by 1 mg/kg U, if the P2O5 fertiliser contained 100 mg/kg U. Consequently, the trace element contents should be representative of background levels.
At each sample site, approximately 20 g of soil was collected from a depth of about 20 cm or termite mounds (where they exist) in plastic sample bags to avoid the effects of both surface and metallic contaminations. The samples were later air-dried and screened for large rock particles in the laboratory. The locations of these samples and other samples (for other studies) are indicated in Figure 2.
3.2. Sample Preparation and Analyses
The samples were prepared in the geochemistry laboratory of the Ashaka Cement Company, Gombe. The preparation involved grinding, pulverisation and quartering to obtained representative samples. The samples so prepared were later shipped to Activation Laboratories, Canada for both trace and major elements determinations. The trace and major elements in soil were determined by BioLeach-MS methods. It is within ActLabs standards to ensure that analyses are conducted with adequate control on precision and accuracy of the results obtained. The quality control is usually done through analysis of standards, blanks and duplicate samples, done under the same conditions with the samples submitted. All these were done by ActLabs, Canada and submitted along with the analytical results. The standard comparison is excellent, making the results to be reliable and in conformity with highest industry standards.
4. Results and Discussion
Table 1 shows total contents of 49 trace and major elements in soils of MAUTECH Campus.
Figure 2. Map of MAUTECH Campus and the neighbouring villages  .
Table 1. Trace and major elements content in soils of MAUTECH campus.
In general, background elemental contents for the soil vary by factors ranging from about 3 times (As, V), about 4 times (Ni, W), about 6 times (Cd, Rb, Be), about 10 times (Cr, Ba, Br), about 7 times (Se), about 18 times (Mo), about 30 times (Co) and about 45 times (Pb) (Table 1).
An examination of geologic map of the region (Figure 1) shows a predominance of granites over migmatite-gneisses with isolated areas of basalts (ultramafic volcanics) in the region. While the gneisses are mostly ferromagnesian silicates with minerals such as olivine (MgSiO4-FeSiO4), pyroxenes (MgSiO3-FeSiO3- CaSiO3) and the amphiboles, the granites are mostly non-ferromagnesian silicates composing predominantly of plagioclase (a solid solution between anorthite, CaAl2Si2O8, and albite, NaAlSi3O8) and potassium feldspars (solid solution between albite, NaAlSi3O8 and orthoclase, KAlSi3O8) with quartz (SiO2) and associated Ni, Co, Pb etc.
Soils formed from predominantly granitic rocks would likely have low values of Ni, Co, Pb, etc. The average values of Ni (426 ppb), Co (3783.25 ppb) and Pb (161.52 ppb) (Table 1) in soil of the study area are far less than their average background values of 17 ppm (Ni), 10 ppm (Co) and 17 ppm (Pb) in uncultivated soils   .
The low values of these elements can therefore be explained in terms of the source materials and their chemical behaviours. Ni has intermediate ionic radii and is abundant in the earlier members of differentiation sequence as a result of ready substitution for Fe and Mg, with some strongly enriched with magnesium in ultramafic rocks  . Ni is concentrated in magnesium and olivine (in ultrabasic and basic rocks) and to a lesser extent in biotite in intermediate and acid rocks  . The low values of Ni can be attributed to the paucity of basic and ultrabasic rocks in the area and the predominance of acid granites. In granites, almost all the Ni is contained in biotite, and in an environment such as the study area that is flanked by acid granites, the value of Ni can hardly be any higher. Co is one of the elements occurring in the transitional group of the elements, and like Ni, has an intermediate ionic radii and substitute readily for Fe and Mg and hence its abundance in the earlier members of differentiation sequence. The low content of Co can therefore be explained in terms of the paucity of both the basic and ultrabasic rocks and its chemical behavior during transportation. Co has relatively high mobility but readily scavenged and held by Fe-Mn oxides  . Pb is one of the elements belonging to “large-ion lithophile” group (LIL). It has cations with large radii and low electric charge, which tend to substitute for K; hence its concentration in felsic rather than mafic rocks  . Pb is concentrated in orthoclase, which is the mineral indicator of the geochemical characteristic of acid and intermediate rocks. Maximum concentrations are found in zircon and in some other accessory minerals  . However, as a result of weathering, Pb is released from the various Pb-bearing minerals in the acidic environment and passed into water phase with little, getting co-precipitated or absorbed by clay minerals and organic matter. All these suggest that soil in the study area was derived principally from these sub-adjacent granites with little contribution from the mafic rocks. The processes involved in such derivation are probably weathering, erosion, transportation and deposition. In other words, the soil does not appear to have been significantly sourced from ultramafic rocks as such soil would contain appreciably high content of Ni, Cr, etc resulting from serpentine, a magnesium silicates which dominate the mineral composition of ultramafic rocks  . The above results underline the significance of material composition and soil forming processes on background contents of trace and major elements in soil.
Correlations among elements are shown in Table 2 and summary of their statistics in Table 3. Correlations are significant at the probability level of 0.01. Among the major elements; Mg has a strong positive relationship with Ca (0.88), and Al (0.74) while Fe is also strongly related to Al (0.69). Several trace elements have very strong positive relationship with one another. These include: Ba-As (0.91), Be-As (0.93), Be-Ba (0.91), Cs-Ba (0.91), As-Cs (0.85), Cr-Ba (0.85), Cr-Be (0.85), Cs-Be (0.88), As-Ce (0.94) and Cs-Cr (0.86). Mn and Mo are poorly related with most of the trace elements. Among the rare earth elements, Eu is strongly related to Dy (0.98), Gd (0.99) and Lu (0.96) just as Dy is strongly
Table 2. Correlation between elements in soils of MAUTECH campus.
Table 3. Summary content of selected elements.
related to Er (0.99), Eu (0.98), Gd (0.98) and Lu (0.98). Others with strong positive correlations include Tb-Sm (0.99), Yb-Sm (0.93) and Lu-Er (0.99). These strong positive correlations among elements suggest that chemical and physical factors control elements associations in parent materials and soil forming processes  .
Soil geochemical studies to determine the background levels of trace and major elements in soils of MAUTECH Campus have been carried out. Based on trace and major element data, parent material and soil forming processes have a major influence on the chemical composition of the soil. The low content of trace and major elements in the soil corresponds with granite migmatites gneisses bordering the study area thus suggesting that the soil was derived from these granites with little contribution from the mafic gneisses. The data may have application to the identification of areas of trace elements deficiencies and trace elements toxicity for plant growth and may also be useful in soil genesis studies. Most importantly, the data may serve as a reference data in the assessment and monitoring of possible future environmental issues related to trace and/or major element contamination.
This paper is part of a research sponsored by Tertiary Education Trust Fund (TETFund) under TETFund Research Project (RP) Intervention through Modibbo Adama University of Technology (MAUTECH), Yola. We sincerely appreciate TETFund for the sponsorship and MAUTECH, Yola for support.