Geostatistical exploration of dataset assessing the heavy metal contamination in Ewekoro limestone, Southwestern Nigeria

The dataset for this article contains geostatistical analysis of heavy metals contamination from limestone samples collected from Ewekoro Formation in the eastern Dahomey basin, Ogun State Nigeria. The samples were manually collected and analysed using Microwave Plasma Atomic Absorption Spectrometer (MPAS). Analysis of the twenty different samples showed different levels of heavy metals concentration. The analysed nine elements are Arsenic, Mercury, Cadmium, Cobalt, Chromium, Nickel, Lead, Vanadium and Zinc. Descriptive statistics was used to explore the heavy metal concentrations individually. Pearson, Kendall tau and Spearman rho correlation coefficients was used to establish the relationships among the elements and the analysis of variance showed that there is a significant difference in the mean distribution of the heavy metals concentration within and between the groups of the 20 samples analysed. The dataset can provide insights into the health implications of the contaminants especially when the mean concentration levels of the heavy metals are compared with recommended regulatory limit concentration.


Subject area
Earth, Environment and Planetary science More specific subject area

Environmental Science
Type of data Table and Figure  How

Data format
Raw, Analysed Experimental factors The collected samples went through a drying process in bid to make it air free, it was grounded and sieved again. 2 g of the samples was placed in a beaker, 2.5 ml of concentrated HNO 3 and 10 ml of concentrated HCl was added to them and then covered with a watch glass. The beaker was then placed on a hot plate for 15 min to heat. The digestate from the heated sample was filtered using a Whatman No. 41 filter paper into a 100 ml volumetric flask. The digestate was later diluted to a volume of 100 ml and then analysed using a Microwave Atomic Absorption Spectrometer.

Data source location
Ewekoro limestone formation in the eastern Dahomey basin, Southwestern Nigeria Data accessibility All the data are in this data article.

Value of the data
The data could be used to determine the level of heavy metal contamination in limestone formations.
The methods can be replicated to other rock formations. For example to other two key lithostratigraphic units of the eastern Dahomey Basin namely; Abeokuta and Akinbo Formations.
Findings can be extended to other metal or non-metal elements not considered in this article. The dataset can provide insights on the health implications of the contaminants on the groundwater especially when the mean concentration levels of the heavy metals are compared with recommended regulatory limit concentration.

Data
The data contains geostatistical analysis of twenty (20) samples of limestone obtained from the Ewekoro limestone Formation in the eastern Dahomey basin, Southwestern Nigeria. The samples were purified and analysed for heavy metal concentrations using the MPAS. The heavy metals detected from the samples are Arsenic (As), Mercury (Hg), Cadmium (Cd), Cobalt (Co), Chromium (Cr), Nickel (Ni), Lead (Pb), Vanadium (V) and Zinc (Zn). The detailed composition is shown in Table 1. The presence of these heavy metals causes contamination. The descriptive statistics is shown in Table 3. Further analysis was conducted to deepen our understanding on the statistical relationships of the samples. The analysis can be replicated on other limestone Formations and the mean heavy metal concentrations can be compared with the recommended limits.

Experimental design, methods and materials
Several data analysis has been carried out on the physical, geological and geophysical characteristics of the Ewekoro limestone formation in the eastern Dahomey basin, Southwestern Nigeria. Some of the works include: factors causing differentials in the shear velocities, Lithotype representation by Nuclear Magnetic Resonance (NMR) and blastability properties, investigation of allochemical and orthochemical components of the limestone formation, ground vibration and noise generated during blasting, gas generating potential and prospects, estimation of thermal conductivity, assessment of reservoir potential, isotopic elements composition and diagenesis. Others include: Organic geochemical analysis and appraisal, elemental analysis, distribution of tree oxides and groundwater composition [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28].

Study area
The study area lies between longitude 3°05′E to 3°15′E and latitudes 6°40′N to 6°55′N and situated within the Ewekoro local government area, Ogun state, southwestern part of Nigeria. It is bounded by Lagos state to the South, Osun state and Oyo state to the North, the republic of Benin to the West and Ondo state to the East. It. The map indicating the study area is shown in Fig. 1.
The geology of the study area is that of Eastern Dahomey Basin, a combination of inland/coastal/ offshore basin that stretches from south-eastern Ghana through Togo and the Republic of Benin to south-western Nigeria. It is separated from the Niger Delta by a subsurface basement high referred to as the Okitipupa Ridge which marks the continental extension of the fracture zone. Its offshore extent is poorly defined. Sediments deposition within the Dahomey Basin follow east-west trend, and the stratigraphy consists of six geological formations comprising Abeokuta, Ewekoro, Akinbo, Oshosun, Ilaro and Benin Formations. The Cretaceous Abeokuta Formation is a non-fossiliferous basal sequence resting on the Precambrian basement. The overlying Ewekoro Formation is a shallow marine limestone; Paleocene in age and belongs to the Tertiary-formed Paleocene and Eocene sedimentary formations. The formation is composed of non-crystalline and highly non-fossiliferous limestone. It is also composed of thinly laminated, fissile and non-fossiliferous shale. Ewekoro Formation is overlain by a shale-dominated Akinbo Formation that is of Late Paleocene-Early Eocene.

Sample collections
Limestone samples were collected from some rock outcrops within Ewekoro local government area, twenty (20) samples in total with their GPS coordinates recorded in Table 2. These samples were then filtered using a sieve in order to remove pebbles and other irrelevant materials which may affect the result during the analysis. These samples were then packaged into neat polyethylene bags and labelled orderly for identification.

Samples preparation
The samples went through a drying process in bid to make it air free, it was grounded and sieved again. 2 g of the sample was placed in a beaker; 2.5 ml of concentrated HNO 3 and 10 ml of concentrated HCl was added to it and then covered with a watch glass. The beaker was then placed on a hot plate for 15 minutes to heat. The digestate from the heated sample was filtered using a Whatman No. 41 filter paper into a 100 ml volumetric flask. The digestate was later diluted with a volume of 100 ml and then analysed using a Microwave Atomic Absorption Spectrometer. Concentration levels of the nine different heavy metals elements within the samples were then measured.

Descriptive statistics
The detailed statistical description of all the samples is vital in determination of the basic information about the collected samples. The details are summarized in Table 3. The respective mean can be compared with the recommended limits.

Correlation coefficient
Numerical value of the correlation coefficient determines the degree of strength and nature of relationship between the observed variables. The result of the Pearson correlation coefficient, Kendall's tau and Spearman rho correlation coefficient are shown in Tables 4a, 4b and 4c.   The implications of the nature and strength of the relationships among the variables are almost consistent among the three different methods.

Analysis of variance
Analysis of variance (ANOVA) was conducted and the result was displayed in Table 5. The result showed that there are significant differences in the means of the heavy metal concentrations of the 20 samples collected from the Ewekoro limestone Formation.