Introduction

The vast potential of groundwater and the need to manage it sustainably can no longer be ignored. Groundwater Constitute approximately 99% of Earth's freshwater, groundwater holds the capacity to offer societies significant social, economic, and environmental benefit (Li et al. 2023a; United Nation Educational, Scientific and Cultural Organization 2022a). Groundwater serves as a primary source of drinking water for a significant portion of the global population. Many communities rely on groundwater to provide clean and accessible drinking water. Groundwater is considered the most important natural resource to mankind (Bai et al. 2020; Soujanya and Praveen 2017; Ševčíková et al. 2021). Groundwater constitutes an important part of the hydrological cycle and is more prone to pollution (Soujanya 2016). Dumpsite located in close proximity to groundwater resources is highly susceptible to leachates pollution (Zhu et al. 2023; Omeiza et al. 2023).

Utilizing leachate-polluted water may result to inexplicable illnesses and death. Leachate-polluted water may have a wide range of ailments in humans. It has been reported that consuming contaminated water may result in diseases such as renal failure, blood-related disorders, anaemia, kidney damage, possible prostate and lung problems, acute kidney failure, anaemia, gastrointestinal bleeding, chronic nephropathy, metabolic syndromes such as heart disease and stroke (Jan et al. 2009; Chen et al. 2012; Monisha et al. 2014; Yaw 2018; Yun et al. 2019). Lack of dumpsite leachate management can cause devastating health effects on humans and the environment. The extreme entry of heavy metals into the environment leads to human and environmental health issues (Wen et al. 2024; Saravanan et al. 2022). Even at low concentrations, heavy metals affect plants, soil, humans, plants, and animals, because these metal wastes are generally toxic in nature (Shang et al. 2023; Priya et al. 2015; Rani et al. 2021). Groundwater pollution from leachate can have far-reaching environmental consequences. The toxic substances in leachate can degrade overall water quality. Moreover, the long-term persistence of contaminants in groundwater can render affected areas unsuitable for future urban land use and development.

Infiltration of open dumpsite leachate through the soil layers can pollute groundwater, due to its varieties of chemical pollutants (Eni et al. 2011; Eshiet and Agunwamba 2012; Hossain et al. 2014; Chen et al. 2016). The pollution of groundwater occurs whenever a reasonable permeable material exists below the soil strata, especially, where no baselining exists, the leachate may carry along some toxic constituents such as heavy metals (Eni et al. 2014; Ejiogu et al. 2018). Dumpsite leachate is a highly complex liquid that can contain contaminants, including heavy metals such as lead, cadmium, mercury, chromium, nickel and arsenic. Eliminating the devastating effects of dumpsite leachate on the environment depends on the materials underlying the dumpsite, the baselining and the interaction of the leachate (Wang et al. 2023; Carter and Parker 2009). In cases where no lining and underlying materials exist, and no drainage to control stormwater, dumpsite leachate can easily migrate to groundwater. Monitoring groundwater pollution requires multiple technical investigations of soil and groundwater quality analyses (Zhao et al. 2024; Akiang and Emujakporue 2023).

Monitoring soil pollution and groundwater quality can be difficult, especially in regions with limited resources and monitoring infrastructure. The United Nation Water Summit on Groundwater, highlight the importance of better monitoring and management of groundwater (Liu et al. 2023; United Nation Educational, Scientific and Cultural Organization 2022b). Additionally, the complex mixture of contaminants in leachate requires advanced modelling of heavy metal transport and costly remediation techniques such as chemical and biological amendment, pump and treat, permeable reactive barrier, and air sparging to restore affected aquifers to safe levels (Lan et al. 2022; Birak and Miller 2009; United State Environment Protection Agency 2012; United State Environment Protection Agency 2023).

Some of the contemporary studies on dumpsite leachate investigated leachate pollution in specific locations in the world (Li et al. 2023b; Cheremisinoff 1997; Ayolabi et al. 2013; Jason et al. 2014: Priya et al. 2015; Akpan et al. 2018; Saravanan et al. 2022; Eze et al. 2022; Akiang and Emujakporue 2023), with a dearth in research focusing on the transport of dumpsite leachate pollution in groundwater around Lemna dumpsite, Cross River State, Nigeria. Therefore, a comprehensive investigation of the transport of heavy metals in groundwater around dumpsites is crucial for understanding the extent of contamination, identifying vulnerable areas, and formulating effective remediation strategies. This study seeks to investigate contaminant transport of heavy metals in groundwater around Lemna dumpsite in Calabar, Cross River State, Nigeria. This will ultimately promote sustainable water resource management and safeguarding the well-being of residence utilizing groundwater in the study location.

The study area

Location

Cross River State is situated in the South-South Geopolitical Zone of Nigeria. The State is positioned between latitudes 4° 30′ and 6° 30′ North and longitudes 8° 00′ and 9° 00′ East (National Bureau of Statistics 2012). It shares boundaries with Benue State in the North, the Atlantic Ocean in the South, Akwa Ibom State in the Southwest, Ebonyi and Abia State in the West, and the Republic of Cameroon in the East. The state covers a total land area of 21,787 square kilometres (National Bureau of Statistics 2012).

The specific geographic coordinates of the Lemna dumpsite showcases latitude 50′, 2′, 1.040928″ and longitude 80′, 21′, 55.727856″. The Lemna dumpsite is situated at 24 m height (Igelle 2024). The geological composition of the study area comprises of underlying aquifer. Both surface and groundwater reservoirs are replenished by substantial rainfall. The aquifer is confined by a limited number of impermeable layers (aquicludes), and the two primary water-bearing strata are the upper and lower zones (Edet and Okereke 2002). The upper zone is more susceptible to surface contamination in comparison to the lower zone (Edet and Okereke 2002). In the vicinity of the Lemna dumpsite, the lithological composition comprises clayey laterite, coarse sand, medium sand, fine sand, light brown fine sand, and brownish-white fine sand (Rural Water Supply and Sanitation Agency 2022) (Figs. 1, 2).

Fig. 1
figure 1

Source: Department of Geography and Environmental Science, University of Calabar, GIS Unit (2022a, b)

Cross river state showing location of dumpsites in the study areas.

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Fig. 2
figure 2

Source: Department of Geography and Environmental Science, University of Calabar, GIS Unit (2022a, b)

Overlay showing dumpsite locations at Lemna dumpsite, Calabar.

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Material and methods

Soil sampling collection

The study employed a Global Positioning System (GPS) to precisely determine the coordinates of sampling points. Soil samples, gathered at depths of 0–30 cm using a soil auger, were strategically collected along a linear trajectory to assess the levels of heavy metal contamination. Sampling points were located at 5 m, 25 m, and 50 m distances at the Lemna dumpsite in Calabar, with corresponding coordinates (Table 1).

Table 1 Coordinates of water and soil sampling collection points in the study areas.
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The soil sampling methodology utilized in this study aligns with the approach described by (Koki 2013). At each sampling point, three soil samples were systematically collected around the dumpsite, maintaining distances of 5 m, 25 m, and 50 m. To serve as a comparative baseline, one control soil sample was obtained at a distance of 1 km from the sampled soils. In total, four soil samples were collected in the designated study areas. The collected soil samples were carefully transferred from the auger to sealed and labelled sampling bags. Subsequently, the samples were stored in a cooler, equipped with ice blocks to maintain a temperature of 4 °C, and expeditiously transported to the laboratory for detailed analysis of heavy metal concentrations (Mulumebet et al. 2023). The soil sampling activities transpired in September 2022, providing a specific temporal context for the study.

The methodology employed in this study adhered to the approach established by (Al-Hamzawi and Al-Gharabi 2023), as well as guidelines from the United States Environmental Protection Agency (2021). For the analysis of heavy metals, an atomic absorption spectrometer (AAS) of the CB-AAS-3520 model, manufactured by Wincom, China, was utilized for the quantification of heavy metal concentrations, specifically (Ar, Hg, Pb, Cd, Cr, and Ni). To prepare the soil samples for analysis, a systematic procedure was followed. Initially, the soil samples were subjected to drying in an electric oven at 100 °C for a duration of 2 h. Following this, the dried samples were grind using a hand mill and sieving through a fine mesh with a pore size of 75 μm, ensuring homogenization in readiness for subsequent laboratory procedures. The prepared soil samples, each weighing 1 g, were then subjected to digestion by the addition of 150 ml of hydrochloric acid (HCl) along with 5 ml of nitric acid (HNO3). This mixture was placed in a sand bath for a period of 60 min. Subsequently, the containers were allowed to cool, followed by the addition of 5 ml of HCl and 50 ml of distilled water to cleanse the container sides from any residue of the dissolved sample. The mixture was then heated to boiling point for 3 min, after which the sample was filtered using a filter sheet. The resultant filtrate, containing a volume of 100 ml, was collected in specialized containers. Prior to sample analysis, a rigorous calibration procedure was executed. The calibration involved the preparation of standard solutions by diluting multi-elemental standard solutions at a concentration of 100 mg/l, in accordance with the methodologies outlined by Al-Hamzawi and Al-Gharabi (2023) and United States Environmental Protection Agency (2021).

Borehole water sampling collection

Water sample collection for this study was conducted both within and beyond the designated 300–500 square metres (m2) buffer zone surrounding boreholes, as stipulated by Mulumebet et al. (2023). The selection of the 300–500-m buffer zone was predicated on the proximity of the boreholes to the Lemna dumpsite, providing a targeted spatial range for sample acquisition. Control water samples, serving as reference points, were systematically gathered at a distance of 1 km from the sampled borehole water locations (Mulumebet et al. 2023). The purposive selection of water samples was focused on five boreholes situated in close proximity to the Lemna dumpsites in Calabar. The coordinates of these boreholes were accurately determined using the Global Positioning System (GPS), as outlined in Table 1. The study involved the collection of five borehole water samples around the dumpsites and an additional control sample within the study areas, resulting in a total of six borehole samples. To ensure sample integrity, the collected water samples were promptly transferred to a cooler, equipped with ice blocks to maintain a temperature of 4 °C. Subsequently, these samples were transported to the laboratory for detailed heavy metals analysis (Mulumebet et al. 2023). It is pertinent to note that the borehole water samples were methodically collected in September 2022, providing a specific temporal context for the study.

Laboratory analysis of heavy metals in borehole water

The methodology employed in this study aligns with the procedures outlined by (Al-Hamzawi and Al-Gharabi 2023), as well as the guidelines set forth by the United States Environmental Protection Agency (2021). For the analysis of water samples, 1 ml of nitric acid (HNO3) was meticulously added to each 100 ml of water sample, followed by filtration through filter paper. Subsequently, the mineralized samples were judiciously preserved by storing them in a refrigerator at a controlled temperature of 4 °C, ensuring their integrity until the time of analysis. The quantification of heavy metal contents, specifically (Ar, Hg, Pb, Cd, Cr, and Ni), in the water samples was conducted through rigorous measurement procedures. Prior to the analysis, a comprehensive calibration process was undertaken using standard solutions. These solutions were prepared by diluting multi-elemental standard solutions, maintaining a concentration of 100 mg/l, in accordance with the methodologies detailed by Al-Hamzawi and Al-Gharabi (2023) and the United States Environmental Protection Agency (2021).

Pumping test

The pumping test parameters include drawdown, Transmissivity, specific yield, specific discharge capacity, slop, static water level, pumping water level, pumping rate, aquifer thickness, hydraulic conductivity in the X, Y, Z directions, hydraulic head, bulk density, effective porosity and screen length and specific storage. These are important data required as impute parameters for assessing contaminants' transport in groundwater.

The pumping test comprised distinct discharge and recovery (recharge) stages, each integral to the comprehensive assessment of borehole aquifer characteristics. In the discharge stage, borehole water was systematically pumped at a constant rate, while concurrent measurements of water levels were meticulously recorded at regular intervals within the same pumped borehole. This discharge stage served to elucidate the behaviour of the aquifer under sustained extraction conditions (Waterloo Hydrologic 2022).

Subsequently, the recharge stage ensued, wherein the pump was halted, allowing the borehole to undergo natural recharge. Concurrent with this recharge phase, water levels were methodically gauged at identical intervals, employing a water level sounder. Instrumentation employed for these operations included a submersible pump and cable, water level sounder, a 20-L container, a generator, stopwatch, and riser pipes, in accordance with the prescribed standards outlined by (Rural Water Supply and Sanitation Agency 2022).

To analyse and interpret the acquired data, semi-logarithmic plots correlating time and drawdown were constructed. The estimation of the aquifer's transmissivity was undertaken following the methodology elucidated by (Cooper and Jacob 1946) and in congruence with contemporary guidelines presented by (Waterloo Hydrologic 2022).

Drawdown

Drawdown refers to the reduction in the water level within a well caused by the extraction of water through pumping. This phenomenon is quantified through drawdown measurements, which entail recording the disparity (expressed in feet or metres) between the static water level (the initial, undisturbed water level) and the level observed during active pumping operations (Cooper and Jacob 1946; Lytle and Markowski 1989; Amah et al. 2012; Waterloo Hydrologic 2022). The mathematical formula is given as follows:

$$\left( {\Delta s} \right) = \frac{s2 - s1}{{\log 120 - \log 25}}$$
(1)

where s, Drawdown; (Δs), Change in drawdown; Log120-log25, Logarithm.

Transmissivity

Transmissivity is the rate at which water passes through a unit width of the aquifer under a unit hydraulic gradient (Brian and Neil 2019). The mathematical formula is given as follows;

$$T = \frac{2.3Q}{{4\pi \Delta s}}$$
(2)

Δs, Drawdown; T, transmissivity; Q, discharge rate.

Hydraulic conductivity

K, T/b; K, hydraulic conductivity; T, transmissivity; b, aquifer thickness.

Specific discharge capacity

The specific discharge capacity, often denoted as flux and velocity, characterizes the flow of water through porous media. It precisely quantifies the volume of water that traverses a unit cross-sectional area of the porous medium within a given timeframe (Cooper and Jacob 1946; Waterloo Hydrologic 2022). The mathematical formula is given as follows:

$$V = \frac{Q}{{{\text{Maximum}}\,{\text{drawdown}}}}$$
(3)

V, Specific discharge capacity; Q, discharge rate (volume of water discharge per day during pumping m2/d); drawdown, maximum drawdown.

Hydraulic head

Hydraulic head is defined as the vertical distance from the free surface of a water body to a specified subsurface point (Cooper and Jacob 1946; Clancy 1975; Driscoll 1986; Waterloo Hydrologic 2022), and it serves as a crucial metric for quantifying the potential energy within a groundwater system.

Within the realm of groundwater dynamics, the total energy comprises three principal components such as pressure, velocity, and the elevation of the water body, commonly known as the elevation head. In instances where groundwater velocities are low, the energy associated with the velocity component approaches zero (Clancy 1975; United States Geological Survey 2017). Bernoulli’s equation is expressed as follows;

$$h = Z + P$$
(4)

where h, hydraulic head; Z, elevation head; P, pressure head.

Hydraulic conductivity

Hydraulic conductivity is a property of both the porous media and groundwater (or other fluid) passing through the pore space due to a pressure gradient based on Darcy's Law, an empirical relationship that relates the fluid flux (specific discharge) to hydraulic gradient, with the conductivity as a constant of proportionality (Waterloo Hydrologic 2022). The conductivity parameters may be defined on a cell-by-cell basis using constant property values (Waterloo Hydrologic 2022). By default, the principal axes of hydraulic conductivity are aligned with the global model-coordinate system (x, y, z), so that in the global model-coordinate system K is represented by a diagonal matrix (United State Geological Survey 2017; Waterloo Hydrologic 2022). The study utilized hydraulic conductivity of clayey sand (0.0015 cm/s) (Waterloo Hydrologic 2022).

Specific storage and specific yield

Specific yield is the volume of water per unit of time produced from a borehole by pumping. Borehole yield is measured in litter per second (l/s) (Lytle and Markowski 1989). Specific storage (Ss) and specific yield (Sy) are material physical properties which characterise the capacity of an aquifer to release groundwater from storage in response to a decline in hydraulic head (Cooper and Jacob 1946; Ingebritsen and Sanford 1999; Amah et al. 2012). Specific Storage (Ss) is defined as the volume of water the aquifer releases from storage under a unit decline in hydraulic head due to aquifer compaction and water expansion (Amah et al. 2012). Visual MODFLOW determines the primary storage coefficient for MODFLOW based on the user-defined specific storage parameter. The primary storage coefficient is given as; specific storage multiplied by the layer thickness (Specific Storage × thickness = Primary storage coefficient) (Waterloo Hydrologic 2022).

Effective porosity

Effective porosity is a dimensionless storage parameter which describes the percentage of the volume of the aquifer material through which flow can occur. An increase in effective porosity increases the volume through which groundwater flow occurs (Waterloo hydrologic 2023). The study utilized effective porosity of clayey sand (0.41) (Waterloo hydrologic 2023).

Bulk density of soil

The bulk density of soil reflects the mass or weight of a certain volume of soil. Soil bulk density determines the infiltration, available water capacity and soil porosity. The study utilized soil bulk density of clayey sand (1.38 g/cm3) (Waterloo hydrologic 2023).

Partition coefficient K d

Partition (or distribution) coefficient, Kd, describes the distribution of a species between a solid and aqueous matrix after equilibration. In groundwater risk assessments, the Kd value describes the degree of sorption of a particular element in the soil or rock to the groundwater. Partition coefficients are expressed in units of ml/g or (1/mg/l). The measured or calculated partition coefficient, Kd, can subsequently be used to predict the effects of retardation on the velocity and travel times of the contaminants being investigated. Substances subject to sorption processes migrate through an aquifer system more slowly than the water or a conservative (non-reactive) substance. Understanding the likely contaminant velocity allows the assessor to locate monitoring boreholes in the most appropriate locations, predict the imminence of any risks to receptors and instigate a timely response (Adey 2005). A study conducted to determine the partition coefficients exhibited lead concentration of 0.1–0.9 g/l, with a corresponding Kd value of 44,580 (ml/g), and 10–99.9 g/l concentration, with a corresponding Kd value of 2380 (ml/g) (United States Environment Protection Agency 1999). Lead at 10 mg/l concentration, with a corresponding Kd value of 317.51 mg/l in groundwater system (Adey 2005). The formula is given as follows.

$$K_{d} = \frac{{\left( {I - F} \right)}}{F} \times \frac{\left( V \right)}{M}$$
(5)

where I, Initial contaminant concentration (soil); Final contaminant concentration (borehole water); Volume of liquid, (l); Mass of solid (kg).

Potential groundwater recharge

Potential recharge (R) is defined as the difference between total precipitation (P) and the potential evapotranspiration (ET0). Climate variables data such as minimum temperature, maximum temperature, radiation and precipitation data were source from National Aeronautics and Space Administration (2024). This allowed the computation of potential evapotranspiration (ET0) using Hargreaves equation (Medici et al. 2019).

$$ET_{o} = 0.0023 {\text{Ra}} \left( {T_{{\text{m}}} + 17.8} \right)\sqrt {{\text{TR}}}$$
(6)

where Ra: extraterrestrial radiation, Tm: daily mean air temperature, computed as an average of the maximum and minimum air temperatures, TR: temperature range between minimum and maximum values.

$${\text{PR}} = {\text{P}} - {\text{ETp}}$$
(7)

where PR: Potential recharge, P: Precipitation (mm/day), ETp: evapotranspiration (mm d−1).

Data analysis

The study utilized statistical analysis to compare heavy metals parameters with WHO guideline. The study utilized paired sampled t test embedded in SPSS version 22, to compare heavy metals parameters with WHO guideline (National Environmental Standard and Regulation Agency 2011; World Health Organization 2017; International Business Management Statistics Package for Social Sciences 2021).

Furthermore, the study utilized Visual MODFLOW to establish contaminant transport of heavy metal around Lemna dumpsite in the study location. Visual MODFLOW version 7.0 brings together industry-standard codes for groundwater flow and contaminant transport. With Visual MODFLOW, groundwater modeler has all the tools required for addressing local to regional-scale water quality, groundwater supply, and source water protection issues (Waterloo Hydrologic 2023).

The Visual Modflow transport engine simulates advection, dispersion, and chemical reactions of contaminants in groundwater systems. The study utilized linear isotherm equilibrium control embedded in Visual Modflow. Linear isotherm is a line drawn on a map or graph connecting points. Isotherms are commonly used on a chart showing steady or constant pressure (Waterloo Hydrologic 2023).

Results

The result of the laboratory analysis of soil around Lemna dumpsite is presented in (Table 2). Lead, Arsenic and Cadmium concentrations at all three distances (5 m, 25 m, and 50 m) are extremely high, significantly exceeding the values recommended in NESREA guideline limits. Mercury, nickel and chromium were below NESREA limit (Table 2) while the paired sample test shows (p < 0.05), indicating significant variation between soil parameters and NESREA recommended values (Table 3).

Table 2 Results of soil heavy metals in Calabar dumpsite
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Table 3 Paired samples test of soil analysis of heavy metals in Calabar dumpsite
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Borehole water parameter values around Lemna dumpsite, Calabar

The result of the laboratory water quality analysis of borehole (BH) water around Lemna dumpsite is presented in (Table 4). The water quality indices tested were arsenic, lead, cadmium, chromium, nickel and mercury for all the boreholes. The water quality analyses of BH2, BH3 BH5 and BH6 around the Lemna dumpsite revealed elevated concentrations of arsenic, lead, cadmium and nickel exceeding the established limits of the World Health Organization (WHO). However, the concentrations of heavy metals in BH 4 were lower than the WHO limit while the paired sample test analysis shows (p < 0.05), indicating significant variation between borehole water parameters and WHO guideline values (Table 5).

Table 4 Results of heavy metals for boreholes around Lemna dumpsite, Calabar
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Table 5 Paired samples test analysis of heavy metals for boreholes around Lemna dumpsite with WHO standard
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Pumping test parameters

The pumping test data is presents in (Table 6). In the pumping test data, drawdown remained low at 0.32 (m). Slope (Δs) stood high at 676.42 (m/day). Specific yield registered a high value of 1.34 (l/s). Pumping rate/Discharge indicates 115.776 (m3/day). Transmissivity measured 0.032 (m2/day). Specific discharge illustrates 43.2 (m2/day). Hydraulic conductivity K measured 4.6296 × 10–8 (m/s). Hydraulic conductivity Kx direction indicates 0.0015 (cm/s), Ky direction indicates 0.0015 (cm/s), and Kz direction indicates 0.0015 (cm/s). Storage measured 0.005 (m/s), and specific storage recorded 0.04 (m/s). Lead sorption concentration for borehole water showcased 0.991 mg/l, and the partition coefficient Kd reached a high value of 687.23 mg/l (Table 6).

Table 6 Summary of pumping test data for Visual Modflow
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Groundwater recharge

Table 7 represents the daily mean values of key climatic parameters, specifically radiation, daily total precipitation, maximum temperature, and minimum temperature, for the study area. In terms of solar radiation, the study area exhibits a mean daily irradiance of 22.78 MJ/m2/day. The daily total precipitation notably recorded at 186.94 mm/day. Maximum Temperature attains a high value of 32.55 °C. Conversely, the minimum temperature records a low value of 20.17 °C.

Table 7 Daily mean Precipitation and Evapotranspiration of the study area
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Table 8 delineates the computation and assessment of groundwater recharge within the study area. Evapotranspiration (ETo) yield a value of 8.140 mm/day. Meanwhile, the precipitation (P) showcases high value of 186.94 mm/day. The culmination of these computations results in the determination of groundwater recharge (R), calculated as the difference between precipitation and evapotranspiration, yielding a value of 178.795 mm/day (Table 8).

Table 8 Potential groundwater recharge in the study area
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Contaminant transport of heavy metals in groundwater

The result of the spatial extent of contaminants is presented in (Table 9). Around the Lemna dumpsite, the predicted solute transport exhibited a spatial spread of 259.2000 m2/day, with the contaminant having the potential to travel up to 94,608 m2/year (Fig. 3).

Table 9 Spread of heavy metal contaminant transport, and the extent of the concentration
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Fig. 3
figure 3

Source: Researcher’s analysis (2022)

Borehole water flow path and contaminants extent of heavy metal around Lemna Dumpsite, Calabar.

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The results revealed that BH2, BH3, BH5 and BH6 designated for drinking purposes were contaminated. The spatial extent of Lead concentration exhibited a maximum of 0.991 mg/l to a minimum of (− 6.72 × 10–18 mg/l), with concentrations decreasing as the plume extend (Fig. 3).

Discussion

This study examined contaminant transport of heavy metals in groundwater around Lemna Dumpsite: Implications for residence utilizing borehole water in Cross River State, Nigeria. The findings of the study present a substantial cause for concern regarding the environmental contamination and potential health risks associated with heavy metal such as arsenic, lead, cadmium, chromium, nickel and mercury concentrations in both soil and water samples collected at varying distances from the Lemna Dumpsite. In the soil analysis, concentrations of lead, arsenic, and cadmium at all measured distances (5 m, 25 m, and 50 m) significantly surpass the recommended limits stipulated by the National Environmental Standards and Regulations Enforcement Agency (NESREA). This egregious deviation from NESREA guidelines suggests a severe contamination issue, warranting urgent attention and remediation measures. On the other hand, mercury, nickel, and chromium concentrations in the soil were found to be below the NESREA limits, indicating a potential variation in the sources and mobility of these heavy metals. The results of the paired sample t test underscore the significance of the observed variations, with a statistical significance level (p < 0.05) affirming the substantial disparity between the soil parameters and NESREA recommended values. Handex (2016) asserted that different materials that end up in dumpsites contain pollutants that are finally released into the soil. Essien et al. (2022) asserted that contamination of the soil is facilitated by leachate seeping through deposited waste. Akshay et al. (2017) elucidated that precipitation facilitated soil contamination.

The water quality indices of arsenic, lead, cadmium, nickel, and mercury in boreholes BH2, BH3, BH5, and BH6 exceeded the established limits set by the World Health Organization (WHO). This raises serious concerns about the portability of water from these boreholes, with potential implications for public health. Notably, BH4 exhibited lower concentrations of heavy metals, remaining within WHO limits, suggesting potential differences in geological or hydrological factors influencing water quality. Similar to the soil analysis, the paired sample t test for borehole water quality indices yielded a statistically significant result (p < 0.05), signifying a substantial deviation between the parameters of the borehole water and WHO guideline values. The findings align with a previous study conducted by Olagunju et al. (2018), Olorunfemi et al. (2018). These findings underscore the presence of significant contamination in the borehole water around the dumpsites. The contamination of the boreholes may be due to the unsuitability of the soil and geology in the study areas. Strict adherence to regulatory standards and the implementation of appropriate management strategies are crucial to safeguard the quality of groundwater resources in these areas.

The pumping test data for Visual Modflow is presents in (Table 6). In the pumping test data, drawdown remained low at 0.32 (m), indicating the difference in water level between the two observation points. This could be influenced by factors such as the permeability of the aquifer and the distance from the pumping well. Slope (Δs) stood high at 676.42 (m/day), representing the steepness of the drawdown curve over a logarithmic scale. This steepness may be influenced by the geological characteristics and the hydraulic conductivity of the aquifer (Lytle and Markowski 1989; Amah et al. 2012; Waterloo Hydrologic 2022). Specific yield registered a high value of 1.34 (l/s), indicating the volume of water released from the aquifer per unit area. This value is influenced by the porosity of the aquifer and its ability to store and release water. Pumping rate/Discharge indicates 115.776 (m3/day), representing the volume of water pumped out during the test. This value is influenced by the pumping system and the aquifer's response to extraction. Transmissivity measured 0.032 (m2/day), indicating the ability of the aquifer to transmit water horizontally. It is influenced by the hydraulic conductivity and thickness of the aquifer (Brian and Neil 2019). Specific discharge illustrates 43.2 (m2/day), representing the rate of water flow per unit area. This value is influenced by the hydraulic conductivity and the gradient of the hydraulic head (Cooper and Jacob 1946; Waterloo Hydrologic 2022). Hydraulic conductivity K measured 4.6296 × 10–8 (m/s), indicating the ability of the aquifer to transmit water under a hydraulic gradient. Hydraulic conductivity Kx direction indicates 0.0015 (cm/s), Ky direction showcase 0.0015 (cm/s), and Kz direction indicate 0.0015 (cm/s), indicating equal conductivities in all directions. It is influenced by the size and interconnectedness of the aquifer's pores (United State Geological Survey 2017; Waterloo Hydrologic 2022). Storage measured 0.005 (m/s), and specific storage recorded 0.04 (m/s), representing the ability of the aquifer to store water. It is influenced by the compressibility and storativity of the aquifer material (Ingebritsen and Sanford 1999; Waterloo Hydrologic 2022). Lead sorption concentration for borehole water concentration showcased 0.991 mg/l. This may be influenced by anthropogenic activities and geological characteristics. The partition coefficient Kd reached a high value of 687.23 mg/l (Table 6). This value is influenced by the sorption capacity of the aquifer material (Adey 2005). Pumping test data are valuable input data for predicting contaminant transport.

The analysis of contaminant transport in the observation boreholes (BH) surrounding the Lemna dumpsite provides a discerning insight into the impact of contaminants on groundwater quality, particularly with respect to boreholes designated for drinking purposes. The study encompassed six observation boreholes (BH2, BH3, BH4, BH5, and BH6) situated proximate to the dumpsite, employing robust methodologies to elucidate contaminant movement and concentration. The directional analysis of contaminant flow paths and the concentration extents, illustrated in Figs. 3, serves as a comprehensive portrayal of the contaminant dispersion in the groundwater surrounding the dumpsite. Notably, the focus of the modelling efforts was on Lead, chosen as a representative indicator to model the spatial extent of heavy metal dissemination in the groundwater. The findings revealed that BH2, BH3, BH5, and BH6, designated for potable water supply, exhibited contamination attributed to the influx of contaminants from the dumpsite. This contamination phenomenon was intricately linked to the congruence in hydraulic head levels between the dumpsite and these boreholes, leading to the establishment of shared groundwater flow paths. The spatial distribution of lead concentrations demonstrated a range from a maximum of 0.991 mg/l to a minimum of (− 6.72 × 10−18 mg/l), with concentrations diminishing as the contaminant plume extend. The negative outward flow values signify the termination of heavy metal concentration movement at specific distances per annum, halting further extension (Fig. 3).

The findings of contaminant transport around the Lemna dumpsite disclosed a substantial dispersion of contaminants, indicative of potential hazards to the adjacent areas. The spatial spread of predicted solute transport around the Lemna dumpsite measured 259.2000 m2/day, suggesting that the contaminant had the capacity to travel up to 94,608 m2/year (Fig. 3). The size of the plume contamination increases with increased length of flow and the concentration of the contaminant decreases with increased length of flow. This may be due to the dispersion of the leachate in the aquifer, which causes the concentration of the contaminants to decrease with increasing length of flow (Cheremisinoff 1997; Jason et al. 2014). This is in line with the study conducted by Jason et al. (2014). This implies a consequential dispersal of contaminants from the dumpsite over considerable distances, posing a tangible threat to the environment and the integrity of groundwater resources. The high rate of contaminate transport may be facilitated by the hydraulic head, permeability and porosity, and the groundwater recharge in the areas. The hydraulic head is the driving force behind groundwater flow; it represents the potential energy in the system. Higher hydraulic head gradients lead to increased flow and, subsequently, higher discharge rates (Advanced Geosciences Incorporation 2023). Additionally, the permeability and porosity of the geological formations that make up the aquifer dictate the ease with which water can flow through the subsurface. Higher permeability and porosity facilitate increased discharge rate. The surplus water available for replenishing groundwater in the study location is 178.795 mm/day. This surplus water can influence the transport of heavy metals in Groundwater. Replenishing groundwater through precipitation or surface water infiltration can influence high groundwater discharge rates (Advanced Geosciences Incorporation 2023). The high rate of the specific discharge is in line with the study conducted by Medici et al. (2019).

Conclusion

This study has provided critical insights into the spatial spread of solute transport of Heavy Metals in groundwater around Lemna Dumpsite in Calabar, Cross River State, Nigeria. The findings, obtained through an analysis of soil and water quality indices, and heavy metal transport dynamics, have raised significant alarms. The paired sample t test of the analysis of Heavy metals in soil exhibited a significant difference (p < 0.05) compared to National Environmental Standard Regulation and Enforcement Agency (NESREA) limits. Likewise, the paired sample t test of the analysis of heavy metals in borehole water exhibited a significant difference (p < 0.05) compared to World Health Organization (WHO) limits. The significant level indicates contamination of the soil and borehole water. The water quality analysis revealed that several boreholes, namely BH2, BH3, BH5, and BH6, exhibited heavy metal concentrations exceeding the established WHO limits, particularly for arsenic, lead, cadmium, and nickel. This contamination poses a direct threat to public health and the surrounding environment. The variation confirmed by the paired sample t test underscores the urgency of immediate remediation efforts and stringent monitoring to ensure compliance with regulatory guidelines.

The predicted contaminant transport exhibited a spatial spread of 259.2000 m2/day, with the contaminant travelling up to 94,608 m2/year. The extent of heavy metals concentration exhibited a maximum of 0.991 mg/l to a minimum of (− 6.72 × 10–18 mg/l), representing a decreasing concentration as the plume extend. The solute transport analysis demonstrated a considerable spatial spread of contaminants from the dumpsite, which could potentially jeopardize nearby areas and groundwater resources. Factors such as hydraulic head, permeability, porosity, and groundwater recharge played critical roles in driving this phenomenon. Understanding these flow dynamics is pivotal for addressing the risks associated with heavy metal contamination. The study emphasizes the immediate need for remediation and stringent monitoring to mitigate heavy metal contamination near the Lemna Dumpsite.