Evaluation of Saltwater Intrusion in Some Coastal Areas in Rivers State, Nigeria

The dry season lasts from about November to April and while the rainy season spans May to October with a brief dry spell in August. The study area has characteristic tropical equatorial climate with mean annual temperature of 32.8⁰C and annual rainfall of 2673.8 mm. Located in Benin Formation, Asari-Toru and Degema is typically riverine and has a large number of outlets through which her tributes from River Niger empty their water into the Atlantic Ocean. It is predominantly sandy with a few intercalations of shale beds. The sands are always unconsolidated, granular in texture and characteristically fine to coarse grained size and described as fresh water bearing. At various depths in the formation, both confined and unconfined aquifers are encountered (Figure 1). In this study, electrical resistivity imaging (tomography) profiling was adopted for the subsurface aquifer investigation due to saltwater intrusion into freshwater aquifer. The geophysical equipment used is the ABEM Terrameter 300 C with a Signal Averaging System (SAS), a method whereby consecutive readings are taken automatically and the results are averaged continuously.

In this study, electrical resistivity imaging (tomography) profiling was adopted for the subsurface aquifer investigation due to saltwater intrusion into freshwater aquifer. The geophysical equipment used is the ABEM Terrameter 300 C with a Signal Averaging System (SAS), a method whereby consecutive readings are taken automatically and the results are averaged continuously. A total of eighteen profile lines were established for the 2-D resistivity survey. For this study, electrical resistivity Wenner imaging was adopted (Frohlich et al., 2002). A 200 m multicore cable with a 10 m take-out interval for Profile 1 and Profile 2 and 100 m multicore cable with a take-out interval of 5 m for Profile 3, Profile 4, Profile 5 and Profile 6 were used. After taking the first reading at station one, the cables were moved in a leap-frog manner to the next position of electrodes 2,3,4 and 5 for the second station reading. This procedure was repeated until spacing of 2a, 3a, 4a, 5a and 6a was acquired. A total of 18 measurements points were occupied for the eighteen profiles during the 2-D resistivity survey. The eighteen resistivity profiling data obtained from the field were processed using a 2-D iterative smoothness constrained least square inversion technique to create a subsurface model of the resistivity by inverting the apparent resistivity data. The inverted sections were generation was done using RES2D inversion software.

Results and Discussion

The nature of the fluid present in the pores of a rock greatly influences the resistivity of the lithology. To ascertain the extent of pollution, the resistivity depth section from polluted segment of the study area was compared with the background value of the control section. The interpretation should distinguish the relatively resistive topsoil from the more conductive clayey units and potential saline water bearing zones. Low electrical resistivity may indicate saline water intrusion; however, clay-rich sediments can also exhibit low resistivity because of their high surface conductivity. Consequently, resistivity alone cannot conclusively establish saltwater contamination without supporting groundwater geochemical analyses.

 Figure 2: Resistivity plot for profiles.

 The supplied inverted section of Profile 1-3 extends to approximately 24.2 m depth. The upper zone, approximately 3.5-7 m deep, exhibits resistivity values broadly ranging from 94.6 to above 333 Ωm, representing relatively resistive topsoil or shallow sediments. Beneath this, the central conductive anomaly extends to approximately 24 m, with resistivity decreasing to 26.8-62.1 Ωm, suggesting possible saline water intrusion or conductive clayey materials. Similar coastal investigations demonstrate that saline saturated sediments generally exhibit low resistivity, although lithological verification is essential (Oladunjoye et al., 2021).

Accordingly, the deeper conductive zone represents a potential groundwater pollution hotspot, whereas comparatively resistive zones may indicate less saline conditions. However, saltwater absence cannot be confirmed from resistivity alone. Resistivity plot for profiles inverse model has identified a shallow conductive anomaly (26.8-48.8Ωm) embedded at 1.75-18.2m in the Benin Formation, which is thought to be a saltwater plume migrating continuously downward (Figure 2). There is significant agreement between the computed and measured apparent resistivity pseudosections (RMS error = 2.0%), which supports the idea of inversion fidelity. Around 30% of coastal aquifers in the Niger Delta are affected by salinisation, and this phenomenon is well documented there as well (Udom et al., 2002). It is represented by a saline wedge that uses near-surface permeable pathways, in contrast to higher-resistivity freshwater bearing strata (>134Ωm). From a hydrogeological perspective, the polluted interval does not meet the potability criteria set by the World Health Organization (2000) for total dissolved solids (TDS) and chloride, and the abnormally high levels of sodium (Na⁺) cause the SAR values to be unsustainable, which goes against the standards set by the National Solid Waste Qualifications 2007. The Benin Formation is vulnerable to tidal force and human over abstraction, as shown by the downward migratory track. Sustainability in groundwater management calls for controlled extraction and ongoing geophysical monitoring, according to these results, which are in line with recent ERT based evaluations (Ohwoghere-Asuma et al., 2023; Ekwok et al., 2022) and the hydrogeological synthesis (Nwankwo et al., 2023) (Table 1 and 2).

Statistics of Physico-Chemical Parameters of Irrigation Water

Descriptive statistics (Table 1) reveal a dataset dominated by strong positive skewness, reflecting the coexistence of minimally impacted wells and heavily mineralized boreholes within the same coastal corridor. Temperature was essentially invariant (mean 28.18 ± 0.45 °C; CV = 1.6%), consistent with equatorial shallow aquifer conditions, whereas EC and TDS exhibited the highest dispersion in the dataset (EC: 77-7,490 µS/cm, CV = 132%; TDS: 40-3,750 mg/L, CV = 148%), confirming the spatially heterogeneous salinization documented for the Niger Delta coastal belt (Eyankware et al., 2021; Udota et al., 2022).

Mean pH (6.57 ± 0.49) was marginally acidic and within the 6.5-8.5 window recommended for irrigation (Ayers & Westcot, 1985), although BH5 (Ido, 5.37) fell below it. Sodium (121.12 ± 110.95 mg/L; CV = 92%) and chloride (96.70 ± 42.20 mg/L) displayed wide ranges with three Na exceedances of the NSDWQ (2007) limit. The sodium adsorption ratio averaged 8.68 ± 8.34 (CV = 96%), with BH2 (Tema) recording 26.38 a value within Richards' (1954) S3 ("high sodium hazard") class, indicating that direct use of such water on fine textured deltaic soils risks permeability decline and sodicity. Conversely, hardness parameters (Ca: 6.24 ± 2.08; Mg: 5.80 ± 1.67 mg/L) were low and uniform (CV < 35%), while nitrate (6.63 ± 2.25 mg/L) remained far below the 50 mg/L threshold, suggesting negligible agrochemical loading relative to salinity stress. Organic loading (BOD: 2.58 ± 1.92; COD: 5.25 ± 3.90 mg/L) was moderate but exceeded the COD irrigation guideline of 10 mg/L at BH1 and BH2. The statistics characterize a salinity limit, sodium hazard dominated irrigation water suite, in agreement with regional irrigation suitability assessments by Egbueri et al., (2024), and underscore that salinity management rather than nutrient or pathogen control constitutes the primary constraint on agricultural reuse of these waters.

Spatial Distribution of Groundwater Salinity

The spatial distribution of groundwater salinity across the study area (Figure 4) reveals a markedly heterogeneous contamination regime, with discrete thermal like hotspots of extreme ionic enrichment that substantially transgress the Nigerian Standard for Drinking Water Quality (NSDWQ, 2007) threshold of <500 mg/L. The most pronounced salinity anomalies are concentrated at Tema (Well 2: 3,810 mg/L), Sama (BH1: 3,310 mg/L), and Ido (BH6: 2,712 mg/L), which collectively define a bifocal plume architecture exceeding the regulatory limit by factors of 5.4 to 7.6 (Figure 4). These extreme values are diagnostic of active saltwater intrusion or anthropogenic saline loading, consistent with regional findings that coastal aquifers of the Niger Delta Benin Formation exhibit low longitudinal conductance and high vulnerability to salinization due to permeable overburden and proximity to marine interfaces (Nkpoidet et al., 2025). By contrast, samples from Sama (BH2: 40 mg/L; Well 1: 114 mg/L), Tema (BH3: 50 mg/L), and Buguma (BH5: 50 mg/L) register salinity well within the NSDWQ potable envelope, underscoring extreme spatial heterogeneity at the intra community scale (Table 1 and 2).

 The spatial gradient manifests a distinct NW-SE vector of increasing salinity, with the highest conductometric signatures clustering around low elevation coastal settlements (Buguma and Ido at 0 m elevation) and extending inland toward Tema and Sama. This topographically controlled distribution corroborates time lapse electrical resistivity investigations that document active lateral salinity changes rapidly intruding freshwater aquifers in southeastern Niger Delta coastal environments (Inim et al., 2020). Notably, the elevated salinity values coincide with high electrical conductivity and total dissolved solids, forming a coherent hydrochemical facies indicative of Na-Cl dominance driven by seawater mixing or evaporite dissolution (Omajene et al., 2024). The NSDWQ (2007) threshold, although not exceeded in all sampling points, is breached at four distinct locations, signifying that approximately one-third of the monitored aquifer units are already compromised. The concentration map thus captures a transitional hydrogeological state wherein fragmented freshwater lenses are progressively assimilated by encroaching conductive fronts, necessitating immediate aquifer vulnerability assessment and regulated extraction to avert total potable collapse (Nkpoidet et al., 2025).

The spatial distribution of hydrochemical parameters across the study area reveals a tripartite zonation reflecting differential contamination vectors and aquifer vulnerability within the Niger Delta Benin Formation (Figure 5). The Northwest cluster (Obuama) registers consistently moderate signatures, with Well 6 exhibiting electrical conductivity (596 µS/cm), total dissolved solids (296 mg/L), and salinity (298 mg/L) within or marginally approaching potable thresholds, alongside a low sodium adsorption ratio (SAR = 1.53), indicating minimal sodium hazard. This cluster represents the least compromised hydrogeological regime, likely benefiting from greater topographic elevation (10 m) and distance from saline intrusion fronts.

The Central cluster (Tema-Abalama) manifests extreme spatial bifurcation, a hallmark of point source contamination superimposed on regional salinization. Tema Well 2 registers the most severe deterioration across all parameters (EC 7,490 µS/cm; TDS 3,750 mg/L; salinity 3,810 mg/L; Na^⁺ 365 mg/L; SAR 26.38), exceeding NSDWQ (2007) limits by an order of magnitude, whereas adjacent BH3 maintains pristine conditions (EC 529 µS/cm; TDS 50 mg/L). This intra-community disparity is consistent with hydrochemical investigations documenting that Na-Cl facies in the Niger Delta arise from both saltwater intrusion and anthropogenic effluent discharge, with spatial heterogeneity driven by preferential flow pathways and differential aquifer confinement (Odesa 2024). Abalama exhibits intermediate degradation (BH4: EC 1,120 µS/cm; salinity 556 mg/L), suggesting progressive plume encroachment (Table 2).

Figure 5: comprehensive Spatial Distribution Map of hydrochemical parameters across the study area

 Table 2: Spatial interpolation of critical hydrochemical parameters across the study area

The Southeast cluster (Buguma-Ido) at 0 m elevation demonstrates coastal aquifer vulnerability, with Ido BH6 registering elevated TDS (2,733 mg/L), salinity (2,712 mg/L), and SAR (15.56), indicative of density driven saltwater intrusion. Buguma remains comparatively moderate, though BH5 exhibits anomalous EC (985.8 µS/cm) despite low TDS, possibly reflecting conductive ionic species from surficial contamination. The collective spatial gradient intensifying from northwest to southeast corroborates regional models attributing coastal groundwater degradation to marine intrusion exacerbated by low topographic relief and excessive extraction (Hagage et al., 2024). The tripartite zonation thus captures a continuum from relatively protected inland reserves to actively salinizing coastal aquifers, mandating differentiated management strategies.

The geospatial distribution of sodium and chloride across the six coastal communities delineates a salinization gradient that is strongly conditioned by proximity to tidal conduits and shoreline position. Sodium concentrations exceed the NSDWQ (2007) permissible limit of 200 mg/L at BH1 (Sama, 242.7 mg/L), BH2 (Tema, 365 mg/L) and Well 6 (Ido, 232 mg/L), with the two former sites occupying the western, ocean facing and river channel axis through which Atlantic water propagates during tidal flooding. This spatial correspondence is consistent with the landward salt-influx mechanism documented for the Niger Delta by (Abam 2001 and Abam et al., 2025) and empirically imaged at Okrika and Bonny by (Abija et al., 2021 and Udota et al., 2022). Chloride, although below the 250 mg/L threshold at all stations, exhibits a coherent eastward elevating band (112–148 mg/L) tracking the tidal creek system, while the most inland station (Obuama) records the lowest values a distance decay pattern diagnostic of marine influence. Critically, the decoupled high-chloride/low-sodium signature at BH4 (Buguma; Cl = 112.7 mg/L, Na = 20.5 mg/L) departs from the coupled Na-Cl enrichment expected of seawater mixing (Na⁺/Cl⁻ ≈ 0.86), implying a non-marine chloride source such as connate brackish water, mineral leaching, or anthropogenic input. The anomously high sodium at Well 6 is relatives to its paired borehole (BH5) further suggests localized well specific contamination rather than regional intrusion. Collectively, the dataset indicates spatially heterogeneous salinization in which active tidal intrusion, paleo saline beds and point source pollution interact, underscoring the necessity of ionic ratio and isotopic verification before ascribing salinity to seawater intrusion alone (Figure 6).

Figure 6: Geospatial distribution of sodium and Chlorine

 Conclusion

This study integrated 2-D electrical resistivity tomography, vertical electrical sounding and groundwater hydrochemistry to evaluate saltwater intrusion in some coastal areas in Rivers State, Nigeria. Low-resistivity zones (0.678–48.8 Ωm) were interpreted as saline wedges exploiting palaeo channel discontinuities, and inversion models were reliable (RMS 1.4-7.6%). Hydrochemical results showed marked spatial heterogeneity: Tema Well 2 recorded an EC of 7,490 µS/cm and a salinity of 3,810 mg/L, while adjacent BH3 remained within potable limits. Salinity, sodium and SAR exceedances were concentrated at Tema, Sama and Ido, and a tripartite zonation of water quality was identified. Vulnerability appears controlled by low topographic relief, aquifer confinement and tidal proximity, with isolated freshwater lenses increasingly threatened by diffusive encroachment. Because low resistivity may also reflect clay content, ionic-ratio and isotopic verification are recommended. Regulated abstraction and continuous geophysical monitoring are essential for sustainable groundwater management.