Introduction

The composition of produced water is complex and can comprise several thousand compounds that vary in concentration between wells and over the life time of a well. Dispersed oil, aromatic hydrocarbons and alkyl-phenols (AP), heavy metals and naturally occurring radioactive material (NORM) are of particular environmental concern (Neff et al. 2011). Produced water can also contain large amounts of organic material, particles, inorganic salts and low molecular weight organic acids like acetic acid and propionic acid, and can have high levels of sulfur and sulfide. Furthermore, injected water following produced water, can bring traces of added chemicals such as biocides, corrosion inhibitors, scale inhibitors, emulsion breakers, coagulants/flocculants and oxygen scavengers to the surface (Johnsen et al. 2004; Neff 2002). The quantities of these compounds vary from one field to another, within the field, and during the life span of the well. Generally, however, monocyclic aromatic hydrocarbons (benzene, toluene, ethyl benzene, xylene-BTEX), polycyclic aromatic hydrocarbons (PAH) and high molecular weight phenol are considered major toxicant in produced water (AMAP 2010; Neff et al. 2011). Also, heavy metals are the main inorganic constituents thought to be of environmental concern. According to Neff (1987), barium (Ba), iron (Fe), manganese (Mn) and zinc (Zn) are considered common heavy metals that are found in produced water. Corrosion of galvanized equipment is also believed to be the source of zinc (Zn) and lead (Pb) in some produced water.

According to Sturve et al. (2006), Carls et al. (2008) and Aas et al. (2000), concentrations of PAH in seawater and sediments are of toxicological significance to both benthic and pelagic marine organisms. PAHs and high molecular weight phenols can result in chronic biological effects. PAHs are known to have mutagenic, carcinogenic and teratogenic properties; the higher the molecular weight of a compound in this group, the lower its solubility and the higher its potential for bioaccumulation in marine organisms. Also, high molecular weight phenols exhibit endocrine disruption potential (Priatna et al. 1994; Nimrod and Benson 1996; Arukwe et al. 2000, 2001). BTEX are moderately soluble in seawater and highly volatile, increasing in toxicity with increasing molecular weight. According to Miao et al. (2005), Lin et al. (2011), heavy metals can be accumulated in marine sediments and organisms. High levels of heavy metals in water have caused damages to marine life by reducing cell division rates, oxidative damage, interference with cellular events and cell death causing damage to gills, liver, kidneys and nervous systems (Flemming and Trevors 1989; Stohs and Bagchi 1995). Even at low concentrations, chronic exposure to water or sediments contaminated with heavy metals can result in reduced growth and photosynthesis of flora and changes in the abundance and taxonomic composition of fauna (Morrisey et al. 1996).

The objective of this paper is to characterize and infer the sources, levels, spatial distributions of hydrocarbon, heavy metals pollutants in the seawater and sediments offshore, Niger Delta. The study area is 15 km offshore to the southwest of the mouth of Dodo River, and it is approximately 90 km southeast of Warri (Fig. 1). Its depth ranges from 13 to 25 m, locating the offshore waters within the inner continental shelves (ICS). The hydrograph of the offshore ICS region is greatly influenced by the tides of the Atlantic Ocean and the Guinea currents. As the Guinea currents flow eastward to the Bright of Biafra, they are joined by the equatorial counter currents. The equatorial counter currents in the area are characterized by turbulent and eddy flows [Environmental Report of OPL 98 Offshore Cross River State (Ashland)]. Produced water from the offshore platform is treated and separated from oil using the gravitational separation and dissolved air flotation technique, after which it is discharged directly into the ocean surface. According to Nigeria’s Environmental Guideline and Standards for Petroleum Industry (EGSAPIN), the maximum amount of oil/grease in produced water allowed for discharge into the water body is 40 mg/l in deep offshore but decreases to 30 mg/l in offshore continental shelf (EGASPIN 2002).

Fig. 1
figure 1

Map showing sample locations in the offshore

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Materials and methods

A total of twenty (20) seawater samples and twenty (20) sediment samples were collected and analyzed for dry and wet seasons. Surface water samples were collected using a hydrobios water sampler each of which was opened to fill and close below the water. All containers were rinsed at least three times with the water being sampled before sample collection. Water samples for physicochemical analysis were collected and stored in appropriately labeled 1-liter plastic cans, and water samples for heavy metal analysis were acidified by adding dilute trioxonitrate (v) acid (HNO3) until the pH was two (2). Also, the water samples for hydrocarbon analyses were collected in wide-mouthed bottles and acidified with dilute tetraoxosulfate (VI) acid according to procedures of APHA 1998. The samples were stored and transported in chilled condition to the laboratory for analyses. However, parameters such as pH, temperature, total dissolved solids (TDS), turbidity and electrical conductivity (EC) were measured directly in the field.

The sediments were collected by means of a Van Veen bottom sediment sampler. After manual removal of dirt, debris and draining water, the samples were placed in a clean 1-liter Pyrex glass bottle with glass lid. The sediment samples were analyzed for BTEX, PAH, phenols and TPH, heavy metals—iron (Fe), cadmium (Cd), lead (Pb), nickel (Ni), barium (Ba), zinc (Zn) and chromium (Cr).The sample locations were chosen relative to the point of produced water discharges (Fig. 2). The seawater and sediment samples were collected from discharge point (DP), 100 mN, 100 mS, 100 mE, 100 mW, 200 mN, 200 mS, 200 mE and 200 mW; control sample was taken from 5 km away from point of produced water discharge.

Fig. 2
figure 2

a Dendrogram of heavy metals pollutants in offshore sediments. b Dendrogram of heavy metals pollutants in offshore sediments

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The physicochemical parameters of seawater were analyzed using the procedures and methods for water quality adopted from American Public Health Association (APHA 1998). Temperature, pH, EC and TDS were determined using pH and ISE meter, while TSS was measured photometrically with the HACH DR/2010 spectrophotometer at the wavelength of 810 nm. Turbidity of the samples was analyzed on the same day using a Hach Ratio Turbidimeter, while Biological Oxygen Demand (BOD) was determined using the dilution method. Chemical Oxygen Demand (COD) was determined using the reflux method. Nitrate (dissolved nitrate), sulfate and phosphate (dissolved) were all determined with the Hach DR2000 spectrometer. TOC was determined by the Dichromate wet oxidation. The concentrations of the heavy metals As, Cu, Co, Cd, CN, Pb, Zn, Ni, Fe, Cr, V and Hg in seawater were analyzed using atomic absorption spectrometry. The heavy metals in the sediments were extracted using a mixture of equal volumes of analytical grade of 1:10 sediment to water ratio, shaken for 1 h. Their concentrations in the seawater were determined by atomic absorption spectrophotometer (AAS) using air-acetylene, while the hydrocarbon contents were analyzed using gas chromatography (GC–MS).

Statistical analysis was performed on the results obtained from laboratory analyses of water and sediment samples. In addition, the concentration values of heavy metals in sediments were evaluated using pollution indices and were used to determine the level of pollution. The laboratory results were normalized in order to reduce the effects of difference in units of measurements and large variations in the measured parameter values. The normalization was done using the ‘Geogenic’ software, after which the principal component Analysis (PCA) was performed using the ‘GenStat Discovery Edition 3’ software. The numbers of significant principal components (PC) for interpretation were selected on the basis of the Kaiser criterion with Eigen values greater than one and a total explained percentage of variation equal to or greater than 70%. Cluster Analysis (CA) was done on the laboratory results of the sediment samples using the Statgraphics XVII software in order to group parameters that have close similarities. The geoaccumulation index (Igeo) (Muller 1969) and pollution load index (PLI) (Tomilinson et al. 1980) were used in evaluating the sediment quality/level of pollution with regard to heavy metal content.

$${\text{Igeo}} = \log_{2} M_{x} /1.5M_{b}$$
(1)

where M x  = concentration of metal in the affected sediment sample; M b —background value of metal in unaffected river sediment.

The factor 1.5 was introduced to include possible variation of the background values that are due to lithogenic variations.

$${\text{PLI}} = \sqrt[n]{{({\text{CF}}_{1} }} \times {\text{CF}}_{2} \times {\text{CF}}_{3} \times \ldots \times {\text{CF}}_{n} )$$
(2)

where CF is contamination factor and n is the number of heavy metals.

The contamination factor (CF) for the metals was calculated as given by Wu et al. (2011):

$${\text{Cif}} = {\text{Ci}}/{\text{Cin}};$$
(3)

where Cif is the single-element contaminant factor; Ci is the mean concentration of the element in samples Cin is a reference value for the element (concentration value of element in unpolluted sediment taken from control point—5 km upstream away from point of produced water discharge point).

The PLI provides simple, comparative means for assessing a site or estuarine quality: a value of zero (0) indicates perfection, one (1) indicates only baseline levels of pollutants present and above one (>1) would indicate progressive deterioration of the site and estuarine quality.

Results and discussion

Physicochemical parameters of the seawater from Offshore

The results of the physicochemical parameters of the seawater (Table 1a, b) show that temperature of seawater samples ranged from 29.02 to 30.05 °C in the dry season and 27.1–28.0 °C in the wet season. This slight increase in temperature during the dry season is probably as a result of the effect of weather. Total dissolved solids (TDS) ranged from 21,250 to 23,180 mg/l in the dry season and 22,500–23,800 mg/l in the wet season. Depending on the ionic properties, excessive total dissolved solids can produce toxic effects on fish and fish eggs. Conductivity ranged from 42.5 to 46.36 ms/cm in the dry season and 45–51.6 ms/cm in the wet season. According to Ademoroti (1990), these values are within normal range for open sea.

Table 1 Physicochemical properties of offshore water samples
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Produced water organic constituents (benzene, toluene, ethyl benzene and xylenes (BTEX), polycyclic aromatic hydrocarbons (PAH) and phenols) were below the detection limits in the dry season water samples; even total petroleum hydrocarbon (TPH), where detected, recorded very low concentration values (Table 1a). However, during the wet season, BTEX ranged from 0.001 to 0.005 mg/l with a mean value of 0.001 mg/l. PAH ranged from 0.001 to 0.004 mg/l with a mean value of 0.002 mg/l. TPH ranged from 1.08 to 18.02 mg/l with a mean value of 9.35 mg/l, recording the highest concentration values at the discharge point. It was observed that the concentrations of PAH are way below the reported concentration range of 25–350 ng/l in the North Sea (Durell et al. 2006). Among the heavy metals (Ni, Fe, Cr, V, Zn, Cu, Cd, Hg, As, Pb and Ba) analyzed for both seasons, As, Cd and Hg were not detected during the dry season, while only As was not detected during the wet season test. This very low concentrations of the organic particulates and heavy metals during the dry season could be attributed to low volume of discharge during that period and also attributed to the fact that the concentrations measured were of those entrapped in the environment (media). Zn, Fe, Cr, V and Ni concentrations in the seawater during the dry season investigation were found to decrease from the discharge point. However, it was observed that concentrations of Pb increased away from the discharge point. This decrease in heavy metals concentrations away from the discharge point could be attributed to attenuation of the heavy metals due to dilution. The observed increase in Pb concentration away from the discharge points suggests a different source for Pb apart from the produce water.

The principal component analyses of the physiochemical parameters of seawater samples produced four components which accounted for 75.04% of total variance for the dry season samples (Table 2a) and three components which accounted for 74.46% of total variance for the wet season samples (Table 2b).The first principal component (PC1) of the dry season samples explained 29% of total variance and had loading for chemical oxygen demand (COD), chromium (Cr), phosphate (PO4), lead (Pb), reflecting inorganic pollution. The second principal component (PC2) which accounts for 19.81% of total variance of dry season water samples (Table 3a) had loading for conductivity (EC), copper (Cu), total dissolved solids (TDS), total petroleum hydrocarbon (TPH), zinc (Zn) and salinity. The third principal component (PC3) had loading for total nitrogen (N), nitrite (NO2), nickel (Ni), temperature and pH. PC4 (11.04%) had loading for dissolved oxygen (DO), iron (Fe), ammonia (NH4), sulfate (SO4) and total suspended solids (TSS). For the wet season water samples (Table 3b), PC1 which accounted for 39.34% of total variance had loading for BTEX, PAH, Co and Zn, while PC2 which accounted for 22.57% of total variance of wet season samples had loadings for chemical oxygen demand (COD), nitrite (NO2), total dissolved solid (TDS), salinity, conductivity and temperature. PC3 had loading for total nitrogen (N), sulfate (SO4), phosphate (PO4) and pH. Physical parameters of seawater such as temperature, pH, conductivity and salinity are factors that influence the distribution and toxicity of the pollutants. Benzene, toluene, ethyl benzene and xylenes (BTEX), polycyclic aromatic hydrocarbons (PAH) and total petroleum hydrocarbon (TPH) most probably have their sources from produced water. The presence of nitrogen in the seawater is considered to be from scale inhibitors which usually are nitrogen containing phosphate esters. These scale inhibitors are soluble and tend to remain in the produced water.

Table 2 Principal component analysis for offshore water
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Table 3 Principal component scores for offshore water
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The principal scores of the dry season samples (Table 3a) show the loading of the four principal components on seawater from each of the sample points. PC scores equal or greater than 1 were considered significant. PC1 had high loading on water samples from the discharge point, 100 mE, 100 mS, 200 mE, 200 mS, 200 mW and 200 mN. PC3 which reflects nitrogen pollution and nickel pollution had high loading at discharge point, while PC4 had high loading on water samples from 100 mN, 200 mE, 200 mS, 200 mW and 5 km away from discharge point. Also, PC scores of wet season samples (Table 3b) reveal that PC1 (which comprise produced water constituents) had extremely high loading on water samples from discharge point. PC2 had significant loading on water samples from discharge point, 100 mE, 100 mS, 100 mW and 200 mE, while PC3 has significant loadings on seawater from discharge point, 100 mE, 100 mS, 100 mN, 200 mN and 200 mE.

Sediment pollution characterization

The concentrations of the organic constituents (total petroleum hydrocarbon-TPH, polycyclic aromatic hydrocarbon-PAH and benzene, toluene, ethyl benzene, xylene-BTEX) during the dry season were generally lower than the detection limit except TPH which ranged from 2.1 to 28.4 mg/kg with a mean value of 18.57 mg/kg. However, in the wet season TPH ranged from 1.18 to 42.48 mg/kg, PAH from 0.001 to 0.014 mg/kg and BTEX from 0.001 to 0.015 mg/kg. Phenols were recorded only at discharge point (Table 4b). These parameters recorded their highest concentration values at the discharge point. The measured concentrations of the organic constituents were found to be below the interim sediment quality guidelines (ISQG). Also, heavy metals such as iron (Fe), cadmium (Cd), nickel (Ni), zinc (Zn), vanadium (V) and arsenic (As) recorded their highest concentrations in sediments at the discharge point. The spatial distribution of pollutants away from the discharge point, as observed by their concentrations in sediments, shows that they have been dispersed by oceanic turbulent diffusion before sedimentation (Niu et al. 2011). The heavy metals concentrations in the sediments were compared with interim sediment quality guidelines (ISQG) and reference values (Effect range low-ERL; effect range medium-ERM) for heavy metals as developed by the US national oceanic and atmospheric administration (NOAA 2012). According to the NOAA, effect range low (ERL) is understood and used as an estimation of the concentration below which toxicity is improbable. These standards consider all adverse biological effects observed in association with elevated metal concentration (Long et al. 1995; MacDonald et al. 1996).As found out, all the heavy metals in the sediments were below the ERL values, implying that the presence of these heavy metals are not high enough to be of acute threat to the aquatic lives.

Table 4 Results of chemical analysis of offshore sediments
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The produced water hydrocarbons namely PAH, BTEX, TPH and heavy metals were subjected to cluster analysis in order to distinguish the heavy metals that would cluster with the hydrocarbons and invariably be associated with produced water discharges. The dendrogram for dry season sediment samples (Fig. 2a) produced two main clusters. The first cluster comprises total petroleum hydrocarbon (TPH), V, Pb, Cr and Zn, while the second cluster comprises Cu, Ni, Ba and Fe. The dendrogram for the wet season samples (Fig. 2b) has three main clusters. The first cluster comprises phenols, BTEX and Zn, the second cluster consists of PAH, Zn, Fe and V, while the third cluster consists of TPH, Pb Cu and Cr. Pollutants (phenols and PAH) in the first and second clusters are evidently produced water constituents. Pollutants in the first and second clusters are also produced water constituents but in addition to some heavy metals. These occur under two overlapping sub-clusters as shown in Fig. 2b. The overlapping sub-clusters imply that some of the heavy metals, although may be from produced water, were probably deposited under different conditions.

The principal component analysis of the hydrocarbons and heavy metals in the sediment samples produced components that accounted for 70.63 and 76.09% of cumulative variance for dry and wet season sediment samples (Table 5a, b). The principal component (PC1) for the dry season sediments accounted for 28.62% of total variance and had loading for barium (Ba), iron (Fe), nickel (Ni), total organic carbon (TOC) and vanadium (V) indicating heavy metals pollution. Similarly, the PC2 which accounted for 27.24% of total variance had loading for chromium (Cr), copper (Cu), lead (Pb) and zinc (Zn), while PC3 (14.77%) comprised barium (Ba), chromium (Cr), copper (Cu), total petroleum hydrocarbon (TPH) and zinc (Zn). The PCs show that the quality of the offshore sediments is influenced by hydrocarbon and heavy metals pollution (In Table 5, *asterisks signify parameters that are not statistically analysed because their concentration values are not detected. Values in bold represent significant Principal component loading).

Table 5 Principal component analysis of offshore sediments
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The first principal component (PC1) for the wet season sediments accounted for 56.24% of total variance and had loading for benzene, toluene, ethyl benzene, xylene (BTEX), polycyclic aromatic hydrocarbon (PAH), phenols, arsenic (As), cadmium (Cd), iron (Fe), vanadium (V) and zinc (Zn), while PC2 which accounted for 19.85% of total variance had pollution loading for heavy metals chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and total petroleum hydrocarbon (TPH). This indicates that the offshore sediments were influenced by produced water hydrocarbon and heavy metals pollution in the wet season.

Table 6a, b shows the principal component scores for the dry and wet season samples. For the dry season samples, PC1 had high loadings on discharge point (DP), 100 mS, 100 mW, 200 mS, 200 mW, while PC2 had high loading at 100 mS, 100 mW, 100 mN, 200 mW implying that the quality of sediments at these locations were influenced by heavy metals pollution. PC3 which indicates heavy metal pollution had significant loading on sediments at discharge point (DP) and also at 100 mE, and 200 mN away from the discharge point. For the wet season samples, the PC scores (Table 6b) show that PC1 (produced water hydrocarbon pollution) had very high loading at discharge point, while PC2 had high loading on samples from 100 mS, 500 mDP, 100 mW, 200 mE, 200 mS, 200 mN, 200 mW which indicates that sediment quality at these sample points are predominantly affected by heavy metals pollution. It was observed generally that the concentrations of the organics and heavy metals in both water and sediments during the wet season are higher than those of dry season and similar results as this were also observed by Ololade et al. (2008), Dan et al. (2014), Nenibarini and Benard (2015) in the Niger Delta region and other regions outside the Niger Delta (Yao et al. 2014; Khwedim 2016).

Table 6 Principal component scores of offshore sediments
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The index of geoaccumulation (Igeo) shows the contribution of individual heavy metals to the pollution of the offshore sediments. In the dry season (Table 7a), for example, the sediments are unpolluted to moderately polluted with barium (Ba) and moderately polluted by vanadium (V), while in the wet season (Table 7b) sediments are also moderately polluted with vanadium.

Table 7 Index of geoaccumulation of offshore sediment
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The pollution load index (PLI) which is an integrated pollution index gives the overall state of the offshore sediments with respect to heavy metal pollution. In the dry season, sediments were polluted at discharge point, and at 100 mE, 100mW, 200 mE, 200 mS, 200 mW and 200mN, while by the wet season, sediments had become even more polluted (Table 8).

Table 8 Pollution load index (PLI)
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Conclusions

This study has shown that the offshore waters of the Niger Delta are polluted with total petroleum hydrocarbon (TPH), nitrite, sulfate, phosphate, nickel and zinc in the dry season and benzene, toluene, ethyl benzene, xylene (BTEX), polycyclic aromatic hydrocarbon (PAH), cobalt, nitrite, total nitrogen, sulfate, phosphate and zinc in the wet season. The sediments are also polluted with barium, chromium, copper, iron, nickel, lead, vanadium, zinc, petroleum hydrocarbon and by benzene, toluene, ethyl benzene, xylene (BTEX), polycyclic aromatic hydrocarbon (PAH), phenols, arsenic, cadmium, chromium, copper, iron, nickel, lead, vanadium, zinc with total petroleum hydrocarbon (TPH) in the wet season. Based on these findings, it is clear that the composition and quality of seawater and sediments offshore are being altered by hydrocarbon constituents and heavy metals from produced water discharges. The level of pollution is such that it poses chronic threat to the ecosystem of the aquatic life. Continuous monitoring is therefore recommended as pollution can, over time, rise from chronic to acute threat of the aquatic lives.