Characterization of heavy metal in soils as affected by long-term irrigation with industrial wastewater

Heavy metal (HM) contamination of agricultural soils and vegetation is a worldwide ecological problem. The application of domestic and industrial effluents to agricultural fields is a common practice in Pakistan (Lone et al. 2000; Ensink et al. 2004). Effluent from city sewage and industrial wastewater (WW) can have a greater impact on irrigation water quality. These effluents have been considered as a rich source of organic matter (OM) and appreciable amounts of plant nutrients. Wastewater originated from industrial estates contains toxic substances such as HM, recalcitrant organics and other undesirable pollutants which may accumulate in the edible parts of food crops and pose serious threats to human life. These are hazardous to water resources, agriculture, ecosystems and the human population (Gray et al. 2006). Wastewater contains HM which may be lethal for animals and humans if their concentrations exceed the permissible limits (Noureen et al. 2015). Heavy metal contamination of agricultural soils has also become a significant environmental problem (Chand et al. 2012). Wastewater irrigation, solid waste disposal, sludge applications, vehicular exhaust and industrial activities are the most important sources of HM contamination of soil and food crops grown on contaminated soils (Singh & Kalamdhad 2013). Mapanda et al. (2005) also reported that wastewater irrigation caused HM accumulation in soil. Research reports on the assessment of HM in WW, their accumulation in soils and uptake by plants, especially in the Hattar area, have been reported (Hussain 2000, 2010; Sial et al. 2005; Sial et al. 2006; Aziz 2007). There is consensus that the quality of food production remains a major concern for crops cultivated in polluted soils. (Chiroma et al. 2003). The understanding of the behavior of HM in soil-plant systems seems to be particularly significant. There is considerable variability of uptake of HM by plants depending on soil properties (Nyles & Ray 1999). HM have been reported in crops grown in abandoned polluted areas (Ihenyen 1991; Jeanne & Sidle 1991; Okoronkwo et al. 2005).

Noureen et al. (2015) reported that vegetable crops have been contaminated with HM in the Hattar areas, Pakistan, and extractable HM in soils exceeded the prescribed limits. Food chain contamination is one of the major routes for entry of HM into the animal system, therefore, monitoring the available pools of metals in contaminated soils has generated huge interest (Datta et al. 2000; Yadav et al. 2002). HM, once accumulated in soils, are generally difficult to remove (Smith et al. 1996). Therefore, efforts are made to immobilize the metal concentrations in soils so that the accumulated HM may be curtailed to prevent them entering the food chain, contaminating surface or underground water. Several studies have been carried out on the impacts of wastewater irrigation on the accumulation of HM in soil and plant systems around the world (Noureen et al. 2015). Reports on heavy metal pollution in soil-water-plant systems in the vicinity of Hattar industrial estate of Haripur-Pakistan, where farmers have been irrigating their fields for several years, are insufficient and scanty (Noureen et al. 2015). Studies on the fractionation of HM in long-term WW-irrigated soils are also poorly documented. Therefore, the study was aimed at characterizing the HM in soils of arable fields in the Hattar area after long-term irrigation with industrial WW.

A field study was conducted in the farmers' fields to determine the severity of HM pollution in the soils irrigated with Hattar industrial wastewater (WW) for longer periods. Wastewater irrigation has been practiced over many years (>15 years) in these areas. The wastewater originated from the Hattar industrial estate of Haripur, Pakistan. The chemical composition of wastewater is given in Table 1. Soil samples (0–20 cm) were collected randomly from four adjacent fields irrigated with WW and four nearby non-irrigated (rainfed) arable fields. These fields were located in the adjoining area. The major agronomic practices, for instance method of cultivation, cropping pattern, fertilization etc., were observed and reported by the farmers as identical. Soil samples were collected from an area of 20 × 20 m² as composite samples after thoroughly mixing in plastic bags. Soil samples were analyzed for physico-chemical properties (texture, OM, pH and cation exchange capacity (CEC), HM fractions). Total carbon content was measured by using a dry combustion method (Nelson & Sommers 1982). Total OM content was calculated by multiplying the total carbon values by 1.72 (Nelson & Sommers 1982). The pH of compost suspension with soil:water at the ratio of 1:5 was measured by using a pH meter (HANNA HI 8520). Electrical conductivity (EC) of the soil suspension was determined with an EC meter (4320 JENWAY). Soil texture was determined using a pipette method (Gee & Bauder 1986). Five grams of soil sample was shaken with 25 mL of 1 M ammonium acetate (NH₄OAc) for 1 h, then centrifuged and filtered to determine exchangeable elements (Ca, Mg, K, Na) using an atomic absorption spectrophotometer (AAS). HM in soils were fractionated by a modified procedure of Amacher (1996). Soil samples (5 g) were sequentially fractionated into: (i) exchangeable metals (including water soluble) extracted with 0.1 mol L^–1 Mg(NO₃)₂ (shaking for 2 h at 80 cycles/min); (ii) carbonate associated metals extraction with 1 mol L^–1 sodium acetate at pH 5 (shaking for 5 h at 80 cycles/min); (iii) oxide associated metals extraction with 0.2 mol L^–1 ammonium oxalate (0.2 mol L^–1 oxalic acid +0.1 mol L^–1 ascorbic acid) at pH 3.3 (in a boiling water bath for 30 min); (iv) organically bound associated metals extraction with 30% H₂O₂ at pH 2 (in a boiling water bath for 30 min); and (v) the residual metals extraction by digestion with nitric acid and perchloric acid. After each successive extraction, the supernatant solution was centrifuged and then filtered. Heavy metal contents were determined using AAS. A complete weight balance system was used to determine the amounts of metal extracted by each extraction. Taking into account the volume of water carried over in the next extraction, each metal fraction was corrected for the aqueous phase elements in lieu of washing the sample with deionized water after centrifugation. Industrial wastewater samples collected from the drain (used for irrigation) were also analyzed for chemical properties (Table 1).

The data were statistically analyzed using the Stat View software. Means of HM fractions were separated using Fisher's least significant difference (LSD) test at P value <0.05.

Soil sampled from agricultural fields irrigated with wastewater (WW) exhibited higher values of CEC, OM, EC and pH as compared to the rainfed fields (Table 2). This showed that WW irrigation for longer periods profoundly affected the soil chemical composition. The texture of the soils remained unchanged after WW irrigation. The study revealed that WW irrigation increased the pH values of soils. The pH of the WW-irrigated fields ranged from 7.6 to 8.7 whereas its values ranged from 6.4 to 7.1 in the rainfed fields. The EC of soils ranged from 554 to 728 μS m^–1 in WW-irrigated fields whereas in the non-irrigated fields the EC values ranged from 182 to 368 μS m^–1. Elevated EC values indicated that the use of industrial WW for irrigation can lead to an accumulation of salts in the soils. Total carbon was achieved between 2.2 and 4.2% in the WW-irrigated fields. Carbon contents were lower in the non-irrigated fields, i.e. in the range of 1.2–2.1%.

The results showed that HM fractions were significantly (P < 0.05) higher in fields irrigated with industrial WW as compared to the rain-fed fields. The metal pollution of soils in Hattar showed that the area is highly contaminated. Schmidt (1997) also reported that HM were present in higher concentrations in WW. Irrespective of the fraction, HM concentrations varied in the order of Pb > Cu > Ni > Zn > Fe > Cd > Mn in the soil after long-term irrigation with WW. This is likely due to the higher concentration of Pb, Cu, Ni and Zn than those of the other HM in the WW. In rainfed fields the HM concentrations differed in soils as Fe > Zn > Mn > Pb > Cd > Cu > Ni. Heavy metal fractions differed in the order of residual metal > oxide associated metal > carbonate associated metal > organically bound metal > exchangeable metal. HM predominantly appeared in the residual form followed by oxide associated form and carbonate associated form. The residual fractions in the most HM were predominant in WW-irrigated soils, which may reflect the geological distribution in the study sites. The residual fraction is the most stable fraction in soils and it is affected by the parent material, soil properties and climate condition (Han 2007). An increase in soil HM in organic fraction was likely due to the increase in soil OM content through long-term WW irrigation. Soil OM has a higher affinity for HM adsorption (Han 2007). Singh et al. (2009) reported that the availability of nutrients and other metals were greatly affected by the physicochemical properties of soil.

Pb concentration was apparently highest among the HM in the WW-irrigated soils (Table 3). The extractability of Pb varied considerably depending on WW irrigation. The concentration of Pb was varied with WW irrigation. Exchangeable Pb ranged from 28 to 72 mg kg^–1 among the sampling sites irrigated with WW. In the rainfed sites the concentration of exchangeable Pb varied from 0.001 to 0.01 mg kg^–1. Carbonate associated Pb ranged from 123 to 283 mg kg^–1 in the fields irrigated with WW. The Pb concentrations associated with carbonate were found to be between 3.4 and 6.7 mg kg^–1 in the rainfed fields. Oxide associated Pb was higher in field 2 (410 mg kg^–1) and lowest in field 4 (286 mg kg^–1) after WW irrigation. The rainfed fields gave significantly lower concentration of oxide associated Pb. Organically bound Pb ranged from 76 to 102 mg kg^–1 in the WW irrigated fields whereas Pb concentrations varied minutely from 0.01 to 0.06 mg kg^–1. Lead was found to be dominant in the residual form which ranged from 398 to 582 mg kg^–1 in the long-term WW-irrigated fields. In the rainfed sites the residual form of Pb ranged from 44 to 82 mg kg^–1. The higher concentrations exceeding admissible levels of metals in cultivable soils may pose a potential risk of HM to the food chain and may restrict the cultivation of crops. Fractionation of HM in soil may help to assess the available metal fractions and the risk of mobilization of metals bound to particular soil constituents (Narwal & Singh 1998). Noureen et al. (2015) reported concentrations of Pb in brassica (128.2 mg kg^–1), spinach (107.3 mg kg^–1), coriander (98.7 mg kg^–1) and radish (103.2 mg kg^–1) in the Hattar area where wastewater has been used for irrigation. The Pb content in these vegetables significantly exceeded the maximum permissible limits of WHO (1992).

Long-term WW irrigation on agricultural lands significantly increases the concentration of Ni in soils (Table 4). The Ni forms in WW treated soils were residual > oxide associated > carbonate associated > organically bound > exchangeable form. Among the WW-irrigated fields, the residual form of Ni was highest in field 3 (295 mg kg^–1) and lowest in field 4 (198 mg kg^–1). Residual Ni ranged from 10 to 21 mg kg^–1 in the rainfed fields. Oxide bound Ni ranged from 62 to 92 mg kg^–1 in the irrigated fields and from 2 to 8 mg kg^–1 in the rainfed fields. Carbonate bound Ni ranged from 62 to 92 mg kg^–1 in the WW applied fields and in the rainfed fields the carbonate form of Ni remained similar among the fields (0.6–0.8 mg kg^–1). Organically bound Ni and exchangeable Ni ranged from 11.6 to 21.5 mg kg^–1 and from 2.4 to 10.5 mg kg^–1 in the WW-irrigated fields, respectively. These forms of Ni were found in very minute amounts in the rainfed sites when compared with the standard values. Dougherty & Hall (1995) reported that the long-term application of WW to agricultural land resulted in HM contamination in soils. Masona et al. (2011) reported that long-term irrigation with WW on agricultural lands significantly increased HM concentrations in soils, and accumulated HM in maize plants. A regular assessment of soil in the study area should be carried out to avoid the excessive build-up of HM in soils to safeguard the environment and the health of the people who consume the crops grown over these lands.

Manganese fraction was affected by the irrigation with industrial WW (Table 5). In soils sampled from the fields irrigated with WW, the average concentrations of residual Mn was 159.5 mg kg^–1, oxide bound Mn was 72.5 mg kg^–1, carbonate associated Mn was 34 mg kg^–1, organically bound Mn was 13 mg kg^–1 and the exchangeable form of Mn was 2.3 mg kg^–1. Among the rainfed fields (non-irrigated), the average concentrations of residual Mn was 66 mg kg^–1, oxide bound Mn was 22 mg kg^–1, carbonate associated Mn was 4.5 mg kg^–1, organically bound Mn was 0.006 mg kg^–1 and the exchangeable form of Mn was found in negligible quantities.

Copper concentrations were substantially higher in WW contaminated fields and profoundly lower in the non-contaminated (rainfed) fields (Table 6). The Cu fractions differed among sampling sites after long-term WW irrigation in the order: residual Cu (262–354 mg kg^–1) > oxide bound Cu (126–182 mg kg^–1) > carbonate bound Cu (62–112 mg kg^–1) > organically bound Cu (22–54 mg kg^–1) > exchangeable Cu (3.6–11.5 mg kg^–1). The contents of Cu in rainfed soils were observed in smaller quantities, regardless of the type of Cu fraction. Ma & Rao (1997) and Hickey & Kittrick (1984) reported that the majority of Cu contents in the soils and sediments were associated with the residual fraction. Vegetables cultivated in wastewater-irrigated soils take up HM in large enough quantities to cause potential health risks to the consumers (Ghosh et al. 2012). The consumption of HM-contaminated food can seriously deplete some essential nutrients in the body that are further responsible for decreasing immunological defenses, intrauterine growth retardation, disabilities associated with malnutrition and a high prevalence of upper gastrointestinal cancer rates (Türkdogan et al. 2003).

The ranges of Cd concentration in soils irrigated with industrial WW were apparently wide (Table 7). Among the sampling fields applied with wastewater, the average concentration of residual Cd was 178 mg kg^–1, oxide bound Cd was 81.6 mg kg^–1, carbonate associated Cd was 31 mg kg^–1, organically bound Cd was 15.5 mg kg^–1 and the exchangeable form of Cd was 1.6 mg kg^–1. Among the rainfed fields (non-irrigated), the average concentration of residual Cd was 29.6 mg kg^–1 and oxide bound Cd was 10 mg kg^–1. The other fractions of Cd, namely carbonate, organically bound and exchangeable forms, were found in smaller quantities. Usman & Ghallab (2006) reported that irrigation of soil with WW resulted in the transformation of metals from the carbonate fraction towards the exchangeable, Fe–Mn oxide, and organic fraction for Zn, towards the exchangeable, organic, and residual fraction for Cu, and towards the exchangeable fraction for Cd. Higher levels of HM in soil and plants in the study area is quite alarming. Noureen et al. (2015) reported the concentrations of Cd as 20.9 mg kg^–1 in coriander, 16.3 mg kg^–1 in brassica, 15.5 mg kg^–1 in spinach and 12.4 mg kg^–1 in radish. This indicated that the consumers were at higher risk after the consumption of these vegetables, which may lead to adverse health effects. Metals can cause serious health risks to the consumers through intake of the contaminated food crops if these metals exceed the permissible limits (Zhao et al. 2014).

The Fe concentrations were surprisingly found to be higher in the rainfed fields as compared to the water contaminated fields (Table 8). Among the WW contaminated fields, the residual form of Fe was highest in field 3 (264 mg kg^–1) and lowest in field 1 (154.6 mg kg^–1). Residual Fe ranged from 132 to 392 mg kg^–1 in the rainfed fields. Oxide bound Fe ranged from 64 to 92.5 mg kg^–1 in the WW-irrigated fields and from 86 to 174 mg kg^–1 in the rainfed fields. Carbonate bound Fe ranged from 22.5 to 92.5 mg kg^–1 in the WW applied fields and in the rainfed fields the carbonate form of Fe ranged from 32 to 96 mg kg^–1. Organically bound Fe and exchangeable Fe ranged from 12.8 to 18.7 mg kg^–1 and from 0.9 to 4.2 mg kg^–1 in the WW-irrigated fields, respectively. In the rainfed sites, the Fe fractions differed as residual > oxide bound > carbonate associated > organically bound > exchangeable. The average concentration of residual Fe was 237 mg kg^–1, oxide bound Fe was 120 mg kg^–1, carbonate associated Fe was 59 mg kg^–1, organically bound Fe was 15 mg kg^–1 and exchangeable Fe was 0.2 mg kg^–1.

Extractability of Zn was the function of sampling sites and the extraction reagents used (Table 9). The average Zn concentrations among WW-irrigated fields were 206, 93.5, 50.9, 17.4 and 2.2 mg kg^–1 in residual, oxide bound, carbonate associated, organically bound and exchangeable form, respectively. The average Zn concentrations among non-irrigated fields were lower than wastewater contaminated fields. The concentrations found were 143.6, 66.6, 21, 11 and 0.13 mg kg^–1 in residual, oxide bound, carbonate associated, organically bound and exchangeable form, respectively. HM contamination in soil is also dependent on properties of soils. Several soil properties, such as CEC, OM and clay minerals, the content of the oxides of iron (Fe), aluminum (Al), and manganese (Mn), and the redox potential regulate the retention and immobilization of HM in soils. These factors regulate the bioavailability of HM and result in the appearance of plant toxicity. Long-term application of sewage WW led to substantial accumulation of Zn in the oxidizable fraction (Fitamo et al. 2007) or exchangeable, reducible, and oxidizable forms (Usman & Ghallab 2006). The availability of HM is influenced by pH, OM, CEC, calcium carbonate, Fe and Mn oxides (Vamerali et al. 2010). Higher levels of HMs in agricultural fields marginalizes soil quality and impairs food security (Kelepertzis 2014). The long-term WW irrigation leads to accumulation of MnO/FeO- and OM-complexes, which are less bioavailable and less mobile (Salbu et al. 1998). Incubation studies using HM-spiked soils indicated that the HM pools were transformed into more stable fractions over time (Jalali & Khanlari 2008). Mapanda et al. (2005) reported higher concentrations of HM (Cu, Zn, Cd, Ni, Cr and Pb) in soils irrigated with WW for ten years. Kabala et al. (2011) reported that the largest fraction of Cu, Pb, and Zn in WW-irrigated soils was strongly bound in a residual form, while exchangeable and the other labile fractions were negligible. Municipal WW applied soil showed considerable amounts of OM, macro- and micronutrients, which were partially taken up by plants, and partly accumulated in soil (Wang et al. 2003; Singh et al. 2009). Heavy metal concentrations in soil irrigated with contaminated water and the maximum admissible concentrations of toxic metals in soils from the USA, UK and EU are given in Table 10.

This study investigated the HM fractions in agricultural soils irrigated long-term with wastewater (WW). The fractionation of HM indicated that the HM in WW-irrigated soils significantly increased in the residual form, followed by oxides bound and carbonate associated HM fractions as compared to non-irrigated soils. Transformation of HM in WW irrigated soils were varied as residual metal > oxide associated metal > carbonate associated metal > organically bound metal > exchangeable metal. Due to the increasing demands for food safety and the risk associated with consumption of food products contaminated with HM, we suggest a restricted use of WW for the prevention of potential health hazards in arable land. Interval monitoring of HM in soil and crops irrigated WW is required to control their excessive accumulation in the food chain.

Funding for this research through the national research program for universities (project No. NRPU-4831) by the Higher Education Commission of Pakistan is gratefully acknowledged.