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

Investigation of heavy metals (HM) fractions in soils irrigated with wastewater (WW) would ascertain their bioavailability and contamination level in soils. This study investigated HM fractions in soils after long-term WW irrigation. WW irrigation profoundly affected HM fractions in soil. The ranges of HM concentrations in soils irrigated with WW were apparently wide. All fractions were signi ﬁ cantly higher in the ﬁ elds irrigated with industrial WW than rain-fed ﬁ elds. HM concentrations varied in the soils as Pb > Cu > Ni > Zn > Fe > Cd > Mn after WW irrigation. In rainfed ﬁ elds, HM concentrations differed in soils as Fe > Zn > Mn > Pb > Cd > Cu > Ni. The HM fractions were dominant in the residual form followed by oxides bound and carbonate associated fractions in WW-irrigated soils. Lower contents of HM in the soil were obtained in the exchangeable fraction. WW irrigation resulted in the transformation of HM into different fractions as residual > oxide associated > carbonate associated > organically bound > exchangeable form. Repeated WW irrigation increased pH values of the soils. The higher EC of soil indicated an accumulation of salts in the soils due to WW irrigation. Mitigation of HM contamination in Hattar industrial ef ﬂ uent is required before irrigation.

Heavy metal contamination of agricultural soils has also become a significant environmental problem (Chand et al. concern for crops cultivated in polluted soils. (Chiroma et al. ). 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 ).  fields for several years, are insufficient and scanty (Noureen et al. ). 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.

MATERIALS AND METHODS
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 m 2 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 ). Total OM content was calculated by multiplying the total carbon values by 1.72 (Nelson & Sommers ). 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 ). Five grams of soil sample was shaken with 25 mL of 1 M ammonium acetate (NH 4 OAc) for 1 h, then centrifuged and filtered to determine exchangeable elements (Ca, Mg, K, Na) using an atomic absorption spectrophotometer (AAS). HM in soils 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).

Statistical analysis
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.

RESULTS AND DISCUSSION
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 () 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 >

Lead (Pb)
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

Nickel (Ni)
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

Manganese (Mn)
Manganese fraction was affected by the irrigation with industrial WW (

Copper (Cu)
Copper concentrations were substantially higher in WW contaminated fields and profoundly lower in the non-contaminated (rainfed) fields (

Cadmium (Cd)
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

Iron (Fe)
The Fe concentrations were surprisingly found to be higher in the rainfed fields as compared to the water contaminated fields (

Zinc (Zn)
Extractability of Zn was the function of sampling sites and the extraction reagents used (   Table 10.

CONCLUSIONS
This study investigated the HM fractions in agricultural soils irrigated long-term with wastewater (WW). The