Bandargaz city is located in the of Golestan province, Iran. The direct distance of Bandargaz municipal wastewater treatment plant from the Caspian sea (Gorgan Gulf) is about 1.7 Km. Also, the distance where the wastewater discharged into the Caspian sea to the Miankaleh protected zone is 35 Km (Fig. 1). Plant was launched in 2005 with a capacity of 3100 m³.day^-1. Wastewater using concrete pipe reached to the earth channels and then emptied to the sea (Fig. 1). In Bandargaz region, irrigation water scarcity in the summer season, which coincides with the peak crop water requirement period, has caused the farmers interested to usage of treated wastewater as an unconventional water resource for supplying a large portion of water requirement in more than 700 ha of paddy field in around of plant [27]. Rice cultivation is dominant in this area and irrigation season is approximately 3-month (late-May to late-August) along with the peak of irrigation water requirement within July. There are more than 700 ha paddy farm that are located near Bandargaz wastewater treatment plant. The majority of the flow in these farmlands is dependent on effluent of treatment plant during the dry season. Within the last 12 years, farmers remove of manhole doors and directly pumping the treated wastewater for irrigation. Most farmers ignore the hazards of the indiscreet reuse of wastewater for irrigation. During the dry season, which is the most intensive agricultural irrigation period in the region, a large portion of paddy irrigation water supply in these areas depends on discharge from the plant.

Fig. (1). Location of Bandargaz wastewater plant and fields irrigated by freshwater (blue) and wastewater (red).

To study the effect of reclaimed wastewater (WW) on growth traits and yield of paddy crop and its comparison with the tube well freshwater (FW) farms in Bandargaz, 40 paddy farms were randomly selected (Fig. 1). Half of these farms have been irrigated by freshwater and the rested half by the reclaimed wastewater in recent 12 years. A similar study method was used by Jang et al. [7]. All the farming operations, conducted under farmers’ management during growth season, were recorded precisely including farm area, the value of used seed for planting, dates of planting and harvesting, growth season length and application amount of nitrogen, phosphate, and potassium fertilizers. During growth season, 7 samples from each water resource (totally 14 samples) were taken coincident with different phonological stages.

Water samples were taken from June to August 2016 with an approximate interval of two weeks according to phonological stage of rice crop. 14 water quality parameters including pH, Total Dissolved Solids (TDS), Electrical Conductivity (EC), calcium, magnesium, sodium, potassium, chloride, sulphate, carbonate, bicarbonate, nitrate (NO₃^-), phosphate (PO₄^-) and Total Hardness (TH) and four target heavy metals (Pb, Ni, Cd, and Cr) were measured. Total water properties were measured based on APHA [29]. Some irrigation water index such as Potential soil Salinity: PS [30], Sodium Adsorption Ratio: SAR [31], Kelley’s Ratio equal to Exchangeable Sodium Ratio: ESR [32], sodium percentage 1: Na%1 [32], sodium percentage 2: Na%2 [31, 33], Magnesium Ratio [32], Ca:Mg ratio [34], Calcium Ratio [34], Residual Sodium Carbonate: RSC [31, 32], Residual Sodium Bicarbonate: RSBC [33] and Permeability Index: PI [32] were calculated. Values obtained from each water resource were treated as replicates. The concentration of heavy metals in samples was estimated by using atomic absorption spectrophotometer (Model AA-10, Varian Inc., Australia) fitted with a specific lamp of particular metal using appropriate drift blank. For minimizing time changes in water quality, samples were collected at 10:00. In order to assess the water resources for irrigation, different irrigation water quality standards including United State Salinity Laboratory: USSL [31], FAO 29 [34], FAO 47 [6], Shainberg and Oster [35], Indian irrigation water quality [36], Oster and Schroer -after [37]- and Indian Council of Agricultural Research [38] and different organizations and countries guideline for reclaimed wastewater reuse were considered.

The heavy metal pollution index (HMPI) was calculated by the following formula to show the level of contamination in water [39].

HMPI = C_i/S_i

Where C_i and S_i are heavy metal content in a water sample and permitted standard of the same metal (μg.Lit^-1), respectively. When the HMPI values exceed than 1.0, water is said to be contaminated by anthropogenic inputs and requires continuous environmental monitoring of the area [39].

At the time of maturity, three plots (1*1 m²) were randomly selected in each farm. Then, 10 samples were collected from each plot. Identical amounts of each field were obtained by average of samples. Values obtained from different farms were treated as replicates. Samples were oven-dried separately at 80ºC until a constant weight was achieved [39]. Yield and growing traits including plant height, spike length, number of seed per spike, thousand seed weight, seed yield, biological yield and harvest index (ratio of seed yield to biological yield) were measured.

Data normality was evaluated and proved by one-sample Kolmogorov-Smirnov test at 5% probability level [40]. The data of water quality and crop traits were subjected to t- student test for assessing the significance of differences. Statistical test and calculating of some descriptive statistics were performed using SPSS software (SPSS Inc., version 21).

The results of t-test at 5% probability level on water quality parameters are presented in Fig. (2) and Table 1. Carbonate was not found in any sample of FW and WW, and no significant difference was observed between FW and WW based on pH and bicarbonate concentration. Calcium, magnesium, sodium, chlorine and sulfate concentration in FW are significantly more than WW which can be resulted by the adjacency of this region wells with Caspian sea and seepage of brackish seawater into groundwater resources. This matter is discernable particularly from chlorine and sodium concentrations than the other ions in FW.

Fig. (2). Mean comparison of different water ions and metals in FW and WW.

The standard of FAO [6, 34], Australia [41], EPA, Jordan and South Korea recommend the appropriate pH range for irrigation to be 6.5-8.4, 6.5-8.5, 6.0-9.0, 6.0-9.0 and 5.8-8.5, respectively [2]. Outside of the normal range, water might be suitable for irrigating, but has the potential to cause an imbalance of nutrients, corrosion or sedimentation of irrigation facilities, mobility of heavy metals in the soil and poisonous ions [2, 34]. In this research, the pH values of FW and WW are in the permitted range for irrigation based on different standards.

Salinity is the most important factor of irrigation water quality that can create a hostile environment. The salinity of FW samples was significantly greater than WW because of more concentration of cations and anions in freshwater. The allowed irrigation water EC for paddy crop was reported to be 2.0 dS.m^-1 [2]. In this regard, wastewater and freshwater salinity were less and more than the threshold salinity of paddy, respectively.

Nitrogen (N), phosphorus (P) and potassium (K) are major nutrients for the crop. It was reported that reclaimed water has more essential nutrients (N, P, K) and some micronutrients for plant growth than freshwater [4, 42]. However, those Nutrients can give negative effects such as nutrient imbalances, groundwater contamination, over-growing and lodging, excessive vegetative growth, failure to ripen, increased susceptibility to pests and disease, reduced fruit set for crops, delays in maturation and decreases in food nutrient quality especially for paddy rice [2, 4, 6]. In consideration of the above negative effects of nitrogen on paddy rice growth, Taiwan, Japan and Jordan have set standards for allowable N in reclaimed water for paddy rice [2, 4, 43]. The amount of potassium, nitrate, and phosphate in the reclaimed wastewater is significantly more than freshwater that can lead to soil fertility (Fig. 2). Mass loads of N, P and K can be calculated by multiplying irrigation water volume and the corresponding concentration. Considering the irrigation water requirement of paddy crop in the studied area (13000 m³.ha^-1), 193, 145.6, and 22.8 Kg of potassium, nitrogen, and phosphorus are added to per hectare of rice during total growth season that is 3.3, 15.3 and 8.5 times more than irrigation by freshwater, respectively. Therefore, it seems that the main part of paddy nutrition needs to potassium and nitrogen, and some phosphorus requirement is supplied through reclaimed wastewater.

The significant difference of FW with WW treatment in various irrigation water quality indexes is related to the difference of the dominant ions concentration (Table 1). The value of PS index is dependent on chlorine and sulfate concentration. The less amount of this index means the better quality for irrigation water; so that water with PS index greater than 5 is harmful for irrigation [44]. According to this, although using wastewater for irrigation doesn’t have the potential risk of salinity, freshwater quality has this hazard.

It has suggested that the sodium problem in irrigation water could be very conveniently worked out on the basis of the values of ESR or Kelley’s ratio. Generally, water is considered as unfavorable for irrigation if ESR be more than one [32]. Therefore, FW and WW are unfavorable and favorable water for irrigation, respectively.

The amount of Na%₁ and Na%₂ were very close to each other because of low concentration of potassium. As per the Bureau of Indian Standards the sodium percentage of 60 is the maximum recommended limit for irrigation water [32]. Thus, FW is at the border of unsuitable irrigation water. However, WW has good quality because the water with sodium percentage between 20 to 40 percent is described as good water [44].

Excess of magnesium (high Magnesium Ratio) in water affects the quality of soils, which causes poor yield of crops [32]. In magnesium dominated water (Ca:Mg ratio less than 1), the potential effect of sodium may be slightly increased. In other words, a given SAR value will show slightly more damage if the Ca:Mg is less than 1 [34]. Also, if irrigation water has calcium to total cation ratio less than 0.15, a further evaluation is needed. Such water may pose a potential problem related to plant nutrition [34]. The result showed that both water resources have good quality in the aspect of Magnesium Ratio, Ca:Mg ratio and Ca Ratio, although the WW is significantly better than FW.

High concentration of CO₃²⁻ and HCO₃⁻ represents alkaline nature of water. Use of such water promotes the precipitation of calcium and magnesium present in the soil solution which causes an increase in exchangeable sodium. For this reason, waters with high level of RSC or RSBC are unfavorable for irrigation uses. When RSC and RSBC are less than 2.5 and 5.0 meq.lit^-1 respectively, irrigation water is safe in the aspect of alkaline hazard [31-33].

Regarding the relative equity of bicarbonate concentration in wastewater and freshwater, the value of these two indicators in freshwater is less affected by the more concentration of calcium and magnesium in the tube well water and this difference is statistically significant. Nonetheless, for both water resources, these two indexes are within safe limits for irrigation and there is no limitation in using them.

Irrigation water is divided into 3 classes by amount PI [32]. In this study, both FW and WW belong to the first class (PI >75%) considered as good for irrigation.

Total hardness (TH) of the FW is significantly more than WW because of more concentration of calcium and magnesium. When the concentration of chloride in irrigation water is more than 4 meq.lit^-1, toxicity problems can occur, especially for sensitive crops [33]. In terms of chlorine concentration, FW and WW are brackish and fresh, respectively, indicating that FW is not desirable for irrigation due to the high concentration of chlorine.

Classification of Bandargaz FW and WW based on USSL [31] were C4S2 and C3S1, that represent FW has very high salinity and medium- sodium hazard while WW has high salinity and low- sodium hazard, respectively. Irrigation water was classified by Shainberg and Oster [35] based on EC and SAR to good, moderate and bad. In this study, FW and WW have good quality. Criteria of FAO 29 [34] and FAO 47 [6] guides showed that FW has severe limitation in the aspect of EC and Chloride. Nevertheless, WW is suitable for crop irrigation (Table 2). Comparing water quality with Manual of Indian Council of Agricultural Research [38] indicated that FW cannot be used for irrigation of sensitive crops. However, Bandargaz WW has not any limits for irrigation of all crops. Indian assessment of irrigation water quality by EC, SAR, RSC [36] showed that FW and WW have marginally salinity (suitable for coarse textured soils) and good quality (suitable for all soils and crops), respectively. Oster and Schroer [37] considered EC and SAR for determination of potential of infiltration problem due to sodium in irrigation water. Both FW and WW have no negative effect on infiltration.

Table 3 lists the levels of studied heavy metals detected in FW and WW and compare them with different national and world standards. The results showed that the average of heavy metal concentrations in both FW and WW samples were in the order of Cr˂Cd˂Pb˂Ni. This finding was very close to order of Cd˂Cr˂Pb˂Ni that reported by Chopra and Pathak [45] for wastewater and tubewell water. However, the study of Huong et al. [46] on surface water and Rhee and et al. [25] on FW and WW showed this order as Cd˂Ni˂Pb˂Cr. Also, the paired two-sample t test showed that there was a significant level of Cd, Cr, Pb and Ni (P<0.01) concentrations in WW as compared to FW.

The concentration of the Cr, Pb and Ni were found to be within safe limit in both FW and WW used for irrigation so that the maximum HMPI index for these metals in freshwater was 0.01, 0.14, and 0.12, respectively and in wastewater was 0.05, 0.28, and 0.16, respectively. However, in both water resources particularly in wastewater, Cd concentration was near to the maximum permission limits of different standards and even it was more than the maximum permission level based on Italian standard (Table 3) so that the maximum value of HMPI index for this metal was 0.67 and 1.01 in freshwater and wastewater, respectively.

Comparison of the permitted concentration of these four metals in different standards shows that FAO 47, EPA and WHO standards are exactly equal, and within surveyed national standards, Iranian and Italian standards are the easiest and the most rigorous national standards. For this reason, the concentration of studied heavy metals satisfied the Iranian wastewater quality standards for agriculture [20] and those were within the recommended maximum concentrations. The values of the heavy metal concentration of FW and WW of this research were far less than the values observed by Chopra and Pathak [45].

When wastewater is used, farmers make changes to farm management due to the awareness of wastewater benefits, especially the presence of nutrition elements that are required by crop. The effect of irrigation water resource on farm management and crop growth and yield is shown in Table 4. There wasn’t any significant difference between freshwater and wastewater irrigated farms in terms of farms area and amount of seeding per hectare. On average, date of planting was two days earlier in wastewater irrigated farms than freshwater ones while date of harvest was two days later. Consequently, the difference between two farms type, because of increasing about 4 days (equal to 3.8%) growth season length affected by wastewater, was significant at 10% probability level.

Farmers in the Bandargaz region have found that wastewater leads to fertile soil due to the presence of nutrients. Therefore, they reduced application of chemical fertilizers in paddy farms as this reduction of nitrogen and phosphorus fertilizers was significant at 5% probability level (Table 4). Application of nitrogen, phosphorus and potassium fertilizers in WW irrigated farms was 23, 22 and 20 percent less than FW irrigated farms, respectively which is a kind of economic and environmental management. However, if the amount of nitrogen, phosphorus and potassium in wastewater added to the amount of direct consumed fertilizers, it is found out that total imported nutrition materials to the WW irrigated farms are more than the ones under FW irrigation. This point was also emphasized by Jung et al. [26]. For example, considering the ratio of 46% for net nitrogen to nitrogen fertilizer, N added to fields by both fertilizing and irrigating ways was 52.1 and 178.4 Kg.ha^-1 in FW and WW farms, respectively. One of the effects of high nitrogen consumption in paddy field is a significant prolonged growth period by 6% [50] which is consistent with the results of this research.

The appropriate nitrogen fertilizer demand depends on characteristics of soil, farming pattern and cultivated variety. So, the total amount of nitrogen required during the growth and maturity period needs to be reviewed [43]. In general, paddy rice requires 90 Kg.ha^-1 of net nitrogen in a complete cycle [43]. With respect to irrigation water requirement of 13000 m³.ha^-1 in Bandargaz region, the average required nitrogen concentration is about 7 mgN.lit^-1. Meanwhile, the average nitrogen concentration in the reclaimed wastewater was 11.2 mg.lit^-1. So, original nitrogen fertilizer can be replaced by nitrogen in the reclaimed wastewater. The “nitrogen excess” phenomenon in reclaimed wastewater is concerned and a dilution strategy could be adopted when reclaimed water from traditional secondary treatment is used [43].

No significant difference was observed in two type farms based on plant height, spike length, and 1000-seed weight, but this factor was effective significantly at 5% probability level on seed per spike such that it was in farms with WW irrigation about 12.4% higher than FW irrigated farms. Insignificant effect of reclaimed wastewater on paddy crop height [51] and 1000-seed weight [26] was reported.

According to the dependency of seed yield to yield component including 1000-seed weight and seed per spike [26], the effect of water type on seed yield was significant at 10% probability level so that this trait was 10% more in WW irrigated farms than ones with FW irrigation. This conclusion is closely in line with findings of other researches which reported increasing seed yield of paddy by 15-19% under reclaimed wastewater irrigation [7, 26, 28]. It seems that this difference was resulted from lower water salinity, chlorine and sodium concentration, and more nutritional materials in wastewater than freshwater. N fertilizer had a significant effect on paddy yield [50, 52]. Hereof, it was reported that significant correlation between nutrient input in irrigation water (N and P) and paddy seed yield which led to an increasing productivity in reclaimed wastewater irrigated fields [7, 26]. However, insignificant decrease of seed per spike and seed yield and significant decrease of 1000-seed weight were reported because of the adverse effect of excessive salts and high concentration of trace metals in wastewater [51]. There wasn’t any significant difference between irrigated farms with WW and FW in respect of biological yield and harvest index; although, harvest index was 14% more in farms with wastewater irrigation.