Long-term Effect of Mixed Wastewater Irrigation on Soil Properties, Fruit Quality and Heavy Metal Contamination of Citrus

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Three orchards with trees in age of 10 to 12 years were selected in each site. The surface irrigation is applied based on tree water-use, which varied seasonally in response to the climate. The chosen orchards apply chemical fertilization and other agricultural practices as recommended by Egyptian Ministry of Agriculture and Land Reclamation ^[19].

2.2. Water Analysis

Five water samples were collected from the main irrigation canals of each site in 500 ml chemically resistant plastic bottles, filtered, preserved with concentrated nitric acid and stored at 4°C. Electric conductivity (Ec), total dissolved solids (TDS) and pH of water samples were determined directly in the irrigation canals using pH-TDS meter "CRISON-MM40". Concentrations of elements and heavy metals in water samples; including potassium (K), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), cadmium (Cd) and nickel (Ni), were determined using Atomic Absorption "Perkin-Elmer, Model AAnalyst 100"), while concentration of sodium (Na) was measured using Flame Photometer "air-acetylene burner" according to ^[20].

2.3. Soil analysis

Soil samples were collected from nine points in each orchard at a depth of 0-30 and 30-60 cm. Samples were air-dried, crushed and passed through a 2 mm-sieve. Ec and pH of soil samples were measured in the lab according to ^[21]. Then, samples from each orchard were mixed together; 1gm of soil was digested and analyzed for elements and heavy metals concentration as described before in water analysis.

2.4. Fruit Yield and Quality

At harvest time, fruit yield was recorded as yield /tree for five trees in each orchard. Thirty mature fruits were randomly collected from each orchard to evaluate fruit quality. Fruit weight as well as soluble solid content (Brix), titratable acidity (TA), Brix/TA ratio, total ascorbic acid (V.C) and pH in fruit juice were measured according to ^[22].

To determine element concentrations in fruit peel and pulp, Fruits samples were washed and cleaned by distilled water. Twenty grams of peel as well as thirty grams of pulp (four replicates per orchard) were digested with sulphuric, nitric and perchloric acid as described by ^[23]. The concentrations of K, Fe, Mn, Cu, Zn, Cd, Ni and Na in digested samples were analyzed as described previously. The concentration of each element was expressed as mg.100 g^-1fresh weight.

2.5. Statistical Analysis

Data were statistically analyzed according to ^[24]. Mean values of water and soil properties, in mixed and fresh water irrigated sites were compared using t-test. Fruit quality was analyzed as completely randomized design using analysis of variance (ANOVA) test. All statistical analysis was carried out using the SPSS Inc. computer program, at the 0.05 level of significance.

3. Results and Discussion

3.1. Physico-chemical Properties of Mixed and Fresh Water

The mean values of physico-chemical properties of collected water samples are presented in Table 1. Data revealed that MW has higher pH, Ec and TDS by 1, 3 and 13- fold, respectively. These values indicated the saline nature of mixed water. Although, high TDS (over 2000 mg. L^-1) of irrigation water causes severe problems ^[25], farmers in Baltim used this water (8700 mg.L^-1) in crops irrigation many years ago.

Table 1. Physico-chemical Analysis of Water Samples

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The concentrations of K, Mn, Zn and Ni in MW were two times or more than in FW, particularly for potassium which was 5 times greater. These results were noticed by ^[26] who reported higher values of heavy metals in El-Rahawy drain than those in the River Nile water. Iron and cadmium were not detected neither in mixed nor in fresh water. On the other hand, Mn and Cu concentration in both MW and FW exceeded the maximum recommended concentrations according to FAO wastewater quality guidelines for agricultural use. Fresh water contamination may be attributed to the addition of Cu and Mn as micro nutrients in the surrounding orchards.

Sodium is the most troublesome major element in the irrigation water. Mixed water had higher sodium (914.5 mg.L^-1) than recommended concentration (184 mg.L^-1) for agriculture use ^[27]. This excessive concentration of sodium can cause soil physical problem and reduce water uptake by plant roots ^[25].

3.2. Effect of Mixed Water on the Irrigated Soil
3.2.1. Soil pH and Ec

Soil pH is an important factor that affects nutrient availability and absorption by plant roots. Soil Ec and pH had increased significantly by using MW for a long time as compared to FW irrigated soil (Fig. 2). This is probably due to the alkaline nature and high concentrations of chemical substances in MW (Table 1). The same results were obtained by ^{[7, 11, 30]} who reported that application of wastewater increased soil pH due to the alkalization effect of basic cations such as Na, Ca and Mg in the water. The pH values were significantly (p<0.05) lower in the topsoil layer than in subsoil, while soil Ec was higher in the topsoil layer, in both sites.

Figure 2. Mean values of pH and Ec of soils irrigated with mixed (MW) and fresh water (FW) under two depths

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3.2.2. Metals Contamination in Soil

The ANOVA analysis (Table 2) showed that the level of metals in MW irrigated soils were significantly higher (p<0.05) than the reference soils (Kafr El-Sheikh city) these results indicated that the application of mixed water had seemingly enriched the soils in metals. My findings are in harmony with ^[31] who reported that heavy metals contamination in agricultural environments attributed to atmospheric fall-out, pesticide formulations and contamination by chemical fertilizers, in addition to irrigation with water of poor quality that contaminated with trace elements.

Moreover, Data in Table 2 demonstrated that all elements concentration were higher in soil than water. Although the heavy metals concentration was relatively low in the irrigated wastewater; its continuous application for many years could lead to accumulation of heavy metals in the soil ^[12]. The average concentration of Cu, Zn, Cd and Ni in the soil irrigated with MW is higher than the acceptable level for Germany maximum limited concentration ^[32] that could result in toxicity of plants and affecting food quality and safety. Elements such as; K, Fe, Mn, Cu and Zn, are necessary for plant growth as macro or micro nutrients, so we must reduce fertilizer applications when we use mixed water irrigation to protect soil and crops from trace elements accumulation.

Table 2. Mean values of element concentrations in soils irrigated with mixed and fresh water

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Mixed water irrigation increased sodium concentrations by 23 and 2.5 times in water and soil samples, respectively compared with those of FW. The decrease of sodium level in the soil compared to its level in the water may be due to leaching properties of sandy soil, so sodium is more readily leached from the soil profile. Other researchers reported similar increases in the salt content in the soil after wastewater irrigation ^[33].

3.3. Effect of Mixed Water Irrigation on Citrus
3.3.1. Yield and Fruit Quality

Fruits collected from the two sites were looking apparently healthy. The effect of MW irrigation on yield parameters of Navel orange was reported in Table 3. Mixed water irrigation increased yield parameters by about 15 and 18% for fruit weight and about 25 and 29% for fruit yield/tree in the first and second season, respectively. These results are expected as mixed water contains nutrients and dissolved organic matter that enriched soil and absorbed by plants ^[7].

Fruit weight in the 2^nd season and yield/tree in both seasons were significantly differed among orchards in the same site. In addition, the interaction between water source and orchards was significant in both seasons. These results proved that there were other agricultural applications that had affect on fruit weight and yield in addition to the source of irrigation water.

Table 3. Yield parameters affected by two irrigation sources in 2012 and 2013 seasons

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Table 4. Navel orange fruit quality affected by two irrigation sources in 2012 and 2013 seasons

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The results of the fruit quality in first and second seasons presented in Table 4 revealed that using mixed water resulted in significant changes in fruit chemical compositions. Soluble solid contents increased and titratable acidity % decreased, that lead to a significant increase in the Brix/acid ratio. Earlier reports revealed that SSC content increased in citrus fruit irrigated with wastewater ^[7] and also in tomato irrigated with high saline water ^[34]. On the other hand, significant differences in SSC among orchards in both seasons indicated that there are other factors affecting fruit SSC in addition to the water source. Using mixed water in irrigation did not produce significant effects on Juice pH and total ascorbic acid content.

3.3.2. Elements Concentration in Fruit Peel and Pulp

Table 5. showed the mean value of elements and heavy metals concentration (mg.100gm^-1 fresh weight) in the peel and pulp of Navel orange fruit affected by MW and FW irrigation. All metal concentrations were several fold higher in fruits collected from MW irrigated orchards compared to FW irrigated ones. The mean values of K, Fe, Zn and Na in fruit irrigated with MW were two or more times higher, especially for iron which was 7 times of its concentration in FW irrigated fruits. In addition, Mn and Ni had been detected only in plants irrigated with mixed water, while Cu and Cd are not detected with both sources of water either in fruit peel or pulp. Data presented declared that the mixed water irrigation had a significant effect on the elements concentration in Navel orange fruit. In this respect, ^{[33, 35]} found significant increase in the concentrations of Fe, Mn, Zn, Cu, Ni, and Cd in wastewater irrigated plants as compared with FW irrigated plants. Also ^{[36, 37, 38]}; proved higher absorption of some risk elements by various crops irrigated with municipal wastewater.

Table 5. Mean values of element concentrations in Navel orange fruit affected by two irrigation sources

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