Risk assessment of irrigated lacustrine & calcareous soils by treated wastewater

Peer review under responsibility of National Water Research Center.

Abstract

The objectives of this study were to explore the effect of irrigation by treated wastewater (TWW) on some chemical characteristics of cultivated lacustrine and calcareous soils, the growth and macronutrients contents of soybean, corn, faba bean and wheat; and the chemical composition and quality of drainage waters from these soils. For this, greenhouse experiments, using PVC tank of 50 kg soil capacity were carried out. The soils were irrigated by FW, TWW or 1:1 FW/TWW. The results suggest that tested plants can be irrigated with reused water since visual damage is minimal, which seems to be related to the plant's low accumulation of saline ions. The dilution of TWW with FW reduced the negative effects observed. The results have also shown a significant increase in the concentration of EC and in the counts of TC and FC in soils of the upper layer (0–20 cm) than in those of the lower layer (20–40 cm).

Keywords:

• Water reuse
• Risk assessment
• Fecal coliform
• Soil salinization

1 Introduction

In many places, fresh water is not sufficient to meet high demand; therefore alternative water resources must be found. Taking into account that, high water consumption increases the volume of wastewater generated (Quadir et al., 2010; Meneses et al., 2010), treated municipal wastewater could be a significant alternative water resource. Not only can this practice reserve significant amounts of fresh water, but it can also reduce the volume of wastewater being discharged to the environment (Pedrero et al., 2010). Wastewater reclamation and reuse (water recycling) constitutes an increasing practice in areas of deficient water balance (Angelakis et al., 1999; Oron, 2003), such as in European and Mediterranean regions. Reuse projects also exist in Japan, USA and Australia (European Commission, 2004). On the other hand the use of wastewater in agriculture is often associated with significant health risks because of the presence of high concentrations of human pathogens, enteric in origin, such as bacteria, viruses, protozoa and helminthes (Toze, 2006).

Wastewater reuse may require further treatment of the effluent to meet several quality criteria. Some countries have neither regulations, nor guidelines to guide reuse practices, yet they still apply raw or partially treated wastewater (Carr, 2005), while some others have either implemented their own regulations/guidelines or have adopted quality criteria based on international regulations. In Italy, recent guidelines allow unrestricted crop irrigation with a bacteriological effluent quality of 10 Colony Forming Units (CFU) per 100 ml of E. coli (Decree No. 185, 12/06/2003, Ministry for Environment, Italy). No restrictions exist for surface waters, which are often badly contaminated with total coliform load usually ranging from 10⁴ to 10⁵ CFU 100 ml⁻¹ (Bonomo et al., 1999). These are used for unrestricted irrigation of vegetables normally eaten uncooked. However, many farmers regularly use untreated wastewater unlawfully to satisfy crop water needs under extreme water shortage conditions (Ait Melloul et al., 2001; Campos et al., 2002; Capra and Scicolone, 2004). A different approach was adopted by the World Health Organization (WHO) which recommends the more liberal threshold of 1000 CFU 100 ml⁻¹ of fecal coliforms for unrestricted irrigation of crops to be eaten uncooked, sports fields and public parks (Ayers and Westcot, 1989). Although there are no hygienic standards concerning restricted irrigation of cereals crops, industrial and fodder crops, pasture and trees (Ayers and Westcot, 1989). Blumenthal et al. (2000) suggested a threshold from ≤10³ to ≤10⁵ fecal coliform bacteria 100 ml⁻¹ on treated wastewater depending on the exposed groups and irrigation techniques.

As a result of the limited quantity of fresh water available for irrigation and other vital sectors in the country, the Egyptian government policy plan was oriented to the reuse of unconventional water resources, as supplements, for irrigating the new reclaimed lands (Elsokkary and AbuKila, 2012).

The objectives of this study, therefore, were to (1) investigate the influence of irrigation by treated wastewater on both the chemical characteristics of cultivated lacustrine and calcareous soils, the growth and macronutrients contents of four plant crops grown on these soils and the chemical composition and quality of drainage waters from these soils; (2) assess whether mixing with well water may reduce the negative effects of using reused water.

2 Material and methods

2.1 Used soils

Two soil types: lacustrine (typic torripsamment) and calcareous (typic calciorthids) were used in this study. These two soils were collected from Abis Region, south Alexandria city and from Bangar El-Sokkar Region, 65 km west Alexandria City. The major soil properties of the two soil types are presented in Table 1. Soil analysis showed that the lacustrine soil is characterized by relatively lower levels of total CO₃²⁻ (3.65%) and relatively higher levels of both OM (2.18%), available N, P and K are 65.0, 8.5 and 460 mg kg⁻¹ soil, respectively than those of the calcareous soils 28.40%, 0.90%, and 50.0, 4.80 and 200 mg kg⁻¹ soil, respectively. However, the amounts of available macronutrients (N, P and K) in the lacustrine soil are within the low range while those in the calcareous soil are within the deficient range (Chapman, 1966). On the other hand, the calcareous soil is more saline (EC_e = 3.30 dS m⁻¹) than the lacustrine soil (EC_e = 0.70 dS m⁻¹) but its salinity is less than the critical limit (less than 4 dS m⁻¹). It is also clear that the two soils had slightly alkaline reaction since the average pH values of the lacustrine and calcareous soils were 8.0 and 8.2, respectively.

Table 1 The main chemical and physical characteristics of the soils under study.

Soil parameter	Lacustrine soil	Calcareous soil
pH	8.0	8.2
ECe (dS m^−1)	0.7	3.3
Total CO3 (%)	3.65	28.4
O.M. (%)	2.18	0.90
Available N (mg kg^−1 soil)	65	50
Available P (mg kg^−1 soil)	8.5	4.8
Available K (mg kg^−1 soil)	460	200
Particle size distribution
Sand (%)	57	79
Silt (%)	20	9
Clay (%)	23	12
Soil texture	Sandy clay loam	Sandy loam
Field capacity (%)	56	45.0
Bulk density (g cm^−3)	1.74	1.65

The lacustrine soil contains relatively higher percentages of clay (23%) and lower percentage of sand (57%) fractions than those of the calcareous soil (12.0 and 79%, respectively). Thus, the lacustrine soil had relatively fine texture (sandy clay loam) as compared with the calcareous soil (sand loam). These data point out, in general, that the lacustrine soil is relatively more fertile than the calcareous soil with respect to its content of relatively higher levels of OM and available macronutrients.

2.2 Water used for irrigation

Water used for irrigation are fresh water (FW) from El-Mahmoudiya Canal at western Nile Delta of Egypt and secondary treated wastewater (TWW) from the western wastewater treatment plant of Alexandria City were used for irrigation. The analysis of irrigation water treatments applied in this study are presented in Table 2.

Table 2 Some water quality parameters of irrigation water.

Parameter	FW	TWW	1:1 FW/TWW	Limits
pH	7.95	8.1	7.45	6.5–8.5^a
TDS (mg l^−1)	665	2300	1525	450–2000^a
SAR	3.65	8.85	7.7	Linked to TDS^a
Total N (mg l^−1)	13.10	38.5	29.5	–
NO3 (mg l^−1)	2.06	6.33	3.43	0–30^b
Total P (mg l^−1)	1.95	12	8.4	0–15^b
Total K (mg l^−1)	170	335	230	–
COD (mg l^−1)	33.5	210	92.5	50^c
BOD5 (mg l^−1)	7.25	77.5	31.5	30^c
FC (cfu/100 ml)	10	470	220	0–1000^b

Sodium adsorption ratio: SAR = [Na²⁺]/([Ca²⁺] + [Mg²⁺])^1/2.

n = 12.

a FAO (Ayers and Westcot, 1989).

b World Health Organization (Cairncross and Mara, 1989).

c Egyptian Ministerial Decree No. 92 of 2013 amending the Ministerial Decree No. 8 of 1982 on the executive Regulations of Law No. 48 of 1982 concerning the Protection of the Nile River and water channels from pollution.

2.3 Cultivated plants

Four plant crops were used as tested plants: soybean (Glycine max L.), variety Crawford and corn (Zea mays L.), variety Giza 323 as summer field crops, while faba bean (Vicia faba), variety Rena-Blanka and wheat (Triticum aestivum L.), variety Giza 69 as winter field crops.

2.4 Experimental layout

Two kilograms water washed gravels (1–2 cm diameter) were placed in PVS pots of 40 cm inner diameter and 50 cm in depth with 5 holes (1 cm diameter) in the bottom, followed by 50 kg lacustrine soil or 45 kg calcareous soil. The pots were placed in the greenhouse at the agricultural research farm of the Faculty of Agriculture, Alexandria, Univ., Egypt.

Ten seeds of each plant crop were sown in each pot, and daily irrigated in summer and every 2 days in winter by fresh water for 2 weeks. The seedling were thinned to four plants per pot and then irrigated by the different water treatments as follows: fresh water (FW), treated wastewater (TWW) or a mix of 1:1 FW/TWW.

The experimental pots were fertilized by N, P and K chemical fertilizers as follows: superphosphate (15.5% P₂O₅) was applied before seeds sowing at a rate of 450 kg ha⁻¹ to the four plant crops, ammonium nitrate (33.3% N) was applied in two equal doses, after tillering and 1 month after with a rate of 120, 240, 120 or 240 kg ha⁻¹ for soybean, corn, faba bean and wheat, respectively, and potassium sulfate (52% K₂O) was applied, in one dose, at a rate of 120 kg ha⁻¹ after tillering for the four plant crops.

2.5 Sampling

2.5.1 Soils

Composite soil samples original soils and soil samples from each pot were collected, air-dried then grounded and passed through 2 mm sieve for analysis. Moreover, soil samples were collected from each pot before plants harvest at two depths (0.20 and 20–40 cm) for analysis.

2.5.2 Water

Irrigation water samples were collected every 2 weeks for analysis. The results shown in the tables are the mean of each water character. Eight samples of drainage water were collected from each pot, 1 day after irrigation treatment, during the growth period for analysis according to the methods of water analysis described by (APHA, 1998). The results indicated in tables are the mean of eight-water analysis for each treatment.

2.5.3 Plants

The shoots of plants were collected at 2 weeks before plants harvesting, washed with tap water, then by distilled water, oven-dried at 70 °C for 48 h, grounded using stainless steel mill and kept for analysis (Chapman and Pratt, 1961). In addition, after 85 days from plant sowing, the shoots of the plants were harvested and the plants are weighted and expressed as shoot dry weight (g plant⁻¹).

2.6 Analysis

2.6.1 Soils

The main soil chemical and physical characteristics were determined as follows: ECe in the extraction of soil paste, pH in 1:2.5 soil water suspension, organic matter (OM) by Walkly and Black method (Page et al., 1982), total CO₃ by calcimeter (Black, 1965), the amount of available phosphorus by Olsen method (Page et al., 1982), the available potassium by NH₄OAC method (Page et al., 1982), available nitrogen by micro Kjeldahl method (Page et al., 1982), and the particle size distribution (sand, silt and clay) by hydrometer method (Black, 1965).

Total coliform and fecal coliform counts were carried out as bacteriological examination. This was carried out by using the fermentation technique; lauryl tryptose broth was employed in the multiple-tube test. For FC counts, the fermentation technique using EC medium (20 g tryptose, 5 g lactose, 1.5 g bile salts mixture, 4 g K₂HPO₄, 1.5 g KH₂PO₄ and 5 g NaCl) in 1 l reagent grade water (APHA, 1998). The bacterial counts for TC and FC were expressed as MPN/100 g soil.

2.6.2 Water

The main chemical, physical and microbiological characteristics of irrigation waters and of drainage waters were determined according to the methods outlined in APHA (1998). The concentrations of Mg²⁺ and Ca²⁺ were measured by Na₂EDTA method and those of K⁺ and Na⁺ by flamephotometer (Chapman and Pratt, 1961) and the value of SAR was calculated (Richards, 1954). Total coliform (TC) was measured by the Lauryl Tryptose Broth Technique and the Multiple Tube Fermentation Method was used for detection coliform bacteria counts (APHA, 1998).

2.6.3 Plants

A half gram of oven-dried plant material was subjected to wet-digestion in H₂SO₄/H₂O₂ (Jones, 1989). The concentrations of N, P and K were determined according to the methods outlined by Chapman and Pratt (1961).

2.6.4 Statistical analysis

The data obtained were statistically analyzed for the least significant difference (LSD_0.05) according to Snedecor and Cochran (1967).

3 Results and discussion

The analysis of types of irrigation water showed clear differences in their composition (Table 2). The analyzed elements in TWW's irrigation water showed a higher concentration than in FW. The high TDS of TWW water was mainly due to the high chloride, sulfate and sodium contents. The ions potassium, calcium, magnesium and phosphates also contributed to increasing the TDS, although to a lesser extent (Banon et al., 2011). In general, as salinity increases in the treated wastewater used for irrigation, the probability for certain soil, water, and cropping problems increases. Under such conditions, good drainage is essential in order to allow a continuous movement of water and salt below the root zone. Long-term use of reclaimed wastewater for irrigation is not generally possible without adequate drainage. Where drainage water salinity exceeds crop threshold levels the water can be blended with fresh water. Blending, which can be done before or during irrigation, enables farmers to extend the volume of water available (Rhoades, 1999; Oster and Grattan, 2002).

In TWW the average SAR observed (Table 2) were lower than the threshold of restriction on use recommended in the FAO-bulletin on water quality for agriculture (Ayers and Westcot, 1989). In addition to their effects on the plant, sodium in irrigation water may affect soil structure and reduce the rate at which water moves into the soil as well as reduce soil aeration. If the infiltration rate is greatly reduced, it may be impossible to supply the crop plants with enough water for good growth. A permeability problem usually occurs in the surface few centimeters of the soil and is mainly related to a relatively high sodium or very low calcium content in this zone or in the applied water (Ayers and Westcot, 1989). At a given SAR, the infiltration rate increases as salinity increases or decreases as salinity decreases. Therefore, SAR and TDS should be used in combination to evaluate the potential permeability problem (Pedrero et al., 2010). Sometimes, treated wastewaters are relatively high in sodium, and the resulting high SAR is a major concern in planning wastewater reuse projects.

In both, TWW and canal water the pH was high (Table 2), especially the former due to the presence of bicarbonates (Banon et al., 2011).

No indication is given, either in World Health Organization (Cairncross and Mara, 1989) or the US Environmental Protection Agency (USEPA, 2004) or FAO-bulletin (Ayers and Westcot, 1989) about the maximum allowable concentration of BOD₅ and COD in reused water. However, the maximum allowable concentration of BOD₅ and COD in reused water is 30 and 50 mg/l, respectively, according to the Egyptian Ministerial Decree No. 92 of 2013 amending the Ministerial Decree No. 8 of 1982 on the executive Regulations of Law No. 48 of 1982 concerning the Protection of the Nile River and water channels from pollution. The high levels of BOD₅ and COD were observed in TWW (Table 2).

Wastewater is a carrier of bacteria, viruses, protozoa and nematodes, which can cause various diseases, a situation found especially in some developing countries, where they use partially processed wastewater for crop irrigation (Asano and Cortuvo, 1998). In the present study, use fecal coliform concentration to measure the microbiological quality of the treated effluent. Both fecal coliforms and E. coli are indicators of effluent fecal contamination (Salgot et al., 2006). E. coli is part of the group of fecal coliforms and the most predominant species of the group. The E. coli bacterium prevails in human digestive tract and for this reason it can be considered one of the best available fecal contamination indicators (Molleda et al., 2008). Worldwide, there are many regulations referring to wastewater reuse that propose E. coli as a fecal indicator (NRMM-EPHC-AHMC, 2006), while others suggest fecal coliforms to be the representative pollution indicator (USEPA, 2004; Marecos do Monte, 2007). Since more than 90% of fecal coliforms are E. coli (Dufour, 1977), in this experiment, it has been shown that the practice of blending TWW with FW assured suitable irrigation water which considerably decreased pathogenic hazards. Therefore, the fecal coliforms concentration in TWW was 470 MPN 100 ml⁻¹, not exceeded the limits of use restriction recommended by the World Health Organization (>1000 MPN 100 ml⁻¹) (Cairncross and Mara, 1989), while the concentration in 1:1 FW/TWW was 220 MPN 100 ml⁻¹ (Table 2).

When the TWW was diluted, the resulting water (mixed water) had a lower saline content, pH, BOD₅, COD, total N, total P and fecal coliform than the TWW, which mitigated the negative effect on plant growth. This has been reported by Pandey and Soni (1994), Gori et al. (2008), Pedrero and Alarcón (2009), Pedrero et al. (2010). In this experiment, it has been shown that the dilution of the TWW with FW led to a 34, 56, 59 and 53% decrease in the TDS, COD, BOD₅ and fecal coliforms, respectively.

3.1 Effect of irrigation water on soil characteristics

3.1.1 Electrical conductivity (EC)

The data tabulated in Tables 3 and 4 showed significant increases in the values of EC_e of the cultivated lacustrine and calcareous soils as a result of irrigation by TWW and 1:1 FW/TWW as compared with those irrigated by FW. Several studies reported that irrigation by treated wastewater had increased soil salinity as compared to irrigation by normal water. These results coincided with that found by Rusan et al. (2007), Kiziloglu et al. (2008), Mojiri (2011), and Morugan et al. (2011). The values of EC_e of cultivated lacustrine soil irrigated by TWW were almost less than the upper critical limits of non-saline soil (less than 4.0 dS m⁻¹), while those of cultivated calcareous soil had exceeded this limit, especially in soils cultivated with faba bean and wheat. It is also clear that the EC_e values of cultivated lacustrine soils were generally lower than those of cultivated calcareous soil. The EC_e values of soil of the upper layer (0–20 cm) were lower than those of the lower (20–40 cm). This clearly observed in both cultivated lacustrine and calcareous soils irrigated by FW, TWW and 1:1 FW/TWW.

Table 3 Some chemical and biological characteristics of lacustrine soil cultivated with soybean, corn, faba bean or wheat as influenced by treatment wastewater (TWW), fresh water (FW) and mix of TWW and FW in equal proportions (mixed water).

Water of irrigation	Soil depth	pH	ECe (dS m^−1)	SAR	OM (%)	Coliform (MPN/100 g)		Available (mg/kg^−1)
						TC	FC	N	P	K
Soybean soil
FW	0–20	7.22	0.66	1.29	2.10	4 × 10^3	400	96	7.85	440
	20–40	7.12	0.70	1.32	2.00	5 × 10^2	200	90	8.00	485
TWW	0–20	7.54	1.36	4.70	2.70	7 × 10^7	3000	114	9.50	780
	20–40	7.35	1.88	4.80	2.40	5 × 10^3	1200	120	8.80	790
1:1 FW/TWW	0–20	7.20	0.70	1.43	2.32	3 × 10^6	2300	110	7.83	488
	20–40	7.15	0.78	1.50	2.20	4 × 10^3	1000	110	7.70	560
LSD 0.05		0.17	0.04	0.60	0.06	–	260	13	0.74	85
Corn soil
FW	0–20	7.15	0.73	1.33	2.10	5 × 10^3	450	85	7.90	550
	20–40	7.40	0.80	1.45	2.00	4 × 10^2	250	84	7.70	665
TWW	0–20	7.40	0.93	1.77	2.55	5 × 10^7	3300	105	9.35	780
	20–40	7.80	0.97	1.95	2.30	3 × 10^3	1300	104	8.50	880
1:1 FW/TWW	0–20	7.00	0.80	1.74	2.16	3 × 10^6	2600	100	7.80	730
	20–40	7.20	0.90	2.00	2.04	3 × 10^2	1200	100	7.80	870
LSD 0.05		0.20	0.03	0.95	0.09	–	290	14	0.72	125
Faba bean soil
FW	0–20	7.11	0.68	1.06	2.19	5 × 10^3	500	88	8.80	475
	20–40	7.25	0.75	1.25	1.95	3 × 10^2	250	89	8.25	530
TWW	0–20	7.24	1.35	3.58	2.89	5 × 10^7	3400	110	11.35	700
	20–40	7.20	1.45	4.10	2.30	5 × 10^3	1400	112	9.60	890
1:1 FW/TWW	0–20	7.23	1.30	3.27	2.35	2 × 10^6	2900	110	7.73	560
	20–40	7.30	1.45	4.00	2.08	5 × 10^3	1300	110	7.90	700
LSD 0.05		0.17	0.13	0.85	0.06	–	290	19	0.77	135
Wheat soil
FW	0–20	7.25	0.70	1.21	2.00	5 × 10^3	500	86	8.15	480
	20–40	7.15	0.85	2.00	1.98	3 × 10^2	250	91	8.00	590
TWW	0–20	7.35	1.45	4.57	2.40	5 × 10^7	3400	108	10.00	790
	20–40	7.15	1.95	5.10	2.10	5 × 10^3	1400	110	9.15	820
1:1 FW/TWW	0–20	7.30	1.25	4.00	2.15	2 × 10^6	2900	110	8.23	615
	20–40	7.40	1.45	5.00	2.02	5 × 10^3	1300	115	8.00	780
LSD 0.05		0.14	0.18	1.40	0.07	–	290	13	0.75	140

Table 4 Some chemical and biological characteristics of calcareous soil cultivated with soybean, corn, faba bean or wheat as influenced by treatment wastewater (TWW), fresh water (FW) and mix of TWW and FW in equal proportions (mixed water).

Water of irrigation	Soil depth	pH	ECe (dS m^−1)	SAR	OM (%)	Coliform (MPN/100 g)		Available (mg/kg^−1)
						TC	FC	N	P	K
Soybean soil
FW	0–20	8.11	3.15	4.80	0.84	8 × 10^2	200	70	3.90	340
	20–40	8.10	3.55	4.95	0.80	0	60	74	3.60	480
TWW	0–20	8.25	3.65	5.60	1.45	5 × 10^4	2000	98	6.25	630
	20–40	8.10	4.00	5.80	1.20	8 × 10^2	300	100	5.50	750
1:1 FW/TWW	0–20	8.11	3.20	5.10	1.00	4 × 10^3	1000	85	4.67	475
	20–40	8.16	3.68	5.50	0.93	4 × 10^2	180	84	4.50	650
LSD 0.05		0.22	0.14	0.85	0.07	–	80	12	0.77	38
Corn soil
FW	0–20	8.10	2.95	5.20	0.92	4 × 10^2	200	75	3.75	280
	20–40	8.00	3.10	6.00	0.80	0	60	76	3.30	340
TWW	0–20	8.20	3.60	5.54	1.35	6 × 10^4	2100	95	5.75	500
	20–40	8.10	4.10	6.40	1.10	4 × 10^2	300	92	5.25	620
1:1 FW/TWW	0–20	8.00	3.00	5.71	1.15	5 × 10^3	1100	86	4.45	450
	20–40	8.15	3.50	6.55	0.98	3 × 10^2	170	84	3.80	560
LSD 0.05		0.19	0.14	1.00	0.08	–	90	13	0.42	44
Faba bean soil
FW	0–20	7.95	3.10	4.95	0.95	6 × 10^2	240	80	3.56	300
	20–40	8.00	3.30	5.50	0.90	0	60	84	3.50	330
TWW	0–20	8.15	5.85	6.75	1.55	1 × 10^5	2100	125	6.50	620
	20–40	8.00	6.95	7.00	1.25	4 × 10^2	300	120	5.00	700
1:1 FW/TWW	0–20	8.05	4.64	5.81	0.98	5 × 10^4	1100	95	4.85	460
	20–40	8.00	5.00	6.50	0.92	4 × 10^2	180	92	4.30	520
LSD 0.05		0.23	0.17	0.85	0.08	–	90	12	0.67	58
Wheat soil
FW	0–20	7.95	3.50	4.56	0.94	5 × 10^2	260	79	3.90	400
	20–40	8.00	3.65	5.10	0.90	0	60	85	3.50	460
TWW	0–20	8.15	6.80	6.63	1.40	7 × 10^5	2200	115	5.85	600
	20–40	8.00	7.00	7.25	1.18	4 × 10^2	300	110	5.10	680
1:1 FW/TWW	0–20	8.25	5.16	6.13	1.10	3 × 10^4	1200	95	5.50	560
	20–40	8.30	5.65	7.00	1.00	5 × 10^2	170	96	5.40	660
LSD 0.05		0.23	0.13	0.75	0.08	–	90	16	0.69	48

This distribution of salinity, in soils of the two layers, indicates a significant transport and leaching of salts from soil of the upper layer to that of the lower layer. In the case of cultivated soils irrigated by TWW, the average values of relative increase in EC_e of lacustrine soils of the upper and lower layers were 84 and 101%, respectively and those of calcareous soils were 57 and 62%, respectively, as compared with that irrigated with FW. The data revealed that, irrigation by TWW would increase salinity of lacustrine soil with higher rate than would be predicted with respect to calcareous soil (Figs. 1 and 2).

Fig. 1 Relative increase (%) in soil salinity of lacustrine cultivated soils as influenced by TWW and 1:1 FW/TWW irrigation treatments.

Fig. 2 Relative increase (%) in soil salinity of calcareous cultivated soils as influenced by TWW and 1:1 FW/TWW irrigation treatments.

However, it is important to point out that the accumulation of salts in soils is affected by many factors, including the quality of irrigation water, soil properties and plant characteristics. Therefore, the degree of soil salinization risk varied greatly under these cases. In the case lacustrine soil cultivated with corn has low salinity than soybean, faba bean and wheat as influenced by TWW. This has been reported in other species by Palacios-Díaz et al. (2009) and Qian and Mecham (2005).

When the reused water was diluted, the resulting water (1:1 FW/TWW) had a lower saline content than the reused water (TWW), which mitigated the negative effect on soil salinity in both soils. Establishing a net downward flux of water and salt through the root zone is the only practical way to manage a salinity problem (Ayers and Westcot, 1989). Under such conditions, good drainage is essential in order to allow a continuous movement of water and salt below the root zone. Long-term use of reclaimed wastewater for irrigation is not generally possible without adequate drainage. Where drainage water salinity exceeds crop threshold levels the water can be blended with fresh water. Blending, which can be done before or during irrigation, enables farmers to extend the volume of water available (Rhoades, 1999; Oster and Grattan, 2002).

3.1.2 Coliform bacteria in soils

The poor microbial quality of the irrigation water used in TWW caused an increase in the microbial levels in lacustrine or calcareous soil samples collected at this treatment, represented by an increase in the fecal coliforms counts (Tables 3 and 4). Regardless of the different microbial load of TWW and 1:1 FW/TWW irrigation water, the microbial count in all soils under this study was always above the recommended range (<1000 MPN 100 g⁻¹) (Cairncross and Mara, 1989). It was also observed that the accumulation of coliform bacteria in soils of the upper layer relative to that of the lower layer indicates low downward movement and transport of bacterial cells through soils. The retention of bacterial cells in the upper soil layer could be due to the presence of higher OM in this layer than in that of the lower layer (Tables 3 and 4). The movement and transport of coliform bacteria in soils have been investigated in several publications. Malkawi and Mohammad (2003) found that surface soil had retained high bacterial counts than deeper soil. They recorded values for TC varying from 2.1 × 10³ to 4.2 × 10³ MPN/g soil and for FC varying from 1.2 × 10² to 4.2 × 10² MPN/g soil in the first few centimeters of soils irrigated with wastewater. Aiello et al. (2007) found that the mean bacteriological counts in soil irrigated with wastewater were higher three times in the 0–10 cm soil layer than in deeper soil (40 cm depth). Deshmukh et al. (2011) reported that between 98 and 99% of FC counts, from wastewater irrigation is retained in the vadose zone. Other authors (e.g. Reneau et al., 1989; Aiello et al., 2007 and McLain and Williams, 2012) have suggested microbial cells from wastewater irrigation can be percolated through the soil to a distance up to 3 m and therefore; contaminate groundwater. The potential of microbial cells movement and transport within the soil has been found to be dependent upon several factors such as temperature, soil moisture, soil type, and vegetation (Howell et al., 1996). Microbial transport via unsaturated flow was almost lower than in saturated flow and, therefore, microbial downward movement was decreased by lowering the saturation percentage (Schafer et al., 1998; Powelson and Mills, 2001; Pang, 2009; Travis et al., 2010; Aislabie et al., 2011).

Data presented in Tables 3 and 4 showed wide differences between coliform bacterial counts in the lacustrine and calcareous soils with the three types of water used for irrigation. This is due to the occurrence of relatively higher levels of OM, available P and clay contents and also to lower both salinity and total carbonate contents in the lacustrine soil than the calcareous soil (Table 1). These soil properties are considered important factors contributing to the favorable condition for stimulating high bacterial population. On the other hand, the presence of high salinity, total carbonate and sand fractions besides low OM and available P contents would adversely affect the survival of bacterial coliform in the calcareous soil as compared with the lacustrine soil.

Figs. 3 and 4 illustrate the higher values of relative increase in FC counts in lacustrine or calcareous soils cultivated with soybean and corn (summer field crops) than in those cultivated with faba bean and wheat (winter field crops). This may be due to more the relatively higher temperature in summer than in winter, which would stimulate the bacterial population, especially in the soil of the upper layer relative to that of the lower layer and also related to a higher concentration in the water used for the irrigation.

Fig. 3 Relative increase (%) in FC of lacustrine cultivated soils as influenced by TWW and 1:1 FW/TWW irrigation treatments.

Fig. 4 Relative increase (%) in FC of calcareous cultivated soils as influenced by TWW and 1:1 FW/TWW irrigation treatments.

3.1.3 Organic matter (OM)

Soil organic matter content is an important factor controlling the microbial activity. Large inputs of OM enhance the growth of microbial organisms, and therefore, clog the soil pores causes reduction in soil's infiltration and favor anaerobic microbiological growth in the soil due to aeration problems (Compton and Boone, 2002; Nunes et al., 2007). The TWW provides more dissolved organic carbon than other irrigation treatments (77.5 mg BOD₅/l) (Table 2). Consequently, the organic matter content is larger in the cultivated lacustrine and calcareous soils as a result of irrigation by TWW (Tables 3 and 4). The accumulation of OM in the surface soil as a result of irrigation by wastewater are in agreement with that found by Rusan et al. (2007), Wang et al. (2007), and Singh et al. (2011).

The results have also shown that the organic matter in both lacustrine and calcareous soils decreased with depth. On the contrary, ECe, SAR and coliform bacteria increased with depth (Tables 3 and 4). Annually monitored organic matter and soil structure to detect the deterioration is very essential in minimizing the structural damage.

The overall average values of the OM in the cultivated upper layer (0–20 cm) of lacustrine and calcareous soils were 2.66 and 1.44%, respectively. While they were 2.28 and 1.18% in cultivated lower layer (20–40 cm) at the same soils, respectively as influenced by TWW irrigation treatments.

The pH of all cultivated soils samples within the range of 7.00–8.30, which is the most desired range in agricultural soils (Tables 3 and 4).

3.2 Effect of irrigation water on plants

Statistically significant differences in the shoot dry weight, N, P and K contents in leaves of the four plant crops were observed between irrigated plants with TWW and other irrigated treatments. Generally, plants irrigated with TWW attained the greatest increase in the shoot dry weight, N, P and K contents in leaves (Fig. 5). In general, it could be concluded that the growth of plants irrigated with TWW was significantly favored, owing perhaps to the beneficial effect of the nutrients present in the treated wastewater. Therefore, the high accumulation of N, P and K in cultivated soils irrigated with TWW was incorporated in the leaf and reflected in shoot dry weight (Tables 3 and 4). Mixing of TWW and FW in equal proportions (1:1 FW/TWW) led to decrease in shoot dry weight, N, P and K contents in leaves (Fig. 5). However, it is important to point out that the leaf mineral concentration is not only influenced by the chemical constituents of the water used for irrigation, but also by the facility of individual salts to leach and transported by plants, the ability to compete with other ions.

Fig. 5 The effect of irrigation water treatments, soil type on plant dry weight (A), leaf N-content (B), leaf P-content (C) and leaf K-content (D).

3.2.1 Shoot dry weight

Fig. 5A showed significant increase in the dry weight of shoot of plants grown on lacustrine and calcareous soils as a result of irrigation by TWW and 1:1 FW/TWW as compared with that irrigated by FW. It is also clear that the shoot dry weight, of each plant crop, was higher when grown on lacustrine soil than on calcareous soil whether irrigated by FW, TWW or 1:1 FW/TWW. Also, these shoot dry weights were almost higher when plants were irrigated by TWW than 1:1 FW/TWW.

The results showed that the shoot dry weight is affected by many factors, including the quality of irrigation water, soil properties, and grown plants. Shoot dry weight of soybean and corn are heavier in lacustrine soils than calcareous soils, while shoot dry weight of faba bean and wheat are heavier in calcareous soils than lacustrine soils.

Therefore, the relative increase of shoot dry weight of the four plant crops grown on lacustrine soil irrigated by TWW were 70% soybean, 78% corn, 84% faba bean and 77% wheat; and of those grown on calcareous soil were 67% soybean, 67% corn, 93% faba bean and 88% wheat, as compared with FW. These data points out that irrigation by TWW would be a supplemental source of P.

3.2.2 Nitrogen content in leaves

The N contents in leaves of soybean and corn plants grown on lacustrine and calcareous soils and irrigated by TWW and 1:1 FW/TWW were within the adequate range (more than 1.25% according to Chapman, 1966). On the other hand, N in leaves of faba bean and wheat grown on lacustrine soil irrigated with TWW were within the adequate range and those grown on calcareous soil were within the deficient range (less than 1.25%). Irrigation of faba bean and wheat plants grown on calcareous soil by 1:1 FW/TWW did not supply these plants by adequate amounts of N and therefore, the N contents in leaves of these two grown plants were in the deficient range (Chapman, 1966). The accumulation of N in leaves is affected by many factors, including the quality of irrigation water, soil properties, and grown plants. Faba bean leaves accumulated more N than other plant leaves in both soils. Corn Leaves more accumulated N in lacustrine soil more than that grown in calcareous soil. While the accumulated N in wheat leaves was more pronounced in calcareous soil than that grown in lacustrine soil (Fig. 5B).

Thus, the relative increase of N in the leaves of the four plant crops grown on lacustrine soil irrigated by TWW were 51%, 68%, 151%, and 48% for soybean, corn, faba bean and wheat, respectively. On the other hand, the same crops grown on calcareous soil showed relative increase of 92%, 19%, 142% and 89%, respectively as compared with that irrigated with FW.

3.2.3 Phosphorus content in leaves

Fig. 5C showed significant increase in P contents in leaves of the four plant crops grown on lacustrine and calcareous soils as a result of irrigation by TWW and 1:1 FW/TWW as compared with those irrigated by FW. The levels of P in leaves of plants grown on the two soils used in this study and irrigated by TWW were within the sufficient and adequate range (0.2–0.5%, according to Chapman, 1966), except faba bean and wheat grown on calcareous soil because P contents in their leaves were within the low range (less than 0.2%). Also, irrigation by 1:1 FW/TWW increased the levels of P in leaves of soybean, corn and faba bean plants grown on lacustrine to levels within the adequate range of P in plant leaves (Chapman, 1966).

The results have shown that the accumulation of P in leaves is affected by many factors, including the quality of irrigation water, soil properties, and grown plants. Faba bean leaves accumulated more P than other plant leaves in both soils under this study (Fig. 5C). Corn and wheat leaves showed more accumulated P in lacustrine soil than in calcareous soil, while in soybean leaves was more pronounced in calcareous soil than in lacustrine one (Fig. 5C).

Therefore, the relative increase of P in the leaves of the four plant crops grown on lacustrine soil irrigated by TWW were 83% soybean, 131% corn, 140% faba bean and 100% wheat; and of those grown on calcareous soil were 111% soybean, 124% corn, 125% faba bean and 70% wheat, as compared with FW. These results emphasize that the applying of TWW for irrigation can be used as a supplementary source of phosphorus.

3.2.4 Potassium content in leaves

Fig. 5D showed significant increases in the amounts of K in leaves of plants grown on lacustrine and calcareous soils as a result of irrigation with TWW and 1:1 FW/TWW as compared to those irrigated with FW. The concentration levels of K in leaves of the four plants grown in this study were within the adequate range of K (0.15–0.3% according to Chapman, 1966). These data proved that TWW and 1:1 FW/TWW irrigation can be used as a supplementary source of potassium to plants grown on both lacustrine and calcareous soils.

The accumulation of K in leaves is affected by the quality of irrigation water, soil properties, and plant grown. Soybean leaves accumulated more K than the other plant leaves in lacustrine soil and it accumulated less K than other plant leaves in calcareous soil Corn grown in calcareous soil accumulated more potassium in their leaves than that grown. lacustrine one. Whereas, wheat grown in calcareous soil accumulated more nitrogen in their leaves than that grown in lacustrine one (Fig. 5D).

Thus, the relative increase of K in the leaves of the four plants grown on lacustrine soil irrigated by TWW were 155%, 58%, 55% and 92% for soybean, corn, faba bean and wheat, respectively. Whereas, those grown on calcareous soil were 74%, 324%, 206% and 106% as compared with that irrigated with fresh water for the same sequence of crops, respectively.

The above-mentioned results prove that irrigating both lacustrine and calcareous soils with TWW or 1:1 FW/TWW could be applied as a complementary and valuable source of NPK.

3.3 Chemical and biological quality of drainage waters

The effect of using different irrigation water treatments applied in this study on chemical and biological drainage water quality is investigated. Affections to drainage waters in terms of pH, TDS, SAR, NO₃⁻, TP, BOD₅, COD, TC and FC have been identified in all irrigation treatments (FW, TWW, 1:1 FW/TWW). Statistically significant differences in the chemical and biological quality of drainage waters were observed between all irrigation treatments. TDS, SAR, NO₃⁻, TP, BOD₅, COD, TC and FC contents are higher in drainage water received from soils irrigated with FW or 1:1 FW/TWW treatments.

Mixing of TWW and FW in equal proportions (1:1 FW/TWW) led to a decrease in the TDS, SAR, NO₃⁻, TP, BOD₅, COD, TC and FC contents in drainage water. However, it is important to point out that quality of drainage waters is not only influenced by the chemical constituents of the water used, but also by soil characteristics and the grown plants.

The pH, NO₃-N, TP and SAR values in the all drainage waters samples never exceeded the limits recommended by FAO organization (Ayers and Westcot, 1989) and the World Health Organization (Cairncross and Mara, 1989).

Therefore, the overall values in all drainage waters samples were found to be within the range 7.50–8.73 pH, 4.34–14.00 mg NO₃-N/l, 3.16–9.51 mg TP/l and 1.88–6.94 SAR. While the recommended values were 6.5–8.5 pH, >30 mg NO₃-N/l and <15 mg TP/l.

Also the concentration of BOD₅ in all drainage waters samples never exceeded the limits recommended by Ministerial Egyptian Decree No. 92 of 2013 (>30 mg l⁻¹) (Egyptian Ministry of Water Resources and irrigation, Decree No. 92/2013, Egypt). Therefore, the overall values in all drainage waters’ samples were found to be within the range of 7.6–25.6 mg BOD₅/l. However, it is important to point out that the BOD₅ values of drainage water resulted in irrigating the soils with FW is less than that irrigated with TWW. This can be explained by the fact that, large inputs of organic matter can enhance the growth of microbial organisms. Irrigation with TWW provides more dissolved organic carbon. Consequently, the organic matter introduced with TWW could get retained into the soil and be degraded by microorganisms, thus reduce the concentration of BOD₅ in the drainage water.

The TDS values of drainage water received from soils irrigated with TWW were higher that irrigated with fresh water or 1:1 FW/TWW (Fig. 6A).

Fig. 6 The effect of irrigation water treatments and soil type on TDS (A) and FC (B) in drainage water.

On the other hand, this can be explained by terms of larger solubility of the most significant ions are the cations of calcium, magnesium, sodium and the anions of carbonates, sulfates, and chlorides, which are leached, reaching the drainage water (Duchaufour, 1978; Miguel et al., 2013). The study carried out by Weber and Juanico (1996) also showed that irrigation by wastewater has negative effects on the quality of drainage water which is mainly due to the accumulation of high concentrations of salts especially Na⁺ and Cl⁻.

The concentration of TDS in the all drainage waters’ samples of cultivated lacustrine did not exceed the limits of use restriction recommended by the Food and Agriculture Organization (>2000 mg l⁻¹) (Ayers and Westcot, 1989). While the TDS level in the drainage waters’ samples of cultivated calcareous below the survey pot irrigated with TWW or 1:1 FW/TWW exceeded this limit (Fig. 6A). According to Food and Agriculture Organization (Ayers and Westcot, 1989) drainage water above 2000 mg TDS/l can be used regularly only for tolerant plants on permeable soils.

From Fig. 6A, it can notice that the concentration of TDS in drainage water is affected by the quality of irrigation water, soil properties, and the grown plants. Thus, the relative increase of TDS in the drainage water of lacustrine soil irrigated by TWW were 132%, 27%, 136% and 70% for soybean, corn, faba bean and wheat grown on lacustrine soil, respectively. While, these values were 34%, 82%, 61% and 143% for the same sequence of crops grown on calcareous soil, respectively as compared with that irrigated with FW.

The results illustrated in Fig. 6B showed that the FC counts in drainage water resulted in irrigating the soil with TWW are higher than that irrigated with FW or 1:1 FW/TWW. The counts of FC in all drainage water resulted in irrigating the soil with TWW or 1:1 FW/TWW exceeded the limits of use restriction recommended by the World Health Organization (>1000 MPN 100 ml⁻¹) (Cairncross and Mara, 1989). Large inputs of organic matter and nutrients can enhance the growth of microbial organisms. Irrigation with TWW provides more dissolved organic carbon. Consequently, the microbial populations are expected to be larger in the parcel irrigated by TWW. So FC is reaching the drainage water was higher compared to drainage water resulted irrigating with FW or 1:1 FW/TWW (Duchaufour, 1978; Compton and Boone, 2002; Nunes et al., 2007). Fig. 6B has shown also that the counts of FC in drainage water are affected by many factors, including the quality of irrigation water, soil properties, and the grown plants.

4 Conclusion

The water deficit in the Egypt, together with the ever increasing demand due to the continued urban growth and the major demand from intensive agricultural activity, has made it necessary to use treated wastewater for irrigation. Therefore, it must also ensure that public health and environmental concerns are fully addressed. By combining results from field investigations, we systematically analyzed the benefits and risks associated with TWW irrigation. Results showed that TWW may generate a huge economic benefit. Risks associated with TWW irrigation were mainly due to its high salt content and FC. Potential risks of soil salinization, FC and drainage water polluted by salt over supply and FC should be addressed, while the soil pollution risk from nitrate and total phosphorus was generally low. The degree of risk is dependent on many factors including the quality of the TWW, soil and plant type, and environmental conditions. The risk can be controlled through proper field management practices; good drainage is essential in order to allow a continuous movement of water and salt below the root zone. The practice of blending TWW with FW in equal proportions showed that the resulting water (mixed water) had a lower saline content, pH, BOD₅, COD, total N, total P and fecal coliform than the TWW; which mitigated the negative effect on plant growth and soil salinization.

Notes

Peer review under responsibility of National Water Research Center.