Seasonal dynamics and hydrogeological assessment of Najafgarh drain and its irrigation channels for agricultural purposes in Southwest Delhi, India

The agriculture sector uses over 70% of global water resources. However, growing water demands from energy and municipal sectors in urban and peri-urban areas are reducing the availability of fresh groundwater for agriculture (Gola et al. 2016; Bisht et al. 2024). Surveys by various government agencies predict a significant increase in water demand by 2050, totaling 1,447 km³, with agriculture remaining the highest-demand sector. This competition for water resources poses a challenge to sustainable agricultural practices and underscores the need for effective water management strategies to balance the demands of different sectors while ensuring the availability of water for agricultural needs (Marrugo-Negrete et al. 2017). Severe water stress at present and in the near future highlights the need for alternatives, such as wastewater, to fulfill irrigational requirements. The continuous use of poor-quality irrigation water may decrease agriculture productivity and have negative effects on farmers, product consumers, and the environment (Gautam et al. 2013; Krishan et al. 2021; Bisht et al. 2024). Hence, the potential of the water sources (both surface and ground water) for irrigation purposes must be studied so that the risk from geochemical and anthropogenic contaminants can be reduced by an appropriate treatment method (WHO 2006; Bisht et al. 2023).

In the study area, the largest drain in Delhi is the Najafgarh drain, which is responsible for approximately 60% of the total wastewater released from Delhi into the Yamuna River (Adhikari et al. 2012; CGWB 2021; Vaid et al. 2022). It was also reported that this drain releases approximately 287.5 million liters per day of wastewater into the Yamuna River. Furthermore, the river's condition is worsened by the presence of 23 significant drainage channels, with 19 of them directly emptying into the river and the remaining four discharging into the river through the Agra and Gurgaon canal. These drains carry substantial amounts of untreated domestic and industrial sewage, serving as primary sources of pollution (CGWB 2019). The Najafgarh drain extends for 51 km before it eventually meets the Yamuna River. Notably, a significant portion of this drain, approximately 31 km, lacks a lining (Shekhar & Sarkar 2013; CGWB 2017). The water from the Najafgarh drain is also employed for irrigation purposes in both urban and peri-urban areas of Delhi, especially in situations where there are no alternative sources of irrigation water (Gautam et al. 2013). In the study area, i.e., Southwest Delhi, over an extended period, the use of drainage water for irrigation has been extensively utilized. This is primarily because it is consistently accessible throughout the year and offers an attractive advantage of containing natural nutrients (nitrogen, phosphate, potassium) that support crop growth without the need for chemical fertilizers (Bhattacharya et al. 2015; Keesari et al. 2020).

Although wastewater contains numerous beneficial nutrients for plant growth (Bhattacharya et al. 2015; Mohanty & Das 2023), it may cause alteration of soil chemistry (Schacht et al. 2014; Bisht et al. 2024). It also reduces the fertility of agricultural land which is harmful to human and animal health through the consumption of infected vegetables and crops (Bhattacharya et al. 2015; Gola et al. 2016; Mohanty & Das 2023). Additionally, the Najafgarh drain has been documented to carry substantial levels of heavy metals such as lead (Pb), copper (Cu), cadmium (Cd), zinc (Zn), iron (Fe), and others (Kolothumthodi & Pulikkal 2022). These metals are present in the fields irrigated with sewage and sludge from the drain, and they can ultimately find their way into the vegetables grown in that soil (Kaur & Rani 2006; Vaid et al. 2022; Bisht et al. 2024). According to a recent investigation, fruit and vegetables grown with contaminated wastewater or water from the Yamuna River and its neighboring drains were found to have heavy metal ions (Table 1) (Adhikari et al. 2012; Paul et al. 2014; Vaid et al. 2022).

Therefore, the present study focuses on a detailed evaluation of drain water quality for irrigation purposes in the peri-urban area of southwest Delhi. It comprehensively analyzes physico-chemical parameters, heavy metals, and irrigation water quality indices during pre-monsoon (PRM) and post-monsoon (POM) seasons. The study also employs innovative techniques like Gibbs plots and Piper diagrams to understand water chemistry, rock–water interactions, and hydrochemical facies in the Southwest peri-urban setting of Delhi.

The NCT of Delhi has a total geographical area of 1,483.7 sq km, out of which 728.4 sq km (49%) constitutes urban regions. Approximately 38% of the total land area, primarily in the Southwest and Northwest districts, is designated for agricultural use. Agriculture is the primary occupation in the study area, with two main crop cultivation seasons: Kharif (July to December) for crops like maize, pearl millet, rice, cotton, lentils, green gram, pigeon pea, and sugarcane, and Rabi (December to April) for crops such as barley, chickpea, wheat, camelina, mustard, and rapeseed (CGWB 2021). The major source of irrigation in the region is groundwater and drain water. However, due to groundwater salinity issues in most villages, farmers often rely on drain water to meet their irrigation needs (Adhikari et al. 2012; Paul et al. 2014; Bisht et al. 2024). Table 1 summarizes the source of heavy metals and its deposition in many parts of vegetables.

In the study area, drain water is commonly used for irrigation, particularly in regions where cabbage and other horticultural crops are predominant. High-quality irrigation water is crucial for maximizing crop yields while maintaining soil health (Batarseh et al. 2021). Therefore, it is important to assess the suitability of drain water for agricultural use. To do this, various irrigation indices are utilized to evaluate the quality of the water.

Based on SAR value, irrigation water can be classified into four classes as SAR < 6 (low; ideal or excellent), 6–12 (medium; good), 12–18 (high; doubtful), and >18 (unsuitable). High sodium concentration in water increases the risk of becoming alkaline, whereas Ca and Mg predominate over Na and cause less risk. High SAR values in irrigation water cause sodium cation exchange complex saturation, which destroys soil structure (Jain & Vaid 2018). SAR also affects percolation time of water in the soil and the soil becomes hard to plough (Keesari et al. 2020). Therefore, low value of SAR of irrigation water is desirable for irrigation purposes.

Higher RSC in water increases sodium adsorption on soil, affecting plant growth. Irrigation water with RSC above 5 meq/L is harmful, while values over 2.5 meq/L are unsuitable for use. An RSC between 1.25 and 2.5 meq/L is marginal, and 1.25 meq/L is the safe limit. Negative RSC indicates excess Ca²⁺ and Mg²⁺ in the water.

Cation concentrations are expressed in meq/L. Water with KR < 1.0 is suitable for irrigation, while KR > 1 indicates potential alkali hazards and is not recommended. High KR leads to increased sodium absorption by clay particles, displacing calcium and magnesium ions, which reduces soil permeability and internal drainage, as noted by Ramesh & Elango (2012) and Jain & Vaid (2018).

The value of MH > 50 means harmful and unsuitable for irrigation, while it indicates suitable and not harmful for irrigation if MH < 50.

The heavy metals concentration, including chromium (Cr), copper (Cu), cadmium (Cd), nickel (Ni), zinc (Zn), iron (Fe) and lead (Pb), in the water samples was assessed through atomic absorption spectroscopy standardized method (Szabolcs 1964; Eaton 2015).

The study area relies heavily on drain water for irrigation, making it essential to assess the quality of this water. The suitability of drain water for irrigation is influenced by factors such as salt concentration, soil types, and existing cropping practices. Using poor-quality water for agriculture can negatively impact crop yields by reducing soil fertility, soil permeability, and aeration (Singh et al. 2021; Sharma et al. 2024). Based on ions (cations and anions) concentration in water, an effort was made to evaluate the quality of irrigation water by analyzing various parameters, including SAR, Na%, KR, MH, RSC, and PI. The results of the drain water samples were compared with FAO standards to assess their suitability for irrigation in Table 3. However, Table 2 provides a statistical summary of the water chemistry of drain water during the PRM and POM seasons and compared with BIS standards.

Table 2

Descriptive summary of hydrochemical data drain water in PRM and POM seasons

Parameters .	Indian standard .	PRM .		POM .
	Permissible limit (PL) .	Below the PL .	Above the PL .	Below the PL .	Above the PL .
pH	6.5–8.5	10	1	10	1
EC	750–2,000	6	5	9	2
(mg/L)	200	2	9	6	5
Ca2+ (mg/L)	75	0	11	0	11
Mg2+ (mg/L)	30	0	11	0	11
Na+ (mg/L)	200	1	10	0	11
K+ mg/L)	10	3	8	2	9
(mg/L)	200	3	8	2	9
Cl−(mg/L)	250	0	11	0	11
(mg/L)	50	9	2	10	1
F−(mg/L)	1–1.5	6	5	9	2

Parameters .	Indian standard .	PRM .		POM .
	Permissible limit (PL) .	Below the PL .	Above the PL .	Below the PL .	Above the PL .
pH	6.5–8.5	10	1	10	1
EC	750–2,000	6	5	9	2
(mg/L)	200	2	9	6	5
Ca2+ (mg/L)	75	0	11	0	11
Mg2+ (mg/L)	30	0	11	0	11
Na+ (mg/L)	200	1	10	0	11
K+ mg/L)	10	3	8	2	9
(mg/L)	200	3	8	2	9
Cl−(mg/L)	250	0	11	0	11
(mg/L)	50	9	2	10	1
F−(mg/L)	1–1.5	6	5	9	2

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The pH is a crucial parameter for assessing the acidity, neutrality, or alkalinity of water. In the study area, the pH of collected water samples ranged from 6.7 to 8.7 during the PRM season and 6.6 to 8.7 in the POM season, with average values of 7.4 and 7.5, respectively. According to FAO standards, the optimal pH range for irrigation is 6.5–8.5, indicating that the drain water is slightly acidic to moderately alkaline. The higher pH levels may be due to significant amounts of sodium, magnesium, calcium, and carbonate, as well as industrial activities such as detergent use and manufacturing operations in the area (Bhattacharya et al. 2015; Bisht et al. 2024).

In the PRM season, the total dissolved solids (TDS) in drain water ranged from 837 to 3,568 mg/L, with an average value of 1,825 mg/L. However, in the POM season, the TDS values range from 487 to 1,353 mg/L with an average value of 827 mg/L, respectively (Table 2). However, the maximum permissible limit of TDS in water according to FAO for discharge in agricultural lands should not exceed 500 mg/L. Based on TDS measurements, all 11 samples of the study area show the elevated TDS value during the PRM season, but after rainfall, some salt concentrations decrease TDS of these sample location. The EC (μS/cm) value is 1,250 to 5,526 with average value 2,721 μS/cm in PRM and 1,298 to 3,600 with average value 2,258 μS/cm in the POM season. It was reported that continuous irrigation with drain water can potentially result in the seepage of soluble salts beneath the crop roots, leading to the accumulation of salts in the upper layer of the soil. This accumulation can have detrimental effects on soil productivity, the activity of soil microorganisms, and subsequently, the growth of plants (Kaur & Kumar 2021; Singh et al. 2021). Therefore, it is suggested, some treatment is required in particular water samples before using irrigation. The salinization risks is associated in DRW originating from concentration of dissolved ionic species, likely coupled to high evaporation effects (Kumar et al. 2007; Adhikari et al. 2012; Singh 2015; Aravinthasamy et al. 2020).

One of the key findings is the mean abundance of the main cations in the drain water samples, which follows the order: Na⁺ > Ca²⁺ > Mg²⁺ > K ⁺. Sodium (Na⁺) emerges as the most prevalent alkali metal in the drain water, with concentrations ranging from 86.1 to 547 mg/L in the PRM season and 77 to 350 mg/L in the POM season. The average values are 292 and 192 mg/L, respectively. This high concentration of Na⁺ can be attributed to various factors, including the weathering of sodium-bearing minerals and potential contributions from anthropogenic sources (Adhikari et al. 2012; Bisht et al. 2024). Calcium (Ca²⁺) concentrations exhibit a notable presence, ranging from 157 to 456 mg/L (average 231 mg/L) in the PRM season and 123 to 320 mg/L (average 198 mg/L) in the POM season. Limestones and dolomites, formed by the dissolution of carbonic acids in water, are identified as the primary sources of calcium in the drain water (Aravinthasamy et al. 2020). Magnesium (Mg²⁺) concentrations range from 56 to 354 mg/L (average 194 mg/L) in the PRM season and 60 to 410 mg/L (average 183 mg/L) in the POM season. The presence of magnesium in drain water can be attributed to various geological and anthropogenic factors (Aravinthasamy et al. 2020; Singh et al. 2021). In contrast, potassium (K⁺) is found to be the least common alkali metal in the drain water samples. Its concentration ranges from 19.7 to 123 mg/L (average 33.7 mg/L) in the PRM season and 5.65 to 21.1 mg/L (average 125.7 mg/L) in the POM season. The reduced K⁺ concentration in drain water could be due to its fixation of K⁺ by clay minerals and the formation of secondary minerals (Paul et al. 2014). However, agricultural operations and the weathering of potash and silicate minerals may contribute to the rising K⁺ concentration in PRM.

The study also examines the abundance of major anions in the drain water, revealing the following order: Cl⁻ > > > > F⁻. Bicarbonate (⁠⁠) concentrations range from 250 to 2,250 mg/L (average 880 mg/L) in the PRM season and 120 to 874 mg/L (average 348 mg/L) in the POM season. The presence of is attributed to the dissolution of CO₂ in water (Ayers & Westcot 1985; Bisht et al. 2024). Chloride (Cl^–) concentrations vary between 220 and 1,792 mg/L during the PRM season and from 210 to 1,430 mg/L (with an average of 653 mg/L) in the POM season. The presence of Cl⁻ in drain water is attributed to urban sewage, industrial discharges, and leaching processes. High levels of chloride not only give water an unpleasant taste but also combine with sodium, which originates from the weathering of granitic terrains, forming sodium chloride (NaCl). An excess of NaCl renders the water saline, making it unsuitable for both irrigation and drinking purposes (Disli 2017).

The SAR in drain water ranged from 2.29 to 33.5 (average 20.45) in PRM and 1.75 to 32.6 (average 15.2) in POM. Table 3 categorizes 11 drain water samples into excellent, good, doubtful, and unsuitable categories during PRM. High SAR is due to a higher ratio of Na⁺ to Ca²⁺ and Mg²⁺ ions, with Na⁺ displacing Ca²⁺ and Mg²⁺ in irrigation water. Studies (Adimalla 2019; Aravinthasamy et al. 2020) report variable SAR values, particularly in areas like Bhatinda, Punjab, due to sodium-based fertilizers and industrial effluents. Gypsum is used to mitigate high sodium levels' effects. The KR for drain water ranges from 0.15 to 0.65, averaging 0.45 in the PRM season, and from 0.14 to 1.56, averaging 1.32 in the POM season. Table 2 shows that 64% of the samples fall within the suitable range in the PRM season, while 91% are suitable in the POM season. The analysis suggests a moderate effect of rainfall on KR values. Additionally, MH value of drain water varies from 27.06 to 77, with an average of 56.10 in PRM, and from 23.10 to 76.30, with an average of 54.40 in POM. The fluctuation in MH values between PRM and POM indicates possible dolomite mineral leaching or dissolution after the monsoon (Singh et al. 2021).

The RSC value of drain water ranges from −15.06 to 7.09 with an average value of −2.75 in the PRM season, while in the POM season the RSC value ranges from −9.10 to 8.60 with an average value of 0.96, respectively (Table 2). Table 3 shows drain water samples (eight suitable, 73% + four marginally suitable 8% + three not suitable 9%) fall under suitable, marginally suitable and non-suitable category for irrigation use during PRM season. Whereas, in POM season, 11 drain water samples (four suitable, 36% + four marginally suitable, 36% + three not suitable, 27%) fall under suitable, marginally suitable and non-suitable category for irrigation purposes during POM. The higher range of RSC in some water samples indicates higher sodium absorption in the soil during irrigation (Jain & Vaid 2018; Adimalla 2019).

The Na % is a key indicator for evaluating sodium hazard in water. In the PRM season, the Na % of drain water ranges from 15.19 to 42.3%, with an average of 31.75%. In the POM season, the Na % ranges from 12.5 to 35.8%, with an average of 23.69%. Table 3 categorizes 11 drain water samples for irrigation during the PRM season, with 9% suitable, 73% good, and 18% permissible. In the POM season, 27% of the samples are suitable, and 73% are good. High Na concentrations in water can lead to sodium absorption by clay particles, displacing Ca and Mg ions, and reducing soil permeability and internal drainage capacity (Ghislain et al. 2012; Sharma et al. 2021).

Similarly, the PI value of drain water ranges from 22.70 to 50.29 with an average value of 39.60 in the PRM season, while in the POM season the PI value ranges from 17.6 to 44.7 with an average of 31.5, respectively (Table 2). From the measured value, the effect of rainfall over PI is not showing much significance. Table 3 shows 11 drain water samples (10 good, 91%+ one unfit, 9%) fall under excellent, good and unfit category in PRM. Whereas, in POM season, Table 3 shows 11 drain water samples (seven good, 64% + four unfit, 36%) fall under excellent, good and unfit category in POM. From Table 2, the effect of rainfall over PI is not showing much significance. Therefore, it is suggested that the soil permeability is affected by the extensive use of irrigation water as it is influenced by Na⁺, Ca²⁺, Mg²⁺ and contents of the water (Gautam et al. 2013).

The MR value of drain water ranges from 27.06 to 77 with an average value of 56.10 in PRM while in POM season MR value ranges from 23.1 to 76.3 with an average of 54.4, respectively. Table 3 shows 11 drain water samples (seven fit, 64% + four unfit, 36%) fall under fit and unfit category for irrigation in POM. The fluctuation in MR value during PRM and POM clearly indicated some degree of leaching process of Mg²⁺ from rock to groundwater after rainfall (Sharma et al. 2021).

Conversely, 9% of drain water samples are placed in the excellent to good category. Moving on to the POM season, 63% of samples are categorized as doubtful or undesirable, while 37% are classified as excellent to good category due to the dilution effect of monsoon. The dilution effect of monsoon rains helps lower the salinity and sodicity of the water, making it more suitable for irrigation. However, the presence of a substantial proportion of samples in the undesirable category still calls for careful management practices to avoid long-term soil health issues (Disli 2017).

Pearson correlation analysis measures the linear relationship between two variables in water chemistry, such as pH and dissolved oxygen, using the correlation coefficient (r) ranging from −1 to +1. A positive r indicates that both variables increase together, while a negative r shows an inverse relationship. This analysis identifies key physicochemical parameters, helping researchers focus on the most influential factors for effective monitoring and treatment, ultimately leading to cleaner water and improved water quality outcomes (Singh et al. 2021; Vaid et al. 2022). In the PRM season, strong positive correlations were found between various ion pairs, such as EC-TDS, EC–Na⁺, EC–Cl⁻, TDS–Na⁺, and TDS–Cl⁻. There were also moderate correlations between EC-Ca²⁺, EC–Mg²⁺, K–Mg²⁺, and Mg²⁺–Cl⁻. These positive correlations indicate the significant contribution of these ions to EC. The increased ionic strength from NaCl salt dissolution enhances the solubility of and enriches the water with Ca²⁺ and Mg²⁺. A relatively low correlation coefficient between Ca²⁺ and concentrations suggests distinct geological processes for these ions. Ca²⁺ can either precipitate as calcite or engage in Ca²⁺/Na⁺ exchange with clay minerals, while remains typically dissolved. However, there were no significant correlations between various ion pairs involving pH, ⁠, EC, TDS, K⁺, Na⁺, ⁠, and (Table 4).

Table 4

Correlation matrix of various chemical parameters of water samples in the PRM and POM

.	pH .	EC .	TDS .	.	Na+ .	K+ .	Ca2+ .	Mg2+ .	Cl− .	.	.	F− .
pH	1.00
EC	0.19	1.00
TDS	0.25	0.97b	1.00
	−0.39	−0.14	−0.14	1.00
Na+	0.16	0.84b	0.84a	0.12	1.00
K+	−0.23	0.43	0.33	−0.13	0.28	1.00
Ca2+	−0.29	0.63a	0.50a	0.30	0.17	0.17	1.00
Mg2+	−0.26	0.54a	0.54a	0.05	0.47	0.56a	0.02	1.00
Cl−	0.25	0.89b	0.79b	−0.31	0.77a	0.51a	0.32	0.69b	1.00
	0.39	0.38	0.18	−0.34	0.09	0.33	0.39	0.25	0.30	1.00
	0.31	0.28	0.28	−0.25	0.38	−0.10	0.25	0.03	0.36	0.21	1.0
F−	0.19	0.25	0.15	−0.01	0.19	0.39	0.07	0.23	0.24	0.22	0.2	1.00
.	pH .	EC .	TDS .	.	Na+ .	K+ .	Ca2+ .	Mg2+ .	Cl− .	.	.	F− .
pH	1.00
EC	−0.31	1.00
TDS	−0.40	0.99b	1.00
	−0.15	0.13	0.12	1.00
Na+	−0.19	0.92b	0.92	0.12	1.00
K+	−0.22	0.31	0.33	0.16	0.22	1.00
Ca2+	−0.35	0.51a	0.50	0.04	0.42	0.04	1.00
Mg2+	0.01	0.28	0.28	0.09	0.22	0.21	0.36	1.00
Cl−	−0.21	0.87b	0.77b	−0.08	0.78b	0.24	0.67a	0.67a	1.00
	0.03	0.45	0.44	−0.10	0.37	0.02	0.27	0.30	0.42	1.00
	0.19	0.02	0.02	−0.21	0.01	−0.08	0.16	0.28	0.22	0.25	1.00
F−	−0.17	0.05	0.04	0.32	0.21	−0.03	0.29	0.15	0.12	−0.25	0.15	1.00

.	pH .	EC .	TDS .	.	Na+ .	K+ .	Ca2+ .	Mg2+ .	Cl− .	.	.	F− .
pH	1.00
EC	0.19	1.00
TDS	0.25	0.97b	1.00
	−0.39	−0.14	−0.14	1.00
Na+	0.16	0.84b	0.84a	0.12	1.00
K+	−0.23	0.43	0.33	−0.13	0.28	1.00
Ca2+	−0.29	0.63a	0.50a	0.30	0.17	0.17	1.00
Mg2+	−0.26	0.54a	0.54a	0.05	0.47	0.56a	0.02	1.00
Cl−	0.25	0.89b	0.79b	−0.31	0.77a	0.51a	0.32	0.69b	1.00
	0.39	0.38	0.18	−0.34	0.09	0.33	0.39	0.25	0.30	1.00
	0.31	0.28	0.28	−0.25	0.38	−0.10	0.25	0.03	0.36	0.21	1.0
F−	0.19	0.25	0.15	−0.01	0.19	0.39	0.07	0.23	0.24	0.22	0.2	1.00
.	pH .	EC .	TDS .	.	Na+ .	K+ .	Ca2+ .	Mg2+ .	Cl− .	.	.	F− .
pH	1.00
EC	−0.31	1.00
TDS	−0.40	0.99b	1.00
	−0.15	0.13	0.12	1.00
Na+	−0.19	0.92b	0.92	0.12	1.00
K+	−0.22	0.31	0.33	0.16	0.22	1.00
Ca2+	−0.35	0.51a	0.50	0.04	0.42	0.04	1.00
Mg2+	0.01	0.28	0.28	0.09	0.22	0.21	0.36	1.00
Cl−	−0.21	0.87b	0.77b	−0.08	0.78b	0.24	0.67a	0.67a	1.00
	0.03	0.45	0.44	−0.10	0.37	0.02	0.27	0.30	0.42	1.00
	0.19	0.02	0.02	−0.21	0.01	−0.08	0.16	0.28	0.22	0.25	1.00
F−	−0.17	0.05	0.04	0.32	0.21	−0.03	0.29	0.15	0.12	−0.25	0.15	1.00

^aLess significant.

^bMore significant.

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In the POM season, substantial positive correlations were observed between TDS–Na⁺, TDS–Cl^–, Na⁺–Cl⁻, Ca²⁺–Cl⁻, and Mg²⁺–Cl⁻. Additionally, relatively moderate correlations were noted between TDS-Ca²⁺, TDS–Mg²⁺, and Na⁺–Mg²⁺. These ion pairs significantly contribute to EC, as indicated by the strong positive correlations with EC–Na⁺, EC–Cl⁻, EC-Ca²⁺, and EC–Mg²⁺. The weak correlation between Ca²⁺ and Cl⁻ may be due to additional mechanisms, such as ionic exchange, particularly in more highly salinized water with increased Cl^– concentration. However, there were no significant correlations among various ion pairs involving pH, ⁠, EC, TDS, K⁺, Na⁺, ⁠, and (Table 4).

In drain water, the Fe content ranged from 0.98 to 8.65 ppm before the monsoon and 0.004 to 10.32 ppm after the monsoon. The majority of samples exceeded the permissible limit of 5 ppm for Fe, according to WHO (2006) guidelines. The elevated Fe levels are likely due to both industrial discharge and the weathering of geological formations containing iron minerals. This is consistent with findings from other industrial regions (Vaid et al. 2022). High Fe concentrations can lead to soil acidification and toxicity in plants, affecting crop yield and health (Kaur & Kumar 2021).

The Piper diagram is a graphical tool used to classify and compare water types based on their ionic composition. It uses a trilinear plot where the X-axis shows the milliequivalent percentage difference between alkaline earth and alkali metals, and the Y-axis shows the difference between weak and strong acidic anions. The diagram categorizes water into eight types: calcium, no dominant, magnesium, sodium, potassium, bicarbonate, sulfate, and chloride. It comprises three parts: a ternary plot for cations (Mg, Na, K, Ca), a ternary plot for anions (CO₃-HCO₃, SO₄, Cl, NO₃), and a central diamond plot that integrates both (Piper 1944). The data plot on the Piper diagram shows that the majority of drain water samples are located in the center (zone B) of the cation facies, revealing that there is no dominance of any cations in the water samples. However, the anion facies suggest that most of the parameters are concentrated in the right (zone G), indicating the dominance of Cl in the PRM season (Figure 8(a)). Similarly, in the POM season, most of the samples are concentrated in the center (zone B), revealing that there is no dominance of any cations in the water samples, while the anion facies suggests that most of the parameters are concentrated in the right (zone G) as well as in the center (zone B), indicating no dominance of any anions but some samples are dominated by the Cl type (Figure 8(b)). Overall, the drain water samples are mixed, with mixed Ca–Mg during the PRM season and mixed Ca–Mg and CaHCO₃ during the POM season. CaHCO₃-type water, linked with low EC values, is probably caused by rainfall recharge processes (Singh et al. 2021) during the monsoon season.

In the peri-urban area of southwest Delhi, where groundwater resources are often affected by salinity issues, drain water serves as an alternative source of irrigation for farmers. This study comprehensively evaluated the suitability of drain water for irrigation purposes by assessing various physico-chemical parameters, irrigation water quality indices, and concentrations of toxic heavy metals during both PRM and POM seasons. The analysis of major cations revealed that sodium (Na⁺) was the most prevalent alkali metal, followed by calcium (Ca²⁺), magnesium (Mg²⁺), and potassium (K⁺). Among anions, the order of abundance was chloride (Cl⁻) > bicarbonate (⁠⁠) > sulfate (⁠⁠) > nitrate (⁠⁠) > fluoride (F⁻). The measured pH values indicated slightly acidic to moderately alkaline conditions in both seasons. The evaluation of irrigation water quality indices, such as SAR, Na%, RSC, KR, MH, and PI, revealed that the drain water was highly unsuitable for irrigation during the PRM season but marginally suitable during the POM season. This variation was attributed to the dilution effect of monsoon rainfall and changes in evaporation rates. The study employed hydrogeochemical techniques, including Gibbs plots and Piper diagrams to understand the mechanisms controlling water chemistry, rock–water interactions, and hydrochemical facies. The Gibbs plots indicated that the chemical composition of the water was influenced by both evaporation and rock weathering processes in both seasons. The Piper diagrams showed mixed types of drain water samples, with the PRM season exhibiting mixed Ca–Mg composition and the POM season displaying mixed Ca–Mg and CaHCO₃ types. The assessment of toxic heavy metals highlighted the presence of elevated concentrations in certain drain water samples, exceeding permissible limits for irrigation water. These findings underscore the potential risks associated with the use of untreated drain water for irrigation, including soil degradation, crop contamination, and potential health hazards, while the study provides valuable insights into the quality of drain water but did not directly assess the impact of using drain water for irrigation on soil quality, crop productivity, and potential accumulation of contaminants in agricultural produce. The findings of this study demonstrate the potential value of index-based irrigation water quality in decision-making procedures such as routine irrigation water monitoring, crop damage prevention, etc. This will aid farmers in creating an effective management strategy and utilizing groundwater sustainably for irrigation needs. This study underlines the importance of routinely evaluating water quality for irrigation so that the risk from anthropogenic and geochemical contaminants can be decreased by suitable treatment technology. Future research could incorporate such assessments to provide a more comprehensive understanding of the risks and benefits associated with the use of drain water for irrigation.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.