Impact of wastewater irrigation on groundwater in the Lahore region and contamination source identification

Organic pollutants in groundwater samples were analyzed for the very densely populated ‘Lahore’ city and its surrounding ‘Mangamandi’ areas of Punjab-Pakistan, where composite (industrial and urban) wastewater isused forcultivationof crops andvegetables. The sampleswerepre-concentratedusing the Solid Phase Extraction (SPE) technique and analyzed by using High Pressure Liquid Chromatography (HPLC) and a Gas Chromatograph Mass Spectrometer (GC-MS). Organic contaminants like Dichlorodiphenyltrichloroethane (DDT), Dichlorodiphenyldichloroethylene (DDE), Dichlorophenol (DCP), Trichlorophenol (TCP), endrin, and dieldrin were found in certain samples above the permissible limits. Stable isotopes like C, N and Owere analyzed to assess the source of shallowgroundwater channel contamination. Chemical and isotopic data reveal that contamination of shallow groundwater channels in the area is mainly due to wastewater irrigation and, to some extent, by seepage through unlined wastewater drains in nearby areas. The wastewater containing organic pollutants, used for cultivation, filters through the soil to contaminate the shallow groundwater channels.


INTRODUCTION
Groundwater is a significant source of water for many municipal water systems (Bralower & Bice ). In urban areas, people mainly rely upon municipal water systems (Rail ). However, withdrawing water from tube wells and hand pumps is also common practice. In these areas, unnecessary pumping by tube wells and recharge by drain Quality parameters like clarity, odor, and color were recorded in the field. All the samples were colorless and odorless as they were being used for drinking purposes.
Physicochemical parameters such as the pH, electrical conductivity (EC) and total dissolved solids (TDS) of samples were measured in situ. Dilute HNO 3 was added to each sample until the pH was <2 for major cations, then the sample bottles were stored at about 4 C. Measurement of pH was done using a digital pH meter (Adwa, Model

Analysis and measurement
Organic compounds were analyzed using HPLC-UV     (1): The moisture produced during the reaction was removed by a cryogenic trap of À80 C. The CO 2 gas was solidified in a liquid nitrogen cryogenic trap. Other undesired gases were evacuated to get pure CO 2 . The pure CO 2 was collected in an ampoule for 13 C analysis on a Varian Mat GD-150 Mass Spectrometer. The isotope ratio (δ‰) of 15 N, 13 C and 18 O were calculated by using the following relation: The overall analytical errors ±0.01‰ (δ 13 C & δ 18 O) and ±0.1‰ (δ 14 N) were recorded for measurements. To ensure precision, standard deviation of the mass spectrometer results were computed and standard deviation of each sample was ensured to be within permissible limits.

Physico-chemical parameters
In Lahore TDS samples were found between 939 to 1,530 mg/ L and EC varied between 1,439 and 2,054 μS/cm.
In Mangamandi, TDS values ranged between 1,001 to 3,313 mg/L, whereas EC values were observed between 1,662 to 5,052 μS/cm (above the permissible limits), as shown in Table 1. Groundwater with a TDS above 500 mg/L and EC >2.25 to 4 mS/cm is considered not safe for consumption (Harter ).

Analysis of organic compounds in Lahore groundwater
The concentrations of organic pollutants in the Lahore area are shown in Table 2      the shallow groundwater channel is more vulnerable to contamination.
Endrin and dieldrin were found in samples LGW-3 and LGW-4 which were collected from an agricultural area near the wastewater drain (upstream), whereas sample LGW-5, collected from an agricultural area at the wastewater drain In sample LGW-6, collected from an industrial area, only endrin and dieldrin were recorded, whereas more organic pollutants were found in LGW-7, LGW-8 and LGW-9 as these samples were collected from an agricultural area along the wastewater drain. The results suggested that the shallow groundwater channel in agricultural areas is being more affected by organic contamination, as compared to industrial areas, due to wastewater irrigation.
Overall endrin and dieldrin were found in nine samples, δ 13 C values in groundwater samples The Lahore groundwater samples showed δ 13 C values ranged from À0.25 to À6.41‰ PDB, as shown in Table 3. In the Lahore samples, LGW-1 was collected from a deep borehole and its value was À6.35‰. The δ 13 C depleted values shows that deep water has wastewater mixing.
LGW-2 was collected from shallow water and it showed less depleted δ 13 C values, indicating that this point has almost no wastewater mixing.
LGW-3 was collected from a borehole and showed slightly depleted values which suggests that this water is slightly affected by wastewater.
LGW-4 (deep channel) had minor depleted δ 13 C‰ values, however  showed depleted δ 13 C‰ values. This suggested that shallow groundwater channels have more wastewater mixing.
LGW-6 (shallow channel) had ignorable depleted values which might be due to sufficient aeration by plant root respiration or oxidation conditions. LGW-7 and LGW-8 were collected from shallow water and depleted δ 13 C‰ values suggest wastewater mixing.
LGW-9 was collected from a hand pump (shallow water) and its more depleted δ 13 C‰ value proves more wastewater mixing.
In Mangamandi samples, MGW-1 and MGW-2 were collected from deep water (a borehole) and showed depleted δ 13 C‰ values which suggest that these groundwater samples have wastewater mixing. MGW-3 was collected from a shallow wastewater channel and had negligible depleted δ 13 C ‰ values due to plant root respiration or prevailing oxidation conditions. MGW-4 and MGW-5 have higher depleted δ 13 C‰ which suggests higher wastewater mixing.
MGW-6 (deep channel) showed less depleted δ 13 C‰ values due to less wastewater mixing. MGW-7 and MGW-8 were collected from shallow water and they have depleted δ 13 C‰ values, which points towards wastewater mixing.

Source of contamination in groundwater
The source of pollution in the groundwater was assessed by using stable isotope tools. Ratios of δ 18 O and δ 15 N were applied to determine the source of pollutants as shown in Figure 5 and Table 3. ii. Shallow groundwater channels in the Lahore and Mangamndi areas are being contaminated mainly due to wastewater irrigation and, to some extent, through seepage from wastewater drains passing through the area.
iii. The ratios of δ 18 O and δ 15 N of nitrates in water samples suggested that the nitrates in groundwater are mainly coming from fertilizers being used in fields.
iv. The depleted δ 13 C‰ value in some areas in the vicinity of wastewater drains suggested that wastewater drains, to some extent, are contributing to contaminate the shallow groundwater channels.
v. The shallow groundwater channels in the areas of Lahore and Mangamandi are under high risk due to infiltration caused by wastewater irrigation and seepage of unlined wastewater drains flowing through the areas.
vi. Wastewater irrigation in the Lahore and Mangamandi areas should be banned and wastewater drains should be lined to conserve the quality of groundwater channels.

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