Assessment of nitrate and nitrite levels in treated wastewater, soil, and vegetable crops at the upper reach of Zarqa River in Jordan.

The purpose of this study is to examine the impact of treated wastewater irrigation on agricultural soils and vegetables along the upper reach of Zarqa River (Jordan). Multiple samples of reclaimed wastewater, soil pits from farms, and vegetables (spinach, parsley, cabbage cauliflower, radish, and onion) were collected and analyzed for pH, total dissolved solids (TDS), and/or NO3- and NO2- contents. The average levels of NO3- and NO2- in treated wastewater samples varied from 167.2 to 209.9 mg/l for NO3- and from 80.3 to 106.1 μg/l for NO2-. Values of TDS and pH exhibited relatively comparable spatial patterns, with higher values in the downstream channel and lower in the upper reach, adjacent to Al-Samra Wastewater Treatment Plant. The average values of NO3- and NO2- showed broadly decreasing trends down-gradient towards King Talal Dam. In soil pits, a marginal increase of pH values with depth was noted, whereas TDS showed a remarkable decrease in soil profile by ~ 2 to 3 folds. Concurrently, the levels of NO3- and NO2- in all soil pits markedly decreased from top to bottom. In vegetables irrigated with treated wastewater, substantially elevated levels of NO3- were observed, compared with those irrigated with rainwater, with leafy vegetables demonstrating higher levels than the root crops. Spinach exhibited higher capacity for NO3- accumulation (4614.1 mg/kg), while onion showed the lowest content (1722 mg/kg). The highest NO2- level was observed in parsley (1.19 mg/kg), and the lowest concentrations were found in cauliflower (0.25 mg/kg). The markedly high accumulation of NO3- in vegetables is an indicator of pollution activities around Zarqa River and poses potential health risks to humans when they are consumed.