Use of 18O, 2H and 15N to identify nitrate contamination of groundwater in a wastewater irrigated field near the city of Shijiazhuang, China

Wastewater irrigation was initiated as early as 1958 in the suburb of Shijiazhuang, China. This has resulted in groundwater becoming contaminated by nitrate to a depth of about 100 m with an average concentration of 40 mg NO3−/L. The annual average irrigation rate of wastewater in this area is about 430 mm, and the water table is relatively stable in contrast to the surrounding area, where no wastewater has been used. Although different in composition, wastewater and old groundwater are found to be well mixed based on the linearity of δ18O and Cl−. Wastewater has relatively high Cl−, but has no detectable nitrate. Dominance of δ15N in the range of 7–11‰ AIR (referred to atmospheric N2 standard) confirms that the nitrate present in contaminated groundwater originates from the wastewater. Nitrate was then used as a tracer to estimate roughly groundwater movement rates, yielding a vertical rate of 2.0–2.5 m/y and a horizontal rate of 41.8–62.5 m/y. Denitrification could occur in a zone 2 km from the wastewater canal, in which the fraction contributed from wastewater was calculated to be 76%. Health concerns should be raised regarding wastewater irrigation, which is a common practice in the suburbs of principal cities of China, especially in arid and semi-arid regions.