Received: 02 January 2019 Accepted: 12 February 2019 Published: 01 March 2019
Subterranean is the most important method for exploiting groundwater in arid and semi-arid regions. There are 26 subterranean strings in Gonabad plain which 19.5 million cubic meters of underground water are drained from them annually. Low rainfall causes to the absence of permanent river in this districts, and the only living way in this arid area is the use of underground water by the subterranean. Some of the subterranean of this plain date back to 2700 years ago. Currently, subterranean water is used for agriculture, but the subterranean are still used as drinking water by direct harvesting from the subterranean crater. From the past to now, the water of Khoshvyy subterranean is also stored in water cisterns and used as drinking water. Because of very rarely amount of nitrogen compound in geology formations, the level of nitrate in underground water is normally low so that, except in pollution situation, the concentration of nitrogen ion does not exceed to 20 mg/l. The presence of nitrate in groundwater can indicate contamination of water due to contact with polluting sources such as municipal sewage, rural areas and landfills or irrigated agriculture. Due to increased urban activity and the production of more sewage, pollution of nitrate water resources is increasing. In this research, nitrate and nitrite levels of six important subterranean strings in Gonabad plain have been studied. The results show that Nitrogen content of groundwater in six subterranean strings is less than the World Health Organization Standard (50 mg/l). The highest amounts of nitrogen are 20 and 23 mg/l in the Rahn and Bidokht subterranean that both of them pass through the residential context due to leakage of domestic wastewater. High levels of nitrate (20 mg/l) in these two subterranean is indicator of beginning of contamination. The lowest levels of nitrate are in the Khoshvyy subterranean around about 10 mg/l. Nitrite content of subterranean water have also been analyzed which all of them were less than the permitted levels of the World Health Organization (WHO) (3 mg/l).