Estimation the annual recharge of Gonabad Plain qanats using isotopic methods

Gonabad Plain is located in arid and semi-arid region. Qanat is the most important method for the exploitation of groundwater in this region. In Gonabad Plain, 19.5 million cubic meters of groundwater is annually extracted from 26 qanats whose flow rates vary from 10 to 120 lit/s. in this research, six major qanats of Gonabad Plain were sampled for isotopic analyses to determine the annual recharge of the groundwater of these qanats. The deuterium (D) and oxygen (18O) isotopes signatures demonstrate the origin of the groundwater of these qanats is infiltration of precipitation through fractures in the limestone and sandstone formations at the south of the basin and infiltration of surface water in the coarse grain alluvial cones at the south border of the plain. Isotopic gradient line of basin shows the groundwater recharge of these qanats originates from an area between 2000 to 2700 m a.s.l. Discharge amount and isotopic content of the groundwater of these qanats in wet and dry periods and isotopic composition of annual precipitation indicates approximately 28 to 39 percent of the qanats discharge is related to the precipitation of the same year.