evaluation of groundwater recharge-renewability by analysis of hydrological processes, water mass balance, and using tracers(case study: hashtgerd watershed)

this paper has focused on the application of hydro- geochemical techniques in conceptualizing of groundwater and to estimate recharge/discharge and the renewability of aquifer in hashtgerd watershed, iran. at first, the aquifer boundary was defined based on data of existing wells and basin morphologic conditions. estimation of recharge rate at different depth intervals by using tracer mass balance was done for 291 representative groundwater samples which includes 195 samples from soil and water of unsaturated zone in depths of 30 to 90 m (sampling in each meter), 10 samples were taken from rainfall, five samples from qanats and 21 samples from wells and 60 samples taken from surface water in the study area. based on water balance technique applying to a 7 year measured data and evaluation of hydrological and hydro- geological processes in the watershed, average groundwater depletion was calculated at 0.37 m/year that indicated annual recharge rate of 245 million cubic meter (mcm) and over use of storage capacity of the aquifer. the groundwater studies showed that the natural recharge from direct rainfall is about 2% of total recharge. the annual recharge rate over an area of 752 km2 varies from 2.3 to 15.1 mm/yr. the required time for aquifer recharge from rainfall for the aquifer of 30 m thickness in the southern part of the aquifer is 1200 years while for aquifer of 90 m thickness in the north, recharge time is around 1100 years. on the other hand, in north and south of the aquifer, transitivity rates are 2500 and 300 m2/day, respectively with the groundwater renewability of 2.92 years which shows the importance of renewability of groundwater via concentrated/linear recharge from fractured zones. studies of direct recharge in other parts of the world have shown that direct recharge rates may vary from zero to around 40% of mean rainfall volume. the results of this study revealed that around 98% of aquifer recharge occurs via concentrated/linear recharge from fractured zones with the average renewability of three years. then it is recommended to consider the above mentioned geologic features in concentrated recharge of aquifer in saturated zone by watershed management, floodwater spreading and vegetation cover improvement projects.