Introduction
In general, rivers have a hydraulic connection with aquifers and they affect each other quantitatively and qualitatively. River water percolates into the underground along its path and after passing through the subsurface layers, it feeds the aquifers. During the infiltration of water inside the layers, the concentration of contaminants in river water is reduced as a result of river bank filtration or because of some processes in the porous media. This makes the quality of the passing water inside the subsurface layers much better compared to the quality of the river water before infiltration and purification.
In the east of Golestan Province and in the vicinity of the Doogh River, one of the main branches of the Gorganrud River, two Felmann wells have been drilled. The discharge of each Felmann Kalaleh wells is about 200 liters per second, which is equivalent to 5 deep wells drilled for drinking water in the Gorgan Plain. The water of these two Felmann wells is used to supply almost 20% of the water of the urban population of Gonbad Kavous. In addition, using the Felmann well method plays an important role in reducing water treatment costs (especially physical treatment). Therefore, the mentioned method can be of interest to water and sewage companies and water affairs located in the northern provinces of Iran
Previous studies showed that Felmann Kalaleh wells have a good efficiency in improving the water quality of the Doogh River. However, the river discharge effect variations on the amount of water parameters extracted from Felmann Kalaleh wells is still unknown and needs to be investigated. Therefore, the investigation of the factors affecting the quality of the river water feeding the wells of Felmann Kalaleh, as well as its effect on the quality parameters of the wells, is important for the optimal and appropriate use of their water for drinking water. Hence, the current research is aimed to investigate the role of Doogh River discharge fluctuations and its effects on the water quality parameters of the river and Felmann well.

Materials and methods
Doogh River is one of the important sub-basins of Gorganroud River and it drains parts of the three provinces of Semnan, North Khorasan, and Golestan. A part of the Doogh River catchment located in Golestan province has many differences with other parts of the basin in Semnan and North Khorasan provinces in terms of climatic conditions, vegetation, geology, physiography, and geomorphology. It plays a very important role in the agriculture of the region in addition to controlling the floods in the east of Golestan Province. Therefore, the quantitative and qualitative study of Doogh River water is very important in terms of agriculture and drinking.
In general, there are two Felmann wells in Kalaleh City, which play an important role in supplying drinking water to Gonbad Kavous City. These two wells were drilled by the water and sewerage company of the province in 2013 in the vicinity of the Doogh River (one of the main tributaries of the Gorganrud River) and in the vicinity of the two villages of Ajen and Qarakhoja. The approximate depth of these two wells is about 20 meters, the diameter of their main opening is about 3 meters, at the end of which 12 radial galleries with an approximate length of 30 meters are drilled, which bring water into the water supply network of Gonbad Kavous City with a flow rate of about 200 liters per second.
In this research, for the physicochemical and microbial analysis of well and river waters, samples were taken simultaneously from Doogh River and Felmann Kalaleh wells following the existing standard methods. For this purpose, 10 samples of river water and 10 samples of each of Felmann Kalaleh wells were collected at the same time in a period of approximately five months from October 2019 to February 2019. Also, the discharge values of Doogh River were collected from the hydrometric station in the vicinity of Felmann wells.
Correlation test was used at the probability level of 0.05 to investigate the relationship between discharge fluctuations with the physicochemical characteristics of Felmann wells and also the relationship between river discharge fluctuations with the river's physicochemical characteristics. All statistical tests were performed in MINITAB 17 software.

Conclusion
Results revealed that most of the values related to chemical parameters such TDS, sodium, calcium, etc. are within the permissible limit for drinking water according to the 1053 standard of Iran and World Health Organization (WHO). However, the amount of turbidity and total coliform is higher than the permissible limit and needs to be purified for use in the drinking sector.
Based on the p-value of the paired T-test for only 6 factors (including TDS, fluoride, sodium, chloride, nitrite, and manganese) out of 24 qualitative factors between the physicochemical characteristics of river water and Felmann 1 and 2 wells that were collected simultaneously and, no significant difference was observed. But this difference is evident in 18 parameters. This indicates that the riverbank filtration as a result of processes such as surface absorption, aerobic, and anaerobic decomposition by microorganisms and filtration of sediments was not effective on the values of TDS, fluoride, sodium, chloride, nitrite, and manganese parameters.
Based on the results of the removal efficiency by riverbank filtration, most of the removal rate due to riverbank filtration is related to turbidity, total coliform, and color, respectively, at the rate of 97.52, 96.91 and 79.17 for well number 1. However, similar numbers were obtained for well No.2. Also, the results revealed that during the riverbank filtration, the amount of some parameters such as calcium, total hardness, etc. increased in Felmann well due to the change in water chemistry and interactions taking place in the active zone created in the river bed.
Results revealed that riverbank filtration played an important role in reducing some quality parameters, especially total coliform, turbidity, and color. But this amount has decreased significantly in nitrate. While some parameters such as bicarbonate and electrical conductivity not only did not decrease much, but also slightly increased. This can be a result of change in water chemistry and the environment during the filtration process, as well as the change in pressure. Also, the mixing of water obtained from filtration with groundwater water can affect the concentration of these parameters.
Investigation of the relationship between river water fluctuations and the physicochemical and microbial characteristics of river showed that 13 of 24 factors investigated include turbidity, color, TDC, total hardness, calcium hardness, chloride, bicarbonate, nitrate, calcium, magnesium, sodium, potassium, and total coliform. There is a significant relationship between the values of these 13 qualitative factors and the Felmann well, and for 11 qualitative factors, no significant relationship was observed between discharge fluctuations and the quality characteristics of river water.
As expected, there is a direct, linear, and positive relationship between the changes in Doogh river discharge with turbidity, color, sodium, and electrical conductivity. This relationship indicates the existence of geological formations such as marl and clays containing evaporite minerals in a large area of sub-basins of this river in North Khorsan and Semnan Provinces, which the intensification of rainfall and the subsequent increase in the discharge of the Doogh River cause the dissolution of this formations in water, and consequently, increase in the amount of dissolved and suspended solids in Doogh River water.
Contrary to the mentioned parameters, the changes between Doogh River discharge and total coliform of whole relationship are direct, linear but negative. So, with the increase in flow rate, the amount of total coliform of Doogh River decreases significantly.
The results of the correlation test, to investigate the significant relationship between the river discharge and the physicochemical characteristics of Felmann well, revealed that the value of 12 factors out of 24 factors investigated in Felmann number 1 including turbidity, color, EC, TDC, total hardness, calcium hardness, ammonia, fluoride, calcium, magnesium, COD, and total coliform are dependent on river discharge fluctuations. For 12 other qualitative factors, including nitrate, nitrite, pH, sodium, potassium, etc., no significant relationship between discharge fluctuations and qualitative characteristics was observed for this well.
The results of the above test for well number 2 showed that only in 8 of the 24 investigated factors, including turbidity, color, total hardness, calcium hardness, calcium, magnesium, COD, and total coliform, a significant relationship between these parameters and discharge fluctuations was observed. In Felmann number 2, for 16 other factors, no significant relationship between flow fluctuations and qualitative characteristics was observed.
Based on Durov Diagram, there is no specific geochemical evolution direction for water resources, and the distribution of samples related to Doogh River and Felmann wells is at the same location. Water type of samples is bicarbonate type in the recharge areas (margins of the heights). Based on the rectangle related to pH, the value of this parameter in Doogh River (7.75-7.85) is slightly higher than in Felmann wells (>7.55). It seems that after the river bank filtration, the value of this parameter decreases due to the processes that occur in the active zone.