Investigation of River‐aquifer Interactions in Bojnourd Plain Using Reach Measurements and Numerical Modeling

Understanding water budget components is crucial for making decisions regarding water resources planning and management. Surface water–groundwater interactions are commonly investigated at the river reach scale and generally classified as connected or disconnected type systems. Connected systems are either gaining surface water system, where groundwater discharges through the streambed to contribute to streamflow, or losing surface water system which loses (or recharges) water to the local groundwater system. Disconnected systems are defined by an unsaturated zone beneath the surface water system which loses water at a rate related to the hydrogeological properties of the streambed and the aquifer. These interactions have significant implications for both water quantity and quality. Seepage of fresh groundwater into a river can be important in maintaining flows during extended dry periods. This can be critical for supplying the needs of surface water users such as irrigators as well as for aquatic ecosystems. Pumping from an aquifer near a river can dramatically change the amount of this base-flow to the river. In contrast, if the groundwater is contaminated, increased groundwater discharge can have a negative effect on river water quality. The Bojnourd catchment is located in North Khorasan province. The catchment covers an area of about 1265.8 km2. The main river in this area, Firouze River, is approximately 10 km in length, and is hydraulically connected to the Bojnourd alluvial aquifer. The alluvial aquifer of Bojnourd plain with 65.2 km2 area is mostly covered by urban area. Hence, effective management of water quantity and quality issues in the Bojnourd catchment requires quantifying flow between surface water and groundwater. Furthermore, conveying water from the outside of basin caused water table to rise which made some problems for urban buildings and infrastructures. Therefore, the river and aquifer interaction needs to be studied more comprehensively.

Numerous techniques and methods are available to describe and quantify the flow between surface water and ground water. This study combined two methods, numerical modeling using MODFLOW code and reach measurements, to quantitatively evaluate groundwater/surface water interactions under highly transient conditions. The groundwater flow system of the study area was conceptualized based on borehole logs, pumping tests, and available hydrogeological and geophysical information. Moreover, field work, including measuring streamflow in three seasons, was carried out to conceptualize and quantify the groundwater/surface water interactions. Following the conceptual model, the numerical model was developed to simulate flow through the system. The model grid had 1274 active cells with a uniform cell spacing of 250×250 m. The water exchanges between the main regional river, Firouze river, and Bojnourd aquifer was simulated using the River (RIV) package. Both hydraulic head target and flux target were used to calibrate the model. The head targets were compiled from the monitoring network which contains 11 observation wells. The flux targets were located in three measurement points along the Firouze river. The data obtained from the fieldwork were used as observed values for the groundwater/surface-water exchanges. The transient model was calibrated and validated for 15 hydrological years, i.e. from 1 October 2001 to 1 October 2016

Model performance was evaluated using root-mean-square error (RMSE). The model results were in agreement with corresponding observed data, including groundwater heads and measured groundwater/surface-water exchanges. The RMSE values during calibration and validation periods were 0.83 m and 1 m, respectively. Analyzing water balances resulted from transient simulation showed that Firouze river is gaining in some reaches and losing in other reaches. In losing reaches, the total flux into the aquifer is 6.4 MCM per year. In gaining reaches, the volume of groundwater discharges through the streambed is about 4 MCM per year. Furthermore, the effect of several management scenarios, including continuing the existing condition, turning the domestic wells off and implementing a sewage system by 2025, on groundwater heads and groundwater/surface-water exchanges was examined using the numerical model. Results showed that by implementing the sewage system, the volume of water discharged to the river would decrease, but it will prevent aquifer and river contamination caused by sewage water. 

In this study, groundwater budget components in Bojnourd aquifer including groundwater/surface-water exchanges were calculated. The results showed that understanding of these surface water-groundwater interactions, which has been ignored in previous studies, is important for effective management of water quantity and quality issues in Bojnourd plain. Moreover, the methodology used in this study including numerical modeling and measuring flow at multiple points along the stream is effective and easy to apply to estimate the direction and magnitude of seepage on a stream reach basis.