A numerical model to simulate the interaction between the Las Vegas drain and the Aconcagua River for water supply of the city of Valparaiso, Chile

Abstract

Three-quarters of the potable water from the city of Valparaíso, Chile, is obtained from the Las Vegas drain. This drain is located in the aquifer of the Aconcagua River basin at approximately 70 km from the city.The Las Vegas drain has approximately 2,000 m lengths, an average diameter of 1.8 m, and it is buried at a depth of 40 m in a zone of the aquifer that has a strong interaction between surface waters and groundwater. Since its construction in 1960, the hydraulic characteristics of the drain have changed; forcing the water utility company to include additional potable water sources such as the injection of surface waters from a water treatment plant, the use of infiltration ponds for artificial aquifer recharge, and even using wells located upstream the Las Vegas drain in drought periods.Modflow 2000 numerical model was constructed with the aim of developing a tool to optimize the water resource management in the Las Vegas drain. The model has three layers with 325,500 cells of 50 x 50 m2. The recharges of the model are precipitation, irrigation, and infiltration from the Aconcagua River and the infiltration ponds. The discharges of the model are evapotranspiration, the pumping from 12 wells and the Las Vegas drain. To run the model, data from 64 years are available. These data comprises precipitation, streamflow, withdrawal from the wells and water flow at the exit of the Las Vegas drain.The analysis of some specific time periods allowed to establish a direct relationship between the water flow at the exit of the drain and the streamflow of the Aconcagua River, and to define the conductance of the river as a function of the streamflow. The initial conductance of the drain was modified to reflect its change in time. In a similar way, other time periods were chosen to calibrate the infiltration rates from the infiltration ponds. A good agreement between observed and calibrated flows in the drain was obtained in both the calibration and validation periods.The numerical model allows prediction of this complex system comprised of the drain, the river, the water treatment plant and the wells, in a zone with important interactions between groundwater and surface waters at hourly temporal scales. This model is an excellent tool that supports decisions to optimize water management in periods of drought and flood.