Integration of HYDRUS-1D and MODFLOW for evaluating the dynamics of salts and nitrogen in groundwater under long-term reclaimed water irrigation

Abstract

Reclaimed water has been extensively used as an alternative resource for irrigation, but can affect groundwater quality due to salt and nitrogen leaching. We conducted field investigations of a shallow groundwater monitoring well at the Research Center for Eco-Environmental Sciences, China, and irrigated sample sites of turf grass with reclaimed water for 8 years. The HYDRUS-1D and MODFLOW models were integrated to study the transport and distribution of electrical conductivity (ECgw) and nitrate–N (N–NO3−) in the shallow groundwater under long-term reclaimed water irrigation. Model calibration and validation showed that the integrated model could simulate the fates of ECgw and N–NO3− in the shallow groundwater. Field experiments and the model simulation showed that reclaimed water irrigation can increase salinity and N–NO3− concentration in shallow groundwater and predicted, assuming the continuation of current irrigation practices, that the annual average ECgw and N–NO3− would reach a steady level of 0.72 dS m−1 and 2.18 mg L−1, respectively. Because ECgw increased with increasing irrigation water salinity and amount, there is a risk of increased salinity in the shallow groundwater under long-term reclaimed water irrigation. Under all simulation scenarios, annual average N–NO3− concentrations in the shallow groundwater at an equilibrium state did not exceed the class II groundwater quality standard (2–5 mg L−1). After proper calibration and validation, the integration of HYDRUS-1D and MODFLOW models offers an effective tool for analyzing irrigation management of low-quality water in water-scarce regions.