PAPER PODMT3DMS-Tool: proper orthogonal decomposition linked to the MT3DMS model for nitrate simulation in aquifers Roohollah Noori 1 & Farhad Hooshyaripor 2 & Saman Javadi 3 & Mehrnaz Dodangeh 1 & Fuqiang Tian 4 & Jan Franklin Adamowski 5 & Ronny Berndtsson 6 & Akbar Baghvand 1 & Björn Klöve 7 Received: 26 March 2019 /Accepted: 6 January 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract The PODMT3DMS-Tool, which consists of proper orthogonal decomposition (POD) linked to the Modular Transport 3- Dimensional Multi Species (MT3DMS) code for nitrate simulation in groundwater, is introduced. POD, as a statistical technique, reduces a large amount of information produced by the MT3DMS model to provide the main components of the PODMT3DMS- Tool, i.e., space- and time-dependent terms of nitrate. The low-dimensional components represent time- and space-dependent factors in the aquifer response such as hydraulic, hydrogeological and water quality variables represented in the simulation using the MT3DMS model. The PODMT3DMS-Tool is thus a combined statistical and conceptual model with a simple structure and comparable accuracy to MT3DMS. Practical application of the PODMT3DMS-Tool to the Karaj Aquifer in Iran for a period of 6 years revealed agreement between nitrate concentrations simulated by the PODMT3DMS-Tool and MT3DMS, with a mean absolute error of less than 0.5 mg/L in most parts of the aquifer. Moreover, the PODMT3DMS-Tool needed only about 10% of the calculation time required by MT3DMS. The PODMT3DMS-Tool can be used to predict nitrate concentration in the Karaj Aquifer, while its simplicity also makes it potentially useful for other water resources problems. Keywords Nitrate . MODFLOW . MT3DMS . Proper orthogonal decomposition (POD) . Iran Introduction High dependency on groundwater resources and excessive water withdrawal from aquifers have led to significant de- creases in groundwater levels globally. In addition, agricultur- al, industrial and urban activities have resulted in the spread of pollutants to aquifers, resulting in groundwater quality deteri- oration. Specifically, nitrate is a common contaminant in most aquifers, which mainly stems from anthropogenic activities (Canter 1996; Stadler et al. 2008; Nakagawa et al. 2017). Nitrate can be converted to nitrite, N-nitrosamides and N- nitrosamines in the human digestive system, which can result Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10040-020-02114-0) contains supplementary material, which is available to authorized users. * Roohollah Noori noor@ut.ac.ir 1 School of Environment, College of Engineering, University of Tehran, Tehran 1417853111, Iran 2 Department of Civil Engineering, Science and Research Branch, Islamic Azad University, Tehran 1477893855, Iran 3 Department of Irrigation and Drainage, Aburaihan Campus, University of Tehran, Tehran 3391653755, Iran 4 Department of Hydraulic Engineering, State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China 5 Department of Bioresource Engineering, Faculty of Agricultural and Environmental Sciences, McGill University, Montreal H9X 3V9, Canada 6 Department of Water Resources Engineering & Center for Middle Eastern Studies, Lund University, Box 118, SE-221 00 Lund, Sweden 7 Water Resources and Environmental Engineering Research Unit, Faculty of Technology, University of Oulu, PO Box 4300, 90014 Oulu, Finland https://doi.org/10.1007/s10040-020-02114-0 Hydrogeology Journal (2020) 28:1125–1142 21 January 2020 /Published online: