Application of SWAT and MODFLOW models to simulate groundwater flow in a coastal multi-aquifer system in Puck region (northern Poland)

There is an increasing need to develop modeling tools capable of simulating water flow and pollutant transport across different environmental compartments, such as inland surface waters, groundwater and seawater. Such an integrated modeling approach is proposed in the framework of the project "Modelling of the impact of the agricultural holdings and land-use structure on the quality of inland and coastal waters of the Baltic Sea set up on the example of the Municipality of Puck region – Integrated info-prediction Web Service WaterPUCK", funded by National Centre for Research and Development (NCBR), Poland, (BIOSTRATEG3/343927/3/NCBR/2017). The project focuses on water flow and transport of nutrients and pesticides in Puck Bay (part of Baltic Sea, separated from open sea by Hel Peninsula), as well as in the adjacent coastal watershed, which contributes surface and subsurface discharge to the sea. The considered land area is about 300 km2. The geology is typical for young glacial areas, with multiple sand aquifers separated by weakly permeable glacial tills. Groundwater flow is directed from moraine uplands towards the sea, and focuses in deeply cut ice marginal valleys, where the sand aquifers are partly confined by layers of peat. Most of the area is used for agriculture and forestry. We applied SWAT model to represent surface and soil hydrological processes and MODFLOW to represent groundwater flow. The SWAT model includes 17 subbasins and 353 hydrological response units (HRU). Due to complex geological structure, the MODFLOW model has relatively detailed discretization, with 6 layers and more than 700 000 cells. The preliminary calibration of the groundwater model was done for steady state conditions. The values of recharge were taken as average results from SWAT simulations for each HRU, corresponding to the period 2000-2015. The recharge rates calculated by SWAT showed a reasonable agreement with the values estimated by a simplified method, based on multiplying the rate of precipitation by correction factors representing influence of soil type, land use, slope of the ground surface and depth to groundwater table. The groundwater model allowed to estimate the rate of submarine groundwater discharge to Puck Bay. As the next step, the SWAT and MODFLOW models will be coupled using the recently developed SWAT-MODFLOW-RT3D computer code and applied to transient simulations of surface and subsurface flow and contaminant transport.