Missing role of groundwater in water and nutrient cycles in the shallow eutrophic lake Kasumigaura,Japan

A process‐based model was developed, NICE‐LAKE (NIES (National Institute for Environmental Studies) Integrated Catchment‐based Ecohydrology), which includes interactions between surface water, canopy, unsaturated water, aquifer, lake and rivers, and used it to model the shallow eutrophic Lake Kasumigaura in Japan. By estimating the spatial distribution of the hydrological cycle, the model shows that groundwater withdrawal greatly affects groundwater distribution and seepage and indirectly influences lake water level. The simulated seepage agrees excellently with the budget‐derived value calculated from the observed groundwater level, lake level and isotope analyses. The model showed that groundwater seepage and groundwater concentrations are important contributors to the eutrophication of Lake Kasumigaura, an important contribution not recognized in previous studies of the lake. Groundwater entering the lake from the north side is contaminated with high concentrations of nitrate and ammonia from intensive pig and cattle raising and cultivated fields. The simulation showed that this high nitrogen load plays an important role in the eutrophication of the lake (the nitrogen load in inflowing groundwater is 30% of river inflow and 4 times that from wastewater treatment plants) in spite of government policies to prevent overland flow of nutrients into the lake. Our results show that NICE‐LAKE is a powerful tool for forecasting how the water quality of the lake will be affected by the (illegal) disposal of animal excreta in the surrounding open fields. Copyright © 2007 John Wiley & Sons, Ltd.