Abstract
To evaluate the nitrogen pollution load in an aquifer, a water and nitrogen balance analysis was conducted over a thirty-five year period at five yearly intervals. First, we established a two-horizon model comprising a channel/soil horizon, and an aquifer horizon, with exchange of water between the aquifer and river. The nitrogen balance was estimated from the product of nitrogen concentration and water flow obtained from the water balance analysis. The aquifer nitrogen balance results were as follows: 1) In the aquifer horizon, the total nitrogen pollution load potential (NPLP) peaked in the period 1981-1990 at 1800 t·yr-1; following this the NPLP rapidly decreased to about 600 t·yr-1 in the period 2006-2010. The largest NPLP input component of 1000 t·yr-1 in the period 1976-1990 was from farmland. Subsequently, farmland NPLP decreased to only 400 t·yr-1 between 2006 and 2010. The second largest input component, 600 t·yr-1, was effluent from wastewater treatment works (WWTWs) in the period 1986-1990; this also decreased markedly to about 100 t·yr-1 between 2006 and 2010; 2) The difference between input and output in the aquifer horizon, used as an index of groundwater pollution, peaked in the period 1986-1990 at about 1200 t·yr-1. This gradually decreased to about 200 t·yr-1 by 2006-2010. 3) The temporal change in NPLP coincided with the nitrogen concentration of the rivers in the study area. In addition, nitrogen concentrations in two test wells were 1.0 mg·l-1 at a depth of 150 m and only 0.25 mg·l-1 at 50 m, suggesting gradual percolation of the nitrogen polluted water deeper in the aquifer.
Highlights
People have a great concern about groundwater quality if they use the groundwater for municipal, industrial and agricultural purposes
The second largest input component, 600 t·yr−1, was effluent from wastewater treatment works (WWTWs) in the period 1986-1990; this decreased markedly to about 100 t·yr−1 between 2006 and 2010; 2) The difference between input and output in the aquifer horizon, used as an index of groundwater pollution, peaked in the period 1986-1990 at about 1200 t·yr−1. This gradually decreased to about 200 t·yr−1 by 2006-2010; 3) The temporal change in nitrogen pollution load potential (NPLP) coincided with the nitrogen concentration of the rivers in the study area
To conduct the water and nitrogen balance analysis, a two layer model was applied consisting of a soil/channel horizon and an aquifer horizon with exchange of water with the river
Summary
People have a great concern about groundwater quality if they use the groundwater for municipal, industrial and agricultural purposes. The availability of unpolluted water is a very important factor to their daily life. In the last few decades, the use of chemical fertilizer applied to farmland has markedly increased. Nitrogen from fertilizer not taken-up by crops percolates through the ground eventually reaching, and polluting, groundwater. The use of wastewater treatment works (WWTWs) has markedly grown in recent decades in Japan, and nitrogen pollution of groundwater has occurred because wastewater treatment was usually limited to secondary stage processes which do not remove nitrogen. Nitrogen, not removed by WWTWs flows into the environment eventually reaching and polluting groundwater
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