Long-Term Assessment of Nitrogen Pollution Load Potential for Groundwater by Mass Balance Analysis in the Tedori River Alluvial Fan Area, Japan

Abstract

<p align="justify"> <span style="font-family:Verdana;font-size:12px;">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</span><span style="font-family:Verdana;font-size:12px;">-</span><span style="font-family:"Times New Roman","serif";font-size:10pt;"><span style="font-family:Verdana;font-size:12px;">1990 at 1800 t·yr^-1</span><span>; <span style="font-family:Verdana;">following this the NPLP rapidly decreased to about 600 t·yr</span>^{<span style="font-family:Verdana;">-1</span>}</span><span style="font-family:;"><span style="font-family:Verdana;"> in</span> the </span><span style="font-family:Verdana;font-size:12px;">period 2006</span></span><span style="font-family:Verdana;font-size:12px;">-</span><span style="font-family:"Times New Roman","serif";font-size:10pt;"><span style="font-family:Verdana;font-size:12px;">2010. The largest NPLP input component of 1000 t·yr^-1</span><span style="font-family:Verdana;font-size:12px;"> in the period 1976</span></span><span style="font-family:Verdana;font-size:12px;">-</span><span style="font-family:"Times New Roman","serif";font-size:10pt;"><span style="font-family:Verdana;font-size:12px;">1990 was from farmland. Subsequently, farmland NPLP decreased to only 400 t·yr^-1</span><span style="font-family:Verdana;font-size:12px;"> between 2006 and 2010. The second largest input component, 600 t·yr^-1</span><span style="font-family:Verdana;font-size:12px;">, was effluent from wastewater treatment works (WWTWs) in the period 1986</span></span><span style="font-family:Verdana;font-size:12px;">-</span><span style="font-family:"Times New Roman","serif";font-size:10pt;"><span style="font-family:Verdana;font-size:12px;">1990; this also decreased markedly to about 100 t·yr^-1</span><span style="font-family:Verdana;font-size:12px;"> between 2006 and 2010</span></span><span style="font-family:Verdana;font-size:12px;">;</span><span style="font-family:Verdana;font-size:12px;"> 2) The difference between input and output in the aquifer horizon, used as an index of groundwater pollution, peaked in the period 1986</span><span style="font-family:Verdana;font-size:12px;">-</span><span style="font-family:"Times New Roman","serif";font-size:10pt;"><span style="font-family:Verdana;font-size:12px;">1990 at about 1200 t·yr^-1</span><span style="font-family:Verdana;font-size:12px;">. This gradually decreased to about 200 t·yr^-1</span><span style="font-family:Verdana;font-s...