Risk assessment of nitrate pollution in groundwater based on the environmental nitrogen-assimilation capacity of agricultural lands in Hokkaido [Japan]

A trial calculation was performed of the environmental nitrogen-assimilation capacity and the amount of nitrogen input based on various statistical data, which were compiled from each city, town and village in Hokkaido prefecture. The relationship between the excess quantity of nitrogen, after nitrogen input, and the environmental nitrogen-assimilation capacity and the nitrate-nitrogen concentration of the groundwater was considered. Environmental nitrogen-assimilation capacity = nitrogen output by the crops + acceptable level of residual nitrate in the soil profile. It is calculated by the amount of nitrate (precipitation-evapotranspiration) x 10 (mg L-1). 1) The average value of the environmental nitrogen-assimilation capacity in Hokkaido Prefecture was observed to be 183 kg ha-1. The maximum and minimum values of the environmental nitrogen-assimilation capacity were 308 kg ha-1 and 94 kg ha-1, respectively. When the average value of the environmental nitrogen-assimilation capacity with respect to main agricultural land use was compared across municipalities, it was largely in the following order: grassland (218 kg ha-1), upland (169 kg ha-1, and paddy land (157 kg ha-1). 2) The average value of the amount of nitrogen input in Hokkaido Prefecture was 185 kg ha-1. This value was largely in the following order: grassland (212 kg ha-1), upland (186 kg ha-1), and paddy land (151 kg ha-1). 3) The average value of the excess quantity of nitrogen, i.e., the difference between the nitrogen input and the environmental nitrogen-assimilation capacity, in Hokkaido Prefecture was 2 kg ha-1. This value was largely in the following order: upland (18 kg ha-1), paddy land (-6 kg ha-1), and grassland (-6 kg ha-1). 4) The average value of the nitrate-nitrogen concentration investigated in groundwater was 4.0 mg L-1 at a depth of 20 m or less. This value was largely in the following order: upland (6.5 mg L-1), grassland (2.5 mg L-1), and paddy land (2.4 mg L-1). 5) Significant positive correlation was obtained between the difference between the nitrogen input and the environmental nitrogen-assimilation capacity and nitrate-nitrogen concentration of groundwater. A case where the nitrate-nitrogen concentration of groundwater exceeded the environmental standards of water quality (10 mg L-1) was in one part of encountered municipality in Hokkaido Prefecture; in such a case, the nitrogen input exceeds the environmental nitrogen-assimilation capacity. 6) Based on these results, it was concluded that the environmental nitrogen-assimilation capacity is an effective guideline for nitrogen input in agricultural lands and helps in the prevention of nitrate pollution in groundwater derived from agricultural land.