Distribution of NO₃-N in groundwater and groundwater flow in a reservoir area of the Sunagawa underground dam, Miyako Island, Okinawa Prefecture, Japan.

Abstract

NO₃-N concentrations, δ¹⁵N and SF₆, in groundwater were measured at 39 observation wells in 2009 and 2012 in a reservoir area of the Sunagawa underground dam, completed in 1994, to evaluate the environmental impacts of construction of the dam in Miyako Island, Okinawa Prefecture, Japan. Groundwater level was lower than the top of the cut off wall and NO₃-N concentrations were high in 2009. On the other hand, groundwater level was higher than the top of the cut off wall and NO₃-N concentrations relatively low in 2012. The reason was presumed that the heavy rain with a dilution was fewer in 2009 than 2012. The distribution of NO₃-N concentrations was roughly the same as the distribution of nitrogen loads in surface because of the operation of the composting plants. δ¹⁵N was smaller than reported in previous studies. Amounts of livestock wastes spread onto fields may have decreased. SF₆ concentrations suggest that a part of the groundwater circulates every several years and old groundwater remains stagnant in other parts. These results show that partial circulation of groundwater results in insufficient mixture in the aquifer. It is necessary to select the monitoring well of the water quality in the reservoir area of underground dam in consideration of these ununiformities.