Abstract
Temporal and spatial dynamics of nitrate (NO3--N) in groundwater are investigated in the southern part of Okinawa Main Island, Japan, where Ryukyu limestone is extensively distributed. Influent factors of NO3--N formation are also examined on the basis of observed data in groundwater of the study area. It is found that probability distribution of NO3--N concentrations agree with normal distribution and the 75 per cent probability of NO3--N concentration is a little higher than the upper value (10mgL-1) of environmental quality criteria. It is considered that NO3--N concentrations are influenced by nitrogen load accompanied with groundwater flow from the upstream to the downstream of the study area. It is also found that NO3--N concentration formation is assumed to depend on two factors, that, mixing-inflow actions and denitrification-dilution actions under conditions of groundwater flow caused by completion of the subsurface dam. It is suggested that nitrogen load from agricultural land in this area influences the long-term fluctuation of NO3--N concentrations in groundwater. It is recognized that groundwater level participates annual fluctuation of NO3--N concentrations in groundwater.
