Time Series Analysis of Influence of Water Cycle on Nitrate Contamination in Miyako Island Ryukyu Limestone Aquifer

This study investigates the complex factors influencing groundwater NO3-N concentrations on Miyako Island, which has a geological structure of highly permeable Ryukyu Limestone over less permeable mudstone. The groundwater NO3-N levels peaked at nearly 10 mg/L in 1989 and have since declined. Our analysis used agricultural statistics, machine learning, and time-series correlation to elucidate the causes of these changes. We found that the decline in concentrations since 1989 was directly linked to a reduction in sugarcane cultivation. However, the mechanism of increase is more complex. A cross-correlation analysis over 60 years revealed two distinct infiltration mechanisms: a rapid one with zero-time lag, responsible for approximately 70% of the NO3-N concentration, and a slow one with a 15-year lag, accounting for the remaining 30%. The slow infiltration is likely due to temporary nitrogen storage in the clay layer. These findings have significant implications for water quality management. The recent shift from summer planting to ratoon cultivation has increased fertilizer use, and this, combined with the 15-year lag effect, suggests that NO3-N concentrations may begin to rise again in about a decade, possibly exceeding the environmental standard. Therefore, continuous monitoring is crucial to prevent future pollution. The methodology developed in this study is also applicable to other islands with similar environments.