Based on shortterm (52 h in consecutive spring and neap tidal cycles) hourly water quality data and longterm hourly electrical conductivity monitoring data of coastal shallow groundwater in northeastern part of Hainan

shortterm and longterm water quality fluctuations are analyzed. The results indicate that: ① Na

+

and Cl

-

are chief ions in shallow groundwater in the research area and vary with quasi-synchronization

which bring about major changes of TDS. Extremely low amount of CO

2-

3

can be found in shallow groundwater. Mutual relations of cation are all positive correlated with distinct intensity at different sites

and are apparently influenced by tidal cycles. Different types of tidal cycles also lead to interchange between positive and negative correlations among anion. ② Soil alkalization degree caused by shallow groundwater increases from the north to the east

from lowmedium level to mediumhigh level. Intensity of seawater intrusion keeps a medium-high level

especially at the spring tidal cycle. Results from water mineralization imply that water quality of the study area is classified between brackish water and salt water. ③ During study period

electrical conductivity vs. mineralization

and electrical conductivity vs.Na

+

+Cl

-

both obey individual linear correlation rules and can be hardly affected in different tidal cycles

which means that long-term monitoring of water mineralization

 Na

+

and Cl

-

can be possible. Also

it means a lot to longterm monitoring of water quality and seawater intrusion. ④ Long-term fluctuations and probability distributions of electrical conductivity follow respective rules which depend more on locations. ⑤ More sharper changes can exist in the amount of Na

+

and Cl

-

than in water mineralization from a long-term view. A great quantity of fresh water discharged from estuary of the Nandu River and occasional seawater intrusions may be the immediate causes for changes in the contents of Na

+

and Cl

-

in shallow groundwater of northern study area.