River and Groundwater Interaction in the mid-stream area of Tama River, Tokyo, Japan

This study aims to evaluate a role of Tama river in groundwater recharge by using multi-tracer approach and statistical analysis in a mid-stream area of Tama River, western Tokyo.

 

We performed an intensive sampling of river water, groundwater and spring water in the mid-stream area of Tama River watershed including tributaries, Akikawa River and Asakawa River from May through September 2022, and totally 21 of river water, 35 of groundwater and 17 of spring water were sampled, and stable isotopic compositions (δ²H, δ¹⁸O), inorganic constituent concentrations were determined on all water samples.

 

The d¹⁸O of Tama River water increases with flow especially after joining of the tributaries. The chemistry of Tama River, shallow groundwater and the spring water is characterized dominantly by Ca-HCO₃ type, whereas the deep groundwater shows a Na-HCO₃ or Na-Ca-Mg-HCO₃ type dominantly.

 

We applied End Member Mixing Analysis to estimate the contribution ratio of Tama River water to the shallow groundwater in the mid-stream area using d¹⁸O and SiO₂ as tracers, and we selected the volume weighted mean of precipitation, mean of Tama river water and the groundwater taken in the border between mountain and plain area. The contribution ratio of river water to the total groundwater recharge ranges from 21% to 83%.

 

Also, we performed a principal component analysis using all analyzed components to evaluate the category of the water chemistry considering the river water and the groundwater interaction. Further, we conducted hierarchical clustering analysis using PCA results.

 

Consequently, the all of water samples are classified in 6 groups, and the shallow groundwater is categorized in the same group as the river water.

 

The results show clearly that the Tama River water contributes dominantly to the shallow groundwater in the mid-stream area, and the river water partly recharges the deep groundwater. It seems to be due to the decline of the major aquifers from south-west to north east directions, which is from river to mid-stream plain area.