11:00 AM - 11:15 AM
[HTT31-01] Geochemical and isotopic map of stream water as a basis of environmental traceability an example of northeastern Japan
Keywords:geochemical map, isotopic map, stream water, traceability, northeastern Japan
In order to elucidate the impact of atmospheric precipitation and the role of chemical weathering and human activities on fresh water, we determined the concentrations of 6 major ions and 46 trace elements and the isotopic ratios of hydrogen, oxygen, and strontium for about 1000 stream waters in the Iwate, Miyagi, Akita, and Yamagata prefectures of northern Tohoku, and compared them with geomorphology, geology and the geochemical map of river sediments.
The δD and δ18O values of stream water tend to decrease with latitude and elevation, but their relation is expressed as δD=6.6δ18O. The slope value of 6.6 is lower than 8.0 in the precipitation and it becomes low in the water of high δD and δ18O values, indicating that river water experienced with evapotranspiration, particularly in southern and coastal areas where both values are high.
The 87Sr/86Sr ratio of stream water in igneous rock watershed ranges from 0.704 to 0.706, which is slightly higher than that of water in the Green Tuff region (0.706-0.707). The 87Sr/86Sr ratios of sedimentary rocks in the north Kitakami region (0.7085-0.7124) are higher than those in the south Kitakami one. The distribution of Sr/Ca ratio of stream water in the igneous rock watershed also resembles to that of watershed rocks. This correspondence of the Sr isotopic ratio and the Sr/Ca ratio of stream water with those of watershed rock reflects the major source of Sr as well as Ca in the water is largely derived from rocks.
However, although the distribution patterns of alkali-earth elements (Sr and Ca) in the water is similar each other, it is different from those in the river sediment. The distribution pattern of other major and trace elements is also different between river water and river sediment. This inconsistent pattern of elements indicates the important role of element fractionation during the chemical weathering of rocks into water, such as the adsorption and desorption of elements on the sediment particles.
Rare-earth elements of river water in the Miyagi prefecture tends to be higher than those in the Iwate prefecture. As REE are present as colloidal form, this tendency suggests that rocks in the Green-tuff region supply colloidal particles than granite and sedimentary rock which widely distribute in Iwate prefecture. Adsorption-desorption reaction play an important role on the water quality of water.