Identification and comparison of bedrock groundwater contribution in headwater streams using Strontium isotope

Abstract:

We applied Strontium stable isotope ratio, ⁸⁷Sr/⁸⁶Sr, to detect the hydrological pathways of the bedrock groundwater and considered the relationship between their contribution and the spatial scales in 4 experimental watersheds. ⁸⁷Sr/⁸⁶Sr value in water will be higher when the water contact with more weathered material. This characteristics is suitable to consider the pathways within the bedrock.

The bedrock material in Kiryu Experimental Watershed (KEW, 5.99ha) and Fudoji Experimental Watershed (FEW, 2.34 ha) is granite. The bedrock seepages from the outcrop along the mainstream were observed at KEW. On the other hand, the bedrock spring was observed at FEW. The ⁸⁷Sr/⁸⁶Sr ratio of the bedrock spring was larger than those of the bedrock seepages, and this fact means the pathways for the bedrock spring are more weathered than those for the bedrock seepages. The ⁸⁷Sr/⁸⁶Sr ratio of the bedrock seepages were larger than the values of stream waters observed in the subcatchments of KEW, and the value in streamwater increased from up- to downstream within KEW. Thus, the bedrock seepages may contribute to the stream. Based on the isotope mass balance and the observation of discharge rate, we estimated the ⁸⁷Sr/⁸⁶Sr of the unknown component which contribute to the stream. The estimates were similar to the values of bedrock seepages, but different from the value of the bedrock spring. Moreover, at the wide-area watersheds of KEW (236ha) and FEW (427ha), the ⁸⁷Sr/⁸⁶Sr of waters flowing in each mainstream were less affected by the contribution of tributaries and were relatively stable around the values of the estimated ⁸⁷Sr/⁸⁶Sr. Therefore, the contributions of the bedrock seepages are especially important for the spatial scaling at granite mountains.

At the other two watersheds, Hiruzen Experimental Forest (HEF, 117ha, tuff breccia bedrock) and Hodaka Hirudani Experimental Basin (85.1ha, complicated by volcanic- and sedimentary rocks), the contribution of groundwater increased at the downstream, however, the ⁸⁷Sr/⁸⁶Sr values were not converged. These facts suggest the existence of deeper pathways within the bedrock in these two sites compared to the sites with granite. Strontium isotope is a useful tool to show the pathways within the bedrock, and to consider the scaling problems of the runoff generation processes.