Long-term observations of bedrock groundwater dynamics and contributions to stream in a Granite catchment - Approach with mean residence time and hydrochemistry
Abstract
Exploration of bedrock groundwater dynamics is one of the latest frontier of hillslope- and catchment hydrology. We have been keeping on monitoring of the chemical and isotopic compositions of bedrock groundwater in Kiryu Experimental Watershed (KEW), Japan since 2003. We set up a hillslope plot in a subcatchment of KEW, and monthly sampled the streamwater of the subcatchment, the outflow from the plot, which occurs as saturated throughflow on the soil-bedrock interface during rainstorms, and groundwater in the soil sediment. Moreover, we excavated the bedrock and installed some tension lysimeters at 0.1, 0.2, 0.4, and 0.8 m deep and boreholes at 12, 15, and 20 m deep below bedrock surface, and sampled them. The streamflow from this subcatchment was perennial. The SiO2 concentration, which is mainly supplied by chemical weathering, increased along with the infiltration process. On the other hand, the NO3- concentration was highest at the surface soil water, and removed along with the infiltration process. The concentrations of both solutes in the streamwater were intermediate between the concentrations in the surface soil water and bedrock groundwater. The mean residence times calculated by delta 18O variations were about 4 or 5 months in the groundwater in the soil sediment and in the shallow (<0.8m) bedrock groundwaters, about 50 months in 12- and 15 m deep, and about 120 months in 20 m deep, respectively. Some studies have pointed out that the mean residence time over 4 years cannot be estimated by the stable isotope methods, because the variations of isotope signals will become flat. However, the isotope signals in 20 m deep borehole showed long-tern variation, and we could successfully estimate the residence time combined with long-term isotope signal of rainfall. That in the streamwater was estimated as about 30 months. These results suggest that a part of the bedrock groundwater can contribute to the stream from the shallower layers. The fact that the streamflow is perennial in this subcatchment means that plentiful supply of groundwater from the relatively shallow bedrock layers exist. On the other hand, other part of the bedrock groundwater infiltrate deeply and less contribute to the stream in this small headwater. As the deeper bedrock groundwater have especially long residence time, we have to keep long-term monitoring to understand the dynamics and roles of this groundwater to hydrological and hydrochemical processes. Moreover, it will be a key of spatio-temporal scaling of these processes, as well as the water yield function of forests.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.H23N..07K
- Keywords:
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- 1804 HYDROLOGY Catchment;
- 1858 HYDROLOGY Rocks: chemical properties;
- 1886 HYDROLOGY Weathering;
- 1831 HYDROLOGY Groundwater quality