Seasonal changes in the discharge, temperature and chemistry of spring water from metamorphic rocks in the southern part of the Owari Hills

A case study on Iwasaki-Ontake hill, Nissin, Aichi, central Japan

Abstract

In the southern part of the Owari Hills, we investigated the seasonal changes in the discharge, temperature and water chemistry of two springs: one from metamorphic rock (hornfels) and the other from gravel layers. Compared to the water from a nearby stream, the water from the two springs had smaller seasonal changes in temperature, was clearer with less turbidity and colour and had a lower concentration of dissolved oxygen (2.6 mgO₂ L^-１). Between the two springs, there was a clear difference in pH and dissolved inorganic substances. The spring water from the metamorphic rock body was weakly acidic (pH 6.0) with a high concentration of dissolved inorganic substances (electric conductivity 16 mS m^-１); the spring water from the sand and gravel layer was acidic (pH 4.7) and had a low concentration (electric conductivity 5 mS m^-１). These differences were attributed to the groundwater infiltration process and acid buffering of the soil. The SiO₂ concentration reflects the residence time of groundwater; it was 26 mg SiO₂ L^-１in the spring water from the metamorphic rock and 10 mg SiO₂ L^-１ in the spring water from the sand and gravel layer. Therefore, the groundwater infiltration rate seems to be slower in the metamorphic rock than that in the sand and gravel layer. On the soil surface, H⁺ is supplied from the nitrification of NH₄ ⁺-N produced by the decomposition of organic matter and CO₂ produced by respiration. H⁺ in acidified groundwater is consumed during the weathering of the bedrock because of acid buffering. Because the metamorphic rock is hard and it has more places inside to be weathered than the sand and gravel layer, the metamorphic rock appear to supply more dissolved inorganic substances than the sand and gravel layer. However, the dissolved inorganic nitrogen in the spring water from the sand and gravel layer was mostly NO₃ ^--N and approximately 50% of that in the spring water from the metamorphic rock. This may be due to nitrate reduction by iron bacteria that prefer the circumneutral and low oxygen conditions of the spring water from the metamorphic rock.