Effects of snow and land modification on an andesite lava aquifer in Chokai volcano, northwestern Japan

Abstract

• Springs and streams on Chokai volcano are strongly affected by winter precipitation. • Combined use of δ 2 H, δ 18 O, and chloride ions can identify groundwater recharge areas. • Foothill springs recharge at low elevations where infiltration is most efficient. • Groundwater at low elevations flows without thorough mixing in lava aquifers. • Land modification and reduced snowfall in warming climate affect foothill springs. Chokai volcano is composed of low-permeability andesite lava, a snowfall-influenced hydrological setting that has been little studied. Snowfall there is expected to decrease due to climate change and expanded quarrying along its southern foot. To evaluate the effects of future environmental changes on the groundwater discharged from springs at the feet of lava flows, which is used for agriculture and domestic purposes, we used isotopic, hadrochemical, geologic, meteorological, and topographic information to elucidate the recharge area, major recharge season, and underground flow of spring waters and surface waters at Chokai. Groundwater and surface water are of NaCl type with high deuterium excess values, indicating a strong contribution of atmospheric deposition of sea salt aerosols during winter. Altitude indicators based on water H and O isotopic compositions and chloride ion concentrations suggest that the foothill springs are recharged mainly on gentle slopes below ∼700 m elevation. This area is generally covered with volcanic sediments, which contain wetlands and perched aquifers that continuously supply groundwater to the underlying lava through fractures and voids without thorough mixing in the lava aquifer. Anticipated decreases in snowfall and the removal of surface rocks may adversely affect not only the foothill springs supplied by snowmelt and the rice cultivation that depends on them, but also the wetland ecosystem.