Usually gases are discharged through gas vents (fumaroles) and we can measure the temperature and composition of these gases by taking regular samples. As magma rises in the volcano, the temperature of fumaroles may increase, and their chemistry will change. Sometimes the gases emerge under a lake, or interact with groundwater in the volcanic edifice. Also the heat from the rising molten material can heat the ground water to form hot springs and lakes.
Changes in the water chemistry of crater lakes and thermal spring waters are used to detect changes in the behaviour of the volcanoes and their associated geothermal systems. Geochemical surveys include sampling of selected springs, lakes and streams at places like White Island; Tarawera (hot water beach); Red Crater and Central Crater on Tongariro; and the Crater Lakes of Ruapehu and White Island.
Monitoring specific water parameters such as temperature, pH, conductivity, and concentrations of dissolved gases can provide insight into the processes expected to accompany unrest or renewed volcanic activity. Changes in groundwater, lake levels, rates of stream flow can also give evidence of unrest within a volcano. Crater lakes in particular are valuable indicators of the status of volcanic systems. For example, at Ruapehu we often see the magnesium concentrations increase if new/fresh rock is made available to the crater lake waters.