Delayed Impact of New Volcanic Ejecta on Ground Water Quality

Abstract

We observed long-term changes in the concentrations of dissolved ions in ground water caused by leachate from new volcanic ejecta deposited on the ground surface of the volcanic Miyakejima Island, Japan. Water samples were collected from nine wells and two rain collectors over a period of more than 10 years, and samples of runoff water were collected periodically. The samples were analyzed for temperature, pH, alkalinity, Cl(-), and SO(4)(2-); some of the samples were also analyzed for delta(13)C. Because the leachate from the volcanic ejecta contained sulfate, we recorded an increase in SO(4)(2) (-) concentrations in the (unconfined) well water. The increase in SO(4)(2)(-) was initially detected between less than 1.4 and 5.2 years after the eruption, showing peak concentrations from 2.4 to 6.4 years after the eruption. This delayed response reflects the transit time of downward-moving SO(4)(2) (-) in the vadose zone, corresponding to an apparent movement rate of 0.4 to 7.2 cm/d. The rate relates to the mean recharge, represented as a fraction of local mean rainfall, and is calculated using the Cl(-) balance method. The magnitude of the recorded increases reflects the volume of volcanic mudflow on the ground surface within the basin. For the management of ground water after an eruption, it is therefore important to know the chemical properties of the volcanic ejecta and the spatial distribution of mudflow to estimate the magnitude of the effect of ejecta on ground water quality.

Hydrothermal and Magmatic System of a Volcanic Island Inferred From Magnetotellurics, Seismicity, Self‐potential, and Thermal Image: An Example of Miyakejima (Japan)

“…Groundwater formed by the mixture of meteoric recharge and hydrothermal fluids is mostly driven toward the ocean due to the water table gradient. The current and past positive self-potential anomalies (Nishida et al, 1996;Sasai et al, 1997), as well as hydrogeological studies (Aoki et al, 1984;Machida & Lee, 2008;Sato et al, 2006), confirm this general fluid circulation. Near the coastlines, groundwater mixes with marine water, as revealed by geochemical analysis of water and 1-D resistivity surveys (Aoki et al, 1984;Sato et al, 2006).…”

“…Intense annual precipitation affects Miyakejima Island (2,950 mm·year −1 , 1981–2010, Japan Meteorological Agency) due to extensive evaporation of the hot ocean, and sea‐breeze convergence over coastal zones (Qian, 2008). A significant amount of such meteoric water (30–50%, after Machida & Lee [2008]) percolates downward through the soil until it reaches a water‐saturated layer. Despite a large amount of recharge, the highly permeable deposits of Miyakejima (10 −9 –10 −12 m 2 after Arai [1978]) keep the main aquifer a few hundred meters below the surface.…”

Groundwater Quality

“…Long-term change in the concentrations of dissolved ions in groundwater was found due to leachate from new volcanic ejecta in the volcanic Miyakejima Island, Japan. Increased sulfate concentration was detected with peak concentrations 2.4 to 6.4 years after the eruption (Machida and Lee, 2008).…”