Machida Isao, Lee Sun-Hoon
Geological Survey of Japan, AIST, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8561, Japan.
Ground Water. 2008 Jul-Aug;46(4):532-7. doi: 10.1111/j.1745-6584.2008.00452.x.
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.
我们观察到日本三宅岛火山地面上新沉积的火山喷发物产生的渗滤液导致地下水中溶解离子浓度的长期变化。在超过10年的时间里，从9口井和2个雨水收集器中采集了水样，并定期采集径流水样。对这些样品进行了温度、pH值、碱度、Cl⁻和SO₄²⁻分析；部分样品还进行了δ¹³C分析。由于火山喷发物的渗滤液中含有硫酸盐，我们记录到（无压）井水中SO₄²⁻浓度有所增加。SO₄²⁻浓度的增加最初在喷发后不到1.4年至5.2年之间被检测到，在喷发后2.4年至6.4年达到峰值浓度。这种延迟反应反映了SO₄²⁻在包气带向下移动的运移时间，对应于0.4至7.2厘米/天的表观移动速率。该速率与以当地平均降雨量的分数表示的平均补给量有关，并使用Cl⁻平衡法计算得出。记录到的增加幅度反映了流域内地表火山泥石流的体积。因此，对于火山喷发后的地下水管理而言，了解火山喷发物的化学性质和泥石流的空间分布，对于估算喷发物对地下水水质的影响程度非常重要。
