Relationships among groundwater flow system, discharge types, and nitrate attenuation zone in upland-lowland boundary region

高原向低地过渡带是地下水中硝酸盐的重要衰减带。我们一直在研究地质、地形、地下水流动和硝酸盐浓度之间的关系，通过密集的水井观测网络和压力计来阐明筑波一个典型的小山坡湿地带硝酸盐的命运。结果表明：1)本区地下水流动系统基本受地形控制，即地下水位与地形一致。2)地下水中高硝酸盐（-100mg/L）在分布在小河谷底部的黑色粘土层附近或以下被衰减。衰减带或锋面沿谷底呈马蹄形分布。3)在厌氧条件下，有机碳作为电子供体进行了反硝化作用。有机含量高的黑色粘土层提供了溶解有机碳和还原地下水。(4)衰减锋在黑粘土层内随水力梯度高低向前或向后移动。即当黑粘土层向砂层以向下流动为主时，衰减发生积极，锋面向后或向上游移动。5)该地区存在渗流型、上升流型和管流型三种类型的泉水。管道流型的硝酸盐浓度普遍高于其他类型。渗漏类型受自然衰减影响。

Transitional zones from upland to lowland have been focused as an important attenuation zone of nitrate in groundwater. We have been studying relationships among geology, landform, groundwater flow and nitrate concentration, in order to clarify nitrate fate in a typical small hillslope-wetland zone in Tsukuba, through intensive observation network of wells and piezometers.The results are summarized as follows :1)Groundwater flow system in this region is controlled basically by landform, that is, water table is consistent with landform.2)High nitrate(-100mg/L) in groundwater is attenuated near or below the black clay layer distributed in the small valley bottom. The attenuation zone or front is distributed like horseshoe-shaped along the valley bottom.3)The attenuation process is attributed to denitrification by organic carbon as an electron donor under anaerobic condition. The black clay layer with high organic contents supplies dissolved organic carbon and reduced groundwater.4)The attenuation front moves forward or backward with low or high hydraulic gradients in the black clay layer. That is, when downward flow is dominant in the black clay layer to the sand layer, attenuation occurs actively and the front moves backward or upstream.5)There are three types of spring, that is, seepage, upwelling, and pipe-flow types in this region. The nitrate concentrations are generally higher in pipe-flow type than others. The seepage type is subject to natural attenuation.