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

地下水中硝酸盐的重要衰减带-旱地向低地过渡带一直受到人们的关注。为了弄清筑波市典型小山坡湿地带地下水中硝酸盐的去向，通过威尔斯和测压管的密集观测网络，研究了地质、地形、地下水流和硝酸盐浓度之间的关系，结果表明：1）该地区地下水流系统基本上受地形控制，即地下水位与地形一致; 2）地下水中高浓度硝酸盐（-100 mg/L）在小谷底分布的黑色粘土层附近或以下衰减。衰减带或衰减锋沿谷底呈沿着马蹄形分布。3）衰减过程是厌氧条件下有机碳作为电子供体的反硝化作用。有机质含量高的黑色粘土层提供了溶解有机碳和还原地下水。4）黑色粘土层中衰减前缘向前或向后移动，水力梯度低或高。当黑粘土层至砂层以下行流为主时，衰减活跃，前缘向后或向上移动。5）该区泉水有渗流型、上升流型和管流型3种类型。硝态氮浓度普遍较高的管流类型比其他。渗流类型为自然衰减型。

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.