School of Planning, Architecture and Civil Engineering, Queen's University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, Northern Ireland, United Kingdom.
Department of Applied Sciences, Dundalk Institute of Technology, Dublin Road, Dundalk, Ireland.
Sci Total Environ. 2015 Aug 1;523:109-19. doi: 10.1016/j.scitotenv.2015.03.117. Epub 2015 Apr 7.
Contaminants discharging from on-site wastewater treatment systems (OSWTSs) can impact groundwater quality, threatening human health and surface water ecosystems. Risk of negative impacts becomes elevated in areas of extreme vulnerability with high water tables, where thin unsaturated intervals limit vadose zone attenuation. A combined geophysical/hydrogeological investigation into the effects of an OSWTS, located over a poorly productive aquifer (PPA) with thin subsoil cover, aimed to characterise effluent impacts on groundwater. Groundwater, sampled from piezometers down-gradient of the OSWTS percolation area displayed spatially erratic, yet temporally consistent, contaminant distributions. Electrical resistivity tomography identified an area of gross groundwater contamination close to the percolation area and, when combined with seismic refraction and water quality data, indicated that infiltrating effluent reaching the water table discharged to a deeper more permeable zone of weathered shale resting on more competent bedrock. Subsurface structure, defined by geophysics, indicated that elevated chemical and microbiological contaminant levels encountered in groundwater samples collected from piezometers, down-gradient of sampling points with lower contaminant levels, corresponded to those locations where piezometers were screened close to the weathered shale/competent rock interface; those immediately up-gradient were too shallow to intercept this interval, and thus the more impacted zone of the contaminant plume. Intermittent occurrence of faecal indicator bacteria more than 100m down gradient of the percolation area suggested relatively short travel times. Study findings highlight the utility of geophysics as part of multidisciplinary investigations for OSWTS contaminant plume characterisation, while also demonstrating the capacity of effluent discharging to PPAs to impact groundwater quality at distance. Comparable geophysical responses observed in similar settings across Ireland suggest the phenomena observed in this study are more widespread than previously suspected.
从现场废水处理系统（OSWTS）排放的污染物会影响地下水质量，威胁人类健康和地表水生态系统。在高地下水位、薄非饱和带限制渗流区衰减的极端脆弱地区，这种负面影响的风险会升高。对位于生产力低下含水层（PPA）上的 OSWTS 进行了地球物理/水文地质综合调查，目的是描述 OSWTS 对地下水的影响。从 OSWTS 渗滤区下游的测压管中采集的地下水显示出空间上不规则但时间上一致的污染物分布。电阻率层析成像确定了一个地下水严重污染区域，靠近渗滤区，与地震折射和水质数据结合，表明到达地下水位的渗透污水排放到更深、更具渗透性的风化页岩区，该页岩区位于更具能力的基岩上。地球物理学定义的地下结构表明，在远离采样点的测压管中采集的地下水样本中遇到的升高的化学和微生物污染物水平与那些测压管靠近风化页岩/基岩界面的位置相对应；那些直接在上游的位置太浅，无法拦截这个间隔，因此是污染物羽流的受影响更严重区域。在渗滤区下游 100 多米处间歇性出现粪便指示菌表明，污水的迁移时间相对较短。研究结果强调了地球物理学作为 OSWTS 污染物羽流特征描述的多学科调查的一部分的实用性，同时也证明了污水排放到 PPA 会对远距离的地下水质量产生影响。在爱尔兰类似的环境中观察到的类似地球物理响应表明，在本研究中观察到的现象比以前怀疑的更为普遍。
