Mapping Spatial and Temporal Heterogeneity of Lake Seepage

利用电阻率和感应电势绘制湖泊渗漏的时空异质性天普大学地质学系的Laura Toran和Jonathan Nyquist要了解地下水与湖泊的相互作用，需要对渗流模式有详细的了解，以确定水量平衡，追踪从湖泊流入地下水或地下水的污染物，并研究营养丰富的地下水对湖泊微生境的影响。但详细测量整个湖泊的渗漏情况是一项艰巨的任务。使用质量平衡和模型计算的通量估计是针对整个湖泊的，但没有指明地下水排放和补给的区域。使用渗流计进行的点测量，或者根据压力计数据和沉积物的水力传导性计算得出的点，通常分布很广，通常很难重现。地质变异性控制渗流，但由于湖底地质变异性很难填图，地质变异性的特征很差。本项目采用调查连续电阻率剖面法(CRP)，这是一种很有前途的海洋沉积物和基岩填图的新技术。在数据反演过程中，通过在船后拖曳电极流光进行的电阻率测量可以与GPS和水深测量数据相结合，从而产生连续的二维电阻率剖面。电阻率的变化与孔隙度、孔隙流体的离子强度和粘土含量的变化相对应。使用最先进的CRP系统在宾夕法尼亚州拉卡瓦克湖收集的初步数据表明，可以直接检测到湖水电导率(2us/cm)和地下水电导率(40us/cm)之间的适度对比。地下水上升流在被阻性湖水饱和的沉积物中表现为一个低阻层。即使是在湖泊和地下水传导性几乎没有对比的地点，如新罕布夏州的镜湖，CRP也可以提供影响渗流模式的沉积物特性变化的信息，如孔隙度和粘土含量。通过快速收集CRP数据，可以收集大量的空间和时间数据，以改进用于直接测量流量变化的渗漏计的位置，从而提供更好的空间覆盖范围、地质特征和渗漏之间的联系，以及检查系统对风暴和季节变化的反应的时间数据。将对CRP数据和使用一系列低成本手动渗漏仪收集的点测量以及水文数据、湖泊沉积物岩心分析和地下水建模进行详细比较，以评估该方法的实用性和局限性。还将通过比较重复地球物理测量与新开发的测井渗漏仪数据之间的差异来研究用于变化检测的实用程序CRP。由于淡水资源面临的压力不断增加，这项研究对社会具有重要意义。更好地了解地下水-地表水相互作用将改善对水平衡和化学转移(养分或污染物)的预测。此外，渗流界面往往是生物活动的热点，在营养和碳循环中起着重要的作用。这项研究旨在转移到其他地点，因为它通过地下水流动模拟考察了地球物理信号和观测到的水文地层学之间的基本关系。这项研究的教育影响发生在多个层面上。将为三名硕士研究生在第二年的学习提供资金。此外，本科生将担任现场助理，并承担研究子项目的所有权。为了进一步增强他们的研究经验，还包括允许本科生参加科学会议并展示他们的研究成果的资金。寺庙经常吸引第一代大学生，少数民族人口占30%；在地质系，学生性别比例很好。因此，坦普尔能够接触到科学界代表性不足的群体。除了大学教育外，该项目还将提供有关地下水-湖泊相互作用对所研究湖泊旅游的重要性的教育材料。

0609827ToranMapping spatial and temporal heterogeneity of lake seepageusing electrical resistivity and induced potentialLaura Toran and Jonathan Nyquist, Department of Geology, Temple UniversityUnderstanding groundwater-lake interactions requires detailed knowledge of seepage patterns todetermine water budgets, to track contaminants flowing from lakes into groundwater, or groundwater into lakes, and to study the influence of nutrient-rich groundwater on lake micro-habitats. But measuring lake-wide seepage in detail is a daunting task. Flux estimates calculated using mass balance and modeling address the lake as a whole, but do not indicate the regions of groundwater discharge and recharge. Point measurements made using seepage meters, or calculated from piezometer data and sediment hydraulic conductivity, are typically widely spaced and often difficult to reproduce. Geologic variability controls seepage, but geologic variability is poorly characterized because it is difficult to map beneath a lake.This project with investigate continuous resistivity profiling (CRP), a promising new technologyfor mapping marine sediments and bedrock. Electrical resistivity measurements made by towing an electrode streamer behind a boat can be combined with GPS and bathymetry data during data inversion to produce continuous 2-D resistivity profiles. Resistivity changes correspond to variations in porosity, ionic strength of pore fluids, and clay content. Preliminary data collected at Lake Lacawac, Pennsylvania, using a state-of-the-art CRP system suggest that modest contrasts between lake water conductivity (2 uS/cm) and groundwater conductivity (40 uS/cm) are directly detectable. Groundwater upwelling appeared as a low-resistivity zone within sediments saturated with resistive lake water. Even at sites with almost no contrast between lake and groundwater conductivity, such as Mirror Lake, New Hampshire, CRP may provide information on variability in sediment properties such as porosity and clay content that influence seepage patterns. The rapidity of CRP data collection permits extensive spatial and temporal data to be collected to improve the placement of seepage meters used for direct measurements of changesin flux, providing better spatial coverage, linkages between geologic features and seepage, and temporal data to examine system response to storms and seasonal change. Detailed comparison will be made between CRP data and point measurements collected using an array of low cost manual seepage meters, as well as hydrologic data, lake sediment core analysis, and groundwater modeling to assess the utility and limitations of the method. The utility CRP for change detection will also be investigated by comparing differences between repeated geophysical surveys with data from newly-developed logging seepage meters.This research is important to society because of the increasing pressures on freshwater resources.Better understanding of groundwater-surface water interactions will improve prediction of water budgets and chemical transfer (nutrients or contaminants). Furthermore, seepage interfaces are often hot spots of biological activity and play an important role in nutrient and carbon cycling. This study is designed for transfer to other sites because it looks at the fundamental relationships between the geophysical signals and the observed hydrostratigraphy through groundwater flow modeling. The educational impact of this research occurs at multiple levels. Funds will be provided for three masters students during their second year of study. In addition, undergraduates will act as field assistants and take ownership of research sub-projects. To further enhance their research experience, funding is included to permit undergraduate students to attend a scientific conference and present their research. Temple often attracts first-generation college students and has a 30% minority population; in the geology department, there is a good gender balance among students. Thus, Temple is able to reach underrepresented groups in the sciences. In addition to university education, the project will contribute educational materials on the importance of groundwater-lake interaction for tours at the lakes studied.