A waste plaster landfill site, established as a high-density polyethylene (HDPE) sheet pile surrounded by a slurry wall to isolate possible contaminants within the landfill, was investigated using electric survey techniques. A sheet pile is an effective barrier wall for contaminated leachate containment, unless there are damaged zones within the pile. However, the perfect insulating properties of the pile make it difficult to apply the electrical resistivity method to measure the containment ability of the pile. To locate damaged zones in the sheet pile, which can act as pathways for the contaminated leachate, a potential difference (PD) profiling technique was applied. Through three-dimensional (3D) modelling, PD profiling demonstrated efficacy in locating damaged zones in the sheet pile. Moreover, a 3D survey was conducted to delineate the leakage pathways through the sheet pile, as well as the leachate distribution throughout the landfill. Before the 3D inversion of measured resistivity data, erroneous data caused by the sheet pile were rejected to minimise distortion of the inversion results. 3D inversion showed the sheet pile to be highly resistive overall, especially at shallow depths; however, further inspection via PD profiling identified several areas of damage in the pile. These damaged zones from PD profiling coincided well with the low resistivity zones present near the surface in the 3D inverted image. Furthermore, major leaks were found below the burial depth of the sheet pile. Thus, it was demonstrated the PD profiling and 3D resistivity survey can locate damaged zones in the sheet pile and delineate leakage pathways over the landfill site.
Electrical resistivity tomography was carried out at seawater intrusion monitoring wells located at watershed in coastal areas. It is difficult to identify the characteristics of resistivity near monitoring well in case of using high signalto-noise ratio array due to the high conductivity condition in coastal aquifer although electrical resistivity survey is well adopted to delineate hydrogeological characteristics with the distribution of electrical resistivity. To improve the quality of electrical resistivity survey for two sites with seawater intrusion monitoring wells, inversion with the results of holeto-surface electrical resistivity tomography using single well was executed. The results of inversion for aquifer near wells were verified with the results of drilling log with the informations of fracture, electrical conductivity logging and normal resistivity logging. The inversion for aquifer near one of two wells was also performed at low and high tide with the same electrodes, respectively. From the inversion result, it is possible to obtain the resistivity images with high resolution and to identify the characteristics of aquifer related to seawater intrusion with tidal fluctuation. From this study, it was demonstrated that the hole-to-surface electrical resistivity tomography method accompanied with drilling log, electrical conductivity logging and normal resistivity logging would be useful to delineate the hydrogeological structures near monitoring wells in coastal areas.
To evaluate the effects of drought on groundwater system in rural areas, the standardized groundwater level index (SGI) was applied to groundwater monitoring wells over S. Korea. Moreover, accumulation period (AP), representing the month with the highest correlation coefficient between SGI and the standardized precipitation index (SPI), was calculated for monitoring wells. In this case, correlation analysis was performed to investigate differences in the response of precipitation and groundwater level to drought using SPI. Groundwater level data from 68 monitoring wells were used for the analysis. The response time of groundwater level to precipitation appeared to be very short, but the groundwater level did not go with SPI during the long-term drought. Results of correlation analysis between reservoir level and SPI show high correlation on the relatively long AP. The results of analysis between SGI and SPI appeared that the AP values ranged from 1 to 3 months for most of the wells indicating that the total amount of groundwater will not decrease significantly in long-term drought periods unlikely it of reservoirs with the high AP values. The nationwide maximum AP values between SGI and SPI were around 4 in the central part of S. Korea, while the minimum AP values were around 2 in the eastern and western part of S. Korea. Consequently, it could be concluded that the wells with low AP value tend to respond to short-term drought, but it has little effect on groundwater system when the long drought occurs. Acknowledgement: This research was supported by the Institute of Planning and Evaluation for Technology in Food, Agriculture, and Forestry (IPET) [Grant number 320046053HD020].
To delineate the spatial extent of seawater intrusion in a small experimental watershed, electrical resistivity surveys including drilling data and chemical analysis of groundwater were conducted. We classified 30 sounding curves applying to H type into 3 categories according to the relative differences of resistivity magnitude by inversion. We also grouped groundwater samples in 15 shallow wells into 2 types based on the HCO3/Cl and Ca/Na molar ratio by TDS. From the comparison results, combined application of VES method and chemical analysis of groundwater samples would be useful in deriving more reliable information on the zone of seawater intrusion in coastal watershed disregarding little drilling results.
Quantitative assessment of groundwater level change under extreme event is important since groundwater system is directly affected by drought. Substantially, groundwater level fluctuation reveals to be delayed from several hours to few months after raining according to the aquifer characteristics. Groundwater system in Jeju Island would be also affected by drought and almost all regions were suffered from a severe drought during summer season (July to September) in 2013. To estimate the effect of precipitation to groundwater system, monthly mean groundwater levels in 2013 compared to those in the past from 48 monitoring wells belong to be largely affected by rainfall(Dr) over Jeju Island were analyzed. Mean groundwater levels during summer season recorded 100 mm lowered of precipitation compared to the past 30 years became decreased to range from 2.63 m to 5.42 m in southern region compared to the past and continued to December. These decreasing trends are also found in western(from -1.21 m to -4.06 m), eastern(-0.91 m to -3.24 m), and northern region(from 0.58 m to -4.02 m), respectively. Moreover, the response of groundwater level from drought turned out to be -3.80 m in August after delaying about one month. Therefore, severe drought in 2013 played an important role on groundwater system in Jeju Island and the effect of drought for groundwater level fluctuation was higher in southern region than other ones according to the regional difference of precipitation decrease.
Leachate from livestock mortality burial is harmful to the soil and groundwater environment and adequate assessment approaches are necessary to manage burial sites. Among the methods used to detect leachate, geophysical surveys, including electrical resistivity and electromagnetic (EM) techniques, are used in many engineering approaches to environmental problems, such as identifying contaminant plumes and evaluating hydrogeological conditions. Electrical resistivity, with a small-loop EM survey, was used in this study as a reconnaissance technique to identify the burial shape and distribution of leachate from livestock mortality burial in five small separate zones. We conducted a multi-frequency small-loop EM survey using lattice nets and acquired apparent conductivity values along several parallel and perpendicular lines over a burial site. We also compared geophysical results to the geochemical analysis of samples from both a leachate collection well and a downstream observation well within the study area. Depth slices of apparent conductivities at each frequency (obtained from the small-loop EM survey data) clearly identified the subsurface structure of the burial shape and the extent of leachate transport. Low-resistivity zones, identified from two-dimensional (2D) electrical resistivity imaging results, were matched to the five burial zones (within a depth of 5 m), as well as high electrical conductivity of the leachate obtained from leachate collection wells, and depth slices of the apparent conductivity distribution obtained from the small-loop EM survey. A three-dimensional (3D) inversion of resistivity data provided a detailed 3D structure of the overall burial site and leachate pathways. Moreover, these zones were widely spread over the burial site, indicating that leachate potentially extended through damaged regions of the composite liner to a depth of 10 m along the downstream groundwater flow. Both the small-loop EM method and the electrical resistivity method were considered suitable for identifying the shape of the livestock mortality burial and the extent of leachate.
ABSTRACT The self‐potential (SP) method is widely used in seepage evaluation hydrological studies to monitor the integrity of infrastructure such as dams, sea dikes, and other types of flood control devices because the electric signals that are measured are directly related to seepage rate. At leaking areas along sea dikes, large SP anomalies can be generated by the rising and falling of tides. Unfortunately, SP data are often contaminated with several types of noise, such as that from drifting electrodes, telluric disturbances, and external electrical noise. Furthermore, SP signals can have high levels of spatial variability due to heterogeneity in lateral resistivity at the locations where the electrodes are installed. Because of these issues, it is very difficult to correlate the measured SP voltages with the streaming potentials associated with groundwater flows at particular points in time. To alleviate these problems, we developed a simple but effective interpretation method for SP monitoring data that involves subtracting consecutive SP voltages collected at different time points from a particular monitoring station. This subtracting procedure is able to effectively reduce spurious SP anomalies caused by electrode drift, change in resistivity, and other types of interference. Therefore, any changes observed in SP measurements over certain time frames were interpreted as resulting primarily from temporal changes in seepage flow. To demonstrate the performance of this method, we analysed SP monitoring data measured at a sea dike located on the southern coast of Korea. Our results confirmed that the SP interpretation method is able to explain changes in streaming potentials depending on the tide change over time and to detect the horizontal location of anomalous seepage zones along the sea dike.
우리나라 서, 남해안에 지하수의 해수침투 현상을 관측하기 위해 설치된 관측망 자료를 이용하여, 해당지역별 대수층을 통한 해수침투의 유형을 분석하였다. 분석에 이용된 관측정은 25개 해안지역 소유역의 총 45개 암반관정으로, 지하수위, 수온, 전기전도도를 대상으로 기본통계분석, 상관성 분석 및 변동유형 분류를 수행하였다. 분석 결과 지하수위의 경우 강우 영향을 받거나 양수에 의한 수위강하가 나타나는 관측정에서 큰 폭으로 변동하고 있다. 지하수온은 대부분의 경우 변동폭이 $0.2^{\circ}C$ 이내로 안정적인 특징을 보여주고 있으며, 전체적으로 평균 온도가 $15^{\circ}C$ 이상으로 나타났다. 전기전도도의 경우 상대적으로 변동폭이 크고 불규칙한 특징을 가지고 있으며, 평균값이 $2,000\;{\mu}S/cm$ 이내의 관측정이 28개, $10,000\;{\mu}S/cm$를 초과하는 관측정은 9개소로 나타났다. 교차상관도 분석에 의하면, 지하수위는 강우의 영향을 받아 변동하는 형태가 많았지만 수온과 전기전도도는 상대적으로 강우의 영향이 없는 것으로 나타났다. 조석의 경우에는 일부 관측정에서 강우에 의한 영향보다 높은 교차상관도가 나타났다. 본 연구에서는 장기관측자료를 변동형태에 따라 정상형, 추세형, 주기형, 충동형, 계단형, 경사형 등 6가지로 분류하였다. 지하수위의 경우 강우나 양수의 영향을 받는 충동형이 가장 많은 73.3%이며, 조석의 영향은 13.4%에 해당되었다. 지하수온의 경우 변동폭이 일정한 정상형태가 51.2로 가장 높게 나타났으며, 강우나 양수에 의한 충동형이 26.7%에 해당되었다. 전기전도도의 경우는 지하수위나 지하수온과 달리 추세형, 계단형, 경사형 등의 변동형태가 우세한 것으로 나타났다. 전체적으로 동일한 해안지역에 설치된 관측정별로 관측자료의 변동 특성이 상이한 경우가 나타남에 따라, 향후 각 소유역별로 관측정 자료의 정기적인 변동 경향성을 분석하는 것이 필요하다. With groundwater data of seawater intrusion monitoring network in coastal areas of Korea's main land, we analyzed types of seawater intrusion through the coastal aquifer. The data including groundwater level, temperature and electrical conductivity obtained from 45 monitoring wells at 25 watershed regions were evaluated. Based on statistical analysis, correlation analysis and variation type analysis, groundwater levels were mainly affected by rainfall and artificial pumping. About 78% of the monitoring wells showed average temperature higher than $15^{\circ}C$ and about 58% of them showed minimum variations less than $0.2^{\circ}C$. Electrical conductivities showed a large magnitude of variation and irregular characteristics compared with groundwater levels and temperatures. Average electrical conductivities lower than $2,000\;{\mu}S/cm$ were observed at 28 monitoring wells while those of higher than $10,000\;{\mu}S/cm$ were done at 9 monitoring wells. From the cross-correlation analysis, groundwater levels were mostly affected by precipitation while temperature and electrical conductivity showed very low correlation. Meanwhile tidal variations strongly affected the groundwater levels comparing to precipitation. We classified the long-term monitoring data according to variation types such as constant process, linear trend, cyclic variation, impulse, step function and ramp. Impulse type was dominant for variations of groundwater level, which was largely affected by rainfall or artificial pumping, the constant process was dominant for temperature. Compared with groundwater level and temperature, electrical conductivities showed various types like linear trend, step function and ramp. According to the discrepancy of variation characteristics for monitoring data at each well in the same region, periodical analysis of monitoring data is essentially required.
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