Geophysical Monitoring of Leachate Recirculation at Orchard Hills Landfill
Investigators: University of Illinois, Chicago
The main objective of this study was to investigate geophysical techniques that can be useful to monitor bioreactor landfills. Several non-invasive surface geophysics techniques were tested at Orchard Hills landfill located in Davis Junction, Illinois. These techniques included electrical resistivity tomography, electromagnetic surveys, ground penetrating radar, and well logging.
Electrical resistivity depends on water content, thus the measured electrical resistivity tomography (ERT) allows determination of spatial distribution of waste moisture (not just isolated locations as with probes). ERT was performed with a Syscal Pro resistivity-meter during several leachate injection periods. From the testing, we observed the leachate distribution along the leachate recirculation lines was not clearly evident, but the leachate distribution around the recirculation line showed a decrease of resistivity around the line. Even though, more comprehensive monitoring are needed, the results of this study showed that ERT method has a great potential to be used as monitoring tool to optimize the leachate recirculation, leading to the best performance of bioreactor landfills.
Electromagnetic surveys were performed to monitor the leachate recirculation. EM31 and EM34, both manufactured by Geonics, Ltd. of Mississauga, Ontario, Canada, were used. Frequencydomain EM conductivity measurements were made along the upstream and alongstream lines, as well as some shorter crosslines. The EM31 allowed faster measurements since both transmitting and receiving loops were mounted on one fiberglass tube, and could be operated by one person (one line is acquired in about 20 min), but the investigation depth was less (between 3 and 6 m, according the coil orientation). The EM34, which consisted of two individual loops that required 2 persons to operate, had an investigation depth of 7.5 to 30 m, depending on the configuration and coil spacing (10 or 20 m respectively). One line was acquired in about 40 min. On the Alongstream Line, the HD showed an increase of conductivity compared to the reference after the first 15 m of the line, most likely reflecting the change in slope of the landfillâ€™s surface. The main changes in conductivity occurred just between the first measurement and the reference, as the variations did not change consequently after the first measurement. The variations observed
above LRL29 were in the same range as the dispersion. Thus, considering the shallow investigation depth (2.8 m) and the measurement dispersion, it seems that leachate recirculation cannot be seen with the EM31 in the HD orientation.
The Ground-penetrating radar (GPR) provides a picture-like display, consisting of radar waveforms plotted as time vs. position (side-by-side) â€œtraces.â€ The display looks like a geological cross-section, but important differences exist. Some signals on the sections may arise from above-ground reflections. Other distortions may also occur (e.g. diffractions from point reflectors, ringing from multiple reflections, etc.). For this reason apparent reflections are called â€œeventsâ€ on the radar sections. Times on GPR sections are usually specified in nanoseconds (ns) â€“ a nanosecond is 1 x 10-9 s. GPR tests conducted over the leachate recirculation cell at Orchard Hills were designed to: (1) determine the penetration depth of GPR signals in highly conductive waste and cover materials, (2) measure the radar wave velocity, and (3) see if the GPR technique could image subsurface targets, such as a leachate recirculation pipe, trench or leachate accumulations within the waste. GPR profiles were made adjacent to, and across two leachate recirculation lines (LRLs) in the eastern part of the cell to meet these objectives. All GPR data iv were collected with a Sensors and Software pulseEKKO IV GPR system with antenna frequencies of 25, 50 and 100 MHz. Surveys included common-midpoint (CMP) lines to assess velocity, south-to-north traverses across LRL29 approximately 10 m west of the LRL29 inlet (east end), where the LRL was approximately 5 m below the GPR line, as well as traverses across LRL29 further upslope where LRL29 was 10 m deep, and, finally, a single south-to-north profile across LRL28, located 22 m west of the inlet for LRL28. The location of the CMP surveys was about 10-20 m south of LRL29, to avoid encountering anomalous ground conditions associated with the LRLs. The GPR signal was recorded at 0.2 m separation intervals to a total
separation of 5.2 m. The CMP record exhibited a ground-wave velocity of about 0.086 m/ns. This value seemed reasonable for clayey materials comprising the cover and underlying refuse/clay mixtures. This velocity was used to convert reflection times to depth on the GPR sections. The optimum antenna configuration was a 50 MHz bistatic antenna since the 100 MHz antenna had too shallow a penetration depth, whereas the 25 MHz antenna had extremely low resolution and produced noisy sections. The 25 MHz antennas were also cumbersome, which may have contributed to the noise problems, since in some cases the antennas were not well coupled with the ground due to bowing, obstruction by vegetation, etc. In some cases the GPR signal may have penetrated to as much as 10 m. GPR profiles under low volume leachate recirculation indicated leachate collection pipes and surrounding areas of increased moisture. The geophysical well logs appear to be strongly influenced by well construction, including bentonite seals and gravel packs surrounding the gas extraction wells. Little to no useful information was obtained. Advanced processing of these logs may reveal additional information. For example, two logs may be subtracted, or divided, to eliminate the effect of the gravel. Electrical conductivity logs appeared to measure conductivity values similar to those recorded on the surface.