Surface geophysical exploration: developing noninvasive tools to monitor past leaks around Hanford’s tank farms

Abstract

A characterization program has been developed at Hanford to image past leaks in and around the underground storage tank facilities. The program is based on electrical resistivity, a geophysical technique that maps the distribution of electrical properties of the subsurface. The method was shown to be immediately successful in open areas devoid of underground metallic infrastructure, due to the large contrast in material properties between the highly saline waste and the dry sandy host environment. The results in these areas, confirmed by a limited number of boreholes, demonstrate a tendency for the lateral extent of the underground waste plume to remain within the approximate footprint of the disposal facility. In infrastructure-rich areas, such as tank farms, the conventional application of electrical resistivity using small point-source surface electrodes initially presented a challenge for the resistivity method. The method was then adapted to directly use the buried infrastructure, specifically the steel-cased wells that surround the tanks, as "long" electrodes for both transmission of electrical current and measurements of voltage. Overcoming the drawbacks of the long electrode method has been the focus of our work over the past 7 years. The drawbacks include low vertical resolution and limited lateral coverage. The lateral coverage issue has been improved by supplementing the long electrodes with surface electrodes in areas devoid of infrastructure. The vertical resolution has been increased by developing borehole electrode arrays that can fit within the small-diameter drive casing of a direct push rig. The evolution of the program has led to some exceptional advances in the application of geophysical methods, including logistical deployment of the technology in hazardous areas, development of parallel processing resistivity inversion algorithms, and adapting the processing tools to accommodate electrodes of all shapes and locations. The program is accompanied by a full set of quality assurance procedures that cover the layout of sensors, measurement strategies, and software enhancements while insuring the integrity of stored data. The data have been shown to be useful in identifying previously unknown contaminant sources and defining the footprint of precipitation recharge barriers to retard the movement of existing contamination.

Enhanced resolution for long electrode ERT

“…The scale at which the LE‐ERT method can be applied has been demonstrated clearly at the pilot scale (scale = X) and the environmental site characterization scale (scale = 100X). For example, Rucker et al (2012b) mentioned surveys where the site was on the order of 100 ha and the target was as at least 70 m below ground surface, which is approximately two orders of magnitude larger in scale than presented here. The small to intermediate reservoir scale (1000X) also should easily be accommodated as long as a sufficient density of wells exist to the depth of the target, as shown in Daily et al (2004).…”

“…Although much of the past focus has been on ensuring that targets can be reproduced, little effort has been spent increasing the utility and enhancing the resolution of the long electrode method. Rucker et al (2012b) demonstrated that combining long electrodes with short electrodes on the surface can distinguish between shallow and deep targets, but a large number of electrodes are needed on the surface and around the perimeter of the survey area to be effective. Zhu & Feng (2011) also demonstrated that a large number of short electrodes on the surface, combined with a single long electrode in the centre of the survey area, could be used to determine the proper depth of a pair of relatively shallow targets.…”

Pilot‐scale field validation of the long electrode electrical resistivity tomography method

“…A pilot‐scale validation experiment was conducted to test the ability of the long‐electrode electrical resistivity tomography method to reconstruct a known target. The scale of the validation test was on the order of 1/17 of a typical Hanford nuclear waste tank, where the long electrode electrical resistivity tomography (LE‐ERT) method was applied to image several waste plumes that have developed beneath the tanks from accidental releases (e.g., Rucker et al . 2010, 2011b).…”

“…From a modelling perspective, the larger cell size actually created the highest model resolution values, with the lowest resolution observed from the model with no depth electrodes. Depth electrodes are important when surface electrodes are sparse and arrays of depth electrodes buried in actual tank farm settings have proven to be a valuable means to evaluate historical leaks (Rucker et al . 2011b).…”

“…The target was constructed by hand to simulate a saline contaminant plume using two soils of known properties and dimensions. The electrical resistivity measurement campaign included a combination of surface, long and a few buried point electrodes, similar to the survey designs described in Rucker et al . (2011b).…”