Environmental Works, Inc. (EWI) recently employed the use of Electrical Resistivity Imaging (ERI) in its quest to find 8,000 gallons of product released from a busted fuel line at a service station in southwest Missouri. The advanced geophysical technology helped locate a second sinkhole on the site, as well as several potential pathways for free-product migration, providing insight on the geologic substrate for where the product might have accumulated and been displaced.
ERI is a geophysical technique for imaging sub-surface structures from electrical resistivity measurements made at the surface by a linear arrangement of multiple electrodes set into shallow ground surface soil.
EWI’s four-part ERI profile was conducted by Dr. Kevin Mickus, Distinguished Professor of Geology at Missouri State University (MSU), using an ARES resistivity and IP imaging system. Max Hoffman, environmental scientist at EWI and on-site project manager, asked Mickus, his former professor at MSU, if he could conduct the profiles after Hoffman’s team was unable to locate the free-product.
The project started as an emergency response in December 2018, and has progressed into an environmental investigation. Then, in the midst of the April 2019 deluge of rain endured by the Ozarks, a sinkhole opened up on the property feet from the point of product release.
Hoffman and the EWI drilling team immediately put a bedrock monitoring well in the sinkhole thinking it was by far the most logical place for the product to settle, but groundwater analytical results from the sampled well came back clean and below detection limits regarding chemicals of concern (COCs) .
“I thought that was going to be a smoking gun,” said Hoffman. “I thought we would pull up gallons of product, but it wasn’t meant to be.”
So, with the product still not located, but soil borings confirming contamination in the on-site soil, Hoffman constructed a work plan based on a site conceptual model and the geologic information gained from logged soil borings.
With one sinkhole already located and with the understanding that there could be many more fractures, sinkholes and cave systems on site, Hoffman enlisted Mickus. Hoffman, who has a bachelor’s in geology, and a master’s in geospatial sciences from MSU, conducted his field research in Las Cruces, New Mexico, where he was assisted by Mickus.
Hoffman called the ERI study a “big step” in the project.
The Earth Imager 2D models produced by the four ERI profiles helped locate a new sinkhole west of the previously known sinkhole, said Hoffman. A well was recently installed in the second sinkhole and groundwater from the well will be sampled in the near future.
“The models were very accurate in determining top of bedrock and the elevation of the sink,” said Hoffman.
Mickus and Hoffman produced the profile models by placing a number of electrode stakes in the ground a meter apart from each other in a straight line. The stakes were connected by cables, with eight stakes to a cable, all running back to a receiver. The stakes put current in the ground, and voltage was measured between them to determine the thickness of the soil and the location of potential fractures and sinkholes within the bedrock.
“The electrodes measure Earth resistance, and the resistance is depending on the material underground,” said Mickus. “Bedrock is highly resistant, while soil is less resistant. So you can go along and have high resistance, low resistance, high resistance.”
Mickus brought along enough stakes to go 72 meters long on the profiles, but was limited by on-site space. Still, Hoffman and Mickus had enough stakes to profile almost the whole back side of the service station property.
Each profile took between 30 and 45 minutes to measure, with the ARES system running through a car battery.
“It has been a couple of years since I’ve done a resistivity test,” said Mickus, adding that he was glad Hoffman asked him to join the project.
For his part, Hoffman is only focused on one thing at the site.
“Ultimately our goal is to find the product,” he said.