Geophysical Survey Report, Mt. Baldy Site – Indiana Dunes National Lakeshore Park, Porter, Indiana

Ursic, James; Cooper, Brian

In late July, 2013 the National Park Service (NPS) requested technical assistance from U.S. Environmental Protection Agency’s (EPA) Region 5. This request came after a young boy became trapped in a void in the Mt Baldy dune (Dune) for several hours on July 12, 2013 before he was rescued. The Dune, part of the Indiana Dunes National Lakeshore Park located near Porter, Indiana, was closed following that incident. Geophysical surveys, using two different methods, were conducted on August 12, 26 and September 3, 2013 at the Dune. The goal was to provide an initial impression of whether other anomalies might be present at the Dune. Park officials also sought to locate any sub-surface tree roots balls that may have been buried by dune movements. Ground penetrating radar (GPR) and a broadband electromagnetic (EM) instrument, were used. These instruments are configured for hazardous waste site investigations. The GPR configuration was found to be incompatible with the dune’s surface for mobility reasons, although several lines of data were collected before it was determined that the technology was not compatible with the Dune environment. The EM was better suited for the terrain and was able to collect data more efficiently. Both instruments rely on changes in electrical conductivity properties to detect anomalous areas within the ground. Sand, air, and decaying organic matter all have very low conductivities which make it difficult to detect anomalies. Of the two methods used, GPR, despite limitations due to the terrain, provided the most detail. However, GPR only worked up to depths of approximately thirty feet based on estimations of subsurface electrical properties. Attempts were made to reach deeper depths, but signals were too weak. EM was programmed to collect data at approximate intervals of: 5 to 15 feet; 15 to 25 feet; 25 to 35 feet; and 35 to 45 feet. These depths were based on estimates of subsurface electrical and physical properties. Both methods indicated that there were anomalies in the Dune, however secondary methods would be necessary to confirm these results. Anomalies were detected with both GPR and EM and a majority seemed to be single versus multiple anomalies grouped together. Detection of anomalies using the EM showed only small differences between anomalies and background conditions. More anomalies were detected between 15 to 25 feet below the dune’s surface then at any other depths. Secondary methods could include: test wells, soil cores, and downhole geophysical testing. The data did indicate anomalies in the Dune. However, EPA cannot conclusively confirm what those anomalies are or whether voids exist in the Dune. Furthermore, EPA is not qualified to make a determination on any potential risks in reopening the Dune or on the stability of the Dune.