PLT - Impact of Groundwater Observations on Dam Design

Stantec designed the Eagle Creek Flood Basin, which will reduce the risk of flooding in Findlay, Ohio. Stantec completed over 100 geotechnical borings and installed 29 open-standpipe piezometers along the proposed embankment alignment (3.75 miles) and within the basin footprint (765 acres). Early design phases included a desktop study and limited soil resistivity testing to consider corrosivity to proposed project elements such as cutoff walls and concrete foundations. These results indicated that the soils could be corrosive to steel. During piezometer data collection, field team members documented degradation of the stainless-steel cables used to suspend electronic transducers and noted a black substance that had accumulated within the open standpipes. The engineering design team engaged with Stantec groundwater professionals to perform environmental site assessments (ESA) to evaluate groundwater conditions that may have led to the observations within the piezometers. The initial ESA focused on groundwater sampling and testing for total petroleum hydrocarbons that may have been related to the historical oil production in the area. It was thought that the observed black substance in the piezometers could be degraded petroleum. The initial ESA results indicated that the groundwater did not contain petroleum hydrocarbons. However, during the initial groundwater sampling event, the field crew noted strong odors commonly associated with hydrogen sulfide produced by sulfur-reducing bacteria. Additionally, groundwater geochemical testing results for oxidation-reduction potential indicated highly reducing conditions, which creates an environment conducive to the sulfur-reducing bacteria. Metals can corrode by hydrogen sulfide and iron in the groundwater can form a black precipitate (iron sulfide). Based on field observations and geochemical testing results, it was believed that the site contained naturally occurring hydrogen sulfide. A second ESA was conducted and confirmed the presence of hydrogen sulfide. Because this condition was identified, the engineering team was able to specify construction materials and concrete additives during design so that cutoff walls and concrete foundations would be more resilient to the corrosive conditions within the groundwater. Identification during design reduced the risk for potential future dam safety issues and costly repairs due to the corrosive conditions.