CAFO Logging & Sampling Results

After reviewing the existing site information for the Nebraska
CAFO facility, Team Geoprobe ran a series of HPT logs along
the east end of the lagoon to obtain a clear picture of subsurface
conditions. The HPT system injects water at about 300 mL/min. as the HPT probe is advanced to depth. The field computer monitors injection pressure and flow with depth and provides a graphical log onscreen. These logs were instrumental in determining subsequent sampling locations and depths. Because it’s a fast and reliable way to complete continous, real-time profiling of soil hydraulic properties, the HPT system is a real time-saver when determining sampling locations
or monitoring well installation points.

The Wenner array on the HPT probe provides a simultaneous electrical conductivity (EC) log with each HPT log. The first log at the CAFO site (HPT1) indicated there was a fine-grained layer between 24 and 25 feet. The HPT pressure increased, HPT flow decreased, and EC increased sharply across this layer. The HPT and EC logs confirmed one another. As additional HPT logs were obtained across the area (HPT2 and HPT4), it became apparent that the dominant lithology in the subsurface was coarser-grained materials, and that fine-grained lenses would thicken, thin, and pinch out at different depths. The logs also responded to the weathered bedrock (~ 45 ft) with an increase in HPT pressure and an increase in electrical conductance, suggesting finer-grained materials.

To confirm the HPT log results, a continuous core was collected with the DT325 sampling system at the HPT2 location. Dave Miesbach and Jeff Gottula with NDEQ visually inspected the cores to confirm the HPT logs. Saturated sands gave some problems with heaving and made recovery of the fine-grained material difficult. However, visual inspection of the samples indicated that the HPT logs provided an accurate and detailed representation of the subsurface lithology and hydrostratigraphy. In addition to verifying the HPT logs, the soil cores also indicated there was potential impact to the alluvial aquifer sediments above the fine-grained layer. This was seen as gray staining on the sediments along with the detection of a pungent odor.

HPT1 Log. First HPT log taken at Nebraska CAFO demo site. Moderately high EC and pressure suggest fine-grained soils to about 5 feet. Lower EC and pressure indicate mostly coarse-grained materials to about 45 ft. The spike between 24-25 ft indicates a clay-rich lens.

HPT2 Log. Similar to first log but note clay lens near the 25-ft depth has gotten thicker. Local bedrock was encountered at about 45 ft in each log and evaluation of samples and local geology map indicate it may be weathered Niobrara chalk.

HPT4 Log. The average EC above the clay layer at 30 ft is still a bit higher than below the clay layer. Bedrock contact was at about 44 ft.

SP16 and HPT1. Two SP16 groundwater samplers were installed adjacent to the HPT1 log location. One screen was set from 20 to 22 feet, above the clay lens, and the second was set from 26 to 28 feet, below the clay lens.