Contributed by Forest Stroud, Staff Geologist, PPM Consultants
Winter in the field is a particularly brutal time of year. It was February and PPM was responding to a release of petroleum products to the subsurface. You could tell how cold it was by the younger members on the drill crew huddling by the drilling rig’s exhaust. The first quarter had also been spontaneously busy. The drill crew had been out of town for at least the past month, and it was clear that they were feeling the wear of the road. But we were there to do a job: determine the extent of soil and groundwater impact, and like every project, hopefully learn something along the way.
Assessing the extent of groundwater impact is a fairly logical and straight forward process. Target the source of the spill, hypothesize how the product is moving in the localized area, start drilling, adjust your hypothesis based on the lithologies discovered, and then drill additional borings. But what happens when the drill rig augers are struggling to advance through the densely packed clay? What happens when you finally get the boring to 40 feet deep, hit bedrock refusal, and the borehole is dry as a bone? Do you install a monitoring well in the dry hole and hope for the best? Return to the site at a later date with a drilling rig capable of drilling into the bedrock in attempt to find water? What do you do when this happens multiple times on the same site? It made me start thinking about the differences between course-grained and fine-grained soils and how different particle sizes can change the behavior of the water-bearing zones beneath our feet.
In order to begin answering this question we need to have a basic understanding of the differences between clay, silt, sand, and gravel. All have different grain sizes, and most geologists and engineers use the Unified Soil Classification System (USCS) to evaluate the differences. This is formally done in a geotechnical laboratory where the soil is passed through a sieve to determine the soil type. However, there are visual methods used in the field to estimate the soil types that aids in real-time decision making.
These differences in particle sizes give geologists clues about where the particles originated, how far they traveled and under what conditions they were deposited. Typically, the more rounded and smaller the particle, the farther the grain has traveled, and larger grains are deposited closer to the erosion source. However, in reality you will find a mixture of clays and gravel; gravel and sand; silt and clay; silt and sand; silt, sand and gravel and on and on. And if you really want to get technical you can start busting out the percentages of each. For example, a soil layer may be comprised of 1% gravel, 49% sand, 25% clay and 25% silt, or some other combination. The possibilities are endlessly mind-boggling. It is common for field geologists to debate whether something should be called a “silty clay” or a “clayey silt”, as the only real way to know is via laboratory testing: a luxury not available when you are making boring/well location decisions on the fly when assessing a site.
There’s also the intrinsic characteristics of fine grained and coarse-grained soils, such as permeability and porosity. Permeability is a medium’s ability to transmit water, and porosity is a medium’s ability for water to fill in the pores in between the grains. Fine-grained material like clay and silt are generally able to hold water because of their porosity but are not very effective at transmitting water because the pore spaces are not connected (thus are “impermeable”). Sands and gravels are generally good at holding and transmitting water and are therefore considered both porous and permeable.
Now that we’ve had a crash course on different soil types and the terms and ideas that are generally associated with each material, what was going on that cold day in February? Those wells did end up making water and the water level rose as high as ~8-12 feet below the surface. It took about 24 hours for those wells to initially fill up. This makes sense with the soil described being mostly dense clay. Because clay has a high porosity but is pretty impermeable the water took longer to flow into the well. In addition, the spinning of the drilling augers can smear the sidewalls of the borehole, thus increasing the time for water to breakthrough this artificially created temporary barrier. Thankfully, the time and effort spent to install those wells on a cold day after months of being on the road were not wasted. The wells served as effective points to monitor groundwater conditions, and for recovery of free phase, adsorbed phase and dissolved phase fractions of petroleum hydrocarbons leading to effective remediation of the site.