New Depths for Groundwater Profiling
Posted on Oct 5, 2017 by Cascade
Groundwater Profiling Tools Powered by Sonic Drilling
Successful groundwater profiling is paramount when performing subsurface site characterization. Both field work and academic study have demonstrated that contaminant flow in a saturated zone is based upon the varying transmissivities of each strata through which groundwater (and the contaminant mass) move. In profile or cross-section plans, contaminant plumes do not typically exhibit “teardrop” shapes as previously depicted in earlier site investigations. Instead, they are actually elongated lobes mimicking the mass-flux of the subsurface. This is the primary reason that groundwater samples collected from long-screened monitoring wells may not be representative of actual groundwater quality; data are flow-weighted (based upon the individual transmissivities the good screen intersects) and depth-averaged (a single groundwater sample collected from a long-screened well is more of a composite sample of the screened interval).
Obtaining discrete groundwater samples during borehole advancement helps clarify the dissolved-phase plume anatomy. It permits more targeted placement of well screens within a precise interval and identifies whether the contaminant mass may be confined to high or low transmissive zones. Depending on the sampling tool, it may help answer whether persistently detected impacts are a result of back-diffusion from finer strata, often considered a secondary source.
The Tools Used in Groundwater Profiling
Multiple profiling tools are used to aid in the task of groundwater profiling.
WaterlooAPS™- The Advanced Profiling System
WaterlooAPS is a premier high-resolution site characterization tool for overburden formations using a direct push technology (DPT) rig platform. The tool allows collection of discrete groundwater samples (3-inch sample ports) at varying depths within a single borehole without the need to remove, decon, and re-advance tooling between samples. A corresponding index of hydraulic conductivity is also obtained at each sample interval yielding continuous hydrostratigraphic logging of the subsurface. Water quality parameters are monitored through a flow cell, ensuring groundwater samples obtained are from undisturbed formation water.
*images from GeoProbe
Manufactured by GeoProbeR, these tools present another method to obtain overburden groundwater samples using a DPT rig. DPT rods are advanced to the target sample depth with an expendable point at the base of the rod. A 1.25” stainless steel screen is then inserted and lowered to the base of the rod string. The rod string is then withdrawn to expose the screen to the formation and target interval. The exposed screen length may be varied up to four feet in length to allow sample collection and hydraulic conductivity testing (“slug” testing) inside of the screened interval. Tubing is then inserted to allow for development and groundwater sample collection.
Groundbreaking Profiling Tools From Cascade Environmental
In heaving sand formations, ablation tills with boulders and cobbles, and even bedrock, a sonic drilling rig may be mobilized to advance through the material and obtain groundwater profile data where the above tools could not. Two sonic groundwater profiling tools are available to fill the profiling niche; the Push-Ahead and Packer Isolation methods.
The Push-Ahead Groundwater Profiler
The Push-Ahead groundwater profiler is a sealed, heavy gauge steel point that is threaded to the base of the three-inch sonic drill rod and driven through and ahead of the sonic casing to the target depth. This method works best for coarse-grained deposits were heaving sands have been encountered with conventional drilling methods.
- When a groundwater sample is to be obtained from a specific depth, the profiler is threaded to the base of the drill string and the point is advanced below the sonic casing into the undisturbed formation to the prescribed groundwater sample interval.
- Once the point is at the specified interval, a water level indicator is lowered through the drill string to the base of the profiler to ensure there is no formation water accumulated inside the profiler and drill pipe.
- The threaded portion between the profiler and drill steel is then partially unthreaded to expose the water ports to allow native formation water to enter the profiler.
- A groundwater sample is then obtained using either a stainless steel bailer or pump with tubing depending on sample quality objectives.
- Groundwater samples can be collected from native, undisturbed formations.
- Sonic casing may temporarily remain in place until groundwater analytical results are obtained. Wells may then be constructed so that screened intervals target zones for greatest impacts.
The Packer Isolation Method
The Packer Isolation Method is designed for collection of groundwater samples in dense overburden or bedrock boreholes where the push ahead method may be less effective.
- A sonic drill (typically with a four inch core barrel with six inch override casing) proceeds to the base of the interval from which a groundwater sample is to be collected.
- Once that interval is achieved, the sonic core barrel is removed from the casing, and a stainless steel screen and packer assembly is inserted to the base of the sonic casing with an overlying packer.
- The sonic casing is then extracted to expose the screened interval to the formation and the packer is inflated inside of the sonic casing to isolate the screen.
- Purging of the temporary well proceeds until stabilization parameters are met. Groundwater samples are then collected.
- Screen lengths may be adjusted to match specific intervals to be sampled.
- Samples may be collected within any formation through which a sonic rig can drill.
- Four inch diameter screens may be used which allows for higher purge rates and greater sample depths using larger submersible pumps.
Each of these methods has their advantages and disadvantages; the SP22 option can be deployed relatively quickly with little experience, though multiple boreholes are needed to sample from multiple intervals. Without off-setting with soil cores or use of a dual tube system, samples are collected blindly without obtaining corresponding stratigraphy data. The WaterlooAPS allows near continuous discrete groundwater samples and continuous hydraulic conductivity data with a single push. However, the tool requires an experienced operator and is slightly more expensive.The sonic profiling tools are excellent for obtaining groundwater samples from difficult formations and can be obtained within a single boring, however, the expense of a sonic rig is always a consideration.
Whatever characterization challenges your job site presents, Cascade has both the expertise and the equipment to get the job done.