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.
Multiple profiling tools are used to aid in the task of groundwater profiling.
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 The rod string is then withdrawn to expose the screen to the formation and target interval.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.
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 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.
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.
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.