To assume that groundwater flows downhill is misleading
A common hydrogeologic misconception made by engineers and scientist is that: Groundwater moves through an aquifer homogeneously and isotropically. Unfortunately, this statement in reality is not always true. Groundwater almost never moves uniformly through the subsurface because heterogeneities cause the majority of the water volume and contamination, to flow along preferential pathways, which can be challenging to identify using typical site assessment techniques such as a monitoring well network. Furthermore, conventional assessment techniques can result in overly simplified site models for parameters such as flow direction and seepage velocities. A lack of sufficient understanding of preferential flow pathways at environmentally-impacted sites can significantly affect the accuracy of a site conceptual model and in turn the selection of remedial technologiesand alternatives, through to the success of such remedial actions.
To address this issue, ERM has successfully applied a cost-effective, state-of-the-art assessment technique called Background Fluorescence Analysis (BFA) to identify and understand preferential groundwater flow at sites. The Swiss-based company Nano Trace TechnologiesTM originally developed the BFA technique for preferential flow detection. For more than ten years, ERM has worked to tailor the use of BFA in conjunction with conventional high resolution site investigation techniques on projects with challenging and protracted remediation histories, the fundamental goal being to understand in detail how groundwater and dissolved contaminants move through the subsurfaceand ultimately to use this understanding to design effective corrective actions.
What is Background Fluorescence Analysis and What Do We Get Out of It?
BFA is based on the principle that a mixture of most organic compounds emits a characteristic pattern of fluorescence when exposed to specific frequencies of electromagnetic radiation. The fluorescence patterns can be compared with each other and be used as a fingerprinting tool. Dr. Martin H. Otz developed the application of BFA techniques as a quick and cost-effective tool for assessment of aquifer characteristics and contaminant migration behavior at environmentally-impacted sites.
Notable benefits of BFA include:
- Differentiating between impacted and non-impacted groundwater.
- Locating preferential groundwater flow pathways.
- Determining which areas or monitoring wells are hydraulically connected/disconnected.
- Distinguishing organic plumes resulting from releases at different locations and dates.
- Estimating the degree of affected groundwater within a single plume.
How does it work?
Natural groundwater systems always contain complex mixtures of naturally occurring or anthropogenic organic compounds. These compounds are collectively know as Dissolved Organic Carbon (DOC). Many of these DOCs are capable of fluorescing. The abundance of these fluorescing organic compounds and BFA’s ability to fingerprint their fluorescent patterns give it an advantage over traditional techniques as it uses the information of all fluorescent organic substances regardless of whether they are naturally occurring or manmade.. With the BFA technique, ‘non-detection’ incidents do not exist as many organic substances can be detected in the lower part per trillion (ppt) range. To demonstrate the power of this technique, if used to test several drops of rainwater each one would show its own unique fluorescence fingerprint.
BFA analyses require only two 40 milliliter (mL) unpreserved clear glass vials per sample location (the samples need to be protected from direct sunlight after collection). The samples are analyzed using a spectro-fluorometer that providesfluorescence fingerprints of the dissolved organic fluorescent compounds. Each fingerprint provides details of the graphical 2D-representation of all dissolved organic fluorescent compounds in the water sample, as shown in the figure (with each line representing a separate water sample).
The specialist recognizes whether the fluorescence fingerprints bear an anthropogenic signal (e.g. the blue and pink line), or not (e.g. the yellow line). If the fluorescence fingerprints show a similar pattern then they can conclude that the organic load must be similar, allowing for the supposition that there is a probable hydraulic connection between the two such fingerprints. In a homogeneous isotropic aquifer, all fingerprints would be the same.
If all fingerprints of a site are compared, a specialist can recognize hydraulic connections between sample locations, information about contaminant migration, its degradation products and natural attenuation processes. The results can then be plotted on well location site maps as shown as an example below.
A useful supplement to the basic BFA technique is the introduction of artificial fluorescence drug- and cosmetic-grade dyes in a Fluorescent Dye-Tracing (FDT). A well known tracer is uranine, which is widely used by public water works to identify utility leakages. ERM can use up to 10 different fluorescent dyes to quantitatively evaluate preferential ground water flow paths. The successful outcome of these tracer tests is based on the knowledge gained from BFA, particularly in organic-rich, contaminated settings.
Suitable areas of implementation
BFA and FDT assessments prior to remediation is an effective tool to characterize the local hydrogeology and preferential flow pathways and to help guide towards various effective corrective actions.
A common pitfall with conventional groundwater site assessment methods is that contaminant fate and transport is based on the interpolation of general groundwater flow between the sample locations. This approach often turns out to be inaccurate and can lead to unsuccessful remedial efforts. A primary reason for which is the poor understanding of the preferential groundwater flow paths that dominate the contaminant fate and transport.
Many pump & treat and also in-situ remedial approaches suffer under insufficient knowledge of the preferential groundwater flow paths or unknown hydraulic short-cuts.
Using a Background Fluorescence Analysis (BFA) and Fluorescent Dye-Tracing (FDT) tests prior to an in-situ application helps define the local hydrology in more detail. The risk of an unexpected distribution of remedial chemical compounds can, therefore, be minimized. Additionally, significant application costs can be saved as the affected areas are treated using an optimized surgical manner rather than a uniform grid site-wide application, which also unnecessarily treats unaffected areas. Most importantly, one can reduce clean-up time by targeting the affected areas for treatment.
Summary and conclusions
In all of the more 75+ BFA studies that ERM has done, the technique has delivered definitive results. The main goal of the BFA is to quickly delineate the extent of contaminated areas and to determine the preferential groundwater flow directions and its hydraulic connections. The BFA technique is extremely sensitive detecting most organic compounds in the lower part per trillion levels (ng/L levels).
BFA and FDT, can also be used to assess the functionality of an existing remediation system (e.g. effectiveness of hydraulic controls) and to assess potential migration pathways before the injection of remediation chemicals. Both techniques are highly recommended as early as initial Phase I & II assessment and in particular during the selection of the most appropriate remedial strategy, especially prior to the design of in-situ remedial injection programs, to assess injection risks.
Using the techniques the distribution and effective flow velocities can be predicted with high accuracy. With the knowledge of the preferential flow paths and the hydraulic connections, the compounds of concern can be treated and removed with almost surgical precision. The risk of an unwanted and insufficient distribution of the remedial compounds can be reduced significantly, thereby reducing time and costs and ultimately also reducing the carbon footprint of a remedial endeavor.
Source: Otz, M.H., 2005, Using spectro-fluorometry and fluorescent dye-tracing to investigate hydrologic processes in organic-rich environments: [Ph.D. thesis], Syracuse, NY, USA, Syracuse Uni¬versity, 605 p.
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