For many years, the environmental industry has relied on decades-old technologies for characterisation of contaminated sites. Assessment and clean-up efforts have been criticised for their high cost, lengthy duration, and the overall poor quality of the data used to make decisions. Also, many clean-up efforts do not perform as expected; going over budget and extending well beyond projected timeframes because of a poor understanding of the fate and transport of the contaminants and the subsurface hydraulic system.
In recent years, our team has implemented several new high tech screening tools capable of providing real-time data to determine the presence and concentration of petroleum hydrocarbons and other contaminants, and the hydraulic properties of the subsurface. Advanced software solutions have been developed in tandem to synthesise and present the data. Some of these new tools include laser induced fluorescence (LIF) and membrane interface probe (MIP) instrumentation, 3-D visualisation tools, and vertical hydraulic profiling instrumentation. These tools provide real-time data on a much finer vertical scale than a typical soil boring or monitoring well. Typically, 75 percent or more of the mass of a contaminant plume occurs across just five to ten percent of the total cross-sectional area of the plume. Small changes in subsurface lithologies and hydraulic properties can have a dramatic effect on where and how contaminants migrate through the subsurface. A better understanding of exactly where the contaminant mass is concentrated provides an opportunity for surgically-focused remedial designs that maximise cost-effectiveness and result in more complete clean-ups.
To harness these technological advancements and improve the quality of data, our team overhauled its approach to characterising contaminated sites. Formerly, static sampling and analysis programmes were used to pre-determine the number and location of samples, and which laboratory analyses would be conducted on them. Now, a flexible, dynamic approach is used, which allows the field geologist to make real-time decisions on when and where to collect samples. This approach relies upon the initial development and continual adaptation of a conceptual site model (CSM). The CSM is a written and/or illustrative representation of the physical, chemical, and biological processes that control the transport, migration of contamination (in soil, air, ground water, surface water and/or sediments). As we gather new information, the CSM is refined to reflect our comprehensive understanding of the site.
“a flexible, dynamic approach is used, which allows the field geologist to make real-time decisions on when and where to collect samples”
The new approach has shown the following advantages for contaminated land stakeholders over the traditional approach:
- Expedited schedules – Site characterisations are completed in one or two mobilisations, rather than spread out over months or years. The previous cycle of collecting data, waiting weeks for laboratory results, then remobilising to address data gaps is halted.
- Lower cost – Overall, more data is collected at a lower cost because the engineer can be more selective about where and when to collect samples for laboratory analysis, which also reduces lab costs.
- Reduced uncertainty – Uncertainties and data gaps are identified in the field and can be addressed on the spot.
- Enhanced data quality – The overall quality and usability of the data is significantly enhanced and data collection is geared toward obtaining the exact information needed to design the remediation approach.
We will first review some of the available high-resolution site characterisation tools that are currently in use, and then follow with a briefing on the dynamic site characterisation approach and how it can be utilised to maximise the benefit of the tools to reduce uncertainty, speed up the site characterisation process, and produce better data for remedial decision-making.
“with the possibility of logging as much as 100 metres in a day, the tool can rapidly collect data at multiple locations allowing for a detailed CSM with a 3-dimensional understanding of the NAPL distribution”
High resolution site characterisation tools
Laser Induced Fluorescence (LIF)
LIF technology relies upon the properties of polynuclear aromatic hydrocarbons (PAHs) that fluoresce under laser light. PAHs are present in the vast majority of petroleum fuels/oils, coal tars, and creosotes. When non-aqueous phase liquid (NAPL) oils are present in the soil, the intensity of the fluorescence under laser light can be measured to determine the oil saturation in the soil. A laser light source and sensor are mounted in a probe head attached to a direct push percussion probe, and electroconductivity (EC) sensors are also mounted in the probe head. As the probe head advances, oil saturation (%) and EC readings are collected every few millimetres. Oil saturation can be measured both above and below the water table. EC readings are utilised to determine the soil type, as finer grained soils have higher conductivity than coarse grained soils. With the possibility of logging as much as 100 metres in a day, the tool can rapidly collect data at multiple locations allowing for a detailed CSM with a 3-dimensional understanding of the NAPL distribution.
Figure 1 shows an example LIF/EC log. A monitoring well installed at this location indicated the presence of a 0.5-metre NAPL layer floating on the water table. However, the LIF log indicated that NAPLs were trapped well below the water table in finer grained sediments and that the level of oil saturation varied considerably across the range.
“a monitoring well installed at this location indicated the presence of a 0.5-metre NAPL layer floating on the water table. However, the LIF log indicated that NAPLs were trapped well below the water table in finer grained sediments”
Membrane Interface Probe (MIP)
The MIP is deployed in a similar manner as the LIF probe, but is utilised to detect volatile organic compounds (VOCs) such as benzene, toluene, ethylbenzene, and xylenes (BTEX), as well as chlorinated solvents such as perchloroethylene (PCE) and trichloroethylene (TCE). The MIP technology utilises a thin film polymer membrane with a heater block mounted in the probe head. As the membrane is heated, VOCs in the soil or groundwater are volatilised and diffuse through the permeable membrane. A carrier gas is circulated through the probe head, VOCs diffuse into the carrier gas, and the VOC-laden gas is returned to the surface and routed through a series of detectors, which typically include a photoionisation detector (PID), a flame ionisation detector (FID), and the halogen specific detector (XSD). Each of these sensors is sensitive to a particular class of VOCs.
Figure 2 shows a 3-dimensional model of dissolved chlorinated solvents at a hazardous waste disposal site. The model can be rotated to evaluate the subsurface impacts from any angle.
FLUTe liner system
Flexible Liner Underground Technologies, LLC (FLUTe) offers numerous configurations of their liner system, which are useful for both free phase and dissolved contaminant detection, as well as for profiling of aquifer characteristics. The FLUTe liner is essentially a high strength, urethanecoated fabric “sock” which is everted into an open borehole. As the liner is lowered into the borehole, water is pumped into the sock, pressing the liner against the borehole wall as the liner is gradually inverted into the hole. To detect the presence of NAPL, the liner can also be outfitted with a hydrophobic, dye-impregnated cover, which presses against the borehole wall from the interior liner pressure. Any NAPL in the formation is wicked into the cover. The cover is impregnated with a dye that dissolves on contact with NAPL, creating a “stain” on the liner. The liner is subsequently removed from the borehole and can be inspected for the presence of discrete NAPL zones.
The liner is also useful for determining the hydraulic properties of the formation as it is lowered into the borehole. As the liner is installed, a constant head pressure is maintained on the inside of the liner by adding water to the sock. The water in the borehole is forced into the formation through pores, fractures, solution cavities, or other flow paths. The rate at which the liner is lowered is controlled by the rate at which water can flow from the hole into the formation. As the liner is lowered it covers the flow paths sequentially. When that occurs, the transmissivity of the hole beneath the liner is decreased and the total flow rate out of the hole is reduced, causing a reduction in the liner descent rate. For example, a sudden drop in the liner descent rate indicates that a high-permeability flow path has been covered by the liner. Logging of the descent rate versus depth is useful in identifying important flow paths – which may also be important in the flow and transport of contaminants.
Hydraulic profiling tool (HPT)
The HPT is designed to create a continuous profile of hydraulic conductivity as the tool advances into the subsurface. The HPT is typically advanced by direct push methods where clean water is injected into the subsurface as the tool is advanced at a controlled rate. The injection pressure required to force the water into the formation is monitored with depth. A relatively low-pressure response indicates a relatively high hydraulic conductivity. Conversely, a relatively high-pressure response indicates a relatively low hydraulic conductivity. The HPT is often combined with an EC probe to corroborate hydraulic conductivity with soil grain size. As previously discussed, the presence of vertically-narrow lenses of high- or low-permeability soils may significantly impact the fate and transport of contaminants. Consequently, identification of these zones is critical to understanding the contaminant distribution and to determine cost-effective means to remediate the contaminants.
Advanced molecular microbiology
It is widely acknowledged that biodegradation of organic contaminants may play an important role in the attenuation and ultimate degradation of many contaminants in the environment.
Understanding whether contaminants are biodegrading and the specific mechanisms, rates, and microbes that are responsible for biodegradation allow us to:
- Better predict whether contaminants will continue to migrate
- Design more effective remediation approaches
- Make better decisions on when to conduct active remediation and when to allow natural processes to degrade the contaminants over time
Several new techniques are available to definitively determine whether biodegradation is taking place and which organisms and mechanisms are responsible. One of these techniques is Compound Specific Isotope Analysis (CSIA), which measures changes in the ratio of stable isotopes (e.g. 13C/12C) in a contaminant as it biologically degrades. Physical processes like volatilisation and dilution generally do not appreciably change the isotopic ratios, so CSIA can potentially provide direct evidence of ongoing contaminant biodegradation.
The above tools represent just some of the advances in high resolution site characterisation, but the real key to expedited and better site characterisation is in the approach to planning, implementing, and utilising the data from these real-time data tools.
Dynamic site characterisation approach
For many years, site characterisation efforts have centred around installation of soil borings and monitoring wells, as well as laboratory analysis of soil and groundwater samples collected from these discrete points. One of the primary limitations of this approach is cost. It is simply too cost prohibitive to collect soil samples for laboratory analysis from every few inches, or even every foot, of every soil boring. Yet, we now understand that small seams within the subsurface may hold most of the contaminant mass.
Similarly, laboratory results from a groundwater-monitoring well represent an average concentration of the groundwater throughout the entire screened section of the well. High concentrations of contaminants may be entering the screen through a small section of the aquifer and become diluted by clean water entering the well from above and below. This approach may lead to two false conclusions: an assumption that the worst-case contaminant concentrations are lower than the actual concentrations within the contaminated seam; and that the aquifer is impacted with contaminants over the entire length of the screen. The interpretation of this data becomes even more difficult when NAPLs are present.
The advent of new tools allowing for real-time collection of qualitative or semi-quantitative contaminant and hydraulic data over very short vertical intervals has the potential to yield a better understanding of the true contaminant distribution. However, the approach to planning and implementing the field work must also be altered to allow on-the-fly decision-making.
This new dynamic site characterisation approach has six basic components:
- Systematic planning – Systematic planning is conducted prior to field mobilisation and is intended to identify the key decisions to be made, develop a conceptual site model, identify sources of uncertainty, and develop a plan to address the uncertainties. Part of this planning is a thorough review of existing documents that describe the site history and geologic profile.
- Dynamic work strategies – Dynamic work strategies are flexible work instructions that allow field personnel to adapt the sampling plan based on data or knowledge gained from real-time measurement. Rather than a strict plan that dictates the exact number and location of soil borings/wells and samples, the work plan typically includes a decision logic diagram that allows the field manager to change or adapt the plan as information is gathered, and to make field decisions about which subsequent activities will best resolve remaining data and decision uncertainties, and/or meet clean-up goals.
- Real-time measurement technologies – The use of real-time measurement technologies such as those described above, is essential to the approach. Real-time measurement technologies return results quickly enough to influence the progress of data collection and field activities.
- Selective laboratory analysis – One of the key cost savings of the dynamic site characterisation approach is collecting fewer samples for laboratory analyses than in a traditional approach. While significantly more data is obtained overall, much of the collected real-time data is qualitative or semi-quantitative and can’t be utilised for direct comparison with regulatory clean-up criteria. Laboratory analysis of soil and groundwater samples still has its place in site characterisation, but is typically used to confirm and calibrate the field results.
- Collaborative data set evaluation – One of the important aspects of data interpretation is the ability to layer multiple data sets and interpret them in collaboration to better understand site conditions. For example, where engineers once relied solely upon soil and groundwater results, they may now utilise MIP data, EC data, hydraulic profiling data, and soil and groundwater results in collaboration to minimise uncertainties. Geographical Information Systems (GIS) and 3-D visualisation software have increased our ability to synthesise and make sense of multiple data sets.
- Remedial Decision Data Collection – In the past, it was common for geologists to conduct site characterisation and then hand off the project to engineers to design a solution for site remediation. Despite full delineation of the contaminant plume, the engineers often found that they lacked the information necessary to design a solution – resulting in the need to re-mobilise to collect additional data. The dynamic site characterisation process involves collaboration between engineers and geologists early and continually in the site characterisation process. As information is collected, the engineers begin to formulate alternatives for remediation and identify data gaps that would impact the design and implementation of the alternatives. The data is collected during the site characterisation phase so there is no delay in the design process. For example, if bioremediation is a potential cleanup alternative, conducting groundwater geochemistry analysis and CSIA or other microbiological evaluations may be advisable to support this remedy.
Our team has successfully implemented the dynamic site characterisation approach on multiple sites that spread across a wide range of geologic conditions and contaminant types. The tools utilised from site to site may vary, but the dynamic approach has consistently led to better remedial decision making, less costly assessments and clean-ups, and faster clean-up times to meet remedial goals.