The application of Point-Velocity Probes (PVP) for both groundwater velocity and groundwater-borne contaminant mass discharge quantification was investigated. The PVP is a novel method to directly measure groundwater velocity at the centimeter scale based on a small-scale tracer test (Labaky et al., 2007), and it has not previously been used to quantify aquifer-stream interactions or contaminant mass discharge.
In the spring 2014, 8 PVPs were successfully assembled and installed at the bank of Grindsted stream, located in the Region of Southern Denmark. The stream of around 10 m width and 1.7 m depth is impacted by xenobiotic organic contamination from two large contaminated sites, Grindsted factory site and Grindsted landfill, which are located 1.5 km north and 2 km south of the stream, respectively.
Numerous injection experiments were conducted in the 8 PVPs, as well as in 4 PVPs installed prior to this study. Horizontal flow directions pointed generally towards the stream, and average seepage velocities ranged from 0.3 to 2.5 m/d with standard deviations between 0.05 and 0.66 m/d.
The groundwater seepage velocities obtained from the PVPs were compared to those obtained from temperature profiling and Darcy's law. The Darcy-based seepage velocities were on average 6 times higher than the PVP values, which were in turn 10% higher than the temperature-based values. Differences may be related to scale differences of the methods, temporal variations as well as uncertainties in the estimates of geological parameters. This latter concern does not apply to PVP measurements, which are based on tracer transport times, making PVPs a useful addition to these kinds of investigations. The fact that the PVP-based seepage velocities fall in between those obtained from the other methods indicates that PVPs are capable of measuring groundwater velocity with an accuracy comparable to that of temperature profiling and Darcy's law.
The PVP-based seepage velocities were combined with groundwater contaminant concentrations to quantify the groundwater-borne contaminant mass discharge. Considering various scenarios, mass discharges for vinyl chloride (VC), benzene and total chlorinated solvents of 37-48 kg/y, 18 kg/y and 0.7-1.4 kmoles/y were found, respectively. Up to 80% of the contaminant mass discharge was found within 13% of the total contaminant plume width (hotspot). This indicates that contaminant plumes may be highly heterogeneous even in homogeneous sandy aquifers, hence fine-scale-monitoring is needed.
Observed contaminant stream concentrations of VC, benzene and chlorinated solvents were between 2.0-3.1 times higher than those calculated from the contaminant mass discharge. This suggests an underestimation of the mass discharge, likely caused by large spatial variations in groundwater velocity and contaminant concentrations.
In order to improve the estimates of the contaminant mass discharge, a second measuring round was carried out in the fall/winter 2014, including i) injection experiments in the 12 PVPs as well as in 2 additional PVPs, ii) multi-level measurements of groundwater contamination in a fine monitoring grid along the stream bank, and iii) slug tests along the stream bank. Preliminary results from the second measuring round support a highly focused discharge to the stream. It seems that a narrow plume embedded in a larger plume accounts for most of the contaminant mass discharge to the stream.
In conclusion, this study illustrates the high potential of PVPs for groundwater velocity quantification near streams, as well as for groundwater-borne contaminant mass discharge quantification. The results from the fall/winter campaign, in-depth interpretation of the field data and perspectives for contaminant mass discharge estimations at streams threatened by point sources will be presented at the conference.
References:
Labaky, W., Devlin, J. F., and Gillham , R. W. (2007). Probe for measuring groundwater velocity at the centimeter scale. Environmental Science and Technology. 41(24):8453-8458.
There are currently several methods used to determine the available fraction of organic contaminants in soils and sediments (e.g. mild-solvent extraction, Tenax extraction, cyclodextrin extraction, passive sampling, biosensors…). However, the comparison of these methods often shows discrepancies in the results, which underlines the need of a standardized method for availability measurement.
The aim of this study was to develop a new tool for the determination of the available fraction of organic compounds in contaminated soils. It consists in a thermodesorption – gas chromatography – mass spectrometry/flame ionization coupling (Td-GC-MS/FID). The idea is to link the binding energy between the compound and the matrix with the desorption temperature. In order to test the feasibility of such technique, polycyclic aromatic hydrocarbon (PAH) contaminated soils presenting various levels of contaminant availability were analyzed. For each PAH, the desorption temperature profile was compared to the efficiency of chemical (Fenton-like and KMnO4 oxidations) and biological (microbial incubation) treatments to degrade PAHs.
A gas plant soil, a wood treating facility soil and two coking plant soils were selected. One milligram of each soil was thermodesorbed at 10 °C/min from 100 °C to 800 °C according to the following temperature ranges: from 100 to 300 °C with six 50°C-steps, then from 400 to 500 °C and finally from 500 to 800 °C. The thermodesorbed compounds were subsequently separated by GC, PAH were identified by MS and quantified with the previously calibrated FID.
When considering one compound, the thermodesorption profiles of the studied soils exhibit differences in the thermodesorption temperatures. This observation highlights differences of PAH availability which were confirmed by comparing the efficiency of chemical oxidation and microbial incubation to remove PAH in the different soils. Correlation is observed between desorption temperature and treatment efficiency, the higher the concentrations of the compounds desorbed at high temperature, the lower the treatment efficiency and, consequently, the lower the availability.
The fine division of the temperature range allowed distinguishing between the measurement of bioavailability and the measurement of chemical availability. It is even possible to go further by differentiating between chemical oxidation treatments according to their efficiencies. In this way, the bioavailable fraction of the 2 and 3-ring compounds corresponds to the fraction desorbed up to 200 °C, for the 3 and 4 rings its correspond to the fraction desorbed up to 300°C and up to 250°C for the higher molecular weight PAHs. The lower desorption temperature for higher molecular weight PAHs is due to the recalcitrance of these compounds towards microbial degradation. The same classification was made for the chemical oxidation treatments. For the Fenton-like treatment the available fraction corresponds to the compounds desorbed up to 250 °C, 300 °C and 350 °C for the 2- to 4-ring compounds, the 4- and 5-ring PAHs and the 5- and 6-ring PAHs, respectively. For the KMnO4 oxidation, the available fractions of 2- to 4-ring compounds, the 4- and 5-ring PAHs and the 5- and 6-ring PAHs were desorbed at temperatures up to 300 °C, 350 °C and 400 °C respectively.
These preliminary results showed that Td-GC-MS/FID allowed linking different levels of availability to the PAH desorption temperatures and indicate that the Td-GC-MS/FID is a promising tool for a precise, fast and exhaustive determination of the available fraction of the organic compound in soils and sediments. This new tool could be used to target the most suitable treatment for the remediation of soils contaminated by organic compounds.
Eugen Martac, Axel Oppermann (Fugro Consult GmbH, Braunschweig, Germany)
Background: Applying Direct-sensing probes in combination with in-situ CPT (CPT: Cone Penetration Test) investigations have turned into powerful tools for the investigation of subsurface contamination, identification of hydraulic properties and exploration of natural resources. Originally developed by the U.S. Company Geoprobe, the Hydraulic Profiling Tool (HPT) is a system manufactured to evaluate the hydraulic behavior of subsurface soil. Using Fugro’s HPT sensors with extended capabilities on a CPT basis proved to have outstanding capabilities when it comes to delineation of the hydraulic structuring of highly permeable subsurface media and providing information about structure and setting up of the lithology simultaneously in just one push.
Methods/Activities: In order to deal efficiently with the in-Situ evaluation of the subsurface hydraulics, a new CPT-based HPT investigation probe was originally developed and is presently in use in Germany by Fugro Consult GmbH. The tool is advanced through the subsurface while water is injected at a constant rate through a screen on the side of the probe. An in-line pressure sensor measures the pressure response of the soil-groundwater system to water injection. Both pressure and flow rate are logged versus depth. The pressure response identifies the relative ability of a soil to transmit water. The water flows into the layers in an easier or more difficult way, depending on the hydraulic properties of the medium. The interpretation provides in a preliminary stage a relative profile of hydraulic conductivity. By means of several slug tests the results are site-specifically translated into absolute values of hydraulic conductivity. The probe extends the initial application domain (up to around 1 – 2 x 10-4 m/s) into to high permeability subsurface media (10-3 m/s) while being able to inject water up to 4000 ml/min.
Results: The irregular distribution of the contaminants is governed by the particular site hydrogeology and hydraulics. When the lithologic and hydraulic features of the site as potential migration pathways are identified, the spatial extension of the contaminant body within the source area and, by means of control planes, within the plume may be delineated. The developed site model delivers the key governing parameters in choosing the appropriate remediation methods for every discrete region of the site. The mass fluxes within the plume could serve as later monitoring device of the eventual success of different remediation options tested at the site.
Conclusion: The method allows a quick, continuous and in real-time profiling of soil hydraulic characteristics. Its applicability is not limited from the hydrogeological site particularities, being able to produce valuable information in both fine- and coarse-grained sediments under unsaturated or saturated conditions. Such high-resolution investigation method yields a solid basis for 3D site characterisation as lithological and hydraulic mapping of the underground.
Israel lies in an arid region with water shortage. An essential demand for this country is a high percentage of reused waters to cover water demand and protect water resources. Due to the water deficiency a lot of measures have been taken to ensure the water supply. Soil Aquifer Treatment (SAT) on Shafdan site near Tel Aviv is characterised by highly variable hydraulic conditions in the soil due to temporary infiltration of spreaded water. Pretreated wastewater flows in dry-wet cycles in several basins, which are filled within approx. 1 day. The water in the basins drains away within 12 hours and therefore the water content and moisture tension in the subsurface vary in dependency of the flooding intervals. During percolation, dissolved organic carbon (DOC) is diminished by 90%. The processes responsible for this enormous elimination are still widely unknown. Possibly the aerobic biodegradation could play a role but the proof is lacking. Many parameters of the soil water (redox potential, oxygen concentration, etc.) are usually influenced by the actual hydraulic conditions in the soil. However, no continuous physical or chemical analyses of Shafdan soil water have been available so far, due to the lack of a sampling technology. Therefore, a sampling procedure was developed in a unique 6 m laboratory column, which allows a representative sampling. The column was filled with soil from Shafdan site. It consists of 6 equal modules (1 m long each). Two samplers (suction probes) were installed in each module of the soil column (= 12 probes). Additionally, tensiometers installed nearly in the same layer can determine current moisture tensions. The resulting values are used for steering the vacuum at the suction probes. The preselected pressure difference between current soil tension value and vacuum pressure at the probes enables constant soil water sampling during all phases of percolating water plume and variable soil water contents.
This new method will allow representative sampling in different soil levels and thus help understanding the processes occurring in the subsurface.
BACKGROUND:
Investigating contaminated sites has traditionally involved drilling and collecting soil and groundwater samples for chemical analysis. This technique is both time consuming and expensive, and in most cases a number drilling sequences are required before the investigation is concluded.
Due to a growing number of contaminated sites, the European Commission has funded development of a new faster and dependable methodology for investigating contami-nated sites: Methodology for fast and reliable Investigation and Characterization of Contaminated Sites - or in short, the MICCS Project.
The MICCS Project has been through two project phases; first the development phase of the technologies, and secondly a demonstration phase, where the new system was demonstrated on contaminated sites formerly described by traditional Site Investiga-tions (SI).
The development of the MICCS Projects technics and concept was conducted by a European consortium of universities, technology institutes and private companies and is partly funded by EU FP7 grants. Project partners also developed software for con-trolling the MICCS probe, logging results, visualising the online sensor results in 3D models on laptop and calculating concentrations of contaminants.
AIM:
One of the aims of the MICCS system is to give a more comprehensive description of the contaminants, their concentrations and migrations in the soil and groundwater. This should results in less required certified analysis by laboratories - with a faster turn-around time and less expensive costs, and at the same time improve the quality of the SI.
Therefore, it is the intention of the project to achieve acceptance by the regulatory authorities in the EU to substitute the major part of the required traditional certified laboratory analysis with the MICCS sensor readings.
Another aim of the demonstrations is to verify that the sonic drilling method itself re-sults in an increased release of the volatile contaminants from the soil and groundwa-ter matrices. This should results in identifying lower concentrations.
THE MICCS CONCEPT:
The MICCS concept consist mainly of two phases of the field site investigation: First, the contaminated site is surveyed by a refined system of Ground Penetration Radar (GPR) and resistivity measuring methods (tracer), which identify underground installa-tions (pipes, UST, etc.), and gives a first indication of the areas contaminated at the site. Main geological features are identified also. For further explanation, please see http://miccs.eu
Second phase of the field investigation is utilizing a new probe technology with newly developed state of the art, highly sensitive sensors for volatile organic components - compound specific.
The MICCS probe system is designed for use with fast sonic drilling. The sonic drill-ing/probing required development of a new design to protect the components during the sonic g-force loads.
The probes sensors are during sonic drilling/probing through the soil profile able to continuously identify and measure concentrations of specific volatile compounds. These results are shown real time on a PC, which allow the operator to determine the type and concentration of volatile contamination at the drilling/probing location. Thereby allowing optimization of the planning of additional drilling/probing locations to fully describe the migration of contamination in the soil, groundwater and soil air.
The real time results of the type and concentrations of contaminants allows a compre-hensive investigation of the site in first field round, and ideally results in a 3D model describing the site geology, underground installations along with the contamination bodies.
DEMONSTRATION AND DOCUMENTATION:
After completion of the development phase, which included two field test in 2013, the demonstration phase included field demonstrations of the MICCS system/concept which were conducted during 2014 at different locations in Denmark.
By systematic comparison of the results of the MICCS system with the results of the conventional analysis, we have been able to demonstrate better and more compre-hensive results for describing contaminated sites.
The demonstrations show good correlation between the MICCS investigation results and the results of the traditional SI performed at the same sites. In general, the MICCS measurements show significantly better sensitivity than the traditional soil sampling with accredited analyses for the most frequent types of contaminants. The MICCS de-tection limits for these compounds are often more than ten times lower than the ac-credited measurements.
CONCLUSION:
Investigations conducted with the MICCS system will not only be faster, but also much more detailed than conventional SI methods. In addition, on-site work will be com-pleted continuously in one workflow, and the volume of required laboratory analysis can be reduced, as the advanced sensors in the probe produces very reliable results.