Background/Objectives. Success of large-scale in-situ remedies is often limited by a conceptual site model (CSM) and hydrostratigraphic understanding that is too general and does not allow for location-specific remedy optimization. Without a comprehensive and local understanding of the hydrostratigraphy, delivery of reagent may be inadequate in areas, resulting in incomplete treatment and longer treatment duration. Conventional performance data, typically collected at monitoring wells within the treatment zone, may not detect this incomplete treatment or may not provide adequate information to adapt the remedy to improve performance.
However, “next-generation” characterization tools and three-dimensional modeling can be costeffectively used to refine the CSM, identify zones of high contaminant flux, and optimize
remedy performance. Since 2001, an enhanced reductive dechlorination (ERD) remedy has been implemented to treat a 2,000-foot long trichloroethene (TCE) plume. The original remedial design was based on a pre-existing CSM that relied on conventional investigation methods such as visual borehole logging and groundwater sampling from site monitoring wells. Twodimensional analysis of available data did not capture the lateral continuity of higher transmissivity hydrostratigraphic units responsible for the bulk of solute transport. After four years of full-scale remediation, conditions favorable for ERD had been established in most areas of the site. However, incomplete remediation was observed in some locations of the plume.
Approach/Activities. To further develop the CSM and understand why treatment was not
observed in some areas of the site, traditional performance monitoring data was supplemented with four targeted investigations using multiple “next generation” high resolution characterization techniques including cone penetrometer (CPT), hydraulic profiling tool (HPT), and membrane interface probe (MIP). Historical and the more recent high-resolution data were integrated into a three-dimensional model using Environmental Visualization System (EVS) software to gain an improved understanding of the hydrostratigraphy and contaminant migration pathways, and this new understand provided a basis for modifications to remedy design and implementation.
Results/Lessons Learned. Three-dimensional modeling and visualization of the indicated lateral continuity of hydrostratigraphic units previously thought to be disconnected providing a refined CSM that was used to significantly expand and optimize the current remedial system. Remedy changes included the installation of additional injection wells, abandonment of select injection wells deemed no longer necessary for operation and monitoring, and adjustment of reagent injection volumes. After adjusting remedial operations, improvements in performance were observed within months. This case study highlights how these new tools and approaches can be effectively used to address the challenges of full-scale plume treatment and maximize effectiveness of in situ approaches.
The public water management of Lower Saxony has to provide about 8 million people with fresh water. With a portion of 71% of the public water supply in Germany (BGR 2010), the fresh groundwater is the most important resource for urban, agricultural and industrial activities. Some of the aquifers used for groundwater extraction provide problems with intruding salt water from different sources (GRUBE et al. 2000). Especially the coastal aquifers may be vulnerable for sea water intrusion, which is the landward encroachment of sea water into fresh water aquifers (IVKOVIC et al. 2012). This process could be enhanced or initiated by anthropogenic activities. Because of the importance of the salt water intrusion problems, the State Authority for Mining, Energy and Geology (LBEG) planed, based on a pilot project in the area of Esens (DEUS 2012), to generate a statewide “salt water map” for Lower Saxony with a focus on the coastal aquifers influenced by sea water intrusion.
In Germany the use of fresh water as drinking water is limited through the thresholds for different parameters in the “Trinkwasserverordnung (German drinking water regulation)” (BMJ 2013). Those are 250 mg/l chloride, 250 mg/l sulfate and 200 mg/l sodium (BMJ 2013). The threshold for the electrical conductivity at 20°C is 2790 µS/cm (BMJ 2013) which correlates with an electric resistivity of 4 Ωm. These parameters were used to characterize the groundwater and divide it into salt-/fresh water areas.
For the coastal regions of Lower Saxony we use airborne electromagnetic measurements (HEM) operated by the “Federal Institute for Geosciences and Natural Resources (BGR)” to get the electric resistivity of the underground (sediments and pore fluids) and combined them with groundwater analyses to detect the intrusion of sea water into the aquifers. Therefore, referring to the experiences and results of DEUS (2012), we combined the resistivity distribution with geological 3D-model of the Pleistocene sediments, containing information from geological maps, profiles, wells and groundwater analyses to distinguish low resistivity’s caused by sea water intrusion, from those caused by clay materials which have the same resistivity. The resistivity data from the electromagnetic measurements were integrated in GOCAD® and the area affected by sea water intrusion was visualized as a 3D surface, which represents the salt-/freshwater interface.
BGR (2010): Grundwasseranteil an der öffentlichen Was¬serversorgung der Bundesländer in 2007. http://www. bgr.bund.de/nn_322854/DE/Themen/Wasser/grund¬wasser__gewin__tab.html, Abruf 01.12.2010.
DEUS, N. (2012): Kartierung der Küstenversalzung mit Hilfe geophysikalischer Daten und 3D-Modellierung im Raum Esens (Ostfriesland). Hannover (unveröff. Masterarbeit Univ. Hannover), 92pp.
GRUBE, A, WICHMANN, K., HAHN, L. & NACHTIGALL, K.H. (2000): Geogene Grundwasserversalzung in den Poren-Grundwasserleitern Norddeutschlands und ihre Bedeutung für die Wasserwirtschaft. TZW-Schrift¬enreihe, 9, Karlsruhe, 203 pp.
IVKOVIC, K.M., MARSHALL, S.K., MORGAN, L.K., WERNER, A.D., CAREY, H., COOK, S., SUNDARAM, B., NORMAN, R., WALLACE, L., CARUANA, L., DIXON-JAIN, P. & SIMON, D. (2012): National-scale vulnerability assessment of seawater intrusion: summary report. Waterlines Report Series No 85, Australia, 185 pp.
BMJ (BUNDESMINISTERIUM FÜR JUSTIZ) (2013): Trinkwasserverordnung in der Fassung der Bekanntmachung vom 2. August 2013 (BGBl. I S. 2977), die durch Artikel 4 Absatz 22 des Gesetzes vom 7. August 2013 (BGBl. I S. 3154) geändert worden ist. – Berlin.
Background and objective
The understanding of chlorinated solvents behavior in fractured limestone aquifers is a challenging task because of the preferential flow of contaminants in fractures and the exchange with the limestone matrix. Characterization of the contaminant distribution, particularly in the matrix, is challenged by difficulties in intact sample collection (coring) for sufficiently discretized data. The characterization is important for the development of a conceptual understanding, for risk assessment and for the choice and operation of an appropriate remediation strategy. The FACT (FLUTe activated carbon technique) is an innovative monitoring technique, which allows determining the distribution of a contaminant in the surrounding of a borehole with a higher resolution than conventional monitoring methods. The FACT technique proved to be a helpful tool for characterization of contaminant distribution in the limestone aquifer at Naverland, a contaminanted site in Denmark (Janniche et al. 2013, Broholm et al. 2013, Kerrn-Jespersen et al. 2013). While the sorbed concentration of contaminant in the carbon felt is obviously related to concentrations in the formation, there is no direct relation between measured sorbed concentration (mg/g AC) and the aqueous pore water concentration (mg/L). The objective of the research presented was to develop a tool for the interpretation of FACT measurements and apply it to the Naverland dataset for comparison with concentrations in groundwater samples sampled from the Water-FLUTe multilevels (Janniche et al. 2013 and 2013b) at the site.
Method and technique
The FLUTe Activated Carbon Technique was described e.g. in Janniche et al. (2013). The sorption of chlorinated ethenes on activated carbon was determined in laboratory experiments as described in Sørensen et al. (2014) to obtain equilibrium sorption coefficients (Kd) for individual and mixed chlorinated ethenes on activated carbon from aqueous solution. As the uptake on FACT in limestone aquifers will depend on transport (advection and/or diffusion and retardation by sorption) in the limestone matrix and fractures as well as on the sorption to activated carbon, a modeling tool was developed (Mosthaf et al. 2014) which allows for the interpretation of field data and the analysis of the influence of various aquifer parameters. The model provides a link between sorbed concentrations on the FACT and the prevailing aqueous pore water concentrations for a range of hydraulic parameters and conditions typical for limestone aquifers.
Results and outlook
The sorption experiments showed very strong sorption with reasonably linear sorption isotherms over a very large concentration range for individual chlorinated ethenes. At high PCE concentrations, competition for sorption sites resulted in non-linearity and much lower sorption of the less hydrophobic compounds TCE and particularly c-DCE. The model simulation results demonstrate the influence of common aquifer parameters on the observed sorbed concentrations on the FACT. The influence of the porosity and of the positioning of the FACT with respect to the flow is comparably small (factor 2-3), whereas the influence of sorption coefficients is increasing with the sorption coefficients and is particularly important for Kd-values above 10-3 L/kg. The hydraulic conductivity has only little influence for values below 10-5 m/s, but up to orders of magnitude influence above that until diffusion within the FACT is limiting the transport processes. For given hydraulic parameters, conditions and exposure time of the FACT, a linear relation between activated carbon concentration and aqueous concentration can be established. This allows the FACT-FLUTe technology to be employed for the characterization of contaminant distribution in limestone aquifers. A comparison between the aqueous (pore water) concentrations calculated with the model from the FACT-FLUTE data with groundwater concentrations from the Water-FLUTe multilevels at the Naverland site showed good correspondence. An advantage of the FACT technique is that it provides discretized data for the matrix and is less influenced by the preferential flow in high conductive zones than multilevel water sampling. It can also be applied in a matrix with strong variation in the hardness (e.g. softer limestone with interbedded chert layers). Furthermore, DNAPL presence in hydraulically active fractures can potentially be identified by high concentration peaks on the FACT.
Literature
Broholm, M.M. et al., 2013. Udvikling af konceptuel forståelse af DNAPL udbredelse i moræneler og kalk ved integreret anvendelse af direkte og indirekte karakteriseringsmetoder. ATV Vintermøde, Vingsted, 5-6. marts 2013.
Janniche, G.S., Fjordbøge, A.S., Broholm, M.M., 2013. ”DNAPL i moræneler og kalk. Vurdering af undersøgelsesmetoder og konceptuel modeludvikling. Naverland 26AB, Albertslund.” DTU Miljø og Region Hovedstaden. www.sara.env.dtu.dk.
Janniche, G.S. et al., 2013b. ”Anvendelse af Water FLUTe multi-level vandprøvetagning til DNAPL karakterisering.” Jordforurening.info 2-13, p. 4-8. Videncenter for Jordforurening. www.jordforurening.info
Kerrn-Jespersen et al., 2013. ”Undersøgelsesmetoder til karakterisering af DNAPL i kalk.” Jordforurening.info 1-13, p. 20-23. Videncenter for Jordforurening. www.jordforurening.info
Mosthaf, K., Broholm, M.M., Binning, P., 2014. ”The FACT-FLUTe technology. A modeling tool for interpreting field data.” DTU Environment and Region Hovedstaden. To appear on www.sara.env.dtu.dk
Sørensen, M.B., Broholm, M.M., 2014. ”Sorption af chlorerede opløsningsmidler på FACT.” DTU Miljø and Region Hovedstaden. www.sara.env.dtu.dk.
The old city centre of Delft is sensitive to both pluvial and fluvial flooding, especially specific areas in the eastern part of the city centre. This includes high groundwater levels, overflowing of canals, surcharging of sewers and flooding of streets and buildings due to storm events. In order to reduce flooding impacts, the canals in the city centre have been separated from the main water system around Delft by means of several weirs which can be closed when heavy rainfall is expected. The city centre also contains several soil contaminated sites, which are most probably influenced by the control of the other parts of the water system in the city (sewer, groundwater and surface water). From 2011 till 2015 a research project was carried out to implement smart monitoring. Goal of the project is to improve water and soil management by creating useful information out of the monitoring data, that can be used for decision making for organizations, policy makers and society.
To study the behaviour of the water system and the interactions between the separate parts of this system, including sewer, groundwater, surface water and soil contamination, a monitoring network was installed. The network consists of online sensors to monitor changes of parameters in, and related to, the water system, such as water level, rainfall conductivity, temperature, turbidity, oxygen and redox potential. A location within the city centre that is contaminated with Chlorinated Volatile Organic Compound (VOC) is investigated in more detail. For this site, the sensor network provides real time information on a number of proxies. Additional monitoring rounds were carried out with electrode measurements and groundwater samples were taken for analyses on VOC and additional parameters (nitrate, methane a.o).
However, just measurements do not provide a prediction of the behaviour of the water system including a soil contamination. Therefore, high quality monitoring data and model results were integrated to information covering the complete water system of the city centre. In order to ensure high data quality, automatic validation algorithms were used which account for missing data, outliers, (linear) trends, signal variance and spatial correlations. A site model was developed for the contaminated site with the monitoring data that describes the spatial distribution of the contaminants and the degradation and transportation over time. The information of the complete water system of the city centre, including the detailed model of the contaminated site, is used to improve soil- and water management for the city centre of Delft.
The proposed paper (and presentation) presents the background of the monitoring system, the design and installation, the data acquisition, the development of the site model, the project results and advise for improved soil and water management in city centres.
Ethyl tert-butyl ether (ETBE), used as a fuel additive in motor gasoline to raise the octane number, is a frequently detected contaminant in soil and groundwater. When ETBE is accidentally released into the subsurface, it rapidly disperses in the environment due to its high water solubility and low interaction with organic matter. The low odor and flavor thresholds in water of 1-2 µg ETBE L-1 makes drinking water resources easily unpalatable. Applying Monitored Natural Attenuation (MNA) as a viable strategy to manage ETBE-contaminated sites bases on a comprehensive understanding of the site-specific biodegradation processes. In general, biodegradation of ETBE has been demonstrated by few microorganisms. However, the role of biodegradation in in situ reduction of ETBE contaminant loads has only scarcely been investigated.
In the present study, we investigated the in situ biodegradation of ETBE in a fuel-contaminated aquifer using two stable isotope tools: i) compound-specific stable isotope analysis (CSIA) and ii) in situ microcosms in combination with total lipid fatty acid (TLFA)-stable isotope probing (SIP). CSIA is based on the principle that molecules with heavier isotopes in their reactive position(s) (e.g., 13C, 2H) are generally slower degraded than those with lighter isotopes (e.g., 12C, 1H). The result is a shift in the isotopic composition (e.g., 13C/12C, 2H/1H) as the remaining contaminant fractions becomes progressively enriched in heavier isotopes (e.g., 13C, 2H) in the course of biodegradation. CSIA provides an appropriate tool to assess in situ degradation of individual environmental contaminants both qualitatively and quantitatively in contaminated aquifers [1, 2]. At the field site investigated, CSIA revealed insignificant 13C-enrichment but low 2H-enrichments with isotopic shifts up to +14 ‰ for ETBE, suggesting biodegradation of ETBE along the prevailing anoxic contaminant plume.
Ten months later, oxygen injection was conducted to enhance the biodegradation of petroleum hydrocarbons (PH) at the field site. Within the framework of this remediation measure, in situ microcosms loaded with 13C-labelled ETBE (BACTRAP®s) were exposed for 119 days in selected groundwater wells to assess the biodegradation of ETBE by TLFA-SIP under the following conditions: (i) ETBE as main contaminant; (ii) ETBE as main contaminant subjected to oxygen injection; (iii) ETBE plus other petroleum hydrocarbons (PH); (iv) ETBE plus other PH subjected to oxygen injection. In situ microcosms in combination with 13C-labelled substrates (BACTRAP®s) aim to enrich indigenous groundwater microorganisms on site and subsequent analysis of their community structure and carbon assimilation patterns (for review see [3]). Under all conditions investigated, transformation of the 13C-carbon, derived from the 13C-labelled ETBE, into fatty acids was found, providing clear evidence of ETBE biodegradation at the field site. Based on the hydrochemical analysis, aerobic and anaerobic degradation of ETBE could be expected at the field site.
References:
1. R.U. Meckenstock, B. Morasch, C. Griebler, H.H. Richnow, Stable isotope fractionation analysis as a tool to monitor biodegradation in contaminated aquifers, J. Contam. Hydrol., 75 (2004) 215-255.
2. M. Thullner, F. Centler, H.H. Richnow, A. Fischer, Quantification of organic pollutant degradation in contaminated aquifers using compound specific stable isotope analysis - review of recent developments, Org. Geochem., 42 (2012) 1440-1460.
3. P. Bombach, H.H. Richnow, M. Kästner, A. Fischer, Current approaches for the assessment of in situ biodegradation, Appl Microbiol Biotechnol, 86 (2010) 839-852.