Monitoring Concepts

Above: Initially higher costs for the investigation of natural attenuation will result in efficient, cost-effective remediation strategies (black line).

Below: Insight gained through contaminant degradation monitoring in various rehabilitation phases.

Monitoring Concepts: Natural Attenuation and Technical Measures

The sound investigation of biological processes in contaminated sites is groundbreaking for the remediation planning. Anyone who takes account of natural attenuation in contaminated sites can massively reduce the costs of rehabilitation (Mäurer et al., 2008, KORA 2008a-d, BLU 2015). Environmental authorities are therefore proposing the extensive exploration of natural processes in contaminant plumes (LABO 2015).

Appropriate Proof of Contaminant Degradation in Certain Stages of Remediation

Preliminary investigation to determine remediation strategies
Combination of natural attenuation and active remediation of the damage source
MNA – sole monitoring of natural biodegradation
Success control of in situ stimulation measures (ENA)
Aftercare for possible remobilization of pollutants.

Detection and characterization of pollutant degradation are often insufficient. Isotope studies can be used to determine the sustainable biological elimination of pollutants in situ. In addition, certain methods allow the determination of degradation rates and a prediction of plume behavior. As soon as pollutant degradation is established as an integral part of a remediation concept, several independent test procedures are generally used (multiple-line-of-evidence approach). Isodetect offers a total of ten monitoring methods.

Isotope monitoring of contaminant plumes comprises sampling along specific flow paths and plume areas (main downstream line, plume fringe, transects) – starting from the pollutant source.

Typically, 5-10 measuring points are sampled during a preliminary monitoring campaign. For detailed investigations, depending on the extent of the contaminant plume, up to 30 measuring points can be taken into account.

Preliminary Investigation: Isotope Analysis of Carbon (¹³C/¹²C CSIA)

One of the first steps in remediation planning is testing the site for natural attenuation processes. The most efficient method of detecting biodegradation for most pollutants is the compound-specific analysis of carbon isotopes 13C/12C (CSIA). Depending on the extent and development of the plume, several measuring points should be sampled once along certain transects and flow paths.

Rules for Selecting Sampling Points

Locate pollutant source (s): 2 – 6 measuring points
Define main downstream line: 2 – 6 measuring points
Define transects: 4 – 10 measuring points
Narrow the border of the plume: 0 – 10 measuring points

In most cases, 5-10 measuring points are selected for a preliminary examination. Preliminary information on hydrogeology, contaminat concentration and site history will facilitate the selection of measuring points and later interpretation of isotope data.

Useful Background Information for Isotope Monitoring

Groundwater flow direction and flow velocity
Pollutant concentrations and redox parameters
Sampling well characteristics and presumed position of aquifer layers
Site history and potential sources of pollution
Previous and planned remedial measures

The initial investigation already provides quantitative results on in situ contaminant degradation, which can be extended and verified with repeated CSIA monitoring.

Dominant contaminants in sites investigated by Isodetect since 2005 (n> 200).

Multi-line of Evidence Approach to Establish a Sound Site Concept

Isodetect has investigated more than 250 contaminated sites throughout Europe over the last 12 years by carbon isotope monitoring. An additional method was applied in about 40% of the sites in order to validate and characterize the degradation processes (multi-line-of-evidence approach). Depending on the pollutant group, certain combinations of monitoring tools are particularly suitable for this purpose.

Preferable Combination of Monitoring Tools for Degradation of Contaminant Groups

PAK: Metabolite Analysis – BACTRAPs – Laboratory Microcosms
MTBE, ETBE, TBA: CSIA ¹³C/¹²C – CSIA ²H/¹H – BACTRAPS – CSIA ¹³C/¹²C repeated – Laboratory Microcosms
BTEX: CSIA ¹³C/¹²C – Metabolite Analysis – BACTRAPS – CSIA ¹³C/¹²C repeated – CSIA ²H/¹H – CSIA redox compounds – qPCR – Laboratory Microcosms
CVOC: CSIA ¹³C/¹²C – qPCR – ³⁷Cl/³⁵Cl CSIA – CSIA ¹³C/¹²C repeated – CSIA redox compounds – Laboratory Microcosms

The selection of monitoring tools is based primarily on the pollutant profile as well as the historical, hydrogeological and rehabilitation strategic features of the contaminated site. Experience has shown that every contaminated site requires a specific monitoring concept.

Frequency of complementary methods for the detailed investigation of sites contaminated with chlorinated ethenes, where 13C/12C isotope monitoring was performed routinously.

Frequency of complementary methods for the detailed investigation of sites contaminated with BTEX or MTBE, where 13C/12C isotope monitoring was performed routinously.

Contaminant plume with zones for different remediation targets and rehabilitation technologies (LABO 2015).

Technical Remediation Combined with MNA

The major redevelopment strategy of isotope studies in most cases (80% of Isodetect’s sites) is the combination of natural attenuation and technical removal of contaminants at the source area (e.g., by in situ stimulation of existing degradation activity, spotting, pump-and-treat, drainage). This cost-saving concept can be implemented in cooperation with environmental authorities. Preliminary exploration of degradation intensity using 13C/12C CSIA provides basic information on natural contaminant attenuation processes in the contaminant plume. The understanding of hydrological and biological processes should be substancialized in the further remediation steps in order to align and configure the technical efforts. The pollutant plume can be divided into zones with different remediation targets (see LABO 2015). Finally, by technical procedures, the pollutant source is eliminated as far as possible and, at the same time, the natural attenuation is further proved in the plume area. Overall, the monitoring concept will enable intelligent, sustainable handling of the natural site conditions and thus leads to considerable cost savings.

Strategies for Natural Attenuation in Combination with Technical Remediation Measures

1. Scenarios: Indication of biodegradation processes
2. In-depth investigation: Quantitative proof of biodegradation and zoning of the contaminant plume
3. Process understanding, modelling, freight analysis, suitability of remediation techniques, cost estimates: Selection and alignment of active remediation technology
4. Definition of the key areas for technical remediation and extended areas for MNA

The Rondenbarg landfill (Hamburg) is reconstructed exclusively by monitored natural attenuation, because technical remedial measures would have been ten times more expensive (Mäurer et al., 2008).

Exclusive Application of MNA

In einigen Altlasten ist aufgrund hydrogeologischer Gegebenheiten bzw. unverhältnismäßiger Kosten keine aktive Sanierung möglich. Wenn die natürliche Schadstoffminderung jedoch stark genug ist, reicht es aus ihre nachhaltige Wirkung nachzuweisen und zu überwachen. In diesem Fall wird ein langfristiges Monitoringkonzept erstellt, das einen umfassenden Mehrmethodenansatz und mehrjährige Untersuchungskampagnen und zur Charakterisierung, Quantifizierung und Prognose der Abbauvorgänge vorsieht. Das typische Vorgehen bei der Planung dieser Projekte beinhaltet mehrere Schritte.

In many contaminated sites technical remediation is impossible due to hydrogeological conditions or disproportionate costs. However, when natural attenuation is intense, it is sufficient to demonstrate and monitor its sustainable effects. In this case, a long-term monitoring plan will be established, which will include a comprehensive multi-line of evidence approach and multiannual investigation campaigns in order to characterize, quantify and forecast natural attenuation processes. The typical approach to realize these projects involves four steps.

Step-by-step Procedures and Tools for Exclusive Application of MNA

Preliminary Proof of in situ biodegradation: Environmental parameters, CSIA 13C/12C
Commensurability of costs: Potential investments for technical measures
Detailed investigations and degradation forecast: Comprehensive multi-line of evidence approach
Aftercare: Long-term monitoring concept using various methods

Despite high monitoring efforts and a long time frame for remediation, MNA concepts are much cheaper than active remedial measures (Mäurer et al., 2008). However, only very few sites of Isodetect (e.g., Rondenbarg) were treated a strictly by MNA.

The pollutants in Rondenbarg (CVOC, BTEX) had spread to 90 m below ground level. Several monitoring methods, for instance 13C/12C isotope monitoring, independently showed a significant and widespread biodegradation.

Spatial sampling design for success control of in situ remediation at injection well GWM 21 with three monitoring wells (CMT) in the injection area and two controls outside.

Success Control of Stimulated Degradation Processes (ENA)

More recently, as an active remedial technology, in situ stimulation of biodegradation is increasingly being carried out (enhanced natural attenuation, ENA). For in situ stimulation, the selection of optimal reagents is essential for the later success of rehabilitation. For this purpose, they should be tested with laboratory microcosms and the prevalent in situ redox processes should be checked with isotope methods.

Depending on the pollutant profile and site conditions, the enhancement of degradation is carried out by adding sulfate, iron nanoparticles, oxygen, molasses or other reagents. For many technologies, sound verification of the intensified degradation of pollutants is necessary. As the hydraulic effects of a reagent injection are difficult to control, contentration data have minor validity. A short-term decline of pollution may be a reesult of dilution effects. In other cases, the mobilization of pollutants in the source area can cause a significant increase in pollution. The monitoring of the isotope fractionation (13C / 12C CSIA) over several months allows a clear success control of the biodegradation. An adequate sampling design regards, the spatial and temporal dimensions of the treatment.

Sampling Concept for Success Control of ENA measures

Select measuring points: 1-2 at the injection site, 2-6 in the redevelopment area, 1-2 as outside control
Schedule sampling dates: 1-2 before reagent injection, 4-10 after reagent injection (maybe redundant)

The isotope enrichment of 2H-benzene (black line) in monitoring well CMT1 indicates significant stimulation of biodegradation in April 2014 (after injection of reagents in November 2013) despite of increasing concentration.

Aftercare

The aftercare of remedial measures is mostly neglected, although the remobilization of pollutants (rebound effect) is a common problem in the remediation of contaminated sites.

Especially after completion of pump-and-treat or injection measures for biological or chemical pollutant elimination, there is generally a massive increase in pollutant concentrations. In this case, care should be taken in the post-treatment period, and the intrinsic degradation potential of the contaminant plume should be detected and verified by appropriate investigations.