Nature to the rescue: Natural processes can reduce pollutants in groundwater
When contaminated sites are being remediated authorities and the companies concerned focus on contaminant loads in the groundwater. So-called natural attenuation processes have been increasingly monitored and used over the last few years. A collaborative research project, which has been working for more than six years in Germany, has now developed the scientific fundamentals to better understand the natural degradation processes. As part of the project, 24 contaminated sites were examined in detail.
(01.06.2010) In the 1950s and 1960s the term natural attenuation of waterways smacked of an attempt to justify bad practices because the lack of sewage treatment plants and the serious impacts of industrial residues on rivers left them virtually unable to clean themselves. Now the term has been rediscovered. Natural attenuation (NA) includes all physical, chemical and biological processes that lead to a reduction in the mass, toxicity, mobility, volume or concentration of pollutants in soil and groundwater without any human intervention. The effectiveness of these processes is monitored over time and is known as Monitored Natural Attenuation (MNA). A third term in this connection is ENA (Enhanced Natural Attenuation); this is an in-situ remediation measure with active intervention in the process involving stimulation of or support for the natural attenuation processes by adding substances to the natural reaction areas.
In contaminated site remediation projects the companies obliged to carry out the remediation work are faced with high costs, and especially in groundwater remediation the pump-and-treat measures that have been running for many years and even decades in some cases give us some hope that costs can be saved by using natural attenuation. A decisive point in the consideration of these processes is acceptance by the supervisory authorities involved in the remediation orders and approvals – and this is where there had been a need for action. In addition, for long-term effective handling of soil and groundwater pollution it is necessary to have a better understanding of and be able to evaluate the natural attenuation processes that take place underground. Only then can they be used and supported in the course of risk assessment and remediation; and this is where there were deficits until recently.
In 2000, the German Federal Ministry of Education and Research (BMBF) announced the collaborative project KORA (Retention and Degradation Processes Reducing Contaminants in Groundwater and Soil). From 2002 to 2008 24.6 million euros were spent in 74 individual projects at 24 contaminated sites examining the effects of natural contaminant reduction processes. These sites are representative of many similar cases in Germany. After six years of research, during which scientists were forced to examine the processes and methods in practice and in the field, MNA is now regarded as one of the best researched strategies for handling contaminated sites. Hence, also thanks to the gratifyingly high level of funding, KORA has proved to be one of the key projects in contaminated site remediation and has gained international acclaim. The results of the funding priority – in the form of working aids that are developed from each other successively – will be provided to representatives from the appropriate authorities, consultant engineers and companies responsible for remediation measures as recommendations for action and guidelines.
If contamination exists (contaminated sites or former landfills) with proven pollution of the groundwater and a need for remedial action, based on the current status it will be considered whether natural attenuation can be taken into account during the progressive process. A decision can be made only on the basis of a risk assessment via an MNA concept.
In agreement with the German National/Federal States Committee on Soil Protection (LABO), which has supported the KORA project from the outset and which in 2005 presented a position paper on the consideration of NA processes in contaminated site remediation, various criteria must be fulfilled to make use of an MNA:
* The quantity of pollutants in the source or the discharge from this should be reduced
* The total processes reducing the impact should be much greater than dilution effects
* The results of the investigation should be an "almost stationary" or shrinking plume and should predict long-term effective NA
To implement an MNA, the KORA recommendations for action describe a four-phase procedure:
+ Phase I: Examine the requirements for MNA
+ Phase II: Specific site investigations to prove the effectiveness of NA
+ Phase III: Forecast and decision about MNA
+ Phase IV: Monitoring and final checks
This timetable to consider natural attenuation within the scope of a remediation measure contains several key points. An MNA concept is not possible at all sites. NA processes are especially effective in the pollutant plume; the source of the pollution should be remediated if possible. Therefore, MNA concepts are often used in combination with active remediation processes as so-called integrated remediation measures. Acceptance of natural attenuation concepts, which has not existed in every authority, should improve considerably due to the results of the research project. Besides, authorities apply the principle of proportionality: measures defined by the authorities and their consequences for the companies that are affected must be in relation to the extent of the hazards that are to be prevented. Measures must be suitable, necessary and appropriate (the mildest solution). This means that active remediation cannot be ordered in all cases.
But the four-phase procedure also shows that the threshold for the deployment of an MNA concept is relatively high – in other words, consideration of natural retention and degradation cannot be had for nothing. On the one hand, the expense of modelling and forecasting is considerable, and it also costs a lot more money to investigate the site. But despite many years of monitoring costs, on the whole this solution can be much less expensive than active remediation (e.g. pump-and-treat (P&T), including a groundwater remediation system). Already many groundwater remediation measures that have been running with P&T for several years are being examined in terms of efficiency. There are many known cases where the systems have been shut down in favour of an MNA solution with the agreement of the supervisory authority.
The results of the funding priority, KORA, have been published in a book of "recommended actions" and in the guidelines of the individual topics. The recommended actions with a collection of methods (85 validated and, in parts, newly developed investigation methods) contain the findings to be generalised and describe the four-phase concept. In the guidelines for the six collected topics industry-specific contaminations are considered and the investigations at the 24 reference sites are documented; they also include aids for systematic processing of individual cases. The guidelines deal with sites contaminated with mineral oils, tar oil, volatile chlorinated hydrocarbons from the chemical and metalworking industries, water hazards from current and former landfills, armament sites, mining tips and stockpiles. The guidelines for topic 7 are dedicated to modelling and forecasts. www.natural-attenuation.de
Author: Hans Joachim Schmitz, Stromberg (Germany)
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