Fastlæggelse af oprensningskriterier for grundvandstruende forureninger

Abstract

The objective of remediation of contaminated sites which pose a risk to the groundwater is to protect the resource for drinking water usage. To guide clean-up the Danish EPA requires that groundwater quality criteria must be met at a point of compliance located either 100 m down gradient of the contaminated site, or at a point 1 years transport downstream in the first impacted aquifer. Thus, the EPA guidelines specify both the point of compliance and the acceptable concentration at that point. In order to make this long term objective operational, it is necessary to understand the relationship between the source concentration and the concentration at the down gradient point of compliance, so that the long term criteria can be transformed into a short term local criteria at the source. The short term criteria at the site can then be used as the basis for an assessment of remediation progress and to determine the stop criteria for clean-up. The coupling of downstream concentrations to the source has not yet been standardized in the remediation industry. An industry survey has revealed that is it done in many different ways, especially at larger sites where various advanced models are used. There is a need, especially for small and average size cases, to develop guidelines to assess remediation performance, and thereby systemize the implementation of clean-up criteria. 21 cases were covered in the Industry survey. In most cases the threat of direct groundwater contamination has been used to set clean-up goals. But in some cases the EPA guidelines have not been used and remediation techniques have been selected based on a holistic environmental assessment. In 20 of the 21 cases clean-up criteria have been determined for the source area – though only partly in 7 of the 20 cases. Based on 5 model cases representing typical Danish geologies, hydrogeologies, and the contaminant types commonly found at small and average size contaminated sites (oil and chlorinated solvents), a methodology has been developed to determine clean-up criteria. The methodology considers the following steps: 1. Set-up of a conceptual site model 2. Definition of points of compliance 3. Definition of criteria at the points of compliance 4. Transformation of criteria to the contaminant source a. Choice of model b. Choice of parameters c. Assessment of uncertainties 5. Documentation The definition of the conceptual site model is probably the most critical part of the process, since this is where the governing processes are identified. Consequently conceptual model uncertainty is the most significant contributor to total model uncertainty. It is necessary to do some “backwards” calculations from point of compliance at the receptor (POCrecp) to the source zone (POCsource) when a long term clean-up criteria at the point of compliance (POCrecp) is to be transferred to an operational short term clean-up criteria at the source (POCsource). In principle, this is the reverse calculation of that typically done in a risk 12 assessment, where calculations are done forwards in the direction of transport from POCsource to POCrecp. The calculations consider the processes taking place between the source zone and the receptor (percolation from the source, transport and dissolution in the groundwater down gradient of the source, and possibly degradation and sorption). Unfortunately, no single tool is available to do this calculation for all sites, geologies etc, and so it necessary to combine a number of (relatively simple) calculation tools/models. Once the clean-up criteria have been defined and the remediation completed, it is necessary to document evidence of clean up at its conclusion and that contamination will not reoccur in future. The extent and type of monitoring samples required for the documentation are determined by the local geological and hydrogeological conditions and the remediation technique. In this report, some general guidelines and check lists are provided which can be used to assess the amount of post-remediation monitoring data needed in a given case. A large number of simple modeling tools can be used for risk assessment of small and average sized contaminated sites. At these sites the development of comprehensive geological, hydrogeological and contaminant transport models is too costly. Based on a literature review by Troldborg (2010), 5 modeling tools (JAGG1.5, REMChlor, RISC4, ConSIM and DTUV1D) have been tested for 5 contaminated site case studies, covering typical Danish geologies, hydrogeology and contaminants. The calculations have shown that it can be necessary to allow an “acceptance period” from the start of the remediation to the time when clean-up criteria are met. An acceptance period is needed because there is a delay between the effect of remediation on the concentration at POCsource and its measurement at POCrecp, due to the transport time between the contaminant source and the point of compliance. This delay can be particularly long in fractured media. If an acceptance period is not allowed the costs of remediation will increase because larger areas will need to be remediated. During the acceptance period a pump and treat system can be used to limit downstream contaminant impact. At first glance, the different modeling tools provide different results – however, this is mostly due to differences in the default values of dispersion and degradation. Other factors affecting results are differences in how the source concentration and screen length are defined. When parameters are set to have similar values, similar results are obtained by the different models. A major conclusion of the 5 case studies is that a very high degree of clean-up (more than 99%) is required to meet the guidelines of the Danish EPA. As a result, many technologies will not achieve the required results, and aggressive remediation techniques such as soil removal or thermal remediation are needed. But these techniques are not always practical and life cycle assessment shows that they are often not the most sustainable solutions. The determination of clean-up criteria must therefore be considered in an overall cost-benefit analysis when the Danish Regions prioritize cases and actions. Chapter 5, “Guidelines for setting clean-up criteria” describes some of the issues which should be considered. A conceptual site model of geology, hydrogeology and contamination should be developed. Often simplification of the conceptual model will be necessary in order to employ available modeling tools, and in some cases it will be necessary to divide calculations into several steps. Creating conceptual site models is the most difficult and important part of the process. Here, consideration should be given to the source architecture, source strength, determination of whether transport should be described through a fractured media or through a homogenous equivalent media. Then the model must be selected and the parameters defined (dispersivity, degradation rate and point of compliance). As part of the calculation of clean-up criteria it is important to determine when the criteria should be fulfilled. Some time will pass by before the effect of a remediation will be seen at a down gradient point of compliance, and in many cases some contamination will be left untreated in the fringes of the source. Finally, it is important to evaluate whether a sound relationship exists between the benefits of the remediation and the costs and environmental impacts of reaching the specified clean-up levels.