Content
Basic Scenarios
Basic scenarios
Introduction
The formulation of risk management scenarios for megasites is a complicated and iterative process. It consists of many different steps starting from the first conceptual idea of potential measures to a final cost-efficient risk management scenario that is accepted by all stakeholders.
Starting point is the prediction of the future impact of the contamination on the receptor without taking any measures (0-scenario or autonomous situation). During the project this prediction has continuously been improved by refining the methodology and increasing the amount of field data. According to the most recent predictions the first aquifer below the harbour is contaminated and will become even more contaminated in the future. On a longer time scale also the pristine groundwater systems outside the harbour will become contaminated. It can be concluded that natural attenuation processes are insufficient to protect the first aquifer below the harbour. Active measures are needed to accomplish the trend-reversal that is needed to meet the requirements of the groundwater directive. The basic risk management scenarios include source-, pathway- and receptor oriented measures. Both the risk reduction effect and the costs of the basic management scenarios will be assessed.
Basic scenarios
In the past several years a large variety of different remediation measures have become available. Many of these are highly innovative and applicable to very specific contaminant situations. For the Rotterdam megasite the number of feasible and effective measures is limited. First of all the geohydrological situation is complex and heterogeneous, which complicates the applicability of many techniques. Secondly, risk management focuses on a specific group of contaminants; the volatile organic contaminants. This further diminishes the number of possible measures.
Based on previous assessments and several meetings with experts, the following source-, pathway- and receptor-oriented management scenarios have been selected:
- Source removal and Pump and Treat (P&T) in the anthropogenic top layer, subdivided in: (a) source removal related to private obligations and permits, and (b) removal of specific sources that cause a high impact to the deep groundwater
- Pump and Treat (P&T) in the first aquifer
- Enhanced Natural Attenuation (ENA) in the first aquifer
- Monitoring and isolation measures at the border of the megasite to protect the pristine groundwater systems
Source removal in combination with pump and treat in the anthropogenic top layer is comparatively costly and only leads to a limited improvement of the quality of the deep groundwater systems (figure). It can be concluded that these measures are therefore ineffective for achieving environmental goals, but particularly effective for fulfilling private obligations for individual sites and obtaining permits. In fact this is the only scenario in which this is the case.
Scenarios are expressed at different ambition levels, depending on accepted chance of exceeding the I-value:
- very high: > 1%,
- high: > 5%,
- medium: > 10%,
- low: > 25%
Effect of source oriented measures in top-layer
(impact on 2nd plane of compliance)
Effect of source oriented measures in top-layer
(impact on 3rd plane of compliance)
With Pump and Treat (P&T) measures in the aquifer environmental objectives can be achieved for the deep groundwater systems at reasonable costs. Trend reversal at the 2nd plane of compliance can be achieved in 2015 for 3.7 M€/year, and at the 3rd plane of compliance in 2030 for 6.3 M€/year. Especially when large volumes of groundwater are involved, P&T measures might lead to setting problems in the subsurface. In addition, the treatment of extracted groundwater involves dilution and drainage towards the sewer systems, which eventually end up in the surface water.
Effect of P&T in 1st aquifer
(impact on 2nd plane of compliance)
Effect of P&T in 1st aquifer
(impact on 3rd plane of compliance)
The effectiveness of Enhanced Natural Attenuation (ENA) measures can be considerable provided that sufficient time and space is available. Risk reduction will therefore only be observed further down flow in the deeper groundwater systems. At the 3rd plane of compliance trend reversal will be achieved in 2050 for a yearly budget of 2 M€. Due to the fact that ENA is innovative and not yet applied at a wider scale within the Rotterdam harbour area, the performance and cost-estimations are relatively uncertain and need to be further investigated.
Effect of ENA in 1st aquifer
(impact on 3rd plane of compliance)
Monitoring and isolation measures at the 3rd plane of compliance are initially rather expensive with 7 M€/year in 2005 to compensate for the already existing contaminant impact at the 3rd plane of compliance and the steep increase of the impact after 2005. In time the effort decreases to 3 M€/year in 2020 and 1 M€/year in 2040 due to a decline of the increase of the impact. The 3rd plane of compliance is completely and instantaneously protected. The major drawback of this scenario is that the deeper groundwater systems below the harbour remain unprotected and will deteriorate in time.
Effect of MNA and isolation at border
(impact on 3rd plane of compliance)
Conclusions
Based on the assessment of the basic risk management scenarios, the following conclusions can be taken:
- The risk reduction effect and the costs that are associated with the basic risk management scenarios differs strongly.
- None of the basic risk management scenarios alone will lead to the achievement of the environmental objectives as formulated by the EU Water Framework Directive and Groundwater Directive, as well as the fulfilment of private obligations for individual sites.
- Therefore a combination of basic risk management scenarios needs to be developed to achieve all objectives in a cost efficient way (see: potential scenarios).
Potential Scenarios
Potential scenarios
Initiation of combined scenarios
The basic risk management scenarios described in step 3.1 have provided insight in the risk reduction effects, the achievement of objectives and the overall costs involved. It has been concluded that none of the basic risk management scenarios alone would lead to a situation in which all of the objectives can be fulfilled. This is only possible by an optimal combination of different risk management scenarios. Based on the costs and effects of the individual risk management scenarios, stakeholders have discussed the cost-efficiency and feasibility of a combined scenario, or the so-called mixed strategy.
Three combined scenarios (A, B and C) have been selected by the stakeholders based on the different ambition levels of the basis risk management scenarios (table underneath). The source-oriented measures are related to the fulfilment of private obligations, permits and liability, and are therefore fixed for each scenario. These measures have to be taken anyhow in all situations. Pathway-oriented measures have been selected in such a way that trend-reversal is achieved at both the 2nd and 3rd plane of compliance. This can be done by combination of Pump and Treat and Enhanced Natural Attenuation in the aquifer (scenario A), or by Pump and Treat alone (scenario B and C). Different ambition levels have been considered for the Pump and Treat scenario. The effect of the source- and pathway-oriented measures on the 3rd plane of compliance determines the extent to which receptor oriented measures are needed. The more effort is taken in the source and pathway-oriented measures, the higher the risk reduction at the 3rd plane of compliance and the less effort remains for receptor-oriented measures (and vice versa).
Overview of the combined scenarios that are selected, indicating the ambition level for each individual risk management scenario
| Scenario | Source-oriented | Pathway-oriented | Receptor-oriented | |
| . | Source removal and P&T* | P&T aquifer | ENA aquifer | Monitoring and isolation |
| A | Fixed according to actual need | Medium ambition level | Medium ambition level | Depending on predicted need |
| B | Fixed according to actual need | High ambition level | - | Depending on predicted need |
| C | Fixed according to actual need | Medium ambition level | - | Depending on predicted need |
Assessment of cost-efficiency
The assessment of the cost-efficiency of the three mixed strategies consist of an evaluation of the costs versus the risk reduction achieved. In principle, the strategy that offers the highest degree of risk reduction at the lowest costs, is the preferred strategy. Risk reduction is defined in this study as the reduction of the impacted extent of the deep groundwater systems, which is expressed at the 2nd and 3rd plane of compliance. In addition to risk reduction and costs other criteria play an important role in the selection of the preferred risk management scenario.
An overview of the costs of the three mixed strategies is provided in the following figure.
Specification of the total costs of the three mixed strategies
The risk reduction achieved by the mixed strategies is expressed as the reduction of the impact at the 2nd and 3rd plane of compliance compared to the autonomous situation.
Mixed strategy A
The reduction of the impact achieved in mixed scenario A is shown in the following two figures, respectively at the 2nd and 3rd plane of compliance. After initiation of the measures in 2005, the area impacted above I-value at the 2nd plane of compliance first increases until 2015 to a maximum of 9.5% of the total aquifer. After that it decreases and stabilises to a minimum of 6.5% after 2040. The impacted length at the 3rd plane of compliance increases to more than 7% in 2040 from where it gradually decreases. For both the 2nd and 3rd plane of compliance the reduction compared to the autonomous situation is significant.
Risk reduction achieved by mixed scenario A at the 2nd plane of compliance. The thin lines indicate the uncertainty ranges at the 25th and 75th percentile and the thick lines the most likely situation at the 50th percentile.
Risk reduction achieved by mixed scenario A at the 3rd plane of compliance. The thin lines indicate the uncertainty ranges at the 25th and 75th percentile and the thick lines the most likely situation at the 50th percentile.
Mixed strategy B
Due to the focus on pump and treat measures in this scenario, the impacted area at the 2nd plane of compliance is strongly decreased from 9% in 2015 to 3% after 2050. In absolute terms a high degree of reduction is achieved compared to the autonomous situation. At the 3rd plane of compliance the reduction is comparable to mixed scenario A, with a gradual decrease after 2040 to 6%.
Risk reduction achieved by mixed scenario B at the 2nd plane of compliance. The thin lines indicate the uncertainty ranges at the 25th and 75th percentile and the thick lines the most likely situation at the 50th percentile.
Risk reduction achieved by mixed scenario B at the 3rd plane of compliance. The thin lines indicate the uncertainty ranges at the 25th and 75th percentile and the thick lines the most likely situation at the 50th percentile.
Mixed strategy C
In mixed strategy C, where the pump and treat measures are less extensive than in scenario B and no Enhanced Natural Attenuation is taken into account, a decrease of the impacted area is achieved at the 2nd plane of compliance but not at the 3rd plane of compliance. A gradual increase to 8% in 2100 is expected. In this case the improvement compared to the autonomous situation is minimal. Monitoring and isolation measures at the 3rd plane of compliance will be needed to sufficiently protect the pristine groundwater systems outside the harbour area.
Risk reduction achieved by mixed scenario C at the 2nd plane of compliance. The thin lines indicate the uncertainty ranges at the 25th and 75th percentile and the thick lines the most likely situation at the 50th percentile.
Risk reduction achieved by mixed scenario C at the 3rd plane of compliance. The thin lines indicate the uncertainty ranges at the 25th and 75th percentile and the thick lines the most likely situation at the 50th percentile.
Final Scenario
Final scenario
Selection of final scenario
The selection of the final risk management scenario includes an assessment of all relevant decision criteria, such as costs versus the risk reduction achieved. In addition to cost-efficiency other considerations play an important role in the final selection of the risk management scenario, such as the compliance with EU legislation and the fulfilment of private obligations for industrial sites.
For the basic risk management scenarios and the mixed strategies an evaluation of the decision criteria has been made. The results of this evaluation are indicated by different colours in the following tables. The green, yellow and red colours indicate that the corresponding criteria perform respectively well, average and poorly.
Evaluation of relevant decision criteria for basic risk management scenarios.
| Criteria | Basic risk management scenarios | ||||
| . | Source * |
Source ** |
P&T ** |
ENA ** |
Mon. + iso. |
| Cost | 16 M€/y | 25 M€/y | 6.3 M€/y | 2.0 M€/y | 3.3 M€/y |
| Compliance EU legisl. (trend reversal) | . | . | . | . | . |
| Fulfilment private obligations | . | . | . | . | . |
| Reduction impact POC2 | 22% | 39% | 68% | 0% | 0% |
| Protection pristine groundwater systems | . | . | . | . | . |
| Impact to other compartments | . | . | . | . | . |
| Technical limitations (setting problems) | . | . | . | . | . |
| Innovative potential | . | . | . | . | . |
**at high ambition level
Based on this information the statement which has been made earlier on the unsuitability of the basic risk management scenarios as a realistic solution for the Rotterdam megasite is confirmed. As indicated by the red colours for several criteria none of the basic risk management scenarios is sufficient to achieve the objectives.
Evaluation of relevant decision criteria for the mixed strategies.
| Criteria | Mixed strategies | ||
| . | A | B | C |
| Cost | 15.7 M€/y | 17.2 M€/y | 15.1 M€/y |
| Compliance EU legisl. (trend reversal) | . | . | . |
| Fulfilment private obligations | . | . | . |
| Reduction impact POC2 | 43% | 71% | 43% |
| Protection pristine groundwater systems | . | . | . |
| Impact to other compartments | . | . | . |
| Technical limitations (setting problems) | . | . | . |
| Innovative potential | . | . | . |
For the mixed strategies objectives are either completely fulfilled or partly fulfilled. The costs of the three scenarios are in the same order. In all cases the same source-oriented measures are implemented, which are all sufficient to fulfil the private obligations. Also the protection of the pristine groundwater systems is guaranteed in all mixed strategies by adopting a thorough monitoring system and taking immediate isolation measures at the 3rd plane of compliance as soon as the contaminants are detected.
Scenario C is the cheapest with 15.1 M€/y but does not lead to trend reversal at the 3rd plane of compliance and is therefore not fully in compliance with the EU legislation. Since scenario C is primarily focussed on source- and receptor-oriented measures and not so strongly on the pathway-oriented measures pump and treat and ENA, the reduction of the impact at the 2nd plane of compliance and the innovative potential are limited.
Due to the extensive pump and treat measures, mixed strategy B is the most expensive scenario. It might lead to the highest degree of impact reduction at the 2nd plane of compliance, but performs worse on the last three aspects. Pump and treat measures cause a relatively high impact to other compartments if the contaminated groundwater is diluted and wasted on the sewer systems. In this scenario large volumes of groundwater are extracted, which in some cases will lead to potential setting problems and potential damage to infrastructure. Further the innovative potential of this scenario is limited because no innovative enhanced natural attenuation technologies are applied.
Scenario A combines pump and treat measures with enhanced natural attenuation measures. Together with source- and receptor-oriented measures these pathway-oriented measures provide an optimal solution that fulfils most objectives. Due to the enhanced natural attenuation measures the innovative potential is high. It is expected that in the near future the performance can be further increased and the costs reduced provided that a research program is initiated.
Based on these considerations, mixed strategy A is considered as the preferred scenario. The Rotterdam stakeholders are recommended to further investigate and implement this scenario. The implementation phase is further described in a megasite management plan (deliverable 2.7).
