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<span class="midtitle">Basic scenarios<br /><br /></span>
 <b>Introduction</b><br><br>
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. 
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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. <br>

<b>Basic scenarios</b> <br><br>
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.
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Based on previous assessments and several meetings with experts, the following source-, pathway- and receptor-oriented management scenarios have been selected: 

<ol type="1">
<li>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 </li>
<li>Pump and Treat (P&T) in the first aquifer</li>
<li>Enhanced Natural Attenuation (ENA) in the first aquifer</li>
<li>Monitoring and isolation measures at the border of the megasite to protect the pristine groundwater systems</li>
</ol>

The listed management scenarios have been worked out separately in terms of the amount of risk reduction and the costs. This has been done at different ambition levels, which are related to the acceptable impact on the receptors. The ambition level varies between an acceptable chance of exceeding the intervention value in the first aquifer of not more than 1% (high ambition level) and not more than 25% (low ambition level). The higher the ambition level the higher the risk reduction and the higher the corresponding costs. Also the factor time plays an important role. The timeframe at which risk management scenarios lead to risk reduction is crucial information for making decisions.

<u>Source removal</u> 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.
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Scenarios are expressed at different ambition levels, depending on accepted chance of exceeding the I-value:
<ol>
<li>very high:  > 1%,</li> 
<li>high: > 5%,</li> 
<li>medium: > 10%,</li>
<li>low: > 25%</li>
</ol><br>

<i> Effect of source oriented measures in top-layer <br>(impact on 2<sup>nd</sup> plane of compliance)</i><br><br>
 

<i> Effect of source oriented measures in top-layer <br>(impact on 3<sup>rd</sup> plane of compliance)</i><br> 

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With <u>Pump and Treat (P&T)</u> measures in the aquifer environmental objectives can be achieved for the deep groundwater systems at reasonable costs. Trend reversal at the 2<sup>nd</sup> plane of compliance can be achieved in 2015 for 3.7 M€/year, and at the 3<sup>rd</sup> 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. 

<i> Effect of P&T in 1<sup>st</sup> aquifer<br>
(impact on 2<sup>nd</sup> plane of compliance)</i><br>
 

<i> Effect of P&T in 1<sup>st</sup> aquifer<br>
(impact on 3<sup>rd</sup> plane of compliance)</i><br>
 
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The effectiveness of <u>Enhanced Natural Attenuation (ENA)</u> 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 3<sup>rd</sup> 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.

<i> Effect of ENA in 1<sup>st</sup> aquifer<br>
(impact on 3<sup>rd</sup> plane of compliance)</i><br>

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<u>Monitoring and isolation measures</u> at the 3<sup>rd</sup> plane of compliance are initially rather expensive with 7 M€/year in 2005 to compensate for the already existing contaminant impact at the 3<sup>rd</sup> 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 3<sup>rd</sup> 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. 

<i> Effect of MNA and isolation at border<br>
(impact on 3<sup>rd</sup> plane of compliance)</i><br> 
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<b>Conclusions</b><br><br>
Based on the assessment of the basic risk management scenarios, the following conclusions can be taken:
<ol type ="-">

<li>The risk reduction effect and the costs that are associated with the basic risk management scenarios differs strongly. </li>
<li>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. </li>
<li>Therefore a combination of basic risk management scenarios needs to be developed to achieve all objectives in a cost efficient way (see: <a href="megasitee29c.html?index=220&sec=3&meg=rotnet">potential scenarios</a>). </li>
</ol>