THE POWER OF PLANTS The application of phytoremediation in the Broekpolder: an interdisciplinary assessment

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THE POWER OF PLANTS

The application of phytoremediation in the Broekpolder:

an interdisciplinary assessment

ABSTRACT: In this interdisciplinary assessment the application of phytoremediation in the Broekpolder in discussed. A plant list with potential species and preliminary distribution in the area is offered. And the project is interpreted as Brownfield Development upon which an analysis regarding criteria of an assessment model of successful Brownfield Development is given. We conclude that remediating the entire Broekpolder with phytoremediation seems impossible. Because the bioavailability of heavy metals present is too low due to the high pH and clay content of the soils.

Course: Interdisciplinary Project

Students: Marte Siebinga (Earth sciences, 11003154) Joost Kingma (Biology, 11020202)

Nina van Lierop (Urban planning, 000000) Coordinator: Dr. Coyan Tromp

Dr. Crelis Rammelt Tutor: Fenna Hoefsloot

Client: Balance projectmanagement

Date: 30/05/2018

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1. Introduction

The Broekpolder is a nature reserve and recreational area of around 350 hectares located immediately to the north east of Vlaardingen, a suburb of Rotterdam. In the period from 1958 to 1976 the area was utilized for disposal of dredged materials from the Rotterdam harbor area (Osté, 2016). A major part of the Broekpolder was raised with 4-7 meters of contaminated silt, resulting in severe soil pollution and the contamination of ground and surface waters in and around the area (Osté, 2016). Chemicals present in excessive amounts are aldrin, endrin, dieldrin, Barium (Ba), Copper (Cu), Chromium (Cr), Cadmium (Cd) and Zinc (Zn) (Grontmij, 2009) (Oste, 2016).

Soil research by Grontmij (2009) calculated that in a worst-case scenario at 19 of their in total 25 evaluated measurement points in the Broekpolder toxic effects from contamination were expected on half of the organisms in the soil. Especially heavy metals contributed to these predictions.

However, bioassays (mainly on bacteria) demonstrated that none of the investigated samples showed toxicological effects as predicted. Grontmij (2009) concluded that, based upon current research, ecological risk in the Broekpolder is exclusively determined by risks of secondary poisoning (i.e. toxification due to increased concentrations of a substance in organisms higher in a food chain) from drin pesticides (especially endrin).

The wet Bodembescherming (Wbb) is an act that aims to regulate the protection of the dutch soils. It recognises the importance of preventing, limiting or reversing changes in the characteristics of the soil that represent a reduction or threat to the functional

properties that the soil has for humans, plants or animals (Artikel 1 Wet Bodembescherming, 2018).

Regarding areas that are polluted before 1987 (i.e. the Broekpolder), the Wbb contains a regulation that prohibits future developments of the area until is recognised that the area is not polluted. And in 2010, based upon research by Chemielinco (1997) that confirmed the polluted state of the area, a decision was issued in context of the Wbb to urgently sanitize the Broekpolder (Grontmij 2011).

The province of South Holland wants to combine the sanitation project with development plans intended to enhance the current function as a nature and recreation area. As a consequence current functionalities of the area have to be maintained while implementing a remediation project.

Also, evaluating the site-preparation costs, market oriented appraisal and expected contribution of planned future land use to regional development is important when developing a brownfield site (Schädler, 2011) like the Broekpolder. According to Schädler (2011) these three pinnacles determine whether a brownfield development project becomes successful or not. Since brownfield development is characterized by its high and uncertain clean-up and preparation cost Paull (2008).

To sanitize the area, different remediation methods can be used. One promising and environmentally friendly remediation method is phytoremediation. Phytoremediation is commonly used as a generic term for the use of plants for remediating soils and water contaminated with organic and inorganic contaminants (UNEP, 2018).

Beneficial to visitors of the Broekpolder is that phytoremediation is unobtrusive (Lelie, 2001) and that the area could remain accessible while sanitized.

Furthermore, the application of this strategy creates an opportunity to research the fundamental mechanisms involved in plants and the soil. This aids the development of more

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effective, transgenic plants with improved phytoremediation abilities.

Therefore, in this report, the application of phytoremediation in the Broekpolder is researched. This is done by answering the following research question and sub-questions: How can phytoremediation be used for sanitation (soil and groundwater) of the Broekpolder, while maintaining current functionalities of the area?

To answer the research question, three sub questions are formulated.

-To what extent can successful brownfield development models give an indication on the level of successfulness by using phytoremediation in the Broekpolder?

-What plant species are applicable for phytoremediation in the Broekpolder and how should these plants be distributed over the area?

-How do soil properties influence the efficiency of phytoremediation?

These questions reflect the interdisciplinary nature of sanitizing the Broekpolder. In the first section of this report relevant theories and concepts in Brownfield Development and phytoremediation are discussed. Then is elaborated upon the research problem and interdisciplinary integration. This is followed by an analysis of the chosen research methods, findings and at last an conclusion with recommendations.

2. Theoretical Framework

In this chapter relevant concepts and assumptions of this research are discussed. First, the importance of the concept brownfield development is being discussed. The assessment model of Schädler (2011) is the starting point of this research and will be elaborated on in the theoretical framework. Secondly, the chemical selection and intervention is being discussed. Thirdly, this research explores the option of being a successful phytoremediation method for the Broekpolder, the framework will elaborate on the concept

phytoremediation. Lastly, the importance of bioavailability for phytoremediation will also be discussed.

2.1 Brownfield development

Multiple definitions of brownfield development are being discussed in scientific literature. Environment Canada (2016) uses the term “properties where past actions have caused environmental contamination, but which still have potential for redevelopment or other economic opportunities” while Alker (2000) refers “ to land that is has been developed before with industrial purposes”. The Broekpolder is a type of brownfield since the soil is polluted caused by industrial purposes, in addition the land is going to be developed and remediated which makes it brownfield development. Brownfield development is

characterized by its uncertainty and risk, which are higher than in other types of development for example mega-projects, regeneration, transformation and re-usage

heritage (Hissink-Muller, 2017). For this reason brownfield development has a higher risk to become a failure or a disappointing achievement than other types of development (Hissink-Muller, 2017) scientific literature about brownfield development mainly focuses on how to ensure successful development or how the development and its success can be measured (Schädler, 2011 ; Grimski, 2001).

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According to Silverthorne (2006) the definition of successful brownfield varies between countries, academic disciplines and regeneration projects, recognising that different concepts of success emerges from the attachment of different values. Schädler (2011) and Grimski (2001) have both created an assessment model concerning successful brownfield development. Schädler (2011) identifies three pinnacles of brownfield

development revitalization: (1) subsurface remediation and site preparation costs, (2) market-oriented economic appraisal, and (3) the expected contribution of planned future land use to sustainable community and regional development. While Grimski (2001) identifies the four factors: (1) site preparation, (2) future use, (3) economic viability and (4) legal framework. Schädler’s (2011) model evaluates redevelopment options of large contaminated brownfields which is focussed on the early stage of the brownfield

redevelopment process and is characterized by limited data availability and by flexibility in land use planning and development scope. In contrary, Grimski (2001) has not created an in-depth assessment model but has created a model consisting of four factors that determine the potential success or failure of brownfield projects. Grimski (2001) more or less discusses why the four factors determine the outcome of a brownfield revitalization project. Both Grimski (2001) and Schädler (2011) focus on brownfield development in general however the Broekpolder is planned to become a greening project and serve as recreational area. Therefore these models by Grimski (2001) and Schädler (2011) could be less applicable in this specific case.

So, Doick (2009) proposes a ‘logic’ model for the evaluation of success of brownfield greening projects. His focus is more on the inputs, processes, outputs and outcomes from and for stakeholders in the area while Grimski (2001) and Schädler(2011) seem to include and lay the focus on the financial part of the development.

So for the Broekpolder, how and to what extend the stakeholders are involved could be more relevant for successful development instead of focussing on the financial part of the development. However the stakeholders will not be included in this research on request of Balance Office therefore the model of Schädler (2011) will be used since both models from Grimski (2001) and Schädler (2011) take the same pinnacles into account. Assessing this model in the Broekpolder case is the starting point in this research based on the outcome whether phytoremediation is a successful method for remediating the area.

2.2 Chemical selection and intervention

The soil in the Broekpolder contains many chemicals. Among others, 16 heavy metals and 5 drin pesticides (Grontmij, 2009). However, only a selection exceeds the intervention value. The intervention value is a concentration that indicates an official threshold-value above which remediation is obliged. This research is limited to the remediation of those pollutants, which are 3 drins (endrin,dieldrin & aldrin) and four heavy metals (Barium (Ba), Chromium (Cr), Cadmium (Cd) and Zinc (Zn)) (Grontmij, 2009).

Drins are organochlorine insecticides. These insecticides were historically widely used but nowadays restricted in most countries due to their persistence in the environment (Jager, 1970). Although no clear definition for heavy metals exist, they are generally defined as molecules with a density of more than 5 g/cm3 (Järup, 2003). Appropriate plant species for phytoremediation in the Broekpolder must be able to bioaccumulate, degrade or reduce the contaminating impact of these chemicals. Phytoremediation is discussed more

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2.3 Phytoremediation

As stated previously, phytoremediation is used as a generic term for the use of plants for remediating soils and water contaminated with organic and inorganic contaminants (UNEP, 2018). Phytoremediation makes use of the innate ability of certain plants to bioaccumulate, degrade or reduce the contaminating impact of particular chemicals (Rhodes, 2013).

This sanitation method comes with advantages and disadvantages.

2.3.1 Advantages and disadvantages of phytoremediation

Firstly, phytoremediation is considered a relatively cost effective method, as the installation and maintenance costs are low compared to alternative remediation methods (Ali et al., 2013; Azubuike et al., 2016; Mench et al., 2009). Sometimes the costs of

phytoremediation are as little as 5% of alternative methods (Prasad, 2003). Other

advantages are conservation of soil structure because plants extract contaminants without affecting the topsoil and possible improved soil fertility due to input of organic matter (Ali et al., 2013; Mench et al., 2009). There is even the possibility of reclaiming valuable metals after harvesting the remediative plants (Rhodes 2013). A more general benefit of the establishment of plants on polluted soils is a reduction in leaching of contaminants to surrounding area because soil erosion is reduced (Ali et al., 2013).

However, large-scale implementation of phytoremediation requires sites with low to moderate levels of pollution since plant growth is often not sustained on heavily

contaminated soils (Ali et al., 2013). Another disadvantage is that phytoremediation is a relatively slow process and the extracted contaminants, when stored in the plants biomass, can enter the food chain through consumption via foraging animals (Ghosh & Singh, 2005). And lastly, if the contaminants are not within the rooting zone of the remediative plants, phytoremediation is not effective (Ghosh & Singh, 2005). Since the Broekpolder area is raised with 4-7 meters of contaminated material, finding plants that remediate this entire layer is potentially difficult. Another important consideration in applying phytoremediation in the invasiveness of plant species.

2.3.2 Invasive plant species

An invasive species is a non-native species whose introduction to the ecosystem under consideration is likely to cause economic or environmental harm or harm to human health (Mullin, 2000). Invasive plant species tend to proliferate rapidly and cause

disturbances to other plants species. For example, through competition with native plants for nutrients and space invasive plants can drive natives out of an ecosystem and potentially even to extinction.

Introduction of invasive species can lead to high costs because, among other reasons, the impact of invasive crop weeds on monoculture practices can lead to economic losses. Ter illustration, in the United States environmental and economic costs associated to invasive plant species is approximately 21 billion euros (25 billion USD) (Pimentel et al., 2005).

Therefore, the usage of potential invasive species for phytoremediation in the

Broekpolder must be minimized and ideally prevented. This is done by researching the origin of remediative plants and selecting for species that are confirmed by van der Meijden (2005) to already exist in the Netherlands without harmful invasive-effects.

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According to Sarma (2011), 500 plant species are identified to have properties potentially suited for phytoremediation. Verbeek (2017) has provided a selection of suitable plant species for phytoremediation in the Broekpolder based on general environmental conditions and non-invasiveness.

This list is used and further elaborated upon. However, even with suitable plant species phytoremediation is not always effective. Because an important factor affecting the efficiency of phytoremediation is the bioavailability of the contaminants in the soil. The next subparagraph elaborates on this subject.

2.4 Bioavailability

One factor influencing the rate of bioaccumulation of contaminants by plants is the

bioavailability of the particular contaminant. Bioavailability can be defined as the fraction of a chemical that can be taken up or transformed by living organisms from the surrounding bio-influenced zone (Wenzel, 2009). If bioavailability is low, the fraction of the contaminants that can be taken up by plants is also low. This will influence the efficiency of

phytoremediation as the ability of plants to bioaccumulate contaminants is lower.

Soil properties have a major influence on the bioavailability and mobility of metals and drins in soils and thus affect the efficiency of phytoremediation (Ali et al., 2013; Wenzel, 2009). For heavy metals, the bioavailability is dependent on the solubility of the metals. Metals absorbed onto negatively charged sites are less mobile and cannot be taken up by plants easily. Soil characteristic like CEC, pH, clay content and organic matter content are all factors influencing the absorption of metals and thus bioavailability (Young and Crawford, 2004; Ali et al., 2013; Meagher, 2000). Soil texture also influences the bioavailability as smaller particles contain a larger surface area for absorption (Rieuwerts, et al., 1998). For organic pollutants (including drins), pH, CEC, organic matter content and clay content are factors influencing the bioavailability as well (Eevers et al., 2017; Nam & Kim, 2002; Wenzel et al., 2009). Besides, bioavailability depends on diffusion and mass transport of the organic pollutant towards the plant roots. Therefore, interconnected soil factors such as water content, porosity and permeability controlling the transport of water and solutes, are also important controls of the bioavailability of organic pollutants (Wenzel et al., 2009). Lastly, increasing soil-pollutant contact time leads to a decreasing bioavailability of organic pollutants due to a process called ageing (Nam & Kim, 2002; Wenzel et al., 2009).

Due to low bioavailability, the potential timescale for phytoremediation processes may be too long. However, the manipulation of soils to increase the uptake of pollutants by plants could enhance phytoremediation (Meagher, 2000). Several studies showed that bioavailability could be optimised by soil manipulation based on the application of chemical agents (Ali et al., 2013; Mench et al., 2009; VanGronsveld et al., 2009). First, lowering the soil pH by the application of elemental sulfur or physiologically acid fertilizers can improve the bioavailability (VanGronsveld et al., 2009). Moreover, the use of complexing agents such as EDDS, EDTA or citric acid promote desorption of pollutants and improve bioavailability (Ali et al., 2013; VanGronsveld et al., 2009). However, there are downsides on artificial soil acidification and the use of complexing agents as these chemical treatments can cause secondary pollution problems (Ali et al., 2013).

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3. Problem description

In the Broekpolder the soil has been contaminated from 1958 to 1978 due to disposal of dredged material from the Rotterdam harbor area. According to the wet Bodembescherming (Wbb) areas that have been polluted before 1987 will have to be cleaned in order to allow future development. In 2010 Chemielinco (1997) confirmed the polluted state of the area and the decision was made to remediate the area. The remediation process has to be organized in such a way that at least it does not disturb and preferably blends in very well with the existing functions of the area. Various aspects can be taken into account like technical feasibility, a limited budget, the interests of various groups of stakeholders, and ecological conditions and uncertainties. For this interdisciplinary project the focus will lay at the exploration and possibility of using phytoremediation as sanitation method for the Broekpolder. The choice for phytoremediation has been made based on previous research results which introduced the possible option of phytoremediation as remediation method for the Broekpolder.

The remediation task in itself for the Broekpolder is very complex, the area is divided among multiple stakeholder with different budgets and ideas for the future use of the Broekpolder, it can cause disagreement which represents the complexity in this case. Interdisciplinary research can tackle the complexity of exploring phytoremediation as

possible remediation method by a certain extend. A discipline determines what type of view a researcher will use for conducting research. In this case three disciplines are combined which brings along three different types of views which adds complexity. It can be difficult to understand each others discipline and research since every discipline might use different definitions of concepts or different ways of methodology. Interdisciplinary research

combines multiple disciplines, combining multiple type of views and different definitions to certain concepts brings along complexity in the research. For this research the integration technique; expansion will be used.

4. Interdisciplinary integration

In this research different disciplines are integrated into one framework to provide an insight into the complexity of the subject. By this integrated framework, the linkages between the different studied factors in this research are made clear. To make the framework the integration technique ‘extension’ is used. This technique expands the meaning of an idea beyond the domain of one discipline into the domain of another discipline (Crelis, 2018). In order to understand which factors influence the applicability of phytoremediation in the Broekpolder, the concept of success is visualized and approached from all contributing disciplines.

The concept of success can be defined as the efficient phytoremediation of the top layer soils in the Broekpolder under a successful brownfield development. In this research a successful phytoremediation of the Broekpolder depends on different factors.

First of all, the remediation rate influences the success of phytoremediation. A higher remediation rate will lead to a more successful phytoremediation as a larger fraction of the contaminants will be bioaccumulated. One factor influencing the remediation rate is the bioavailability of pollutants. A relatively low bioavailability of the pollutants will have a negative effect on the efficiency of phytoremediation. As the diagram shows, bioavailability

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of pollutants is dependent on several soil properties such as pH, CEC, clay content and soil texture. In the Broekpolder case the relatively high pH and clayey soils have a negative influence on the bioavailability of the pollutants, particularly of the heavy metals. The remediation rate also depends on the plant physiology and development. If the used plants are unable to develop in the area, they will not be able to remediate the soils. Consequently, for a successful phytoremediation, plants need a tolerance for heavy metals and drins. Besides, the development of the plants is dependent on both abiotic factors such as

temperature, rainfall and wind, and biotic factors like organic matter content. Furthermore, the use of GMOs or the treatment of soils can have a positive influence on the remediation rate. Another important aspect of a successful phytoremediation is the invasiveness of the used plant species. As invasive species are relatively hard to manage, the usage of invasive species needs to be prevented in order to achieve a successful phytoremediation.

Overall, the three pinnacles of brownfield development determine the successfulness of phytoremediation. For a successful phytoremediation the project needs low preparation costs of the area, a sustainable future use of the area and economic viability. Moreover, low preparation costs are also dependent on the costs of soil treatments or the use of GMOs.

Figure 1: Integrative framework showing the different factors affecting a successful phytoremediation.

5. Methodology

This section describes the methodological framework to answer the sub questions and consequently provide an answer to the main research problem. Due to the time limitations

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in this research it is not possible to gather and analyse core samples in the study area. Therefore, the predominant methods are a literature research and spatial analysis.

The literature research provided an overview on current knowledge, concepts and theories regarding brownfield development, phytoremediation and the ecological conditions in the Broekpolder. Consequently, the distribution of the contamination and corresponding remediative plants were visualized using a spatial analysis.

The following paragraphs elaborate more on the methodological approaches taken in researching Brownfield Development, Phytoremediation and creating visual representations.

5.1 Brownfield Development

The results of the possibility of phytoremediation as remediation method in this research will be interpreted in the assessment model by Schälder (2011) on successful brownfield development based on the following three pinnacles: evaluating the site-preparation costs, market oriented appraisal and expected contribution of planned future land use to regional development is important when developing a brownfield site.

5.2 Phytoremediation

To determine which plant species are applicable for phytoremediation in the Broekpolder, previous research by Verbeek (2017) regarding this topic specifically and available scientific literature was consulted. Research by Osté (2016) about the surrounding contaminated surface-waters and rapports from Grontmij (2009, 2011) about the

contamination and soil properties in the area were used to determine the spatial

distribution of present contamination. Soil research from Grontmij (2009) also provided information on properties like soil Ph, soil structure, soil fauna and the bioavailability of present drin pesticides and heavy metals.

5.3 Visual representations with ArcGIS

In order to gain useful knowledge about the spatial distribution of the pollutants and their bioavailability, a spatial analysis was conducted. The spatial analysis of this research was carried out in ArcGIS, which is a geographic information system that is often utilized to create and analyze geographic maps. The software is used to create maps of the Broekpolder to visualize potential plant distribution in an attractive lay-out. Total amounts of relevant contaminants and corresponding bioavailability were also displayed in maps.

To conduct the spatial analysis, the geographic coordinates of the sample points (Appendix A) and associated biochemical measurements initially published by Grontmij (2009) were adopted. First, the RD coordinate system was changed into longitude latitude by using Matlab and the exact locations of the sample points were added to ArcGIS.

Subsequently, the Broekpolder was subdivided into 11 sectors to assist the development of a plant distribution (see Figure 2) (i.e. these subdivisions are identical to sectors in previous rapports (Osté, 2011) (Grontmij, 2009 & 2011). Those 11 subdivisions were added as

polygons to ArcGIS by using the create features tool. Thereafter, the bioavailability values of the different pollutants were calculated and added to the Attribute Table. Also, the total values of contamination were added to the Attribute Table. Subsequently, a visualization of the total values and the bioavailability of the pollutants was made. Finally, a map of the

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spatial distribution of different plant species was created based on those visualizations and the conducted literature research.

However, some limitations on the methodology exist. First of all, most sectors contain two measurement points. In developing a spatial plant distribution and ArcGIS maps these measurements were extrapolated to the corresponding sector. Likewise it was

assumed that the toxicity concentrations are homogeneously spread regarding depth. As mentioned, a distribution was made that links the phytoremediation characteristics of the selected plant species to appropriate sectors (i.e. sectors with contaminants present that correspond to remediative characteristics of plants previously selected for phytoremediation in the Broekpolder). By making this distribution it was assumed that the remediative plants survive the inherent toxic conditions. However, as mentioned in the introduction, plant growth is often not sustained on heavily contaminated soils (Ali et al., 2013).

Figure 2: The sample points and sectors in the Broekpolder

6. Results

6.1 Selecting plants

In this section the selected plants in the area are discussed. Table 1 shows the different plant species that are able to remediate the heavy metals and drins present in the

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Broekpolder. All these plants are, as mentioned, selected for their phytoremediative capabilities while also not being invasive to the Dutch environment.

In the first column their scientific name is presented, followed by the Dutch name in the second column. Then, six heavy metals each have a column. Followed by three columns, each for one drin pesticide (endrin, dieldrin & aldrin). An ‘x’ sign in the column below these chemicals indicates that there is scientific data confirming phytoremediative capabilities for the species on the corresponding row regarding this particular chemical. Ter illustration,

Ricinus Communis can remediate barium, cadmium, lead and zinc. A ‘+’ sign behind an ‘x’

emphasizes these capabilities for a particular plant-chemical interaction. Ter illustration, the species Eleocharis acicularis also remediates cadmium, copper and zinc while being

especially effective in remediating lead.

In the next section the findings in table 1. are combined with information about the contamination in the Broekpolder to formulate a recommendation for particular plant species per sector.

Table 1. Phytoremediation characteristics of selected plant species

6.2 Distributing plants

Here findings from the previous paragraph about phytoremediation characteristics of particular plant species is used to match each sector with species that show remediation capabilities for the contamination present. In table 2 there are four columns. The first column indicates a sector. As mentioned in paragraph 5.3 (visual representation with ArcGIS) and shown in X, the Broekpolder is divided into 15 sectors. Each sector contains one or more measurement points, as shown in column 2.

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Then, for most measurement points, certain chemicals cross the intervention value. The intervention value is a concentration that indicates an official threshold-value above which remediation is obliged. The chemicals that cross the intervention value guide the plant recommendation in the last column. These plants have remediative capabilities for the chemicals that cross the intervention value in column 3.

Ter illustration, in sector 1 there is one measurement point. Measurements taken by Grontmij (2009) showed that the intervention value for drins was crossed. Therefore, the plant recommendation is the last column consists of plants species from the Cucurbita & Cucumis family. Because, as shown in table 1., these species have remediative characteristics for drin pesticides. The information in table 2. is visualized in Map 2 on page 15. In the next section other visualisations will be discussed as well. Namely, the spatial analysis regarding the bioavailability of the selected pollutants

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Figure 3: Visualisation of the potential plant distribution in the Broekpolder

6.3 Bioavailability

In this section the spatial analysis of the bioavailability will be discussed. The different maps display the bioavailability and the total values of the heavy metals and drins (aldrin, dieldrin and endrin) in the Broekpolder. For larger versions of the maps see Appendix 2-6. The blank areas on the map represent areas with total values which are too low to measure or without contamination of the particular pollutant. Figure 4 shows the bioavailability and total values of the drins in the Broekpolder. As the figure shows, bioavailability of drins is relatively high, ranging from 5% in sector 13Z to 75-100% in sectors 10, 11A, 12 and 5. Figure 5 shows that the bioavailability of zinc is extremely low. Every area has a bioavailability of less than 0.5%. For the other heavy metals (shown in figures 6-8) bioavailability is also low. This low

bioavailability of the heavy metals will reduce the efficiency of phytoremediation since only a small fraction of the heavy metals can be taken up by plants. For drins phytoremediation will probably be more efficient as the bioavailability is higher. Besides, the efficiency of phytoremediation will differ for every sector as the total values and bioavailability also differs per sector (see figures 4-8).

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Figure 4: Spatial visualisation of bioavailability and total amounts of drins

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Left to right: figure 6 & 7: Spatial visualisation of bioavailability and total amounts of chromium and Barium.

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6.4 Interpreting the results by the assessment model on successful

brownfield development

In this research the possibility of phytoremediation as remediation method for the Broekpolder have been explored. The results presented, indicate the following for the assessment model by Schädler (2011) on successful brownfield development; the first pinnacle presented by Schädler (2011) evaluates the site-preparation costs of remediation and development.

In this research an estimation of the site-preparation costs have not been taken into account since the research focuses on exploring the possibility of using phytoremediation as method. However phytoremediation is a method known for its low costs, as one of the few remediation methods according to Reddy (1999). So, for this methods the costs are expected to be low and do not cause any problems or disagreement by the involved stakeholders. On the other side, phytoremediation is a method that takes a lot of time to remediate the area. The question is whether this is desirable.

The second pinnacle takes into account the market oriented appraisal. In this specific case the Broekpolder will become a green space which often lacks the possibility to generate money to complement the costs made by the remediation method chosen. However since phytoremediation is low cost, the need for generating money in the Broekpolder might not be necessary. On the other side in the future on the developed areas could be placed a restaurant or water activity facilities. Nonetheless these ideas could be carried out by using multiple remediation methods, it is not phytoremediation bound.

The last pinnacle discusses the future use in the Broekpolder. The choice for phytoremediation for this pinnacle could affect a change in the landscape then initially planned, secondly phytoremediation in the Broekpolder could take several decades in certain sectors. So by choosing phytoremediation the landscape will be decided by the contaminated soil and the plants that are able to clean that particular part of soil. The question is whether that is desirable compared to the landscape design plans that have been made. Also, the plants that could phytoremediate in the Broekpolder produce contaminated vegetables. These vegetables cannot be consumed. So, this consumption by humans and animals will have to be prevented in the following years.

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7. Conclusion and discussion

To conclude, the remediation of every sector of the Broekpolder by phytoremediation seems impossible. This can be justified by the fact that bioavailability of the heavy metals is too low in each sector due to the high pH and clay content of the soils. The low bioavailability will decrease the remediation rate and will make phytoremediation less efficient and thus less successful (see Figure 1). However, for drins bioavailability is relatively high in most sectors. Therefore, applying phytoremediation only in the sectors with high drin bioavailability might be a more promising approach. Unfortunately, one downside of this approach is that only few plant species remediating drin pesticides exist. In our conclusion it is assumed that the influence of bioavailability of contaminants on the efficiency of phytoremediation in the Broekpolder is estimated correctly.

Another approach to make phytoremediation more successful is to take measures that will increase the bioavailability. Artificial acidification or the use of complexing agents might be promising, however, secondary pollution problems could arise. Therefore, more research about the potential side effects of those soil treatments needs to be conducted. Interpreting these results for the assessment model on successful brownfield development shows that the preparation costs of phytoremediation are low however the method could take years to remediate, the question is whether this is desirable for the involved

stakeholders. In addition phytoremediation does not generate money, nor the planned development. Furthermore, phytoremediation will cause a change in landscape for the following decades since the method takes a lot of time. It affects the plans created on the design of the landscape and the plants use for phytoremediation produce vegetables that cannot be consumed. So this consumption by humans and animals will have to be

prevented.

8. Recommendations

 Focus on the phytoremediation of drin pesticides in the sectors with high bioavailability

 Conduct more research about the side effects of artificial acidification and the use of complexing agents

 Conduct research on the opinions of the involved stakeholders about phytoremediation, including the change in landscape and the production of contaminated vegetables

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Appendix A - Coordinates and sectors

Tabel 3. Coordinates of measurement points and corresponding sectors in the Broekpolder (Grontmij, 2009, bijlage 2).

THE POWER OF PLANTS The application of phytoremediation in the Broekpolder: an interdisciplinary assessment