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Passive samplers (PS) for the assessment of dredging activity impacts

From October 2021 to January 2022, researchers from AZTI (Basque Country, Spain) used passive samplers (PS) for the assessment of dredging activity impacts on the quality of port waters in the Royal Maritime Club of Abra (Bilbao Port, N. Spain) and supported by the Port Authority of Bilbao.

Following the guidelines developed by the European MONITOOL project[1], Diffusive Gradients in Thin- films (DGT) PS were deployed (5 days) at five stations at increasing distance from a dredging area, in 3 different periods: before, during and after dredging activities. Simultaneously, spot water samples were collected at the same stations, at the DGT deployment and retrieval time. Hydrographic parameters were measured in situ: temperature, oxygen, salinity and pH. Priority (Cd, Ni, Pb) and other specific metals (Cu, Co, Zn) were analysed in the DGTs and in the water samples by ICP-MS. Total organic carbon, turbidity and suspended matter were also analysed in the collected water samples.

Results will be presented in ECSA 59 Conference next September, in the special session organized by the MONITOOL project entitled “Potential Roles of Passive Sampling in Investigative and Regulatory Monitoring”[2], whose abstract submission deadline has been extended to 29 April.

 Noticia AZTI 04 2022

[1] https://www.monitoolproject.eu/images/Publicaciones/A_Good_Practice_Guide_for_the_Use_of_DGTs.pdf

[2] http://www.estuarinecoastalconference.com/special-session.asp#0033

Interview with Professor Hao Zhang, Lancaster University, UK, and member of the MONITOOL scientific advisory board 

Professor Hao Zhang, Lancaster University (UK), together with Bill Davison, are the parents of the DGTs, whose research was published in the high standing journal Nature, in 1994. After that, they founded the DGT Research Limited in Lancaster in 1997. Her vast knowledge and experience with DGTs and her high interest in collaborating on the project made her an essential member of the MONITOOL scientific advisory board.

Photo3 for MonitoolOn the basis of the results of Monitool project, how do you see the future of DGTs? Is there any real chance for DGTs to be finally adopted for the routine monitoring in the WFD?

Monitool is the first major international passive sampling project on a large scale involving 8 partners and 9 associate partners, covering the Atlantic region, Mediterranean and North Sea. It is a pioneering project that systematically investigates and develops the potential of using DGT passive samplers for monitoring the chemical status of transitional and coastal waters under the WFD. A consortium of dedicated experts working closely together on focused objectives has ensured a successful project. The encouraging outcomes of Monitool will be followed by other scientists in other regions in the world. I am very impressed with the amount and quality of data obtained in the project. The extensive data set of 6500 concentrations of metals by DGT and spot sampling provides a solid foundation and a big step towards adopting DGT as an accepted routine monitoring tool in the WFD. There is no doubt that DGTs will eventually be accepted by the regulators, but it is a question of time, as these things tend to move incredibly slowly.

What do you think would be the best approach to move from research to legislative level? As an expert, do you have any recommendations to involve stakeholders and authorities?

I think that DGT will be accepted progressively rather than in a single step. There are two ways of doing this. One approach, as Monitool so ably does, is to stress the need to move away from spot sampling which is likely to be unrepresentative. From an operational perspective, Monitool is absolutely the right to i) use the data base to derive EQS DGT values and ii) use a model to convert the DGT concentration to the total dissolved, so that EQS marine water values can be still used. The other way forward is to promote the increasing recognition that standard EQS measurements should be regarded as a first step in assessment. Failure to meet the EQS should trigger further study with measurement by DGT being the simplest and possibly the best next step. This is moving towards the tiered system used in Australia. The argument needs to be made more strongly that EQS are based on biological response tests where organisms were exposed to dissolved and bioavailable metal. Measurement of metals in natural water by spot sampling with 0.45 micron filtration includes colloids and complexes which may not be bioavailable. DGT excludes those species. It can therefore be argued that DGT relates more directly to the original lab data on which EQS is based.

For DGT to be accepted at the legislative level, it is very important to involve stakeholders and authorities. Monitool could influence the agenda by hosting workshops and setting up stakeholders forums to let the stakeholders share their views and engage directly with the scientific team.

Considering the decline of wild mussels that could be used for biomonitoring studies, do you foreseen a worldwide monitoring network similar to 'Mussel Watch' but based on passive samplers? Could the obtained data be used for the long-term assessment of contaminant trends? What are the strengths and limitations? 

Mussel Watch has been a useful programme, but it is not a state-of-the-art way to monitor the seas. Mussels do not occur everywhere, there are confounding factors because of size/age differences of the mussels, and they also metabolise and alter some contaminants.

Worldwide monitoring of contaminant trends with a passive sampling tool like DGT would be very beneficial. The advantages are ease of use, providing time integrated data, controlled exposure times (unlike mussels), good comparability between labs and with respect to time and high relevance to potential biological impact. Many limitations of Mussel Watch could be overcome by using standardised DGT samplers. Together with more standard chemical measurements, important additional information on the role of colloids and complexation would be obtained. The DGT technique has the necessary features for establishing a worldwide monitoring network programme, with unified deployment, sample treatment and analysis protocols.

Deep knowledge of DGT is now based on over 1100 publications in the peer-reviewed literature.  A DGT network would certainly provide the necessary data for long-term assessment of contaminant trends globally. Co-deployments of different DGTs would ensure nutrients, metals, radionuclides and different classes of organics could all be monitored, giving much more complete data than Mussel Watch.

A major limitation would be ensuring a well-supported network of sites for the secure deployment of the samplers. While DGT may be a good surrogate for a biological assessment, as in Mussel water, it is not a biological measurement, and some may argue that only a biological tool can truly assess impact on an ecosystem.

How do you see the possibility of having a Monitool 2? How would you approach it?

Monitool 2 would be very worthwhile. Personally, I would have thought directing Monitool 2 towards adopting DGT for EU monitoring is the way to go. The funders probably expect that now.  To do that, I suggest playing on the ‘complete service’ that DGT can provide (metals, organics, nutrients, and radionuclides) would be very appealing.  The practical issues like cost, ease of use, site selection, stakeholder buy-in etc. could be addressed. For the research element of Monitool 2, perhaps it should include an investigative element that explores whether DGT measured labile concentrations are better related to some of the biological response and toxicity measurements. An obvious next step would be to extend the study to a wider suite of metals, including metalloids and rare earth elements.

The WFD also lists many organics. DGTs have been developed for many of them. Monitool 2 could extend to organics, with a systematic approach to assess its suitability for screening and full quantification of the WFD target compounds.

Monitool 2 should have a strong element of stakeholder involvement, and make a broad comparison of other issues in adopting DGT for EU monitoring, like cost, site selection, deployment recommendations, logistics etc. 

Which are the research fronts you are working in your laboratory now? According to you, what will be the target compounds of interest in the near future?

As I said earlier, there are over 1100 DGT publications in peer reviewed journals covering method developments and applications in waters, soils and sediments. There are many innovative developments of DGT technique, such as biological response studies, combining DNA-based sensing and simplifying analysis using direct gel colorimetry etc.

Our research group has been working on bio-DGT (combining DGT with biosensors), deployment systems for monitoring and application of DGT in investigation of biogeochemical processes. We have several projects on nutrients such as phosphorus, and on organic pollutants such as pesticides, antibiotics, PPCPs and other drugs. I think there is great potential in DGT for organic chemicals in the environment, not just as a passive sampler for monitoring, but also as a novel research tool for advancing our understanding of the behaviour, transport and fate of organic pollutants.  

Designing and development of an additional experiment performed by IFREMER at the laboratory

Monitool results point out that, even in suspected highly contaminated sites (harbours, marinas, etc.), concentrations of metals (Cd, Ni and Pb) in marine water (dissolved fraction) are far below the AA-EQSmarine water. In an attempt to solve this and other matters, a new study appeared to be necessary. Therefore, Ifremer proposed to carry on an additional experiment to get more data in laboratory conditions in order to:

  • Improve the generic relationship between the metal concentration measured in DGT and the concentration measured in the dissolved fraction (MDGT/Mdissolved), by eliminating the bias due the fact that we are comparing metal results by spot sampling (dissolved metal) versus time integrated measurement (labile metal - DGT).
  • Reach concentrations close to the AA-EQS marine water in order to check the validity of that relationship with higher concentrations.

The approach is based on measuring the concentrations of Cd, Ni, Pb and Hg in the water-samples consisted of 10 L from previously selected different sampling locations. These metals were determined in the 10 L containers (stirred throughout the duration of the experiment) by the DGT technique (exposed during 5/7 days in the water samples) and by the analysis of the filtered water samples by ICPMS, ASV and cold vapour atomic fluorescence spectroscopy for Hg.

AZTI samples DGT1

Marine water samples, on a stirring table during DGT deployment.

Different physico-chemical parameters were also analysed in the same samples: salinity, alkalinity, "ligands" concentration, suspended matter concentration, particulate metals, chlorophyll, pheophytin, dissolved and particulate organic carbon (DOC and POC respectively), as well as nutrients concentrations. In addition, ecotoxicological tests were carried out in these samples.

All the analyses were performed by Ifremer, except the ASV, POC, chlorophyll, pheopigments and DOC, which were carried out by LEMAR (Université de Bretagne Occidentale).

The ten selected sampling locations were the ones sampled by the Partners during MONITOOL campaigns, where the highest concentrations of Pb, Cd or Ni were determined, or those with specific environmental conditions (high salinity, low suspended particular matter). The selected locations and partners responsible for collecting the samples were:


Additionally, 3 samples of filtered (0.2µm) and sterilized seawater were included: 2 of the samples were spiked to have quantifiable concentrations in Cd, Ni and Pb of the order of EQS and EQS /2.

The experiment started on July 15th and the results, which will be available by December 2020, are expected to provide interesting conclusions.

ICES ExpertsAZTI presents MONITOOL results at different scientific events during 2020

The work conducted and the knowledge gained along MONITOOL lifetime have been presented by AZTI at four different scientific events. One of them was attended face-to-face and the other three events took place virtually due to the COVID-19 restrictions.

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Instituto Tecnológico de Canarias (ITC) carried out a wastewater sampling campaign following the MONITOOL standardized methods

Detecting and monitoring the level of contaminants in water bodies is a key element of the Water Framework Directive, WFD (2000/60/EC). The Directive 2013/39/EU of the European Parliament and of the Council of 12 august 2013 includes some heavy metals as priority substances: Cd, Hg, Ni and Pb. Its transposition to Spanish regulation (Real Decreto 817/2015) also includes Cu, Cr VI and Zn as preferential substances.

Discharges from land-based sources (industries, ships and particularly Wastewater Treatment Plant, WWTP) are one of the main input sources of heavy metals impacting the marine environment, consequently, the evaluation of the metals with quality standards in the effluent before they are dumped into to the coastal waters can help us to the implementation of the WFD, which aims for “good status” for all   water bodies in EU.

Spot water sampling of a discharged effluent at a specific point provides a good snapshot of what is happening at that point in time. However, it does not specify whether that snapshot faithfully reflects what is habitually present in that flow. Given these shortcomings, passive sampling techniques constitute a promising tool since they are continuous sampling methods, providing more representative information about the sampled environment. More specifically, to detect metals in water environments, soils and sediments the Diffusive Gradients in Thin Films (DGT, Research Ltd) are widely used.

Even if the use of DGTs can be extremely advantageous to study the metal concentrations in wastewater effluents due to their high variability, there are not so much studies in the literature in which DGTs are directly deployed in the wastewater discharges pipelines/pit, for the evaluation of metals (Gourlay-Francé, 2011; Thomas, 2009). Some of the reviewed studies consisted on taking wastewater samples and then, at the laboratory, the DGTs were deployed in flasks for a certain period of time, under stirring and temperature controlled conditions (Buzier et al., 2006a, 2006b, 2011; Yapici, 2008).

IMG 8977

In this context, in July 2020, the project partner ITC carried out a study with the objective of comparing the metal concentrations obtained with spot and passive sampling in five wastewater coastal discharges in Gran Canaria Island. Thereby, following the MONITOOL standardized methods, six different wastewater effluents were discrete sampled and simultaneously, DGTs were deployed directly in such effluents, these being:

  • The discharge from one of the biggest WWTP in Gran Canaria
  • Two mixed treated discharges (urban and industrial)
  • The treated discharge from an industrial site
  • The cooling water discharge from a thermal power plant
  • The discharge from a marine fish farm

Seven metals were targeted in these effluents:

  • Cd, Ni and Pb: as priority substances
  • Cr, Cu and Zn: as preferential substances
  • Al, Co, Fe, Mn: for their important role in the marine environment

The discrete water samples and the exposed DGTs are currently still under analysis, but we hope we will soon share the results and to be able to evaluate the DGT sampling techniques as a tool in the monitoring of heavy metals in treated wastewater direct discharges.


  • BUZIER, Rémy, et al. Trace metal speciation and fluxes within a major French wastewater treatment plant: Impact of the successive treatments stages. Chemosphere, 2006, vol. 65, no 11, p. 2419-2426.
  • BUZIER, Rémy; TUSSEAU-VUILLEMIN, Marie-Hélène; MOUCHEL, Jean-Marie. Evaluation of DGT as a metal speciation tool in wastewater. Science of the Total Environment, 2006, vol. 358, no 1-3, p. 277-285.
  • BUZIER, Rémy, et al. Inputs of total and labile trace metals from wastewater treatment plants effluents to the Seine River. Physics and Chemistry of the Earth, Parts A/B/C, 2011, vol. 36, no 12, p. 500-505.
  • GOURLAY-FRANCÉ, Catherine, et al. Labile, dissolved and particulate PAHs and trace metals in wastewater: passive sampling, occurrence, partitioning in treatment plants. Water Science and Technology, 2011, vol. 63, no 7, p. 1327-1333.
  • THOMAS, Philip. Metals pollution tracing in the sewerage network using the diffusive gradients in thin films technique. Water science and Technology, 2009, vol. 60, no 1, p. 65-70.
  • YAPICI, Tahir, et al. Investigation of DGT as a metal speciation technique for municipal wastes and aqueous mine effluents. Analytica chimica acta, 2008, vol. 622, no 1-2, p. 70-76.