The current study evaluates the effect of seawater physico-chemical characteristics on the relationship between the concentration of metals measured by Diffusive Gradients in Thin films (DGT) passive samplers (i.e., DGT-labile concentration) and the concentrations measured in discrete water samples. Accordingly, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to measure the total dissolved metal concentrations in the discrete water samples and the labile metal concentrations obtained by DGT samplers; additionally, lead and cadmium conditional labile fractions were determined by Anodic Stripping Voltammetry (ASV) and total dissolved nickel was measured by Cathodic Stripping Voltammetry (CSV). It can be concluded that, in general, the median ratios of DGT/ICP and DGT/ASV(CSV) were lower than 1, except for Ni (median ratio close to 1) and Zn (higher than 1). This indicates the importance of speciation and time-integrated concentrations measured using passive sampling techniques, which is in line with the WFD suggestions for improving the chemical assessment of waterbodies. It is the variability in metal content in waters rather than environmental conditions to which the variability of the ratios can be attributed. The ratios were not significantly affected by the temperature, salinity, pH, oxygen, DOC or SPM, giving a great confidence for all the techniques used. Within a regulatory context such as the EU Water Framework Directive this is a great advantage, since the simplicity of not needing to use corrections to minimize the effects of environmental variables could help in implementing DGTs within monitoring networks.

This protocol was developed based on the knowledge acquired in the framework of the Interreg MONITOOL project (EAPA_565/2016) where extensive sampling campaigns were performed in transitional and coastal waters covering eight European countries. It provides detailed procedures and guidelines for the sampling of these waterbodies by concurrent collection of discrete water samples and the deployment of Diffusive Gradient in Thin-films (DGT) passive samplers for the measurement of trace metal concentrations. In order to facilitate the application of this protocol by end-users, it presents steps to follow in the laboratory prior to sampling campaigns, explains the procedures for field campaigns (including in situ measurement of supporting parameters) and subsequent sample processing in the laboratory in preparation for trace metal analyze by inductively coupled plasma-mass spectrometry (ICP-MS) and voltammetry. The protocol provides a systematic, coherent field sampling and sample preparation strategy that was developed in order to ensure comparability and reproducibility of the data obtained from each project Partner in different regions.

This study investigates the relationships among Ni, Cd and Pb's different chemical forms determined by different methodologies in coastal and transitional waters across a broad geographical scale. Concentrations were measured in spot samples and through passive sampling (DGT). High variability of metal concentrations was found among sampling sites and methodologies due to natural water fluctuations rather than to a given metal or method. Total dissolved metal concentrations in spot samples were lower than the EQS-WFD values. The labile fractions of Cd and Pb, measured in spot samples by Anodic Stripping Voltammetry and by DGT-ICPMS, were highly correlated. Similar labilities were found for Cd, while for Pb, the ASV labile fraction was ≈50% lower. These results reflect the pool of mobile and labile species available towards each technique kinetic window, and they seem not to be affected by discrete sampling flaws.

Integrative passive samplers, such as DGT (Diffusive Gradients in Thin-films), are identified in European Technical Guidance Documents as promising tools to improve the quality of the assessment, in the context of the WFD (EU Water Framework Directive). However, DGT results cannot yet be used directly in a regulatory framework to assess the chemical status of water bodies, as DGT labile concentrations cannot be directly compared to the metal AA-EQS_{marine water} (Annual Average Environmental Quality Standard) established by the WFD, which are defined in the dissolved concentration. Therefore, prior to using DGT results in a regulatory context, for cadmium, nickel and lead, an adaptation of existing AA-EQS_{marine water} for DGTs should be pursued, ensuring at least the same level of protection. In this sense, in the framework of the MONITOOL project, a robust database of dissolved and labile metal concentrations in transitional and coastal waters, for adapting the existing AA-EQS_{marine water} for DGT technique, was obtained. Building on these results, this study proposes a methodology and provides values and equations for using DGT results for the chemical status assessment of marine waters, by adapting the EQS_{marine water} to adapted EQS_DGT or predicting dissolved concentrations from DGT results. Based on available dataset, a first simulation of “chemical status” assessment per MONITOOL sampling site using DGT measured labile concentrations was carried out and the results were compared to an assessment based on dissolved concentration to check their compliance. These results demonstrate that the use of DGT passive samplers is appropriate for the metal concentrations level encountered in the marine environment. Further work is recommended to test the effectiveness of the methodology proposed in this study under WFD conditions on more sites and to establish common strategy guidelines for the use of DGT passive samplers in monitoring.

This paper describes the results that have been obtained in a real case study of a hybrid constructed wetlands system, which has been in continuous operation for over 11 years. The main aim of the study was to understand the long-term operation and efficiency of the system (which is situated in the municipality of Santa Lucía, Gran Canaria, Spain), which comprises two vertical-flow and one horizontal-flow constructed wetlands for the treatment of urban wastewater. The system, which was originally designed to treat a flow rate of 12.5 m³/day, with a load of 100 equivalent inhabitants, has been operating since its inauguration (July 2008), with a flow rate of almost 35 m³/day and a load of 400 equivalent inhabitants. Despite this, the mean total removal efficiencies during the study period (2014–2019) are optimal for a system of these characteristics, as follows: 92% for 5-day biochemical oxygen demand (BOD₅), 89% for the chemical oxygen demand (COD), and 97% for the total suspended solids (TSS). The system efficiency, with respect to nutrient removal, was somewhat lower, resulting in 48% for total N and 35% for NH₄. It has been confirmed with this study that this type of system is an appropriate, robust, resilient nature-based solution for the treatment of the wastewater that is generated in small communities, especially in zones with a warm climate, stable mean temperatures, and mild winters.