1. The DPSIR causal approach, the conceptual framework and the response issue
The WFD [1] is based on a water systems-level approach, recognizing that water systems are natural systems of river basins that commonly cross multiple national borders and jurisdictions. Water systems may be threatened by the mixtures of chemicals (‘specific pollutants’) that are emitted in significant amounts to the water system. Those result in a highly diverse chemical pollution pattern at the site of emission and/or downstream [6, 9].
To handle this vast diversity of pollution situations, we suggest that water quality assessors employ a systematic approach to diagnose water quality problems and their probable causes, as prescribed in the WFD-Annex II. We therefore combined the WFD-suggested DPSIR approach [8, 16]) with the extended conceptual framework for solution-focused management of chemical pollution in European waters [13, 19]. The result of the combined concepts is shown in Fig. 1. The present paper focuses specifically on early-stage attention for exploring optional Responses (R), that is, to explore the ‘solution space’ when a water quality problem is hypothesized or found. The WFD (Annex VI) does provide already a list of standard measures that can be addressed as potential solutions to be considered for the programs of measures (Additional file 1). The list suggests that the ‘solution space’ is large, but it does not provide a very specific or operational strategy or solution approaches. Figure 1 suggest that the ‘solution space’ encompasses technical abatement options (lower left, ‘Abatement’), but also suggests how to explore the ‘solution space’ further (via the entries ‘Chemicals’, ‘Environment’ and ‘Society’), as detailed below.
Given the conceptual framework of Fig. 1 and the tools and services to characterize water quality problems [20], we aimed to systematically collate abatement techniques and management options and strategies and to make the results available for re-use by others encountering a similar chemical pollution problem. Systematic storage of those—with or without evaluating them—enables a whole community of users to retrieve collated options and experiences, and thus to explore a wide array of options. Users can retrieve options in the ‘solution space’, to derive programs of measures for their specific problem (see below).
As compared to current practices, the combined framework (Fig. 1) encompasses a change from single chemicals per site to a system-level approach, from a problem description-oriented approach to (also) a solution-targeted approach, and from a limited view on the ‘solution space’ to a systematic basis to recognize that the ‘solution space’ is large.
2. The early exploration of the ‘solution space’
The early management attention to the Response-step (R) of the DPSIR causal cycle can be supported by systematic collations of data on technical abatement options and a description of the management strategy. To that end, such information was collated in a database of technical abatement options [21], and in a proposal for the systematic evaluation of non-technical solution scenarios (see Additional file 1). Both were designed to be broadly applicable. This supports users in exploring the ‘solution space’ and may help to inspire them to evaluate options they would never have thought of, and the availability of a database of options helps to avoid that ‘the wheel is re-invented over and over again’.
The technical options are provided as a database of technical abatement options and efficiencies for the application in wastewater and drinking water treatment plant construction and upgrading [21]. The database provides insights into the degree of expected removal of hazardous chemicals from wastewater and raw water for drinking water production for various techniques. This was achieved by an analysis of the installation-specific removal efficiencies of chemicals with different physical–chemical properties. It should be acknowledged that the database can be continuously expanded, based on the experiences gained, which would further improve the value of the technical abatement database.
The non-technical options were found to be highly diverse (Fig. 2). The exploration of prevention and management strategies is currently formatted as a strategy to explore the ‘solution space’ (Fig. 2). Note that this figure is directly derived from and related to the conceptual framework (Fig. 1). It provides a generic scheme that supports end users in exploring the non-technical ‘solution space’. The visualization of the ‘solution space’ in Fig. 2 shows that there are three general levels to approach a pollution problem, going from operational via tactical to strategic options. Note that the discrimination between these levels is not strict. Further details are in Additional file 1. Figure 2 shows how the conceptual framework (Chemicals, Environment, Abatement and Society, Fig. 1) thus in general supports a systematic exploration of the available ‘solution space’ (Fig. 2).
The application of the strategy and the scheme of Fig. 2 are further elaborated in Additional file 1. There are two final remarks on the ‘solution space’ in relation to other (non-chemical pollution) stress. First, it should be noted that the exploration of the ‘solution space’ in the present paper focused on chemical pollution only. However, the diagnosis of impacts of all stressors may show that chemical pollution is only part of the problem, or even negligible, and that the ‘solution space’ for the integrated management plan should also consider the solutions to other stressors. Second, it should be noted that a single solution strategy may help reduce the impacts of multiple sources of stress. For example, zonation (between land use and water systems) helps reduce emissions of both nutrients and agricultural chemicals.
3. Prioritizing the intensity of measures against chemical pollution
Diagnostic results—ranking sites and compounds regarding the relative importance of chemical pollution to cause harm—are needed as a first step to help prioritize the need for and intensity of the measures that can be taken to prevent or reduce chemical pollution problems. As any compound (currently in trade, or produced in the future) can pose harm (alone or in a mixture), the WFD and current research therefore consider all compounds and their mixtures. The diagnostic step is supported by diagnostic tools and services (e.g., [6, 10, 11]) and helps to steer management efforts to those sites and compounds that are most problematic for reaching the WFD environmental goals (good chemical and ecological status). The exploration of the ‘solution space’ might focus on prioritized water bodies and compounds, but would also consider lower-ranked cases where a solution option is relatively easy to implement.
4. Solution-focused practices
So far, the recommended approaches are introduced as novel concepts, with generic schemes to assist water quality assessors in practice. The combination of the solution-focused framework, the diagnostic approaches and the database and strategy for exploring the ‘solution space’ yields a novel flow diagram (Fig. 3). The diagram closely relates to the current WFD-assessment and management cycle, but emphasizes the novel key step (early focus on exploring the ‘solution space’) as well as the aforementioned recommendations to improve current practices (such as to follow the systems-level approach of the WFD).
5. Evidence for improved status
Case studies have shown that the implementation of solution strategies resulted in reduced chemical pollution problems in European surface water systems.
First, the chemical, bioanalytical and ecological tools that are available were used to evaluate chemical pollution in relation to the efficacy of wastewater treatment plants (WWTPs) in removal of chemicals and reducing risks and impacts [22, 23]. The evaluation considered WWTP upgrades with an added activated carbon treatment step and considered up- and downstream and before/after comparisons. It was demonstrated that the improved treatment influenced ecosystem exposure (reduced) and quality (improved). The extra carbon treatment was beneficial for the chemical, biological and ecological status of the receiving water bodies [22,23,24,25].
Second, additional studies considered ten riverbank filtration sites along the River Rhine and its tributaries, and looked at modeling, existing data and additional analytical measurements of trace organic compounds to assess the attenuation potential of selected chemicals present in the surface water by riverbank filtration. For a site with long retention times to the drinking water well, the results enabled the categorization into very persistent, partially removable and fully removable compounds in the given time scales [26]. For three sites with short travel times, a broad analytical screening enabled categorization of the chemicals into “persistent” and “naturally attenuated” classes [27]. For one Dutch site, the efficiency of anaerobic riverbank filtration was assessed before and after reverse osmosis treatment, using a battery of bioassays combined with non-target screening. The treatment process of reverse osmosis was characterized in more detail using spiked anaerobic riverbank filtrate [28].
Monitoring can also directly trigger a solution strategy or method. Daily wide-scope target and non-target screening of water samples using high-resolution mass spectrometry at River Rhine stations triggered successful abatement measures when non-regulated and non-monitored relevant chemicals were detected [29]. Many pollution sources can be located in river catchments via DPSIR analyses and/or monitoring. The example case studies cited above, as well as scenario studies with models [6], show that corrective measures, such as change in industrial production processes or improved waste management, can significantly reduce or eliminate discharges and chemical pollution risks.
6. Exploring future options
The compilation of optional technical abatement and management strategies can be followed by a ‘fitness check’ of expected water quality improvements. Here, the water quality assessor evaluates each option with respect to critical aspects, such as practical implementation, costs and efficacy. Scenario analyses can be run to evaluate the expected improvements in water quality, applying component-based approaches. An example result of such a comparative assessment is shown in a case study of future emission scenarios of chemicals at the European scale under alternative policy strategies [6, 30]. The most remarkable result was a highly positive effect (35% less toxic pressure, expressed as multi-substance potentially affected fraction, msPAF) of the phasing out of 26 substances of very high concern (SVHC) listed on the REACH Candidate List (out of the 1357 chemicals registered under REACH that were included in the ‘future management’ scenario). This clearly shows the high potential of focused regulatory measures to reduce the total chemical burden in general [31]. But specifically, the water quality change in relation to SVHC-focused emission reduction measures appeared to be more than proportional, driven by non-linear exposure–effect relationships (see also [32, 33]).
7. Evaluation and communication of trends: chemical footprints
Communicating the output of the changes following from an implemented solution scenario and/or future management scenarios requires an innovative approach for evaluating trends and communicating results. This is key, given the diverse appearances of the chemical pollution problem. A chemical footprint approach was developed for this, providing summary information of the chemical pollution for an area [34, 35]. The chemical footprint indicator provides summary insights in the net likelihood of chemical pollution to cause harm. Indications for a decreasing chemical footprint were found in a retrospective study of a European basin [35], in line with emission reduction policy objectives and efforts and associated observations made with effect-based methods. The chemical footprint indicator can currently provide insights in the chemical footprint at the level of local water bodies. That is, the management-relevant outcomes of current chemical footprint analysis consist of (1) information whether and in how far upstream ‘source’ areas contribute to a local mixture risk, (2) information on the relative importance of chemical emissions to the local mixture toxicity and (3) information on whether and in how far mixture toxicity from a polluted water body is transported to downstream ‘target’ areas [36]. These types of information are key to define programs of measures against pollution and which actors to address (upstream or local) who have shared responsibility in causing risks (1 and 2) and to inform water managers of the downstream areas.
8. Further developments
The success of water quality protection and management regarding chemical pollution depends on the possibility to identify and implement optimal abatement techniques and management approaches [31, 37]. The implementation of the solution-focused risk assessment paradigm into the practice of European water management is supported by a conceptual framework that guides the assessment process and provides a systematic overview of available abatement and management strategies. The abatement database and the management strategies are continuously expanding, following the continued cycle of water quality management activities and monitoring-based water quality evaluations.