Building a transboundary hazardous substance emission model for the Danube River Basin. Overview of the key challenges of data availability, data uncertainty, knowledge gaps of substance behaviour

Beside common pollutants, such as organic material and nutrients, an ever-widening list of chemicals also put pressure on the quality of our rivers, lakes and the health of the prestigious ecosystems living within them. An investigative work to understand the main sources and pollution pathways of these chemicals is an important task ahead of us. A series of large-scale studies have been undertaken with the cooperation of experts from almost all Danube countries to build a hazardous substance inventory in the first step and then to build an emission model based on this inventory. The inventory has been built within the frames of the Danube Hazard mᶾc InterReg project, the model building is currently ongoing within the frames of the Tethys InterReg project. Three chemical groups are investigated by applying the MoRE (“modelling of Regional Emissions”) model for the Danube River Basin (DRB) represented by key elements and compounds: the group of potentially toxic elements is represented by 6 heavy metals and arsenic, industrial chemicals are represented by the two most common per- and polyfluoroalkyl substances (PFAS), PFOS and PFOA, and human pharmaceuticals are represented by a pain killer (diclofenac) and a psychoactive drug (carbamazepine). Each of these substances are ubiquitous in the environment but linked to different sources and pathways. While the source of pharmaceuticals is fairly well known, the estimation method for their emissions is challenging if one needs to reflect regional differences of it or to account for the effects of the type of sewage treatments applied in the treatment plants. Heavy metals are abundant in soils all over the DRB, while the uncertainties of the emissions from operating and abandoned mining facilities are also key to be addressed if hot spots to be identified. The most difficult, however, is the regionalisation of PFAS substances as beside emissions from point sources and urban runoff, they appear in atmospheric deposition all over the basin and in groundwater around known and potential hot-spots, meanwhile the actual emissions from point sources are also much less documented. A key step to upgrade our inventories for the model is that the emission from industrial facilities to air and water are described in detail as far as data from national and international databases, literature or BAT documents is available. Knowledge gap is indicated by the amount of plants with known discharge (162) compared to all the industrial facility in the DRB (6258 facility identified in the Industrial Emission Directive database). For the most uncertain emission sources and pathways, the study aims an order of magnitude estimation for all the potential estimation sources. Hence, for example tile drain pathway has been introduced for chemicals present in soils, despite the lack of sufficient concentration data in effluents. The estimation of groundwater concentration is not only difficult for diffuse sources but also for hot spots, which may be known only by literature information. The latter (literature values) is applied for landfill sites, which are treated as legacy hot spots for PFAS substances and pharmaceuticals.