Large Anthropogenic Nutrient And Pollutant Loads To The Sea From Small Unmonitored Near-coastal Catchment Areas

Taking Sweden and the whole Baltic Sea region as examples, the study shows also that the reported Swedish loads of nitrogen and phosphorus to the Baltic Sea are significantly smaller than expected from strong correlations between a country’s nutrient loads and its population, area and economic activity (GDP per capita) within the Baltic Sea drainage basin (BSDB), which are found for all other Baltic Sea countries.

Erroneous calculation of the nutrient loads from the relatively large Swedish unmonitored near-coastal areas can explain this significant difference between the reported and the expected nutrient loads from Sweden to the Baltic Sea.

The near-coastal areas that are left without systematic environmental monitoring may be small, but they extend along most of the coastlines and often have a large population proportion. For the whole BSDB, for instance, these areas cover 13% of the total area and 24% of the total population of the BSDB, according to an earlier study⁽²⁾ by the same research group. For Sweden, the corresponding fractions are even larger: 55% of the Swedish population in the 20% unmonitored near-coastal catchment area of Sweden⁽²⁾. The new study⁽¹⁾ shows that, with such a large population proportion, the concentrations of, for instance, nitrogen, phosphorus and organic pollutants in the water flow that are generated within these small near-coastal catchment areas may be much larger that in the systematically monitored main rivers.

The mass load to the sea from a catchment area is the product of the area’s water flow to the sea and the mass concentration in that water flow. The latter may be large in near-coastal catchment areas, both due to a large population and due to increasing seawater intrusion into the near-coastal groundwater, which changes chemical conditions and may re-mobilise pollutants that were previously adsorbed on the solid particles of the groundwater system.

The unmonitored near-coastal catchment areas are not just forgotten when, for instance, the Swedish nutrient loads to the Baltic Sea are estimated. The data gaps are bridged with the help of computer model calculations. However, because the model results cannot be checked against representative field data for the unmonitored areas, they may be significantly wrong.

A series of earlier and parallel, detailed process studies from the SU research group have shown: 1) that near-coastal catchment areas have particularly complex water flow conditions, which are not only possibly, but even probably erroneously estimated where some necessary data is missing(3-4); and 2) that the subsurface water systems (soil, groundwater, sediments) of catchment areas contain large legacies and long-term memories of the cumulative anthropogenic inputs of nutrients and pollutants in the catchments over at least the last decades(5-7), which can now be continuously transported to the sea, without detection, along the long coastlines of unmonitored near-coastal catchment areas.

The new study synthesizes main implications of these earlier and parallel studies and suggests a concrete, improved methodology for interpreting available field data and estimating the mass loading to the sea from unmonitored near-coastal catchment areas.