This project aims to provide a UK-wide assessment of the threat of atmospheric emissions of the toxic heavy metal mercury (Hg) to soil organisms, and to conduct a preliminary assessment of threats to aquatic biota. Such knowledge is needed in order to determine what inputs of Hg can be tolerated without significant damage. The information can be used by Defra to determine emission control policies, i.e. to regulate the amount of Hg that can acceptably be added to the atmosphere by human activities. Because Hg is a pollutant that is transported long distances through the atmosphere, amounts of the metal deposited in one area can depend upon emissions elsewhere. Indeed Hg is especially known as a “global pollutant”. Therefore emission control requires international cooperation and agreement, and the research results will contribute to Defra’s negotiation approach.
Currently, the impacts of other atmospherically-transported heavy metals are quantified by Critical Loads. The critical load is the maximum allowable continuous input to the soil. It is measured in units of kilogram of metal per hectare per year, and is determined by
(a) the toxicity of the metal under the conditions prevailing in the soil;
(b) the rate at which metal is lost from the soil, principally by leaching in runoff water.
Both factors vary with soil type, and so soil properties must be taken into account in the calculation of critical loads. At present, a generally-applicable method for estimating soil critical loads of mercury is lacking, and this research aims to develop one, and to assess whether it can meet the needs of the UK landscape.
Mercury toxicity will be assessed by the collation and analysis of published studies in which Hg effects on soil organisms (invertebrates and microbes), and plants, have been measured for different soil samples. By amalgamating all the data, we can derive a Critical Limit Function (CLF). This is an equation that relates the toxicity of Hg dissolved in soil water to the chemical conditions prevailing in the soil, principally its acidity (pH). We can use the CLF to calculate, for a given soil, the total dissolved Hg concentration that corresponds to the maximum allowable degree of toxicity. This is the Critical Limit – the concentration at which no more than 5% of the organisms are affected, or at which 95% are protected.
We can quantify reasonably well losses of Hg from soils through soil water leaching and removal of metal by the harvesting of trees, but gaseous losses of the metal are less well understood. The project will review publications on this process, and use the information to describe the overall retention and transport of Hg by soil. This will permit the calculation of the Critical Load of mercury for different soils, which will be applied using nationally available soils data for each 1 x 1 km grid covering the UK, to produce maps showing how the Critical Load varies across the country.
We will also compare the Critical Loads with the actual current loads (i.e. deposition) of Hg, to determine whether, where, and by how much, they differ. This will provide a full overview of the current situation with regard to mercury inputs. We will also compare current concentrations of mercury in soil with those corresponding to the Critical Limits, to provide an assessment of the state of UK soils with regard to mercury pollution.
The Critical Load method deals with steady states, i.e. it assumes the inputs and outputs of metal are equal. But this does not provide any information about how long it would take to reach that situation, nor how other environmental factors, such a soil acidification or recovery from it, would affect the behaviour of mercury. We will investigate the time factor, by the use of a dynamic catchment model, CHUM-AM. The model will need to be adapted to deal with Hg, and tested by comparing its predictions of Hg in soil and water with measured values. Then it can be used to explore the likely responses of soil mercury to different future scenarios.
Mercury accumulates in freshwater fish in a fat-soluble form, methylmercury, and this can be toxic to consumers of the fish, including humans. Simple methods to assess accumulation, which depends upon the inputs of mercury and the chemical properties of the water, have been established in other countries, and we will apply these methods to a range of representative UK waters to attempt to identify those at risk.
The results of the project will provide assessments of the current and future levels of mercury in UK soils, and relate these to likely deleterious effects on soil organisms. Information on surface waters will also be obtained. The knowledge about UK Critical Loads will be available for atmospheric scientists to determine the maximum emissions that can be made from Europe and the rest of the world without threatening the UK soils and waters.