The application of strontium isotopes to catchment studies: the Plynlimon upland catchment of Central Wales

The processes involved in streamflow generation have received considerable attention over the last few decades, primarily in response to concerns about acid deposition and the consequent acidification of streams (Mason, 1990; Likens et al. 1996). The effects of acid deposition on water quality as well as on ecosystem balance depend largely on the ability of catchment materials to neutralise such strong acid inputs, primarily through chemical reaction with soil and bedrock. Much work has also focussed on the transport of agriculturally derived pollutants such as nitrate and pesticides, and on the effects of land-use change (e.g. afforestation, deforestation). Fundamental to modelling the response of catchments to these environmental changes is a knowledge of the water flowpaths and the source and behaviour of solutes in groundwater and through the soil and unsaturated zone. The water flowpaths and sources will vary both seasonally and during storm events making solute-transport modelling difficult. Water falling on a catchment undergoes significant chemical changes through reaction with biotic and mineral surfaces. The chemical evolution of this water is strongly influenced by the specific pathways and residence time in the catchment. The complexities of the shallow hydrological system such as large variations in soil type, biomass and geology have meant that there has been considerable, and as yet largely unresolved debate concerning flow mechanisms and solute transport even through relatively simple catchments. Acidic atmospheric deposition (natural and anthropogenic) is neutralised within catchments by a two step process (Johnson et al., 1981): initial uptake of hydrogen ion acidity takes place through exchange with reactive Al in the soil and is followed by exchange of Al and protons by base cations in the soils and due to bedrock weathering. The soil base cation pool is important in buffering acidic inputs to catchments through rapid ion-exchange reactions. In a system at equilibrium, the release of cations through weathering is balanced by acidic inputs. The soil base cation pool is important in the buffering of acidic inputs to catchments primarily through rapid ion-exchange reactions. This pool is, however, limited and excess inputs act to deplete the pool, leading ultimately to serious damage to local ecosystems. Although ion exchange reactions may control short term fluctuations, long term change is controlled by mineral weathering which regulates the primary production and release of base cations to the soil. Therefore, the ability of catchments to buffer acid inputs is fundamentally controlled by the geology and soil type of the catchment. Areas where the bedrock is depleted in base cations or where weathering rates are slow (acid igneous and metamorphic rocks) are most at risk from such pollution. In a study of soils from southern Sweden, Falkengren-Grerup & Tyler (1992) concluded that silicate weathering did not keep pace with biomass uptake and leaching. Weathering rates are, therefore, a fundamental control on the long-term response of catchments to environmental change. In parts of upland Britain, especially where bedrock is depleted in base cations, the soil pool may be strongly influenced by precipitation inputs in addition to weathering (Shand et al., 1997; White et al., 1998).