Silicon concentrations in UK surface waters

Abstract

This paper describes the variations in silicon concentrations in UK waters for a wide range of catchment systems (near pristine, rural, and agricultural and urban impacted systems). The paper largely concerns silicon levels in streams, rivers and lakes based on extensive data collected as part of several research and monitoring initiatives of national and international standing. For a detailed study of an upland catchment in mid-Wales, information on atmospheric inputs and groundwater chemistries is provided to supply background information to cross link to the surface water chemistry. Several hundred streams/rivers and lakes are dealt with within the study, dealing with the main types of freshwater riverine and lacustrine environments. The streams/rivers vary from small ephemeral runoff to the major rivers of the UK. The geographical location of sites vary from local sites in mid-Wales, to regional studies across Scotland, to the major eastern UK rivers entering the North Sea and to acid sensitive upland sites across Wales, the English Lake District, Scotland and Northern Ireland. The surface waters range in silicon concentration from 0 to 19 mg-Si l −1 (average for individual sites vary between 0.7 and 7.6 mg-Si l −1) and there are some clear variations which link to two primary processes (1) the relative inputs of groundwaters enriched in silicon and near surface waters more depleted in silicon and (2) plankton uptake of silicon during the summer months under baseflow conditions. Thermodynamic analysis reveals that the waters are approximately saturated with respect to either quartz or chalcedony except for two circumstances when undersaturation occurs. Firstly, undersaturation occurs at pH less than 5.5 in the upland areas and this is because the waters are mainly sourced from the acidic organic soils which are depleted in inorganic minerals. Secondly, undersaturation occurs in the lowland rivers when biological activity is at its highest and this leads to silicon removal from the water column. Quartz equilibrium can be approached (at pH>5.5) mainly within the upland systems which are not aquifer recharge dominated. However, for the lowland systems that are groundwater recharge dominated, it is chalcedony saturation which is approached, and such saturation is often observed within groundwaters. Similar patterns of undersaturation in response to biological uptake are seen in lakes and the extent of silicon depletion increases with biological productivity. Chalcedony oversaturation can occur for some UK rivers under baseflow conditions and this probably links to a higher rate of weathering.