Acid neutralization capacity measurements in surface and ground waters in the Upper River Severn, Plynlimon: from hydrograph splitting to water flow pathways

摘要

Acid Neutralization Capacity (ANC) data for ephemeralstream and shallow groundwater for the catchments of the upper River Severn show a highly heterogeneous systemof within-catchment water flow pathways and chemical weathering on scales of less than100m. Ephemeral streams draining permeable soils seem to be supplied mainly from shallow groundwatersources. For these streams, large systematic differences in pH and alkalinity occurdue to the variability of the groundwater sources and variability in water residence times. However,the variability cannot be gauged on the basis of broad based physical information collected inthe field as geology, catchment gradients and forest structure are very similar. In contrast,ephemeral streams draining impermeable soils are of more uniform chemistry as surfacerunoff is mainly supplied from the soil zone. Groundwater ANC varies considerably over spaceand time. In general, the groundwaters have higher ANCs than the ephemeral streams. This is dueto increased chemical weathering from the inorganic materials in the lower soils and groundwaterareas and possibly longer residence times. However, during the winter months the groundwaterANCs tend to be at their lowest due to additional event driven acidic soil water contributionsand intermediate groundwater residence times. The results indicate the inappropriateness of ablanket approach to classifying stream vulnerability to acidification simply on the basis of soil sensitivity.However, the results may well indicate good news for the environmental management of acidic and acidsensitive systems. For example, they clearly indicate a large potential supply of weatheringcomponents within the groundwater zone to reduce or mitigate the acidifying effects of landuse change and acidic deposition without the environmental needs for Aiming. Furthermore,the high variability of ephemeral stream runoff means that certain areas of catchments wherethere are specific problems associated with acidification can be identified for focusedremediation work for the situation where liming is required. The case for focused field campaigns and cautionagainst over reliance on blanket modelling approaches is suggested. The results negate the conventional generalizationswithin hydrology of how water moves through catchments to generate streamflow events(from Hortonian overland flow to catchment contributing areas).