Sodium and chloride levels in rainfall, mist, streamwater and groundwater at the Plynlimon catchments, mid-Wales: inferences on hydrological and chemical controls

Variations in sodium and chloride in atmospheric inputs (rainfall and mist) , stream runoff and groundwater stores are documented for the upper Severn River (Afon Hafren and Afon Here catchments), Plynlimon, mid-Wales. The res ults show five salient features. 1. Sodium and chloride concentrations are highly variable and highly correl ated in rainfall and mist. The sodium-chloride relationship in rainfall has a slope close to the sodium/chloride ratio in sea-water, and an intercept that is not significantly different from zero. This indicates that sea-salt is the dominant source of both sodium and chloride in rainfall, which woul d be expected given the maritime nature of the metrology. For mist, there i s also a straight line with near-zero intercept, but with a slightly higher gradient than the sea-salt ratio, presumably due to small additional sodiu m inputs from other sources. 2. There is an approximate input-output balance for both sodium and chlorid e, with the exception of one groundwater well, in which high chemical weath ering results in an anomalous high Na/Cl ratio. Thus, atmospheric depositio n is the dominant source of both sodium and chloride in groundwater and str eamflow. 3. The fluctuations in sodium and chloride concentrations in the streams an d groundwaters are strongly damped compared to those in the rain and the mi st, reflecting the storage and mixing of waters in the subsurface. 4. On all timescales, from weeks to years, sodium fluctuations are more str ongly damped than chloride fluctuations in streamflow. The additional dampi ng of sodium is consistent with ion exchange buffering of sodium in the cat chment soils. 5. Sodium and chloride concentrations are linearly correlated in the stream s and groundwaters, but the slope is almost universally less than the sea-s alt ratio and there is a non-zero intercept. The Na/Cl ratio in streamflow and groundwater is higher than the sea-salt ratio when salinity is low and lower than the sea-salt ratio when salinity is high. This pattern of behavi our is again consistent with ion exchange buffering of sodium in the catchm ent soils. The core features of this study are two fold. Firstly, sodium and chloride concentrations are highly damped within the streams and groundwaters relati ve to the atmospheric input. Secondly, streamflow sodium and chloride respo nd in similar ways across the catchments, except for the added cation excha nge damping of the sodium signal. These findings are remarkable given the h eterogeneous nature of the catchments and the complexity of the chemical ti me series signals in the streams.