The hydrochemistry of the headwaters of the River Severn, Plynlimon

Abstract

Abstract. Water quality data spanning 13 years and covering an extensive range of major, minor and trace elements in rain and stream waters at Plynlimon in mid Wales, are presented. Rainfall water chemistry is highly variable due to varying proportions of marine and pollutant derived constituents associated with patterns of atmospheric circulation. Stream waters, being composed of different proportions of waters from three chemically distinct sources at any one time (atmospheric deposition, the soil system and deeper groundwaters), are also chemically highly variable. For example, components predominantly derived from deposition such as chloride change only in response to sea-salt deposition episodes. Solutes associated with bedrock weathering such as calcium, magnesium, and alkalinity decrease with increasing flow, those associated with the upper soil layers such as aluminium, many transition metals, dissolved organic carbon and hydrogen ions increase with increasing flow. The nutrients (e.g. nitrate, boron, bromide and iodine) exhibit strong seasonal cycles associated with cycles of vegetation growth and decay. The changes in stream water chemistry resulting from tree harvesting in the Afon Hore catchment are shown to have run their course within a period of eight years. Nutrient increases in the first few years following the commencement of felling have returned to or fallen below pre-felling values. Aluminium changes are shown to be complicated by changes in nitrate and calcium. Aluminium concentrations initially increased and have fallen below their pre-felling value. Data for chloride suggest a reduction in capture of dry and mist deposition; this indicates the importance of understanding reduced deposition as a result of felling. Felling has also affected the soil micro-climate which experiences greater fluctuations in temperature and an increase in the concentration of constituents associated with organic matter. Input-output mass balance estimates show that atmospheric inputs of many constituents are retained strongly by the catchment (e.g. ammonium, phosphate, barium, boron, lead and iodine). In contrast, many of the transition elements as well as divalent base cations, aluminium and alkalinity show a net release from the catchment. Conservative constituents such as chloride and sodium show a net input-output balance.

Long-term changes in the water quality of rainfall, cloud water and stream water for moorland, forested and clear-felled catchments at Plynlimon, mid-Wales

“…Chemical analysis techniques have been described previously (Neal et al, 1997a) and only brief reference is made here. On the day of collection, pH, conductivity and Gran alkalinity were determined electrometrically.…”

“…The object of this exercise is to address two issues: has stream water quality improved over the last 17 years; does conifer harvesting perturb acidification trends associated with emissions changes? The catchments used provide one of the longest and most detailed records for water quality in the UK (Neal et al, 1997a) and are perhaps the most actively researched with regards to hydrology, sedimentology and hydrochemistry (Neal, 1997(Neal, , 2000.…”

High-frequency water quality time series in precipitation and streamflow: From fragmentary signals to scientific challenge

“…For this reason, when the very high level of activity required for the collection and analysis of samples from rainfall plus two streams proved unsustainable, it was decided to discontinue intensive sampling of the Lower Hafren after the first year so that sampling could continue at the Upper Hafren for the second year. Details of the sampling sites, analytical protocols, and the broader hydrochemistry based on weekly monitoring are contained in Neal et al (1997Neal et al ( , 2010Neal et al ( , 2012 together with the metadata attached to the data sets accompanying this paper.…”

Groundwater nitrogen composition and transformation within a moorland catchment, mid-Wales

“…Studies in the Plynlimon catchments of mid-Wales have provided long-term records of changes in stream nitrogen chemistry and have enabled the impact of changes in historical deposition and landuse to be assessed (Neal et al, 1997c). Simplistic models, which consider the groundwater Science of the Total Environment version 0.2 system as an impermeable compartment, fail to model changes in stream chemistry successfully and the need to understand and include processes within the groundwater system is clear (Neal et al, 1997b;Neal et al, 1988).…”