P.A. Stevens

Acid Deposition in Wales: the results of the 1995 Welsh Acid Waters Survey

In 1995, bulk precipitation was sampled at weekly intervals from 25 collectors located across Wales. The precipitation chemistry was dilute, acid (overall mean pH 4.88), and dominated by seasalts with a sodium to chloride ratio close to that of seawater. Seventy percent of the sulphate was from non-seasalt sources and the equivalent ratio of sulphate to nitrate was 1.8. Precipitation chemistry varied spatially with a decrease in the proportion of seasalts but an increasing proportion of excess sulphate with distance eastwards. Seasonal maximum concentrations of seasalts occurred in the first 3 months of the year whilst maximum excess sulphate, nitrate and ammonium concentrations were observed during the summer months. Maps of modelled wet plus dry deposition at 20-km square resolution showed that the acid-sensitive uplands of Snowdonia, the Cambrian mountains and the Brecon Beacons received large inputs of excess sulphur (15–25 kg S ha−1 year−1). Total inorganic nitrogen deposition to much of Wales was between 20 and 25 kg N ha−1 year−1. In 1995, Wales received 32 ktonnes of excess sulphur and 42 ktonnes of inorganic nitrogen equivalent to 9 and 11%, respectively, of the UK total deposition (based on 1992–94 figures). Nitrogen deposition contributed 60% of the total acidifying input to Wales. Since the first survey in 1984, the overall mean total sulphate concentration in rainfall has declined by about 28% to 2 mg litre−1; comparable to changes at long-term monitoring sites. Nitrate concentrations also declined by 18% to 0.23 mg N litre−1 whilst there was no statistically significant change in ammonium concentrations. Between 1984 and 1995, wet deposition of total sulphur to Wales decreased by 22% from 36 to 28 ktonnes, whilst wet deposited inorganic nitrogen declined from 27 to 22 ktonnes. These intensive regional data from the far west of Europe provide valuable insight into current deposition trends and the relative importance of sulphur and nitrogen deposition to acidification.

Effects of clearfelling on stream and soil water aluminium chemistry in three UK forests

Data are presented demonstrating how clearfelling has changed soil and stream water aluminium chemistry. For soil waters, a strong empirical relationship was observed between inorganic aluminium (Al(inorg)) and total inorganic anion (TIA) concentrations. Before felling, chloride and sulphate accounted for the largest proportion of the TIA concentration. After felling, in soils where nitrification was active, nitrate became increasingly important. Where this led to an increase in TIA, Al(inorg) concentrations increased. Over five years, nitrate concentrations have fallen, along with TIA, resulting in a sympathetic decline in Al(inorg). Streams draining clearfelled areas initially became more acid, although chloride and sulphate concentrations decreased. Stream water nitrate concentrations increased soon after felling and remained higher than controls for up to four years. While nitrate concentrations were high, Al(inorg) remained unchanged. Subsequently, as nitrate and TIA decreased, Al(inorg) also declined to concentrations below those in the control stream. Clearfelling upland forests will not necessarily result in immediate improvements in water quality, although long-term benefits may be seen before canopy-closure of the next crop.

Factors influencing bulk precipitation chemistry at an upland site in mid-wales

Bulk precipitation at an upland grassland catchment was monitored over a two year period using a continuously open collector. The data were analysed to establish the relative contributions from marine and non‐marine sources and the influence of wind direction and speed on precipitation chemistry. The precipitation is dilute, acidic and dominated by sea derived solutes. The relative contribution of marine‐derived solutes is related to weather conditions dominated by autumnal storms from the west and southwest, whereas periods of calm weather in spring with light easterly winds is an important control on the chemistry of non‐marine ions. An estimate of the rate of bulk deposition of solutes at the site is given, and the possible errors relating to the interpretation of this estimate for a catchment system are discussed.

The effect of sampling frequency on chemical parameters in acid-sensitive streams.

We assessed the effects of different simulated sampling regimes (weekly, fortnightly, monthly and bimonthly) on parameters describing the water chemistry of 72 streams in acid-sensitive areas of Wales. For pH, sulphate, total hardness and aluminium, reduced sampling frequency had no discernible or systematic effect on the apparent annual mean chemistry relative to the values derived from weekly data. Standard deviations and coefficients of variation were either unaffected, or were reduced. However, sampling frequency had a moderate effect on mean pH when the data were separated into seasons: winter mean pH increased on average by 0.13 units and summer means decreased on average by 0.12 units, when using bimonthly data relative to weekly. Extreme values were detected less effectively at lower sampling frequencies, significantly altering the intercept and/or the slope of the strong relationships between the means and minimum pH or maximum Al. These effects almost certainly reflect the exclusion of extreme events (summer drought and winter floods) from low sample frequencies and reveal limitations in the use of mean values from periodic sampling programmes for summarising some aspects of site chemistry. Nevertheless, previously established relationships between mean stream chemistry, land use and stream biology were still strong at all sampling frequencies. Clear recommendations about the needs to fully parameterise episodic fluctuations depend on unanswered questions about: (i) whether biota respond to mean or episodic chemical conditions and (ii) whether baseflow chemistry, episodic fluctuations, or some combination of these, will best reflect trends in acidification.