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Drop of water

Environmental Change > Pollution Pollution Of the many factors influencing change in the Earth’s climate and environment, the impact of pollution caused by human activity in the past 200 years is the most pressing concern. Pollution of water — our most vital earth resource — is a major focus of study. Some highlights of recent research are given below:

Tracing water paths through small water catchments under tropical mountain rain forest in south Equador by an oxygen and hydrogen isotope approach In three steep microcatchments under tropical montane forest, samples of rainfall, throughfall, lateral (organic layer) flow, soil water, and streamflow were collected between 23 August 2000 and 15 August 2001. Water samples were analysed for O and H isotopes to elucidate the preferential directions — vertical versus lateral — of water flowpaths in soils and how they are linked to the precipitation and soil water regime. Additional soil moisture measurements were conducted to support the isotope study. The δ¹⁸O of rainfall shows large variations (-12.6‰ to +2.1‰) related to different air-masses. There is no correlation between δ¹⁸O values in rainfall, temperature, and rainfall amount. Local meteoric water lines for rainfall and throughfall suggested that evaporation was minimal. The δ¹⁸O values of throughfall and lateral flow are similar δ¹⁸O values to those in rainfall. One of our vacuum units for the extraction of soil solution with porous cup lysimeters.		The San Francisco valley west of Zamora (Province of Zamora-Chinchipe, Ecuador) with the research station Estación Científica San Fransisco (ECSF) in the background. Variations in δ¹⁸O values of the soil water and the streamflow are smaller (-9.1‰ to -3.0‰ and -5.8‰ to -8.7‰) than those of rainfall, throughfall, and lateral flow. The δ¹⁸O values in streamwater increased immediately after an intense rainstorm event to isotope values similar to those of rainfall and lateral flow indicating that during elevated rainfall the water flows rapidly in the organic layers to the stream channel paralleling the surface. This finding was confirmed by the higher volume of water in the organic layer than in the upper mineral soil during the rainstorm event. Our results suggest that water flowpaths through the ecosystem are dominated by vertical directions through the soil profile to the stream channels during normal wet conditions, interrupted by short-term flow direction changes to lateral pathways mainly in the organic layers during rainstorm events. (With R Goller and W Wilcke University of Bayreuth, Germany). Goller, R, Wilcke, W, Leng, M J, Tobschall, H J, Wagner, K, V alarezo C, and Zech, W, 2005. Tracing water paths through small water catchments under tropical montane rain forest in south Ecuador by an oxygen and hydrogen isotope approach. Journal of Hydrology, 308, 67–80. Click on the link below to visit the project home page: www.bergregenwald.de

Origins of salinity in the Sherwood Sandstone aquifer The Sherwood Sandstone aquifer represents the second most important source of groundwater in the UK. Although its quality is generally good, water in the northern parts — from the Vale of York to Teeside — exhibits problems of high salinity associated with sulphate. Anhydrite (CaSO₄) and gypsum (CaSO₄.2H₂O) are common as marine evaporite deposits in the Permo-Triassic marls and mudstones which enclose the sandstone aquifer, and sulphate may also be expected as a product of sulphide oxidation in the Carboniferous Coal Measures. We conducted a detailed ³⁴S/³²S ratio (δ³⁴S) and ¹⁸O/¹⁶O ratio (δ¹⁸O) analysis of dissolved sulphate in groundwater from different parts of the Sherwood Sandstone aquifer, and anhydrite/gypsum deposits in the Permian and Triassic rocks. The data allowed the sulphate in the groundwater to be assigned to three different sources (see Figure): sulphate present in low concentrations, and tending to have low δ³⁴S values. This is typical of normal ‘background’, non-marine evaporite sulphate derived from soils or oxidaton of small amounts of rock sulphide marine evaporite sulphate present at high concentrations in waters from the western, ‘feather edge’ part of the aquifer coming from the stratigraphically-underlying Permian rocks marine evaporite sulphate present at high concentrations in waters from the eastern part of the aquifer coming from the stratigraphically-overlying Triassic rocks. (With Paul Shand and Tony Milodowski, British Geological Survey).		Top. Sulphate deposits (gypsum and anhydrite) in Permian and Triassic rocks can be distinguished by their different δ³⁴S values. Bottom. The yellow and orange curved bands represent the concentration and δ³⁴S values which would be expected if a low-concentration, low- δ³⁴S water was contaminated with high concentrations of Permian or Triassic sulphate. From this we can establish that the groundwater sampled from the Sherwood Sandstone (blue circles) is contaminated by sulphate derived from both Permian rocks (water samples in the yellow band) and from Triassic rocks (water samples in the orange band).

Subglacial environment of a High Arctic glacier A recent CASE studentship investigated the provenance of nutrient nitrogen in a glacial basin of the High Arctic. Coastal waters in these regions are typically nutrient limited. They are therefore particularly sensitive to the effect that climate warming is having on the flow of nutrient laden rivers fed by glacial meltwaters. Biological activity used to be regarded as relatively unimportant in the cryosphere, so previous studies of glacial geochemistry have tended to focus on inorganic processes. In this study, however, chemical balances and isotope data point to microbial involvement in major processes affecting nitrogen. These include assimilation of ammonium deposited in snowfall, and denitrification. The latter may be linked with sulphide oxidation and other reactions determining redox conditions beneath the glacier.		Much of this activity occurs in sub-glacial water stored at the ice-bedrock interface, during the winter months, and may include processes releasing geological nitrogen stored within the bedrock. With the start of the spring thaw these sub-glacial waters are released as dramatic ‘upwellings’, with a chemistry very distinct from the water derived from supra glacial ice- and snow-melt. Upwelling of subglacial water, Midre Lovénbreen, Svalbard. (With Peter Wynn and Andrew Hodson (University of Sheffield)).

Nitrate in upland waters Problems associated with the high levels of nitrate in surface and groundwaters tend to have focused on the impact of modern farming methods in more populated parts of the UK, and the restrictions on agriculture required for compliance with the EU Nitrate Directive. However, nitrate also constitutes a potential problem in more remote, upland areas. Here it is nitrate in atmospheric deposition which is of concern, because it has a particular impact on upland areas due to a combination of factors: 1) these are high rainfall areas; 2) low concentrations of base cations in upland soils makes them particularly prone to acidification, and nitrate is soon likely to become the main acidifying agent in ‘acid’ rain; and 3) upland ecosystems are adapted to low nutrient levels and are often N-limited. Concerns centre on the fact that current trends suggest critical loads for acidity and N deposition will be exceeded in many upland areas by 2010, and this threatens future compliance with the EU Waters Directive. Predicting the response of upland areas to nitrate deposition, however, depends on knowledge of the extent to which their soil/plant ecosystems are already N-saturated. In saturated systems, excess soil N is likely to undergo bacterial nitrification, and be released as nitrate into surface waters.		To this end, being able to distinguish between nitrate derived from atmospheric deposition, and nitrate formed by soil nitrification is particularly important. In conjunction with long-term investigations of UK upland waters conducted by the Environmental Change Research Centre at University College London, NIGL has recently undertaken the first combined ¹⁵N/¹⁴N + ¹⁸O/¹⁶O analyses of nitrate in rainfall and stream samples in the UK. Data from four upland catchments are summarised in the Figure: Afon Gwy (Wales), Lochnagar (Cairngorms), River Etherow (Peak District), Scoat Tarn (Lake District). Several very clear results emerge: Whereas δ¹⁵N values do not easily distinguish between atmospheric and bacterial nitrate, there is a clear, c.70‰ distinction in the δ¹⁸O values. Most stream waters have δ¹⁸O values which correspond very closely to the theoretical values for soil bacterial nitrate (based on knowledge of the δ¹⁸O values of atmospheric O₂ and local stream H₂O). Therefore, at least during the sampling period (July–October), very little atmospheric nitrate passes directly through the soils into the streams; the nitrate in the streams is almost exclusively derived by nitrification. Only the outflows of Lochnagar and Scoat Tarn show significant atmospheric nitrate (c. 17%), and this may represent rainfall which has fallen directly onto the surface of these water bodies. (With the Environmental Change Research Centre, University College London).

Origin of volcanic sulphate The majority of volcanoes emit sulphur in the form of gaseous SO₂. However, primary sulphate aerosol has also been observed, and its exact origin has not yet been explained. A combination of sulphur and oxygen isotope ratio measurements of sulphate aerosols, SO₂, and volcanic glass from the Masaya volcano were therefore used to further our understanding of the production of this volcanic sulphate. It was shown that the sulphate aerosol displayed a mass-dependent oxygen isotopic signature, suggesting that it does not contribute to the mass-independent oxygen isotope anomalies found in ambient atmospheric aerosol, and that O₃ and H₂O₂ did not play an important role in its production.		The low δ¹⁸O value (~+5.5‰) of the aerosol, coupled with the low δ¹⁸O value of the magma (+6.6‰), also implied that atmospheric oxygen played no part in the production of sulphate. Measurements of the δ³⁴S values of the SO₂ were affected by isotopic fractionation during collection, but could be corrected to yield a value of ~+6 ±1‰. The higher δ³⁴S value for sulphate aerosol (+7.7‰) implied α SO₄-SO₂ > 1, and comparison with models of fractionation suggest that gas-phase oxidation of SO₂ with OH at ambient, near surface conditions was not responsible for the sulphate formation. Comparison of the δ³⁴S values of the magma (+6.6 ‰) and the aerosol, however, suggest that primary emission of SO₄^2- could not be ruled out as the production mechanism. High-temperature OH oxidation of SO₂ or high-temperature equilibration of the volcanic gases to form sulphate were also possible. With TA Mather and DM Pyle (University of Cambridge), JR McCabe, VK Rai and MH Thiemens (University of California, San Diego), and G Fern (University of Greenwich, Medway).

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