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CHUM-AM was used to investigate the behaviours of atmospherically-deposited heavy metals (Ni, Cu, Zn, Cd and Pb) in three moorland sub-catchments in Cumbria UK. The principal processes controlling cationic metals are competitive partitioning to soil organic matter, chemical interactions in solution, and chemical weathering. Metal deposition histories were generated by combining measured data for the last 30 years with local lake sediment records. For Ni, Cu, Zn and Cd, default parameters for the interactions with organic matter provided reasonable agreement between simulated and observed present-day soil metal pools and average streamwater concentrations. However, for Pb, the soil binding affinity in the model had to be increased to match the observations. Simulations suggest that weakly-sorbing metals (Ni, Zn, Cd) will respond on timescales of decades to centuries to changes in metal inputs or acidification status. More strongly-sorbing metals (Cu, Pb) will respond over centuries to millennia. Catchment turnover times for the strongly-retained metals Cu and Pb are of the order of centuries, whereas those for the more mobile Ni, Zn and Cd are appreciably shorter.

CHUM-AM was used to investigate changes in soil and water chemical variables in four moorland sub-catchments in Cumbria UK, to which non-marine S deposition has declined by 65% since the 1970s. The principal processes represented in the model comprise N and S uptake and release, water movements, the binding of cations by soil organic matter, chemical interactions in solution, and chemical weathering. CHUM-AM reproduced reasonably well the current soil pH and pools of N and S, and changes in streamwater chemistry over the period 1970–2000, notably decreases in the concentrations of alkaline earth cations and sulphate, and increases in pH. The model also predicts streamwater pH–Al relationships in agreement with observations. Predictive calculations suggest that constant atmospheric deposition of N at present rates will lead to N saturation and re-acidification, whereas a 50% reduction in N would stabilise soil and streamwater pH at about the present levels. CHUM-AM accounts for recovery from acidification due to sulphur deposition, but predicts re-acidification if nitrogen deposition is not reduced.

Declining sulphur deposition since the late 1970s has led to increases in pH in lakes and streams of the English Lake District (Cumbria, UK). To determine whether there have been biological responses to the chemical changes, we carried out surveys of stream macroinvertebrates and lake surface sediment diatoms, and compared the results with those from earlier surveys. Macroinvertebrate taxa in five streams (current average pH range 5.1–7.0) sampled in 1999 showed clear changes from those found during 1965/1966 and 1972. For three of the streams, more taxa were present in 1999 than recorded using comparable sampling methods in the 1960s and 1970s, despite lower numbers of individuals being recovered in 1999. Values of the Margalef diversity index could be calculated for both 1965–1972 and 1999 for four of the streams; the index was significantly greater in 1999 (P<0.001) in three streams, and unchanged in one stream (the most acid). The 1999 survey revealed the presence of acid-sensitive taxa that had been absent in the earlier surveys, notably the three stoneflies Leuctra fusca, L. moselyi and Chloroperla tripunctata. Some taxa that had been present during 1965–1972 were absent in 1999, but few of these were acid-sensitive. Diatoms from the surface sediments of six lakes were classified according to their acid sensitivities. In three of the lakes sampled in 1999 there were more diatoms characteristic of circumneutral waters, and less acidophilic species than had been found in 1983–1985. In the remaining three lakes, no noticeable changes had occurred. Overall, the observed biological changes are qualitatively as expected for the observed increases in pH, and there have been no instances of biological change in the opposite direction. The results support the expectation that changes in freshwater ecological status can be reversed by decreasing the remote emissions of acidifying pollutants.

Laboratory studies were conducted to investigate the rates ofrelease of Na, Mg, Al, Si, K, Ca and Fe from 10 samples of metamorphosed igneous rock of the Borrowdale Volcanic Group (BVG), consisting mainly of plagioclase feldspars, chlorite andquartz, with minor amounts of hornblende, epidote and carbonates.Experiments were performed using freshly-prepared rock pieces (ca. 12 mm cubes), in batch mode, but with frequent changes of the solution phase so that accumulation of dissolved solutes wasminimised. The initial element release rates were relatively high, but declined with time to reach approximate (pseudo-) steady state conditions. Element release rates declined withpH in the pH range 2–7, the dependence upon pH decreasing in the order Al Fe > Si > Mg Ca > K Na. Thepseudo-steady state rates for Si were comparable to steady state values for single minerals. Calcium showed the greatestsample-to-sample variation (more than 100-fold). Rock dissolutionwas far from stoichiometric, with Ca and Mg being present in thesolution phase in proportions greater than in the rock, and Al and Si in lower proportions. In some samples the release rates ofMg and Ca were equal, while in others the Ca rate was up to 100times that of Mg. The rate of release of Ca is correlated with the carbonate content of the rocks. The results indicate that the main source of Ca in the faster-dissolving samples is calcite, while the highest release rates of Mg are consistent with dissolution of dolomite. Aluminium, Si and Fe are probablyreleased predominantly from chlorite. The results are discussed in terms of the supply of base cations to stream waters drainingcatchments underlain by rocks of the BVG.

Fifteen freshwater samples containing significant concentrations of dissolved organic carbon (DOC) were titrated with copper under standardised conditions (pH 6 and 7), and concentrations of Cu2+ were measured with an ion-selective electrode. Measured values of [Cu2+], which were in the range 1E-11–1E-5 mol/L, were compared with those simulated using Humic Ion-Binding Models V and VI. It was assumed that copper speciation was controlled by the organic matter, represented by fulvic acid (FA), together with inorganic solution complexation (calculated with an inorganic speciation model). The models were calibrated by adjusting a single quantity, the concentration of FA. The optimised value - [FA]opt - was that giving the best agreement, according to least squares, between measured and simulated [Cu2+]. The calculations took into account competition by other dissolved filterable) metals (Mg, Al, Ca, Fe(II), Fe(III), Zn); in the case of Fe(III) it was assumed either that all the dissolved metal was truly in solution, or that the activity of Fe3+ was controlled by equilibrium with Fe(OH)3. The assumption about Fe(III) had relatively small effects on the fitting of Model V, but was significant for Model VI, because Model VI represents low-abundance, high affinity binding sites in humic matter, which are sensitive to Fe(III) competition. Because of its inclusion of the high affinity sites, Model VI provided better fits of the data than did Model V. Furthermore, Model VI with Fe3+ activity controlled by Fe(OH)3 gave smaller variation in the ratio of [FA]opt to [DOC] than Model VI with all Fe(III) assumed to be in solution. The average [FA]opt/[DOC] found from the Cu titrations was 1.30, which implies that 65% of the organic matter is ‘active’ with respect to metal binding. The average ratio of 1.30 is in reasonable agreement with ratios obtained by applying the model to field data sets for charge balance (1.22), Al speciation (1.56) and base titrations of Cu-amended waters (

Equilibrium dialysis was used to measure Co- and Cu-binding by an isolated peat humic acid (PHA) in controlled laboratory experiments under simulated estuarine conditions: ionic strengths of 0.005-0.7 M in NaCl and mixed Na-Mg-Ca chloride solutions, with trace metal concentrations of ~5x10-7 M, a PHA concentration of 10 mg/l, and at constant pH values of ~7.8 (Co and Cu) and ~4.6 (Cu only). Generally, Co- and Cu-humic binding decreased substantially with increasing ionic strength and, in the case of Cu, with decreasing pH. The presence of seawater concentrations of Ca and Mg had a relatively small effect on Co-humic binding and no measurable effect on that of Cu under the experimental conditions. The binding data were well-described by an equilibrium speciation code (WHAM) after optimising the fits by varying the metal-proton exchange constants for humic acid within justifiable limits (i.e. within 1 standard deviation of the mean exchange constants used in the WHAM database). The main factor producing the observed variations in metal-humic binding at constant pH was the electrostatic effect on the humic molecule. WHAM was used to predict Co- and Cu-humic binding in simulations of real estuaries. Co-humic binding is predicted to be relatively unimportant (generally <5% of total Co), whereas the Cu-humic complex is likely to be the dominant species throughout an estuary. The main factors producing changes in Co- and Cuhumic binding in the real-estuary simulations are the electrostatic effect on the humic molecule, ligand competition (mainly from carbonate species) for metals and to a lesser extent Ca and Mg competition for humic binding sites. Variations in pH are significant only at the freshwater end of an estuary. WHAM simulations also indicated that competition effects between metals are more likely to occur in freshwaters, than in seawater, due to enhanced electrostatic binding at low ionic strength.

The concentrations of Al bound to identified low molecular weight organic acids (LMWOAs), phosphate and humic compounds in soil solutions (O1, O2, E, B1 and B2 horizons) from Fennoscandinavian coniferous forest soils were calculated. Two models were evaluated: WHAM (Winderemere Humic Aqueous Model) and a model treating the natural organic acids as monoprotic. The WHAM model was calibrated to find an average charge balance of unity, and then used for Al speciation. This was achieved considering approximately 80% of the natural fulvic acid to be 'active'. For the monoprotic model, constants obtained from previous fitting of soil solution data were used and the model was calibrated using acid/base titrations. The modelling confirms that the low molecular weight acids are important complex formers, binding on average 11-42% (O), 19-20% (E), and 0-30% (B) of the total Al in solution depending on the model used. The monoprotic model yielded higher concentrations than WHAM. Both models predicted that the major part of Al in solution was organically complexed to humic substances and LMWOAs, which was consistent with analytical values of 'quickly reacting' Al (monomenc inorganic forms; 12-14% of Altot on average in the E and B1 solutions). Both models could predict the magnitude of the analytical values (RMSD=4-5 μM). On average the WHAM model showed a better fit for the E horizon solutions and the monoprotic model for the upper B horizon. The pH dependence of the low molecular weight fraction of Al was modelled in the pH range 3-5.5 for a 'typical' O1 and E solution assuming no other changes in soil solution composition. The extrapolation showed that the models had a different pH dependence. The concentration of Al bound to LMWOAs in WHAM decreased with higher pH while the opposite was seen for the monoprotic model. Also the influence of the concentration of total Al in solution was modelled. The models showed similar trends but for the 'typical' O1 horizon sample the monop

Long-term changes in stream water chemistry in the upper Duddon catchment (southwest Lake District, UK) were investigated. Ten streams were sampled and analysed weekly during 1998, and the results compared with data for the early 1970s and 1986. The waters exhibited a range of pH, average values for 1998 being 5.04–7.04. For all the streams, the average pH in 1998 was greater than that during 1971–73. Statistical analysis was carried out, using the 1970s data to estimate the magnitude of inter-annual variation, and taking discharge into account on the basis of antecedent rainfall. The results showed that for two of the streams the pH increase was significant at the 2.5% level, while for a further three it was significant at the 10% level. Comparison of the 1998 concentrations of nitrate and non-marine sulphate with data obtained for five streams in 1973–74 showed that average nitrate concentration had increased from 11 to 20 μeq dm−3 while that of non-marine sulphate had decreased from 94 to 50 μeq dm−3. For four of the streams, comparisons were also made between the 1998 data and those for 1986. In three cases, pH in 1998 was generally higher, and Al generally lower, than the values for 1986, but in the fourth case little difference was evident. The present results support observations for five nearby standing waters, strengthening the evidence for a general reversal of acidification in the southwest part of Lake District, due to a decline in the deposition of pollutant sulphur.

Water chemistry data are reported for five high-altitude standing waters in the English Lake District, with current average pH values in the range 5–7. The waters show long-term increases in pH, ranging from 0.3 to 0.8 pH units, between 1974 and 1997. The pH of Devoke Water, which was acidified only mildly, has returned to values estimated for the pre-industrial period (1850 and earlier). Alkalinity in Devoke Water increased from ca 20 μeq litre−1 in the early 1980s to ca 70 μeq litre−1 in the 1990s, and alkalinities in three of the other waters have increased by ca 20 μeq litre−1 since the 1970s. For the two intensively monitored sites (Devoke Water and Levers Water), significant decreases in the concentration of non-marine sulphate are demonstrated, which have taken place concurrently with decreases in the atmospheric deposition of pollutant sulphur. Approximate calculations suggest that the catchment of Levers Water was a sink for sulphur in the 1970s, and that the catchment of Devoke Water may currently be a source of sulphate. For neither Devoke Water nor Levers Water is there evidence of a long-term decrease in the concentration of non-marine base cations.

The binding of Eu to fulvic acid (FA) as a function of pH was determined by Schubert's method and by equilibrium dialysis. Data obtained by both methods showed a strong pH dependence, which was reproduced well by the model used to interpret the data (humic ion-binding model VI). The model was also able to account for five other data sets from the literature, and there was reasonable agreement among the values of the characteristic binding parameter for Eu–FA interactions. Data sets investigating the effects of aluminum competition were also explained well by Model VI. The model was used to calculate the speciation of Eu in natural waters as a function of the most-likely competitive solution species (protons, aluminium and calcium). The calculations indicate that under environmentally realistic concentrations both, aluminium and calcium can be significant competitors at low and high pH, respectively, although FA binding appears to dominate Eu speciation under most conditions.