Gran Alkalinity Research Articles

A tracer‐based assessment of hydrological pathways at different spatial scales in a mesoscale Scottish catchment

AbstractGeochemically based hydrograph separation techniques were used in a preliminary assessment to infer how runoff processes change with landscape characteristics and spatial scale (1–233 km2) within a mesoscale catchment in upland Scotland. A two‐component end‐member mixing analysis (EMMA) used Gran alkalinity as an assumed conservative tracer. Analysis indicated that, at all scales investigated, acidic overland flow and shallow subsurface storm flows from the peaty soils covering the catchment headwaters dominated storm runoff generation. The estimated groundwater contribution to annual runoff varied from 30% in the smallest (ca 1 km2) peat‐dominated headwater catchment with limited groundwater storage, to >60% in larger catchments (>30 km2) with greater coverage of more freely draining soils and more extensive aquifers in alluvium and other drift. This simple approach offers a useful, integrated conceptualization of the hydrological functioning in a mesoscale catchment, which can be tested and further refined by focused modelling and process‐based research. However, even as it stands, the simple conceptualization of system behaviour will have significant utility as a tool for communicating hydrological issues in a range of planning and management decisions. Copyright © 2002 John Wiley & Sons, Ltd.

Calcite saturation in the River Dee, NE Scotland.

The spatial and temporal variations in calcite (calcium carbonate) solubility within the Dee basin of NE Scotland were assessed using water chemistry data gathered from a network of 59 sites monitored for water quality from June 1996 to May 1997. Calcite solubility, expressed in terms of a saturation index (SIcalcite), was determined from measured streamwater pH, Gran alkalinity and calcium concentrations and water temperature. In general, the waters of the Dee system are undersaturated with respect to calcite, though the saturation index is higher during the summer months indicating a dependency on flow conditions and biological activity. Under low-flow conditions, the streamwaters are dominated by water derived from the lower soil horizons and deeper groundwater stores and therefore, ions such as Gran alkalinity and calcium are at their highest concentrations as they are derived mainly from bedrock weathering. The influence of biological activity on the carbonate system is also evident as the observed pH and estimated EpCO2 values indicate strong seasonal patterns, with the highest pH and lowest EpCO2 values occurring during the summer low-flow periods. Only at three sites in the lowland region of the catchment, during the summer low-flow period, are the waters oversaturated. As such, the Dee system represents an extreme 'end-member' case when compared to many UK rivers that span both under- and oversaturated conditions during the year. Regression analysis highlights a systematic change in the SIcalcite-pH relationship in a broad east-west direction across the Dee system. At sites draining the relatively impermeable upland areas, the regression of SIcalcite against pH gives a straight line with a gradient in the range 1.6-2.4. Correspondingly, under the most extreme alkaline conditions found at sites draining lowland agricultural areas, a straight-line relationship exists but with a gradient of unity. It is concluded that these changes in the SIcalcite-pH relationship are due to variations in the bicarbonate system induced by the flow conditions and biological activity. Given the waters are undersaturated, then calcite precipitation and hence phosphorus co-precipitation cannot occur within the water column.

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

Abstract. Long term changes in the water quality of rainfall, cloud water and stream waters draining acidic and acid sensitive moorland and forested catchments at Plynlimon, mid-Wales, are examined for the period 1983 to 2001. Atmospheric inputs of chloride and sulphate are influenced by the relative inputs of clean maritime and polluted land based air masses. There is no systematic increase or decrease over time for chloride and non-sea-salt sulphate. Rather, there is a decadal scale process possibly representative of the influence of the North Atlantic Oscillation that affects the maritime and pollution climate of the Atlantic seaboard of the UK. Over 17 years of study, there may be a small decrease in non-sea-salt sulphate of about 10 μeq l-1 and a small improvement in acid neutralising capacity of about 20 to 30 μeq l-1 in rainfall. There is a clear improvement in cloud water chemistry with respect to pollutant components (ammonium, nitrate, non-sea-salt sulphate) and acidity (acid neutralising capacity improved by about 300 μeq l-1) through the study period. Many of the changes in cloud water chemistry are similar to rainfall over the same period except the magnitude of change is larger for the cloud water. Within the streams, there is some evidence for reductions in acidity as reflected by acid neutralising capacity becoming less negative. For one stream, deforestation occurred during the sampling period and this led to large increases in nitrate and smaller increases in aluminium midway through the study period. However, the climate and hydrological variability largely masked out other changes. The current analysis provides only a start to identifying trends for such a complex and variable environmental system. The need for strong statistical tools is emphasised to resolve issues of: (a) hydrological induced water quality variability, (b) changing soil and groundwater "endmember" chemistry contribution to the stream and (c) the non-linear patterns of change. Nonetheless, the analysis is enhanced by examining trends in chemistry for yearly averages and yearly average low catch and high catch rainfall and cloud water events as well as low and high flow stream chemistry. This approach allows trends to be examined within the context of endmember mixing. Keywords: Calcium, aluminium, ammonium, pH, Gran alkalinity, ANC, nitrate, chloride, sulphate, Plynlimon, cloud, mist, rainfall, stream, acidification, North Atlantic Oscillation, trends

On modelling the effects of afforestation on acidification in heterogeneous catchments at different spatial and temporal scales

A modelling approach is presented for simulating and predicting future changes in streamwater Gran alkalinity throughout a large, heterogeneous river system. The methodology is based on integrating End Member Mixing Analysis (EMMA), the Model of Acidification of Groundwater in Catchments (MAGIC) and spatial data describing the catchment characteristics stored on a Geographical Information System (GIS). These are integrated within a Functional Unit Network (FUN) to predict the changes in Gran alkalinity resulting from possible future changes in atmospheric deposition and land use (low intensity afforestation) in the River Dee catchment, NE Scotland. Model results indicate that declining sulphate and constant nitrogen deposition, combined with low intensity Scots pine ( Pinus sylvestris) afforestation are unlikely to contribute significantly to streamwater acidification.

Recovery Pattern from Acidification of Headwater Lakes in Finland

Acid sensitive headwater lakes (n=163) throughout Finland have been monitored during autumn overturn between 1987–1998. Statistically significant decline in sulphate concentration is detected in 60 to 80 percent of the lakes, depending on the region. Median slope estimates are from −1.1 µeq L−1 in North Finland to −3.3 µeq L−1 in South Finland. The base cation (BC) concentrations are still declining especially in southern Finland (slope −2.5 µeq L−1), where every second lake exhibits a significant downward trend. The BC slope is steeper for lakes with less peatlands, more exposed bedrock, longer retention time and southerly location, but these factors are inter correlated. Gran alkalinity slope medians for the three regions range from 1.4 to 1.8 µeq L−1 yr−1. No significant negative alkalinity trends were detected. The similarity in the slopes of SO4, BC and alkalinity in this data compared to seasonal sampling data from Nordic Countries can be regarded as indirect evidence that autumnal sampling is representative for long term monitoring for these ions. There are no indications of increased organic carbon in lakes, as found in some recent trend analyses of similar regional data sets. Although the processes behind the positive development in these lakes have to be revealed with site- specific intensive studies, this data suggests, that the initial recovery from lake acidification in Finland is a regional phenomenon.

Alkalinity measurements within natural waters: towards a standardised approach

A discussion on the measurement of alkalinity in bicarbonate bearing and acidic waters is presented as a move towards developing a standardised approach within the aquatic sciences. The discussion is based on theoretical and practical considerations. Practical illustrations are provided from measurements of calcium bicarbonate bearing waters (River Thames) and acidic to near neutral waters from acidic soil systems with calcium bearing groundwater sources (the mid-Wales region and the Scottish Dee basin). A comparison of single endpoint and Gran procedures is made and the various error terms assessed. It is concluded that single endpoint and Gran methodologies are applicable in the standard pH titration used in aquatic research (pH 4.5–4.0). However, analytical errors increase, particularly for the single endpoint titration, the lower the endpoint/endpoint-range in pH. For the Thames, the value of a single endpoint over a Gran procedure is illustrated. The Thames results show that some buffering components may still be titrated out within the Gran titration range used: this leads to an underestimate in the alkalinity. Indeed, by making a comparison between single endpoint and Gran alkalinity measurements, the Thames results indicated the presence of unexpected buffering components: the interrogative nature of this type of analysis is highlighted. A standardised approach is outlined.

Riverine inputs of major ions and trace elements to the tidal reaches of the River Tweed, UK

This paper examines spatial and temporal variability in freshwater inputs of trace elements and major ions to the tidal reaches of the River Tweed. The relationships between concentrations of major ions and trace elements (in dissolved and acid-available particulate forms) and flow are examined, and a simple two-component endmember mixing analysis performed to link river water chemistry to catchment sources, and to identify whether within-river processes modify concentrations to produce non-conservative behaviour. The results indicate that most dissolved major ions and trace elements behave conservatively in the lower reaches of the River Tweed, the variability in concentrations being dominated by hydrology and the existence of different high-flow and low-flow endmember runoff chemistries. This suggests that the variability in concentration of most dissolved trace elements and major ions in the lower Tweed can be modelled using simple mixing relationships. However, the relationships between pH, Ca, Mg and Gran alkalinity show pronounced non-conservative behaviour, indicating the importance of understanding within-river process for modelling these determinands. The non-conservative behaviour identified is related to biological controls and the resultant diurnal variations in pH which promote daytime removal of Ca from solution by precipitation of calcium carbonate (and corresponding removal of Mg by co-precipitation) during extreme low-flow conditions and algal blooms during the summer.

Acid neutralisation capacity measurements within natural waters: towards a standardised approach

Detailed hydrochemical data for an extensive survey of streams draining small acidic and acid sensitive forested and felled catchments across Wales are used to assess the relationship between Gran alkalinity and acid neutralising capacity (ANC). The waters vary in their ANCs from relatively high (1500 μEq./l) to low values (−319 μEq./l) thus covering a range which encompasses good quality bicarbonate bearing to highly acidic waters. The results show a good relationship when allowance is made for aluminium and organic anion charge associated with dissolved organic carbon. The relationship takes the form: ANC ≈ Alk Gran − Al Total + Δ DOC where [] denotes equivalent and {} molar concentrations, while Δ, a stoichiometric constant, has a value of approximately 0.07 which relates to the deprotonation of hydrogen ions from the DOC. The relationship is of importance to acidification studies as it provides a means of assessing the critical measure of ANC which is infrequently determined from routinely and much more easily measured determinands. The work shows that both aluminium and organic anion charge can make up a substantial part of the ANC and that neither can be ignored.

Modelling streamwater quality under varying hydrological conditions at different spatial scales

A simple and pragmatic technique for modelling streamwater acidity in a major Scottish catchment, the River Dee in Aberdeenshire, is presented. Based on End Member Mixing Analysis (EMMA), the technique is used to predict daily streamwater concentrations of Gran alkalinity and calcium at a range of spatial scales (1–690 km 2). For a given site anywhere within the upland region of the catchment, the streamwater end member compositions are estimated from geological, soil and land cover data using a Geographical Information System (GIS). The utility of the approach is demonstrated by the predicted stream-water Gran alkalinity and calcium concentrations matching with a reasonable degree of accuracy those observed in the field. There are two assumptions: (i) the streamwater is composed of two end members and (ii) the end members are temporally invariant. The accuracy of the model predictions suggests that these two assumptions are reasonable, at least to a first approximation in large ( > 50 km 2) catchments. To model environmental change the technique must be linked to process based models, which predict how end members will change over time.

The influence of catchment characteristics, agricultural activities and atmospheric deposition on the chemistry of small streams in the English Lake District

Six synoptic surveys of 55 streams draining the central English Lake District, Cumbria, were carried out during the period May 1996–March 1997 to measure streamwater chemistry under a variety of flow conditions and relate this to geology and land use. Annual mean flow-weighted Gran alkalinity of the streams in the study ranges from 2 to 452 μeq l−1. Using 200 μeq l−1 alkalinity as a sensitivity threshold, 41 of the streams (75%) can be considered sensitive to acidification via acid deposition. Of the sensitive streams, 12 have annual mean flow-weighted Gran alkalinities less than 50 μeq l−1, suggesting a critical level of sensitivity to acidification. By grouping the streams into categories based on the dominant geology, it is clear that geology plays a major role in determining streamwater chemistry. However, the chemical signature of the streams is significantly modified through the effects of land use. One example is stream nitrate (NO3−) concentration which ranges from 0 to 238 μeq l−1 (flow-weighted mean). All of the seven streams with flow-weighted NO3− values above 50 μeq l−1 drain predominantly agricultural catchments, with cover varying between 63 and 100%. The flow-weighted chemistry data were related to catchment characteristics (geology, land use, deposition and soil type) using multiple regression analysis. The influence of agriculture generally dominated the models. In particular, strong relationships were found between percentage agricultural land and Gran alkalinity (R2=0.53), percentage agricultural land and calcium (R2=0.50) and percentage agricultural land and nitrate concentrations (R2=0.40). This survey confirms that the resistant bedrock of the Lake District generally results in low base cation resupply to the streams. Due to this very low buffering capacity it is likely that many of the streams have been impacted by acid deposition, especially those draining the slow-weathering Skiddaw Slates, Borrowdale Volcanics and granitic bedrock. The easier-weathering Silurian Slates provide higher buffering capacity to drainage waters. Agriculture, which is mostly found on the Silurian Slates, increases the buffering capacity of the drainage water but also results in enhanced nitrate leaching.

An assessment of excess carbon dioxide partial pressures in natural waters based on pH and alkalinity measurements

Methods of estimating excess partial pressures of carbon dioxide in river waters using pH and Gran alkalinity measurements are considered using data from several UK lowland rivers covering a spectrum of industrial, urban and agricultural catchments. Two simple equations are shown to be eminently suitable except for the most demanding accuracies at pH values greater than 9 when carbonate and hydroxide ions as well as calcium complexes with bicarbonate, carbonate and hydroxide become larger. The most basic of these equations, which simply allows for the averaged effects of temperature and ion activity, is EpCO 2=(Alk Gran in μEq/l units +10 6− pH )∗10 6− pH /6.0. The second equation, which allows for variation in temperature and average ionic strength, is EpCO 2= (0.95∗Alk Gran inμEq/l units +10 6−pH)∗10 6−pH (6.46−0.0636 t° C) . Within this equation, the 0.95 term represents the average factor which converts the chemical concentration of monovalent ions into chemical activities and t°C is temperature in degrees Celsius. For more demanding situations, such as at high pHs, the following equation is suggested EpCO 2= (0.95∗Alk Gran in μEq/l units +10 6−pH+10 6+pH+p K water )∗ 10 6−pH (6.46−0.0636∗t °C)∗ (1+2.38∗10 pH−p K2 ) . As in the previous case, the 0.95 term allows for activity concentration relationships for univalent ions. For this equation, p Kwater and p K2 represent minus the logarithm of the equilibrium constants for the respective reactions H 2O=H ++OH − and HCO 3 −=H ++CO 3 2−, where pK water=−6.0843+4471.33/(273+ t° C)+0.017053∗(273+ t° C) and pK2=−6.498+2902.39/(273+ t° C)+0.02379∗(273+ t° C). The (1+2.38∗10 pH−p K2 ) term allows for the effect of the carbonate ion contribution to the alkalinity and it is the ratio of the carbonate ion to the sum of carbonate and bicarbonate with both the numerator and denominator in units of equivalent concentration. The 2.38 term converts the ratio of carbonate to bicarbonate from an activity to an equivalent concentration ratio. Comparisons of measured EpCO 2 for the River Frome agree well with estimates based on the above equations and this adds to the confidence of the methodologies above pH 6. For pH<6, other methods are required owing to interferences from organic acids and aluminium.

Acid neutralizing capacity of solutions containing organic acids isolated from Finnish lakes

Organic vs. bicarbonate contribution to Gran alkalinity was studied using empirical measurements. Distilled water with various amounts of bicarbonate and organic acids was equilibrated with synthetic air during Gran titrations. Natural organic acids isolated from Finnish lakes were used as a carbon source for these solutions. The experimental design corresponded to computer simulations made by Cantrell et al. (1990). The results indicate that the discrepancy between the Gran ANC and the ANC calculated from anion deficit can be explained partly by the organic acids, which remain unprotonated in the titration. The unprotonated fraction of organic acids was on average 5.5 and 6.2 μeq per mg of dissolved organic carbon (DOC), when pH ranges of 3.0–4.0 and 3.5–4.5 for Gran plot regression were used, respectively. The acid neutralizing capacity provided by organic acids varied from 18 to 75 μeq/L, depending on the initial pH and DOC concentration and the pH range of the Gran plot regression. The results of empirical Gran titrations agree with the theoretical calculations made by Cantrell et al. (1990).

Alkalinity regulation in maine headwater lakes: Terrestrial vs. lacustrine contributions in summer

Twenty-seven Maine lakes in felsic terrane were sampled in May and October 1986, bracketing the summer interval of minimum flushing, in order to statistically test a hydrogeochemical process model that separates Gran alkalinity (ANC) production into terrestrial vs lacustrine components. Significant ANC increases over the 5 mo were restricted to relatively shallow lakes with large contiguous watersheds, and/or lakes where contact zones in the local bedrock offered increased potential for summer groundwater inflow. Summer ANC increases depended mainly on terrestrial contributions; in-lake alkalinity generation was 16±5 meq m−2 over 5 mo, of which ca. 11 meq m−2 resulted from biological assimilation of directly-deposited nitrate-N. The remaining 5±5 meq m−2 cannot be assigned with confidence, but is thought to reflect lacustrine sulfate reduction. The over-summer changes in ANC and conductance of Maine headwater lakes in upland granitic terrane approached statistical detection limits resulting from pure analytical and sampling error. This convergence highlights the importance of even small analytical biases when analyzing limnological time series, and confirms the utility (statistical efficiency) of the one-time, comparative sampling design adopted during the U.S. EPA's Eastern Lake Survey (ELS).

Changes in Conductivity, Alkalinity, Calcium, and pH During a 50-Year Period in Selected Northern Wisconsin Lakes

Between 1925 and 1941, E. A. Birge, C. Juday, and co-workers in Wisconsin made extensive water quality measurements on over 600 northern Wisconsin lakes, providing a benchmark against which comparisons can be made if the modern equivalent measurements can be determined. Of these, 149 lakes were sampled from 1979 to 1983. Data on conductivity, alkalinity, calcium, and hydrogen ion (pH) are available for these lakes from 1925 to 1931. The historical methyl orange alkalinity data were adjusted to be comparable to the recent Gran alkalinity data by subtracting 57 μmol/L from the historical measurements. The colorimetric historical pH measurements were adjusted to be comparable to recent potentiometric measurements by a regression model. No adjustments were made to the historical calcium or conductivity measurements. Comparisons of adjusted historical data with recent data show a significant increase in the means of all four variables. The largest increases in all four variables show an association with increases in land use development on the lake perimeters. Statistically significant changes in alkalinity are observed, with 28 lakes increasing and 11 decreasing, compared with 110 lakes showing no change. Possible explanations for chemical changes in those lakes that have experienced little change in land use are discussed. For the lakes in northern Wisconsin which may have been affected by airborne pollutants, it is likely that these effects had their beginning before initiation of the historical lake survey.

Seasonal variations of water chemistry in oligotrophic streams and rivers in Kejimkujik National Park, Nova Scotia

The seasonal patterns of flow and the concentrations of color, Mg, Ca, H+, Na, Cl, organic anions, SO4, and Gran alkalinity are examined for five streams or rivers in Kejimkujik National Park (Lower Mersey River, Atkins Brook, Grafton Brook, Pebbleloggitch Brook, and Beaverskin Brook). These range in organic color and acidity from very darkwater Atkins Brook (average 191 Hazen units, pH 4.2) to clearwater Beaverskin Brook (5 Hazen units, pH 5.5). In general, most dissolved substances are present in a relatively large concentration during the high-flow period of winter-spring (most notably color, Mg, H+, Ca, Na, organic anions, and SO4). In contrast, Gran alkalinity generally occurs in its highest concentration during the lowflow period. These observations suggest that during the high-flow period, substances are “flushed” from the terrestrial watersheds of these rivers and streams.

Patterns of water chemistry among twenty-seven oligotrophic lakes in Kejimkujik National Park, Nova Scotia

A cluster analysis was used to apportion 27 oligotrophic lakes in southwestern Nova Scotia into five multivariate groups on the basis of patterns of covariation of 11 chemical variables (Cl, SO4, Gran alkalinity, organic anions, Ca, Mg, K, Na, H+, and color). These groups are described in terms of the average values of the chemical variables. The relationships among the groups were investigated by an ordination by detrended correspondence analysis. The first, and by far the strongest axis of the ordination separated lakes with relatively large concentrations of alkalinity, from strongly colored lakes with large concentrations of H+ and organic anions. Axis 2 separated acidic lakes, from lakes with large concentrations of Ca and alkalinity.

The detection of Rhizosolenia (Bacillariophyceae) in sediment of Ontario lakes and implications for paleoecology

The detection of the freshwater genus Rhizosolenia (Bacillariophyceae) remains in sediments is affected by the oxidants used in the digestion procedures. The Rhizosolenia counts decrease from 30% H2O2>50% HNO3>50% chromic acid>100% HNO3>100% H2SO4. Rhizosolenia is mostly found in Ontario lakes with summer average pH ranging from 5.6 to 8.3, Gran alkalinity from 0.2 to 120 mg L−1 as CaCO3 and water colour from 4 to 105 Hazen units. Large Rhizosolenia populations are generally found in clear lakes, neutral pH and low alkalinity.

Organic versus anthropogenic acidity in tributaries of the kejimkujik watersheds in Western Nova Scotia

Dilute waters (Ca = 0, 3 to 0.8 mg L-1) respond by depressed pH levels throughout the year to existing atmospheric wet deposition of sulphate (20 kg ha-1yr-1). This occurs in southwest Nova Scotia particularly during the cold, wet season when runoff is high. Colored waters of similar Ca levels receiving runoff from peaty catchments exhibit pH values one unit lower (4.7 to 4.1) than those of catchments of similar Ca levels but free from peat deposits (pH > 5.3). In colored streams sulphate and organic anions show opposing cyclic patterns while the negative gran alkalinity is the near mirror image of sulphate. Sulphate anion peaks during the high spring discharge when the organic anion concentration is lowest. Sulphate suddenly drops during the summer months during periods of high evapotranspiration, low water table and runoff, when the concentration of organic anions is highest. Both Al and Fe follow cyclic patterns similar to that of the organic anions while H+ reflects that of the sulphate anion. Hydrogen ion concentration is always higher than that of sulphate in very colored waters, particularly during the summer months when organic anion concentrations are very high. Analysis of data in the tributaries in the Kejimkujik watersheds indicates that while considerable organic acidity is present in colored waters, anthropogenic sulphate further increases the free acidity of these waters, particularly at times of high discharge.