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 (SI_calcite), 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 SI_calcite–pH relationship in a broad east-west direction across the Dee system. At sites draining the relatively impermeable upland areas, the regression of SI_Calcite 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 SI_calcite–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.     

Evaluation of different approaches to quantify strong organic acidity and acid-base buffering of organic-rich surface waters in Sweden

The role of organic acids in buffering pH in surface waters has been studied using a small brownwater stream (26mg L(-1) TOC) draining a forested catchment in Northern Sweden. Under the conditions of elevated pressure of CO2 stream field pH was changed between 3.5 and 6.1 during the acidification and alkalinization experiment. Acid-base characteristics of the natural organic matter were also determined using a high precision potentiometric method for a concentrated sample from the same stream. We compared the predictions from the Windermere Humic Aqueous Model (WHAM Model V), a model derived from the potentiometric titration (diprotic/monoprotic acid model) and a previously derived triprotic acid model which only uses alkalinity and TOC as input variables. The predicted buffering characteristics of all three models are very similar in the pH range 4.5-7 which suggests that during routine analysis alkalinity and TOC are sufficient to give a good estimate of organic acid anion charge contribution in a large range of surface waters. A slightly adjusted version of WHAM V successfully describes the organic charge contribution in a large number of sampled surface water lakes, which were previously used to calibrate the triprotic model.     

A new instrument for on-line measurement of bicarbonate alkalinity

Alkalinity or buffering in anaerobic digesters may be chiefly attributed to bicarbonate and should be at least 1000 mg CaCO₃/l for successful operation. A simple on-line instrument for the direct determination of bicarbonate concentration, especially for automatic control of anaerobic digesters, is described. The proposed method is based on a continuous measurement of the carbon dioxide flow rate evolved from a stream of sampled solution after saturation with gaseous CO₂ and subsequent acidification with excess acid. This eliminates the need for a pH probe which is subject to fouling.The device was calibrated using sodium bicarbonate solutions (0.01–0.065 M) and the calibration showed an accuracy of ±5%. The device was tested on effluent from anaerobic digesters operating on ice-cream waste supplemented with sodium bicarbonate within a range of 0.02–0.04 mol/l. The results compared favourably with titrimetric measurements, extensively used to determine bicarbonate alkalinity with an accuracy of ±5%. The response of the instrument to variation in alkalinity concentration is sufficiently rapid for most industrial applications.     

Determination of activated sludge biological activity using model corrected CO2 off-gas data

Carbon dioxide (CO(2)) online off-gas monitoring is useful to detect changes in biological activity for activated sludge systems especially under limited oxygen conditions like under simultaneous nitrification-denitrification (SND) where respirometric measurements are not applicable. So far, the influence of the bicarbonate system on the liquid-gas transfer of CO(2) prevented the wider use of off-gas CO(2) for monitoring purposes in wastewater treatment. The objective of the paper is to demonstrate a practical method to correct measured off-gas CO(2) as an indicator of biological activity by taking into account pH shifts (resulting in CO(2) release or retention) and changes in influent alkalinity. The simple model is based on the physicochemical system of the bicarbonate/CO(2) equilibrium and the liquid-gas mass transfer for aerated systems. Standard on-line measurements (pH, temperature, flow rates) and periodical alkalinity titration serve as input data to estimate the influence of the carbonate system on the CO(2) off-gas concentrations measured on-line. For a particular plant the CO(2) mass transfer coefficients are derived from measurements compared to the theoretical calculation from oxygen mass transfer. The model estimates the biological carbon dioxide production rate (CPR; heterotrophic activity) by the correction of the measured carbon dioxide transfer rate (CTR; C-flux by the off-gas) with the calculated inorganic carbon dioxide transfer rate (r(F)) considering bicarbonate consumption (autotrophic activity).     

A regional pH-alkalinity relationship

An explicit equation relating the pH and alkalinity of natural waters is derived as an inverse hyperbolic sine function. For the range of pH generally encountered in lake and stream acidification studies, the parameters of the equation are related to the dissolved CO₂ saturation relative to atmospheric CO₂ and the concentration of weak acid anions, though the relationship is approximate. The functional form allows for statistical parameter estimation given a regional or temporal pH-alkalinity data-set. The relationship simplifies pH estimation in regional aquatic acidification models where alkalinity is computed as the master variable.     

Hydrology and water quality of the headwaters of the River Severn: Stream acidity recovery and interactions with plantation forestry under an improving pollution climate

This paper presents new information on the hydrology and water quality of the eroding peatland headwaters of the River Severn in mid-Wales and links it to the impact of plantation conifer forestry further down the catchment.The Upper Hafren is dominated by low-growing peatland vegetation, with an average annual precipitation of around 2650 mm with around 250 mm evaporation. With low catchment permeability, stream response to rainfall is “flashy” with the rising limb to peak stormflow typically under an hour. The water quality is characteristically “dilute”; stormflow is acidic and enriched in aluminium and iron from the acid organic soil inputs. Baseflow is circum-neutral and calcium and bicarbonate bearing due to the inputs of groundwater enriched from weathering of the underlying rocks. Annual cycling is observed for the nutrients reflecting uptake and decomposition processes linked to the vegetation and for arsenic implying seasonal water-logging within the peat soils and underlying glacial drift. Over the decadal scale, sulphate and nitrate concentrations have declined while Gran alkalinity, dissolved organic carbon and iron have increased, indicating a reduction in stream acidification.Within the forested areas the water quality is slightly more concentrated and acidic, transgressing the boundary for acid neutralisation capacity as a threshold for biological damage. Annual sulphate and aluminium concentrations are double those observed in the Upper Hafren, reflecting the influence of forestry and the greater ability of trees to scavenge pollutant inputs from gaseous and mist/cloud-water sources compared to short vegetation. Acidification is decreasing more rapidly in the forest compared to the eroding peatland possibly due to the progressive harvesting of the mature forest reducing the scavenging of acidifying inputs. For the Lower Hafren, long-term average annual precipitation is slightly lower, with lower average altitude, at around 2520 mm and evaporation is around double that of the Upper Hafren.     

Numerical technique to correct for weak acid errors in gran function analysis of titration data

Ordinary Gran function analysis of titration data from samples which contain weak acids results in overestimates of strong acid acidity and underestimates of weak acid acidity. A numerical technique is developed which solves a general acid-base model from titration data. The technique is an iterative procedure which adjusts Gran functions for the effect of the weak acid components. The model and the method are validated with simulated titration data and laboratory data from test solutions.     

pH control for enhanced reductive bioremediation of chlorinated solvent source zones

Enhanced reductive dehalogenation is an attractive treatment technology for in situ remediation of chlorinated solvent DNAPL source areas. Reductive dehalogenation is an acid-forming process with hydrochloric acid and also organic acids from fermentation of the electron donors typically building up in the source zone during remediation. This can lead to groundwater acidification thereby inhibiting the activity of dehalogenating microorganisms. Where the soils' natural buffering capacity is likely to be exceeded, the addition of an external source of alkalinity is needed to ensure sustained dehalogenation. To assist in the design of bioremediation systems, an abiotic geochemical model was developed to provide insight into the processes influencing the groundwater acidity as dehalogenation proceeds, and to predict the amount of bicarbonate required to maintain the pH at a suitable level for dehalogenating bacteria (i.e., > 6.5). The model accounts for the amount of chlorinated solvent degraded, site water chemistry, electron donor, alternative terminal electron-accepting processes, gas release and soil mineralogy. While calcite and iron oxides were shown to be the key minerals influencing the soil's buffering capacity, for the extensive dehalogenation likely to occur in a DNAPL source zone, significant bicarbonate addition may be necessary even in soils that are naturally well buffered. Results indicated that the bicarbonate requirement strongly depends on the electron donor used and availability of competing electron acceptors (e.g., sulfate, iron (III)). Based on understanding gained from this model, a simplified model was developed for calculating a preliminary design estimate of the bicarbonate addition required to control the pH for user-specified operating conditions.     

过氧化钙释氧过程中碱度调控方法的研究

当采用过氧化物作为原位生物修复污染地下水的释氧剂时,以微生物培养驯化过程中所需的营养组分(NH4)2SO4和KH2PO4作为pH调节剂,同时以电气石和现场土壤的调节、缓冲作用作为辅助手段,对过氧化钙(CaO2)释氧过程中的碱度调控方法进行了研究.结果表明,(NH4)2SO4和KH2PO4的协同作用可将溶液pH值由12.1降至适宜微生物生长的6.5～8.5;电气石和现场土壤可持续稳定地降低溶液的pH值,在一定条件下可减少pH调节剂的使用量,避免地下水的二次污染.     

Natural inactivation of phosphorus by aluminum in atmospherically acidified water bodies

Atmospheric acidification of catchment-lake ecosystems may provide natural conditions for the in-lake control of P cycling. This process is based on the elevated transport of aluminum from acidified soils and its subsequent precipitation in the water body and is described for strongly acidified forest lakes, acidified and circumneutral reservoirs, and a moderately acidified alpine lake. In water bodies with episodically or permanently acidified inflows a pH gradient develops between lake water and tributaries due to: (i) neutralization of acidic inflows after mixing with waters with undepleted carbonate buffering system, and/or (ii) the in-lake alkalinity generation dominated by biochemical removal of NO3- and SO4(2-). With the pH increasing towards neutrality, ionic Al species hydrolyze and form colloidal Al hydroxides (Al(part)) with large specific surfaces and strong ability to bind orthophosphate from the liquid phase. Moreover, Alpart settles and increases the P sorption capacity of the sediment. The presence of Al(part) on the bottom reduces orthophosphate release from sediments after its liberation from ferric oxyhydroxides during anoxia because Al(part) is not sensitive to redox changes. Consequently, the natural in-lake P inactivation may be expected in any water body with elevated Al input and a pH gradient between its inlet and outlet.     

Aluminium complexation by an aquatic humic fraction under acidic conditions

Equilibrium dialysis and acid-base titration were used to investigate the interactions between Al and a fraction of aquatic humic substances (HS), in the pH range 3–5. Binding of Al by the HS increased with Al³⁺ activity and with pH. Under conditions relevant to natural waters, υ (mol Al bound per gHS) varied from 0 to 1.5 × 10⁻³. The data were modelled with an emprical linear logarithmic expression (Model I) and on the basis of the polyelectrolyte nature of the HS, incorporating competitive binding of H⁺ and Al³⁺ (Model II). Both models gave tolerable fits (r = 0.93). Model I is simpler to apply, while Model II allows the calculation of proton release accompanying Al binding, and provides information on the net charge of the Al---HS complexes. The results were used to calculate the distribution of Al between organic and inorganic forms under conditions prevailing in acidic natural waters.     

Acids and bases in fresh-waters: Interpretation of results from gran plots

The determination of acids and bases in fresh-waters by the Gran procedure, using coulometric generation of hydroxide ions, is discussed. To obtain correct results the titration must start at a sufficiently low pH, provided by an acidification of the sample, and be continued until a sufficiently high pH value is reached. Normally, the pH range 3.6–10.3 should be covered. If a too small pH range is covered, erroneous results are obtained, even though the Gran plot may apparently have linear regions.When bases are present in the sample, the amount of these will appear as a negative value for strong acid, and a corresponding positive value for weak acids will be obtained, provided that the bases are not volatilized upon acidification.     

Errors in gran function analysis of titration data for dilute acidified water

Errors that result from applying simple Gran function analysis to titration data from typical dilute acidified surface waters are discussed. Theoretical development and experimental verification of these errors are presented. The results of this study show that strong acid acidity is almost always overestimated and weak acid acidity is underestimated. Further, if CO₂ contamination of the titrant is present, total acidity will be overestimated.     

Calcite saturation in eastern UK rivers

Calcite saturation in eastern UK rivers is assessed in relation to the potential kinetic inhibition of calcite precipitation. Two well established inhibitors are considered: soluble reactive phosphorus (SRP, i.e. inorganic monomeric phosphorus); and dissolved organic carbon (DOC). The rivers show a range of calcite saturation levels from approximately 1 hundredth to approximately 100-fold. The greatest range occurs for the northernmost river considered, the Tweed, where the waters range from highly unsaturated to highly oversaturated. The lowest range occurs for the most southerly rivers (the Great Ouse and the Thames) where the waters are consistently oversaturated with respect to calcite. The contrasting patterns relate to a greater diversity of water quality within the northern regions. Thus, during the winter, the main waters are derived from the upland areas with acidic soils and low weathering rates. During the summer baseflow periods, groundwater inputs are more important and high photosynthesis results in particularly high pHs and calcite oversaturation. In contrast, for the southern rivers, the main source of water during both the summer baseflow and the winter highflow periods comes from calcium carbonate rich aquifer sources. Statistical analysis of pH vs. the logarithm of the calcite saturation index for each river indicates strong linear features with individual gradients of approximately 1. This linearity results from an autocorrelation (the logarithm of the saturation index is calculated from the pH) and this indicates that calcite solubility controls are not operative in any of the rivers examined. A comparison of calcite saturation levels and SRP and DOC concentrations show a pattern inconsistent with kinetic hindrance, although some structure is observed, probably due to the mixing reactions between point and diffuse sources of water with contrasting chemistry.     

Effect of container type on pH and alkalinity stability

Errors in pH measurement of 0.1-0.2 units can be propagate large errors in the predicted aqueous speciation. When accurate on-site analyses are impossible to obtain, changes in pH and alkalinity between the time of sampling and after transport to a laboratory must be minimized in many sampling programs.Several container and cap combinations were evaluated for their ability to preserve the pH and alkalinity of test waters that were out of equilibrium with atmospheric carbon dioxide over time periods representative of mail or other surface transport. Provided that no air bubbles were present when the sample bottle was filled and sealed, polyvinyl chloride bottles with caps having a conical polyethylene insert yielded the most satisfactory results. High density linear polyethylene bottles were also generally satisfactory. Cap design was observed to be very important. Guidelines for analysis and preservation are given, and if followed, laboratory measurements can be reliably substituted for field measurements in many instances.     

Mechanism of degradation of the quality of natural water in the Lakes Region of the Ethiopian rift valley

The natural waters of the Lakes Region in the Main Ethiopian Rift (MER) show serious problems of chemical quality. The high content of fluoride reaches 300 mgl(-1) and affects the health of the population who live in this area. Furthermore, the alkaline and sodic characteristics (8 2(Ca + Mg)]. which results from the weathering and dissolution of the volcanic rocks. The predictive qualities of the model AQUA were tested in the context of this region. A simulation of the concentration of the waters by evaporation has shown chemical variations relatively similar to those obtained with field data which include the minor chemical elements such as the fluoride or the dissolved silica. For field data, enrichment is more variable and may represent localised conditions of mineralization. This model is then used to study a method of defluoridation by supplying these waters with calcium in the form of gypsum. This method also allows the reduction of the alkalinizing and sodifying characteristics of the waters without reaching the fluoride standard concentration accepted for drinking waters. For drinking purpose, waters of the Ethiopian rift should undergo another method of defluoridation.     

Impacts of blending ground, surface, and saline waters on lead release in drinking water distribution systems

The impacts of distribution water quality changes caused by blending different source waters on lead release from corrosion loops containing small lead coupons were investigated in a pilot distribution study. The 1-year pilot study demonstrated that lead release to drinking water increased as chlorides increased and sulfates decreased. Silica and calcium inhibited lead release to a lesser degree than sulfates. An additional 3-month field study isolated and verified the effects of chlorides and sulfates on lead release. Lead release decreased with increasing pH and increasing alkalinity during the 1-year pilot study; however, the effects of pH and alkalinity on lead release, were not clearly elucidated due to confounding effects. A statistical model was developed using nonlinear regression, which showed that lead release increased with increasing chlorides, alkalinity and temperature, and decreased with increasing pH and sulfates. The model indicated that primary treatment processes such as enhanced coagulation and RO (reverse osmosis membrane) were related to lead release by water quality. Chlorides are high in RO-finished water and increase lead release, while sulfates are high following enhanced coagulation and decrease lead release.     

A study of the cycling of manganese and other elements in a seasonally anoxic lake, Rostherne Mere, U.K.

The vertical distribution of sulphate, dissolved sulphide, alkalinity, pH, total iron and manganese, Fe(II) and Mn(II) and humic substances was monitored for a year in a seasonally anoxic lake, Rostherne Mere, U.K. During summer stratification Mn(II), probably of sediment origin, accumulated in the hypolimnetic bottom waters to a concentration of 50μm. Release occurred in two stages and appeared to be related to the supply of organic material to the sediment. No ferrous iron or dissolved sulphide were detected in the water column, although they were measured in the interstitial waters of the sediment. Ferrous sulphide formation in the sediment controls the iron(II) and dissolved sulphide concentrations and so minimises the concentration gradients at the sediment-water interface. Electron acceptors other than oxygen are capable of oxidising ferrous and sulphide ions in the water column. pH and alkalinity were the only other chemical components to show appreciable variation with depth, reflecting the biological processes of photosynthesis, respiration and decomposition. Iron and humic substances simultaneously increased in the surface waters after a major flood indicating that they are both present in the solution in contact with the soils of the catchment.     

Abiotic hydrolysis of the detergent builder tripolyphosphate by hydrous manganese dioxide

The hydrolysis of tripolyphosphate to orthophosphate is facilitated by suspensions of amorphous manganese dioxide. Pyrophosphate is observed as an intermediate. The rate enhancement decreases with increasing pH. The weak sorption of orthophosphate on the oxide also decreases with increasing pH, indicating that the orthophosphate product would be bioavailable. The presence of calcium ions in natural waters increases both the rate of hydrolysis of tripolyphosphate and the extent of sorption of orthophosphate. Under some circumstances this abiotic mechanism could play a significant role in the hydrolysis of tripolyphosphate in the aquatic environment.     

Synergistic inactivation of Cryptosporidium parvum using ozone followed by free chlorine in natural water

The synergistic effect of sequential exposure to ozone followed by free chlorine on inactivation of Cryptosporidium parvum oocysts suspended in natural waters was studied in bench-scale batch reactors. Animal infectivity using neonatal CD-1 mice was used to measure oocyst inactivation. The synergistic effect measured in two alkaline (pH 8.1) natural waters was statistically significant but was considerably smaller than previously reported in buffered de-ionized water at pH 6.0. Temperature, ozone primary treatment level, and water type did not have measurable impacts on the synergistic effect. Efforts to increase the synergistic effect by reducing the pH from 8 to 6 by acid addition were unsuccessful. In the two low alkalinity (pH 6.0) natural waters tested, the measured synergistic effect was greater than in the alkaline waters, but was still less than that measured previously in buffered de-ionized water. It was concluded that the synergistic effect reduction in the natural waters tested was due in part to alkalinity and in part to other unidentified water quality characteristics. Sequential treatment with ozone followed by free chlorine may only be a feasible strategy for achieving synergistic C. parvum inactivation credit for water treatment facilities with natural waters having a low pH (near 6.0).