Acid Neutralizing Capacity Values Research Articles

Seasonal and long-term temporal patterns in the chemistry of Adirondack lakes

There is considerable interest in the recovery of surface waters from acidification by acidic deposition. The Adirondack Long-Term Monitoring (ALTM) program was established in 1982 to evaluate changes in the chemistry of 17 Adirondack lakes. The ALTM lakes exhibited relatively uniform concentrations of SO42−. Lake-to-lake variability in acid neutralizing capacity (ANC) was largely due to differences in the supply of basic cations (Ca2+, Mg2+, K+, Na+; CB) to drainage waters. Lakes in the western and southern Adirondacks showed elevated concentrations of NO3−, while lakes in the central and eastern Adirondacks had lower NO3− concentrations during both peak and base flow periods. The ALTM lakes exhibited seasonal variations in ANC. Lake ANC was maximum during the late summer or autumn, and lowest during spring snowmelt. In general Adirondack lakes with ANC near 100 Μeq L−1 during base flow periods may experience decreases in ANC to near or below 0 Μeq L−1 during high flow periods. The ALTM lakes have exhibited long-term temporal trends in water chemistry. Most lakes have demonstrated declining SO42−, consistent with decreases in SO2 emissions and SO42− in precipitation in the eastern U.S. Reductions in SO42− have not coincided with a recovery in ANC. Rather, ANC values have declined in some ALTM lakes. This pattern is most likely due to increasing concentrations of NO3− that occurred in most of the ALTM drainage lakes.

Influence of organic acids on the pH and acid‐neutralizing capacity of Adirondack Lakes

Past approaches for evaluating the effects of organic acids on the acid‐base characteristics of surface waters have typically treated them solely as weak acids. Analysis of data collected by the Adirondack Lakes Survey Corporation (ALSC) from 1469 lakes throughout the Adirondack region shows that this approach is not valid. While the data indicate that natural organics contain a continuum of acid functional groups, many of which display weak acid characteristics, a significant fraction of the organic acid is strong (pKa < 3). Dissolved organic carbon (DOC) contributes 4.5–5 μeq/mg DOC of strong acid to solution. The associated anions make a negative contribution to Gran acid‐neutralizing capacity (ANC). Because organic anions can produce negative Gran ANC values, the common practice of considering negative values of Gran ANC evidence of acidification solely by mineral acids is not valid. The strength of organic acids also influences the observed deviation between Gran ANC values and ANC values calculated as the difference between base cation and mineral acid anion concentrations (CB – CA). Ninety percent of the deviation is due to the presence of strong organics while the remaining 10% is due to DOC‐induced curvature in the F1 Gran function. Organic acids can also strongly influence pH. Their largest effects were found in the 0–50 μeq/L Gran ANC range where they depressed pH by up to 1.5 units. In addition, a method for predicting changes in pH in response to changes in mineral acidity, DOC, or both without having to rely on inferred thermodynamic constants and the uncertainties associated with them has been developed. Using the predictive method, the response of representative lakes from four sensitive lake classes to a 15‐μeq/L decrease in mineral acidity ranged from +0.17 to +0.38 pH units. If concurrent increases in DOC are considered, the pH changes would be even smaller.

Recovery of the highly acidified Clearwater Lake watershed, Ontario, Canada, simulated with the ILWAS model

The Integrated Lake Watershed Acidification Study (ILWAS) model was used to simulate the recovery of the highly acidified Clearwater Lake, Ontario. Recovery started in the mid-1970s and continued through the 1980s in response to an estimated 30–50% decrease in S deposition from smelter emissions in nearby Sudbury, Ontario. Appreciable recovery of Clearwater Lake between the mid-1970s and 1987 is simulated by the ILWAS model, as indicated by a 50% decrease in SO − 4 concentration, an 80% decrease in total Al concentration, an increase in acid-neutralizing capacity (ANC) from −60 to −17 μeq/l, and an increase in pH from 4.2 to 4.8. These decreases in acidic constituents are in good agreement with monitoring data. Long-term simulations indicate that deacidification may continue until ∼2020, if S deposition rates remain at or less than the 1987 level. Simulations for years beyond 2020 yield yearly average ANC values of 18–40 μeq/l and pH values of 6.2–6.6, which are comparable to the estimated pre-smelter pH of 6.0–6.5 for Clearwater Lake. The agreement between simulated and monitoring data for the first 10 a of recovery and the simulated long-term recovery of the lake to its presmelter level of acidity lend confidence in the capability of numerical models to simulate the reversibility of watershed acidification and suggest that some highly acidic watersheds may eventually recover, given substantial decreases in acid deposition.

The episodic acidification of Adirondack Lakes during snowmelt

Maximum values of acid neutralizing capacity (ANC) in Adirondack, New York lake outlets generally occur during summer and autumn. During spring snowmelt, transport of acidic water through acid‐sensitive watersheds causes depression of upper lake water ANC. In some systems lake outlet ANC reaches negative values. We examined outlet water chemistry from 11 Adirondack lakes during 1986 and 1987 snowmelts. In these lakes, SO42− concentrations were diluted during snowmelt and did not depress ANC. For lakes with high baseline ANC values, springtime ANC depressions were primarily accompanied by basic cation dilution. For lakes with low baseline ANC, NO3− increases dominated ANC depressions. Lakes with intermediate baseline ANC were affected by both processes and exhibited larger ANC depressions. Ammonium dilution only affected wetland systems. A model predicting a linear relationship between outlet water ANC minima and autumn ANC was inappropriate. To assess watershed response to episodic acidification, hydrologic flow paths must be considered.

Evaluation of acid neutralizing capacity data for solutions containing natural organic acids

When the acid neutralizing capacity (ANC) of an aqueous solution is measured by an acidimetric titration, the measured parameter (ANC t) is generally assumed to correspond to the ANC of the sample (ANC i). This paper demonstrates that ANC t actually corresponds to ANC i — ANC e, where ANC e is the ANC of the sample at the titration endpoint. This relationship applies whether the endpoint occurs at a fixed pH or is determined by a Gran plot. When the sample contains organic acids, ANC e is generally non-zero and its neglect can introduce serious errors into the interpretation of ANC titration data. This paper presents computer simulations of ANC titrations in systems containing both carbonate and organic alkalinity, using a realistic thermodynamic model of the acidities of natural organic acids. The necessity of considering ANC e is demonstrated both computationally and by analysis of published results in which discrepancies between Gran ANC values and anion deficits could not previously be explained.

Changes in diatom‐inferred pH and add neutralizing capacity in a dilute, high elevation, Sierra Nevada lake since A.D. 1825

SUMMARY. 1. We developed equations which describe relationships between the relative abundance of diatom assemblages in the surface sediments of twenty‐seven Sierra Nevada lakes and the present pH and acid neutralizing capacity in these lakes.2. These equations were employed to reconstruct pH and acid neutralizing capacity at twenty‐four depth intervals in a 20 cm long sediment core from Emerald Lake, a dilute, high elevation lake, located in Sequoia National Park, California, U.S.A.3. The diatom‐inferred pH and acid neutralizing capacity values indicated that neither pH nor acid neutralizing capacity exhibited significant trends since about A.D. 1825. We conclude from our diatom analysis that Emerald Lake has not been affected by acidic deposition.

The feasibility of monitoring wilderness lake chemistry with remote sensing methods

Wilderness lakes in the U.S. are at risk to the effects of atmospheric pollution. The results of the EPA Western Lake Survey, for example, show that many of the lakes in the West have very low concentrations of dissolved constituents, and are sensitive to acidification. Lakes located in wilderness areas were found to be particularly sensitive: median ANC values of 91.4 μeq/1 compared to 282.7 μeq/1 in non-wilderness areas. Sampling remote wilderness lakes is difficult and expensive. For example, the cost of sampling each lake in the National surface water survey was about $ 4000 per lake. Remote sensing methods may provide an alternative means of monitoring the chemistry of wilderness lakes. This study defines the feasibility of the remote monitoring of lake chemistry in the Adirondack Park region of New York by comparing measured lake chemistry with field reflectance measurements, remote reflectance measurements from aircraft and satellite and laboratory and airborne laser fluorosensor data.Water samples collected from a representative population of Adirondack Park lakes were analyzed for pH, acid neutralizing capacity (ANC), dissolved organic carbon (DOC), total reactive aluminum and plant pigment concentration concurrent with the collection of remote sensing data. Lake parameters estimated from field reflectance measurements which directly effect lake optical properties, total pigments, DOC, and turbidity were estimated with less error and more precision than those parameters not directly related to lake optics, pH, ANC and A1 concentration. Results from airborne MSS and Landsat MSS data produced lower R(2) values than estimates using field reflectance.The concentration of DOC can be estimated remotely by using laser fluorosensing. The spectral-fluorescence properties of DOC were correlated with the pH and aluminum concentrations in the lake water. Remotely measurable DOC fluorescence spectra were used to estimate DOC, H, and A1 concentrations.Of the methods investigated, laser fluorosensing shows the most promise for the remote prediction of lake DOC, hydrogen ion and aluminum concentration in Adirondack lakes while reflectance measures may be used to estimate lake chlorophyll, DOC and transparency. Given the large number of wilderness lakes and the difficulty of sampling in remote wilderness, remote sensing methods may provide an alternative, while less precise, method of monitoring lake chemistry. The applicability of these findings to lakes in other wilderness areas is unknown. Similar studies of the feasibility of monitoring Western wilderness lake chemistry with remote sensing methods are being initiated.

Influence of aqueous aluminium and organic acids on measurement of acid neutralizing capacity in surface waters

ACID neutralizing capacity (ANC) is used to quantify the acid-base status of surface waters. Acidic waters have been defined as having ANC values less than zero, and acidification is often quantified by decreases in ANC. Measured and calculated values of ANC generally agree, except for low-ANC waters. These waters, however, are of primary interest in lake-acidification studies. The discrepancy in ANC values is greatest for waters with high concentrations of aluminium and/or dissolved organic carbon (DOC). The discrepancy due to aluminium increases with increasing concentration of dissolved monomeric aluminium (Alm) and can exceed 50 (μeq 1−1 at low pH and high Alm values. Here we show that this error can be minimized by using a proton reference level for aqueous aluminium of 2+, rather than 0 or 3+, as is done usually. The value of 2+ is consistent with the mean charge on aluminium at the equivalence point of strong-acid titrations. The discrepancy between calculated and measured ANC also increases with increasing DOC concentrations, exceeding 50 μeq 1−1 for waters with DOC concentrations greater than 800 μmol C 1−1 (1000 μmol C 1-1 = 12 mg 1−1 DOC). This error can be decreased by titrating to a low consistent pH value (for example, 3.0). This reduces the systematic underestimation of ANC due to curvature in Gran1 plot analysis, while still allowing accurate measurement of increments in hydrogen-ion concentration. ANC should not be used as a single parameter for characterizing the chemical suitability of surface waters for biota or for assessing the susceptibility of low-ANC waters to acidification by acid deposition.

Chemical and physical characteristics of lakes in the southeastern United States

A survey of the chemical status of 252 lakes, representing an estimated 2424 lakes, was conducted in the Southern Blue Ridge province and in Florida during Fall 1984. This survey was part of the Eastern Lake Survey designed to quantify the number of acidic lakes and lakes with low acid neutralizing capacity (ANC) with surface area greater than 4 ha. Of the 102 lakes sampled in the Southern Blue Ridge, none were acidic (ANC {le} 0) and only 1.4% of the population of 258 lakes were estimated to have low (ANC {<=}50 {mu}equiv/L). In contrast, an estimated 22% of the lakes in the Florida subregion were acidic, and 35.3% were estimated to have low ANC. Almost 90% of the lakes in the Southern Blue Ridge were reservoirs. These lakes generally had moderate ANC values (median = 250 {mu}equiv/L) and had low concentrations of aluminum, dissolved organic carbon (DOC), and sulfate compared to those of Florida, the upper Midwest, and the Northeast. The lakes in the Southern Blue Ridge showed a high degree of chemical homogeneity relative to those of other regions in the East. In contrast, the chemistry of lakes in Florida exhibited a high degree of heterogeneity, indicating themore » presence of several distinct subpopulations. Florida had the highest proportion of acidic lakes in the East, some of which had moderately high concentrations of aluminum, DOC, and sulfate. Some of the Florida lakes are naturally acidic from organic acids. However, the many clear water, acidic lakes exhibit chemical characteristics that are not inconsistent with a hypothesis of acidification from deposition. The geographic separation of the Southern Blue Ridge from Florida and the strong contrasts in physical and chemical characteristics of the respective subregions require that they be treated as distinct regions.« less

Chemical and physical characteristics of lakes in the northeastern United States

A survey of the chemical status of 252 lakes, representing an estimated 2424 lakes, was conducted in the Southern Blue Ridge province and in Florida during Fall 1984. This survey was part of the Eastern Lake Survey designed to quantify the number of acidic lakes and lakes with low acid neutralizing capacity (ANC) with surface area greater than 4 ha. Of the 102 lakes sampled in the Southern Blue Ridge, none were acidic (ANC {le} 0) and only 1.4% of the population of 258 lakes were estimated to have low (ANC {<=}50 {mu}equiv/L). In contrast, an estimated 22% of the lakes in the Florida subregion were acidic, and 35.3% were estimated to have low ANC. Almost 90% of the lakes in the Southern Blue Ridge were reservoirs. These lakes generally had moderate ANC values (median = 250 {mu}equiv/L) and had low concentrations of aluminum, dissolved organic carbon (DOC), and sulfate compared to those of Florida, the upper Midwest, and the Northeast. The lakes in the Southern Blue Ridge showed a high degree of chemical homogeneity relative to those of other regions in the East. In contrast, the chemistry of lakes in Florida exhibited a high degree of heterogeneity, indicating themore » presence of several distinct subpopulations. Florida had the highest proportion of acidic lakes in the East, some of which had moderately high concentrations of aluminum, DOC, and sulfate. Some of the Florida lakes are naturally acidic from organic acids. However, the many clear water, acidic lakes exhibit chemical characteristics that are not inconsistent with a hypothesis of acidification from deposition. The geographic separation of the Southern Blue Ridge from Florida and the strong contrasts in physical and chemical characteristics of the respective subregions require that they be treated as distinct regions.« less