Acid Neutralizing Capacity Values Research Articles

Comparative Assessment of the Acid-Neutralizing Capacity, Price and selected Physical Properties of different Antacids Suspensions available in Benin City, Edo State, Nigeria.

Background: Patients are often faced with the challenge of choosing an antacids suspension that will provide effective and prompt relief from excessive gastric acid secretion. Objective: The aim was to evaluate acid-neutralizing capacity (ANC), price and selected physical properties of twenty (20) brands of antacids suspensions from pharmacies in Benin metropolis, Nigeria. Materials and Methods: United States Pharmacopeia assay method was used to determine the ANC. Visual inspection, pH, relative density and flow times were determined using standard methods. Results: The ANC of all the brands of antacids suspensions was above the US FDA requirement for effective antacids (≥5 mEq per dose). Brand E had the highest ANC of 27.15 mEq/ 10 mL while brand C had the lowest at 11.60 mEq/ 10 mL. All the antacids suspensions had pH range between 7.30 and 8.76. The flow time was found to be highest with sample N (70.11 ± 0.35) min and lowest with sample A (0.27 ± 0.01) min. The relative density was highest with sample N (1.28 ± 0.02) and lowest with sample B (1.03 ± 0.00). There was no significant difference in ANC values between imported and locally manufactured antacids suspensions. However, there was significant difference in their prices. Conclusion: Rapid and effective acid-neutralizing capacity of antacids is achieved with antacids suspensions containing at least two active pharmaceutical ingredients. Most of the antacids contain aluminium hydroxide and magnesium hydroxide. When medication is needed for symptomatic relief from acid reflux disorders, antacids containing these active ingredients will be effective.

Chronic and episodic acidification of streams along the Appalachian Trail corridor, eastern United States

AbstractAcidic atmospheric deposition has adversely affected aquatic ecosystems globally. As emissions and deposition of sulfur (S) and nitrogen (N) have declined in recent decades across North America and Europe, ecosystem recovery is evident in many surface waters. However, persistent chronic and episodic acidification remain important concerns in vulnerable regions. We evaluated acidification in 269 headwater streams during 2010–2012 along the Appalachian Trail (AT) that transits several ecoregions and is located downwind of high levels of S and N emission sources. Discharge was estimated by matching sampled streams to those of a nearby gaged stream and assuming equivalent daily mean flow percentiles. Charge balance acid‐neutralizing capacity (ANC) values were adjusted to the 15th (Q15) and 85th flow percentiles (Q85) by applying the ANC/discharge slope among sample pairs collected at each stream. A site‐based approach was applied to streams sampled twice or more and a second regression‐based approach to streams sampled once to estimate episodic acidification magnitudes as the ANC difference from Q15 to Q85. Streams with ANC <0 μeq/L doubled from 16% to 32% as discharge increased from Q15 to Q85 according to the site‐based approach. The proportion of streams with ANC <0 μeq/L at low flow and high flow decreased from north to south. Base cation dilution explained the greatest amount of episodic acidification among streams and variation in sulfate (SO42−) concentrations was a secondary explanatory variable. Episodic SO42− patterns varied geographically with dilution dominant in northern streams underlain by soils developed in glacial sediment and increased concentrations dominant in southern streams with older, highly weathered soils. Episodic acidification increased as low‐flow ANC increased, exceeding 90 μeq/L in 25% of streams. Episodic increases in ANC were the dominant pattern in streams with low‐flow ANC values <30 μeq/L. Chronic and episodic acidification remain an ecological concern among AT streams. The approach developed here could be applied to estimate the magnitude and extent of chronic and episodic acidification in other regions recovering from decreasing levels of atmospheric S and N deposition.

High elevation watersheds in the southern Appalachians: Indicators of sensitivity to acidic deposition and the potential for restoration through liming

Southern Appalachian high elevation watersheds have deep rocky soils with high organic matter content, different vegetation communities, and receive greater inputs of acidic deposition compared to low elevation sites within the region. Since the implementation of the Clean Air Act Amendment in the 1990s, concentrations of acidic anions in rainfall have declined. However, some high elevation streams continue to show signs of chronic to episodic acidity, where acid neutralizing capacity (ANC) ranges from 0 to 20μeqL−1. We studied three 3rd order watersheds (North River in Cherokee National Forest, Santeetlah Creek in Nantahala National Forest, and North Fork of the French Broad in Pisgah National Forest) and selected four to six 1st order catchments within each watershed to represent a gradient in elevation (849–1526m) and a range in acidic stream ANC values (11–50μeqL−1). Our objectives were to (1) identify biotic, physical and chemical catchment parameters that could be used as indices of stream ANC, pH and Ca:Al molar ratios and (2) estimate the lime required to restore catchments from the effects of excess acidity and increase base cation availability. We quantified each catchment’s biotic, physical, and chemical characteristics and collected stream, O-horizon, and mineral soil samples for chemical analysis seasonally for one year. Using repeated measures analysis, we examined variability in stream chemistry and catchment characteristics; we used a nested split-plot design to identify catchment characteristics that were correlated with stream chemistry. Watersheds differed significantly and the catchments sampled provided a wide range of stream chemical, biotic, physical and chemical characteristics. Variability in stream ANC, pH, and Ca:Al molar ratio were significantly correlated with catchment vegetation characteristics (basal area, tree height, and tree diameter) as well as O-horizon nitrogen and aluminum concentrations. Total soil carbon and calcium (an indicator of parent material), were significant covariates for stream ANC, pH and Ca:Al molar ratios. Lime requirement estimates did not differ among watersheds but this data will help select catchments for future restoration and lime application studies. Not surprisingly, this work found many vegetation and chemical characteristics that were useful indicators of stream acidity. However, some expected relationships such as concentrations of mineral soil extractable Ca and SO4 were not significant. This suggests that an extensive test of these indicators across the southern Appalachians will be required to identify high elevation forested catchments that would benefit from restoration activities.

Long-term changes in acidity and DOC in throughfall and soil water in Finnish forests.

The main objective of this study was to examine if any detectable trends in dissolved organic carbon (DOC), sulphate (SO4-S) concentrations and acid neutralizing capacity (ANC) in throughfall (TF) and soil water (SW) could be found during 1990-2010 and to relate them to recent changes in decreased acid deposition. The study was conducted in seven boreal coniferous forest sites: four of which are managed and three unmanaged forests sites. Generally, temporal trend showed a significant decrease in SO4-S concentrations in bulk precipitation (BP), TF and SW. At some of the sites, there was an increasing tendency in BP and TF in the DOC concentrations. This feature coincides with decreasing SO4-S concentration, indicating that SO4-S may be an important driver of DOC release from the canopy. However, a slightly increased temperature, larger senescing needle mass and consequently increased decaying activity in the canopy may partly explain the increasing trend in DOC. In SW, no consistent DOC trend was seen. At some sites, the decreased base cation concentrations mostly account for the decrease in the ANC values in SW and TF.

Development of a total maximum daily load (TMDL) for acid-impaired lakes in the Adirondack region of New York

Acidic deposition has impaired acid-sensitive surface waters in the Adirondack region of New York by decreasing pH and acid neutralizing capacity (ANC). In spite of air quality programs over past decades, 128 lakes in the Adirondacks were classified as “impaired” under Section 303(d) of the Clean Water Act in 2010 due to elevated acidity. The biogeochemical model, PnET-BGC, was used to relate decreases in atmospheric sulfur (S) and nitrogen (N) deposition to changes in lake water chemistry. The model was calibrated and confirmed using observed soil and lake water chemistry data and then was applied to calculate the maximum atmospheric deposition that the impaired lakes can receive while still achieving ANC targets. Two targets of ANC were used to characterize the recovery of acid-impaired lakes: 11 and 20 μeq L−1. Of the 128 acid-impaired lakes, 97 currently have ANC values below the target value of 20 μeq L−1 and 83 are below 11 μeq L−1. This study indicates that a moderate control scenario (i.e., 60% decrease from the current atmospheric S load) is projected to recover the ANC of lakes at a mean rate of 0.18 and 0.05 μeq L−1 yr−1 during the periods 2022–2050 and 2050–2200, respectively. The total maximum daily load (TMDL) of acidity corresponding to this moderate control scenario was estimated to be 7.9 meq S m−2 yr−1 which includes a 10% margin of safety.

Empirical Critical Loads of Atmospheric Nitrogen Deposition for Nutrient Enrichment and Acidification of Sensitive US Lakes

The ecological effects of elevated atmospheric nitrogen (N) deposition on high-elevation lakes of the western and northeastern United States include nutrient enrichment and acidification. The nutrient enrichment critical load for western lakes ranged from 1.0 to 3.0 kilograms (kg) of N per hectare (ha) per year, reflecting the nearly nonexistent watershed vegetation in complex, snowmelt-dominated terrain. The nutrient enrichment critical load for northeastern lakes ranged from 3.5 to 6.0 kg N per ha per year. The N acidification critical loads associated with episodic N pulses in waters with low values of acid neutralizing capacity were 4.0 kg N per ha per year (western) and 8.0 kg N per ha per year (northeastern). The empirical critical loads for N-caused acidification were difficult to determine because of a lack of observations in the West, and high sulfur deposition in the East. For both nutrient enrichment and acidification, the N critical load was a function of how atmospheric N deposition was deter...

Relating hydrogeomorphic properties to stream buffering chemistry in the Neversink River watershed, New York State, USA

AbstractMonitoring the effects of acidic deposition on aquatic ecosystems in the Northeastern US has generally required regular measurements of stream buffering chemistry (i.e. acid‐neutralizing capacity (ANC) and calcium Ca2+), which can be expensive and time consuming. The goal of this paper was to develop a simple method for predicting baseflow buffering chemistry based on the hydrogeomorphic properties of ten nested watersheds in the Neversink River basin (2·0–176·0 km2), an acid‐sensitive basin in the Catskill Mountains, New York State. The tributaries and main reach watersheds have strongly contrasting mean baseflow ANC values and Ca2+ concentrations, despite rather homogeneous vegetation, bedrock geology, and soils. A stepwise regression was applied to relate 13 hydrogeomorphic properties to the mean baseflow ANC values and Ca2+ concentrations. The regression analysis showed that watersheds with lower ANC values had a higher mean ratio of ‘quickflow’ runoff to precipitation during 20 non‐snowmelt runoff events (referred to as mean runoff ratio). The mean runoff ratio could explain at least 80% of the variability in mean baseflow ANC values and Ca2+ concentrations among the ten watersheds. Greater mean runoff ratios also correlated with steeper slopes and greater drainage densities, thus allowing the prediction of baseflow ANC values (r2 = 0·75) and Ca2+ concentrations (r2 = 0·77) with widely available spatial data alone. These results indicate that hydrogeomorphic properties can predict a watershed's sensitivity to acid deposition in regions where the spatial sources of stream buffering chemistry from the bedrock mineralogy and soils are fairly uniform. Copyright © 2010 John Wiley & Sons, Ltd.

Time series and recurrence interval models to predict the vulnerability of streams to episodic acidification in Shenandoah National Park, Virginia

Acid rain affects headwater streams by temporarily reducing the acid‐neutralizing capacity (ANC) of the water, a process termed episodic acidification. The increase in acidic components in stream water can have deleterious effects on the aquatic biota. Although acidic deposition is uniform across Shenandoah National Park (SNP) in north central Virginia, the stream water quality response during rain events varies substantially. This response is a function of the catchment's underlying geology and topography. Geologic and topographic data for SNP's 231 catchments are readily available; however, long‐term measurements (tens of years) of ANC and accompanying discharge are not and would be prohibitively expensive to collect. Transfer function time series models were developed to predict hourly ANC from discharge for five SNP catchments with long‐term water‐quality and discharge records. Hourly ANC predictions over short time periods (≤1 week) were averaged, and distributions of the recurrence intervals of annual water‐year minimum ANC values were model‐simulated for periods of 6, 24, 72, and 168 hours. The distributions were extrapolated to the rest of the SNP catchments on the basis of catchment geology and topography. On the basis of the models, large numbers of SNP streams have 6‐ to 168‐hour periods of low‐ANC values, which may stress resident fish populations. Smaller catchments are more vulnerable to episodic acidification than larger catchments underlain by the same bedrock. Catchments with similar topography and size are more vulnerable if underlain by less basaltic/carbonate bedrock. Many catchments are predicted to have successive years of low‐ANC values potentially sufficient to extirpate some species.

Model Prognoses for Future Acidification Recovery of Surface Waters in Norway Using Long-Term Monitoring Data

During the past 20 years, acid deposition in Europe has decreased by more than 60%, yet still a large number of lakes and streams in southern Norway have not recovered to a water quality sufficient to support sustainable populations of trout or salmon. Long-term (30 years) monitoring data were used hereto constrain the calibration of the acidification model MAGIC to three Norwegian calibrated catchments. The model accounted for 60-80% of the variance in the year-to-year variations in concentrations of most of the major ions in streamwater. The results support the use of the lumped parameter acid neutralizing capacity (ANC) to link chemical parameters to biological response, as the calibration efficiency for ANC is considerably higher than for other biologically important parameters such as inorganic aluminum (Al(n+)) and pH. Three different scenarios for future deposition of sulfur were run: current legislation, maximum feasible reductions, and an illustrative scenario removing all anthropogenic deposition. These analyses show that much of the potential improvement in water quality has already occurred and that only limited further improvement can be expected from the current legislation. The current legislation is unlikely to produce ANC values sufficiently high to allow self-reproducing populations of trout at two of the three sites. Most of the response in water chemistry to reduced acid deposition has been rapid; the water chemical responses largely occur the same year or a few years after reduction in the input. The soil pool of exchangeable base cations depleted during 150 years of acid deposition, however, requires several centuries for replenishment. The uncertainties in future predictions come from several factors, such as future nitrogen dynamics and impacts from changes in seasalt and precipitation events. The differences in future water chemistry predicted from changed seasalt deposition or nitrogen dynamics are larger that the differences between different deposition scenarios. Hence, these factors must be included in future assessments of recovery from acidification.

Use of stream chemistry for monitoring acidic deposition effects in the Adirondack region of New York.

Acid-neutralizing capacity (ANC) and pH were measured weekly from October 1991 through September 2001 in three streams in the western Adirondack Mountain region of New York to identify trends in stream chemistry that might be related to changes in acidic deposition. A decreasing trend in atmospheric deposition of SO4/2- was observed within the region over the 10-yr period, although most of the decrease occurred between 1991 and 1995. Both ANC and pH were inversely related to flow in all streams; therefore, a trend analysis was conducted on (i) the measured values of ANC and pH and (ii) the residuals of the concentration-discharge relations. In Buck Creek, ANC increased significantly (p < 0.05) over the 10 yr, but the residuals of ANC showed no trend (p > 0.10). In Bald Mountain Brook, ANC and residuals of ANC increased significantly (p < 0.01), although the trend was diatonic-a distinct decrease from 1991 to 1996 was followed by a distinct increase from 1996 to 2001. In Fly Pond outlet, ANC and residuals of ANC increased over the study period (p < 0.01), although the trend of the residuals resulted largely from an abrupt increase in 1997. In general, the trends observed in the three streams are similar to results presented for Adirondack lakes in a previous study, and are consistent with the declining trend in atmospheric deposition for this region, although the observed trends in ANC and pH in streams could not be directly attributed to the trends in acidic deposition.

The influence of total organic carbon (TOC) on the relationship between acid neutralizing capacity (ANC) and fish status in Norwegian lakes

Acid neutralizing capacity (ANC) is the parameter most commonly used as chemical indicator for fish response to acidification. Empirical relationships between fish status of surface waters and ANC have been documented earlier. ANC is commonly calculated as the difference between base cations ([BC]=[Ca 2+]+[Mg 2+]+[N +]+[K +]) and strong acid anions ([SAA]=[SO 4 2−]+[NO 3 −]+[Cl −]). This is a very robust calculation of ANC, because none of the parameters incorporated are affected by the partial pressure of CO 2, in contrast to the remaining major ions in waters, pH ([H +]), aluminum ([Al n+]), alkalinity ([HCO 3 −/CO 3 2−]) and organic anions ([A n−]). Here we propose a modified ANC calculation where the permanent anionic charge of the organic acids is assumed as a part of the strong acid anions. In many humic lakes, the weak organic acids are the predominant pH-buffering system. Because a significant amount of the weak organic acids have p K-values<3.0–3.5, these relatively strong acids will permanently be deprotonated in almost all natural waters (i.e. pH>4.5). This means that they will be permanently present as anions, equal to the strong acid inorganic anions, SO 4 2−, NO 3 − and Cl −. In the literature, natural organic acids are often described as triprotic acids with a low p K 1 value. Assuming a triprotic model, we suggest to add 1/3 of the organic acid charge density to the strong acid anions in the ANC calculation. The suggested organic acid adjusted ANC (ANC OAA), is then calculated as follows: ANC OAA=[BC]−([SAA]+1/3CD*TOC) where TOC is total organic carbon (mg C L −1), and CD=10.2 is charge density of the organic matter (μeq/mg C), based on literature data from Swedish lakes. ANC OAA gives significant lower values of ANC in order to achieve equal fish status compared with the traditional ANC calculation. Using ANC OAA the humic conditions in lakes are better taken into account. This may also help explain observations of higher ANC needed to have reproducing fish populations in lakes with higher TOC concentrations.

Soil Freezing and the Acid‐Base Chemistry of Soil Solutions in a Northern Hardwood Forest

Reductions in the depth and duration of snow cover under a warmer climate may cause soil freezing events to become more frequent, severe, and spatially extensive in northern temperate forest ecosystems. In this experiment, snow cover was manipulated to simulate the late development of snowpack and to induce soil freezing at sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) stands at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire. The objective of this manipulation was to elucidate the effects of soil freezing on the concentrations and fluxes of soil solution H+, Ca2+, Mg2+, K+, and Na+, as well as values of acid neutralizing capacity (ANC). Mild soil freezing events (soil temperatures never decreased below −5°C) resulted in pronounced acidification of soil solutions, driven primarily by nitrification, in the forest floor of sugar maple stands during the growing season. This mobilization of NO−3 from the forest floor of maple stands was accompanied by the leaching of Ca2+ and Mg2+ in Oa horizon solutions. Responses of soil solution acid‐base chemistry to soil freezing were not evident in yellow birch stands or in the Bs horizon of either vegetation type, emphasizing the importance of vegetation type and the mineral soil in determining the effects of climatic disturbance on drainage water chemistry and nutrient loss. These results suggest that models of soil biogeochemistry in temperate forest ecosystems should consider soil‐freezing events when simulating the acid‐base chemistry of soil solutions and the translocation of nutrient base cations between soil horizons.

Patterns of streamwater acidity in Lye Brook Wilderness, Vermont, USA.

Under the United States Clean Air Act Amendments of 1977, a class I designation safeguards wilderness areas from the negative effects of new sources of air pollution. We monitored streamwater chemistry in the class I Lye Brook Wilderness in southwestern Vermont from May 1994 through August 1995. Stream samples were collected biweekly at nine sampling locations throughout the wilderness and were analyzed for major cations and anions, dissolved organic carbon, pH, and acid-neutralizing capacity. Eight of nine sites sampled had mean annual acid neutralizing capacity values below zero. During the study period, decreases in streamwater acid neutralizing capacity values were caused primarily by SO(4)(2-). At some sites, however, NO(3) (-) and naturally occurring, weak organic acids were seasonally important. During high discharge, the low pH and high concentrations of inorganic monomeric Al were at levels that are toxic to acid-sensitive aquatic species. Watershed mass balances were calculated to determine annual gains or losses for measured ions. These budgets indicate that S inputs and outputs were nearly equal, there was a net loss of base cations, and a net gain in N. How long these watersheds can continue to assimilate additional N inputs is unknown.

A practical application of Driscoll's equation for predicting the acid-neutralizing capacity in acidic natural waters equilibria with the mineral phase gibbsite

A practical application of Driscoll's equation for predicting the acid-neutralizing capacity (ANC) in acidic waters equilibria with the mineral phase gibbsite is reported in this paper. Theoretical predication values of ANC are compared with the experimental data obtained from different literatures. The effect of aluminum (Al) on the value of ANC is investigated. It indicates that Al plays an important role in regulating the buffering effects in acidic natural waters. Failure to consider Al in acidic waters may bias assessment results in certain situations so as to overestimate the ANC values in response to increase in atmospheric deposition.

Sulfur Acidity Loading in South Korean Ecosystems

The applicability of critical load (CL) methodology for thedetermination of natural terrestrial ecosystem sensitivity to sulfur acidity loading in South Korea was investigated.The sulfur critical load values, CLmaxS, were calculated for the terrestrial ecosystems of South Korea using the steady-state mass balance approach. The corresponding mapping of CLmaxS was carried out on the scale of 11 × 14 km grid cells. The estimated CLmaxS values depend on the low rate of soil chemical base cation weathering (mainly, 200–400 eq ha-1 yr-1), relativelylow base cation depositions (mainly less than 450 eq ha-1 yr-1) and base cation uptake (predominantly 300–400 eq ha-1 yr-1), and in significant degree on high valuesof acid neutralizing capacity. The latter in turn is connectedwith relatively high values of surface runoff (maximum 9000 m3 ha-1 yr-1). It has been shown that about 75%of CLmaxS values are in the range of 1000–2000 eq ha-1 yr-1 and about 15% are relatively low values(<1000 eq ha-1 yr-1). About 10% of ecosystems haveCL values more than 2000 eq ha-1 yr-1. The sensitiveand very sensitive ecosystems occur in the southeastern part of the country whereas the sustainable ecosystems are wide spread in the northeastern part. In accordance with sulfur critical load and sulfur deposition patterns, in 1994–1997 the CLmaxS values were found to be exceeded in about 40% of total number of Korean ecosystems, mainly in the southeastern part of the country. The average yearly valuesof exceedances varied from 176 to 3100 eq ha-1 yr-1.

Prediction of episodic acidification in North-eastern USA: an empirical/mechanistic approach

Observations from the US Environmental Protection Agency's Episodic Response Project (ERP) in the North-eastern United States are used to develop an empirical/mechanistic scheme for prediction of the minimum values of acid neutralizing capacity (ANC) during episodes. An acidification episode is defined as a hydrological event during which ANC decreases. The pre-episode ANC is used to index the antecedent condition, and the stream flow increase reflects how much the relative contributions of sources of waters change during the episode. As much as 92% of the total variation in the minimum ANC in individual catchments can be explained (with levels of explanation > 70% for nine of the 13 streams) by a multiple linear regression model that includes pre-episode ANC and change in discharge as independent variables. The predictive scheme is demonstrated to be regionally robust, with the regional variance explained ranging from 77 to 83%. The scheme is not successful for each ERP stream, and reasons are suggested for the individual failures. The potential for applying the predictive scheme to other watersheds is demonstrated by testing the model with data from the Panola Mountain Research Watershed in the South-eastern United States, where the variance explained by the model was 74%. The model can also be utilized to assess chemically new' and chemically old' water sources during acidification episodes.

Modelling regional response of lakewater chemistry to changes in acidic deposition: the MAGIC model applied to lake surveys in southernmost Norway 1974-1986-1995

Abstract. Two methods for modelling regional responses of lake water quality to changes in acidic deposition in southernmost Norway were examined. Both methods are based upon the MAGIC model but differ in mode of regional application; one uses site-specific while the other uses Monte-Carlo methods for model calibration. The simulations of regional responses from both methods were compared with observed responses based on data from three lake surveys in southernmost Norway conducted in 1974, 1986 and 1995. The regional responses of the two modelling approaches were quite similar and agreed well with the observed regional distributions of lakewater chemistry variables. From 1974 to 1986 the observed data indicated that despite a decline of approximately 10% in sulphate (SO4) deposition, the mean acid neutralizing capacity (ANC) of lakes in southernmost Norway declined by approximately 6 μeq l-1. Both modelling approaches simulated no change or a very small decline in mean ANC for that period. From 1986 to 1995 the observed data indicated that, in response to an approximate 40% decline in SO4 deposition, the mean ANC of lakes in southernmost Norway increased by 11-16 μeq l-1. The modelling approaches simulated increases of 9-10 μeq l-1 in mean ANC for the same period. Both simulations and observations indicate that &gt; 65% of lakes in southernmost Norway were acidic in 1974 and 1995. Both simulation methods predict that &gt;65% of the lakes in southernmost Norway will have positive ANC values within 10 years of reductions of SO4 deposition to 20% of 1974 levels. Of the two regionalization methods the site-specific method appears preferable, because whereas the Monte-Carlo method gives results for a region as a whole, the site-specific method also reveals patterns within the region. The maintenance of a one-to-one correspondence between simulated and observed systems means that simulation results can be mapped for a geographically explicit presentation of model results. The ability to examine geographic patterns of response is becoming increasingly important in regional assessments.

The response of lake water in the Adirondack region of New York to changes in acidic deposition

The Adirondack region of New York is characterized by a large number of lakes with low values of acid neutralizing capacity (ANC; ∼25% of lakes have summer ANC values <0 μeq/l), which are sensitive to atmospheric deposition of strong acids. Time-series analysis showed that concentrations of sulfate (SO42−), nitrate (NO3−), ammonium (NH4+) and basic cations have decreased in precipitation, resulting in increases in pH. A relatively uniform rate of decline in SO42− concentrations in lakes across the region (1.91±0.27 μeq/l yr) suggests that this change was due to decreases in atmospheric deposition. Despite the marked declines in concentrations of SO42− in Adirondack lakes, there has been no systematic increase in pH and ANC. The limited response of lake water ANC and pH to decreases in atmospheric deposition of SO42− may be attributed to a combination of factors, including: (1) depletion of exchangeable pools of basic cations in soil, (2) additional inputs of SO42− to watersheds in the form of unmeasured dry deposition and/or an internal supply of sulfur (S) from mineralization of soil organic S pools or weathering of S minerals, (3) elevated leaching losses of NO3−, and/or (4) pH buffering associated with elevated concentrations of aluminum (Al) and/or naturally organic acids.

Modeling the acid‐base chemistry of organic solutes in Adirondack, New York, lakes

Data from the large and diverse Adirondack Lake Survey were used to calibrate four simple organic acid analog models in an effort to quantify the influence of naturally occurring organic acids on lake water pH and acid‐neutralizing capacity (ANC). The organic acid analog models were calibrated to observations of pH, dissolved organic carbon (DOC), and organic anion (An−) concentrations from a reduced data set representing 1128 individual lake samples, expressed as 41 observations of mean pH, in intervals of 0.1 pH units from pH 3.9 to 7.0. Of the four organic analog approaches examined, including the Oliver et al. (1983) model, as well as monoprotic, diprotic, and triprotic representations, the triprotic analog model yielded the best fit (r2 = 0.92) to the observed data. Moreover, the triprotic model was qualitatively consistent with observed patterns of change in organic solute charge density as a function of pH. A low calibrated value for the first H+ dissociation constant (pKal = 2.62) and the observation that organic anion concentrations were significant even at very low pH (&lt;4) indicate that naturally occurring organic solutes in these waters have strongly acidic functional groups. Inclusion of organic acidity in model calculations resulted in good agreement between measured and predicted values of lake water pH and ANC. Assessments to project the response of surface waters to future changes in atmospheric deposition, through the use of acidification models, will need to include representations of organic acids in model structure to make accurate predictions of pH and ANC.

Trends and patterns in lake acidification in the State of Vermont: Evidence from the Long-Term Monitoring Project

Twenty-four low acid neutralizing capacity (ANC) lakes in Vermont have been monitored since 1980 to characterize their chemical variability, and to determine if they exhibit temporal trends in acid/base chemistry. Many of the lakes exhibit significant decreasing trends in SO42− and base cation (CB) concentrations, but few exhibit significant changes in pH or ANC. An examination of all trend results (significant and insignificant) suggests a tendency for ANC and pH values in these lakes to be increasing, but either the changes are too small, or the number of observations too small, for these trends to be significant. Data from these lakes suggest that the primary responses of surface waters in this region to declining rates of SO42− deposition are decreases in SO42− concentrations and rates of cation leaching from watershed soils. Decreasing rates of cb deposition may combine with lower rates of cation leaching to produce declines in cB that are very similar to measured declines in SO42− concentration. Vermont lakes exhibit their lowest ANC values in spring, attributable, for the most part, to dilution of cB concentrations during spring snow melt. Concentrations of SO42− are also more dilute in the spring, but cB decreases are greater, and the net effect is a lowering of ANC. One quarter of the Vermont lakes monitored exhibit strong seasonality in NO3− concentrations, with peak concentrations near 70 Μeq L−1. In these lakes, spring increases in NO3− concentrations are more important than CB dilution in producing minimal spring ANC values.