Headwater lake chemistry during the spring freshet in North-Central Ontario

From data on 30 headwater lakes in north-central Ontario we found that, during the spring snow melt of 1981, all lakes underwent serious declines in alkalinity. Generally, SO₄ ^2?, alkalinity, Ca⁺ and Mg²⁺ concentrations were reduced by runoff and rain then recovered to intermediate levels after the major inputs declined. As expected, a range in responses was evident with lower alkalinity systems showing the greatest changes. The observed changes, however, were consistent with acid loading having depleted alkalinity. In calculating an input-output budget for each lake, we found that changes in Cl^?, Na⁺, and K⁺ were consistent with atmospheric inputs being the major source as the difference between the expected input and the actual contribution from rain and snow had a mean near zero. There appears to be a significant, ? 45%, watershed source of sulphate that we hypothesize is from dry deposition occurring prior to snowfall and is eluted with the melting process. With refinements to a mass balance approach explaining the watershed source of SO₄ ^2? and Al, we feel it is possible to predict springtime lake changes given a few chemical and simple morphometric variables.     

Biogeochemistry of a forested watershed in the central Adirondack Mountains: Temporal changes and mass balances

Information on atmospheric inputs, water chemistry and hydrology were combined to evaluate elemental mass balances and assess temporal changes in elemental transport from 1983 through 1992 for the Arbutus Lake watershed. This watershed is located within a northern hardwood ecosystem at the Huntington Forest within the central Adirondack Mountains of New York (USA). Changes in water chemistry, including increasing NO₃ ^? concentrations (1.1 μmol _c , L^?1 yr-1), have been detected during this study period. Starting in 1991 hydrological flow has been measured from Arbutus Lake and these measurements were compared with predicted flow using the BROOK2 hydrological simulation model. The model adequately (r²=0.79) simulated flow from this catchment and was used to estimate drainage for earlier periods when direct hydrological measurements were not available. Modeled drainage water losses coupled with estimates of wet and dry atmospheric deposition were used to calculate solute budgets. Export of SO₄ ^2? (831 mol _c ha^?1 yr^?1) from the greater Arbutus Lake watershed exceeded estimates of atmospheric deposition in an adjacent hardwood stand suggesting an additional source of S. These large drainage losses of SO₄ ^2? also contributed to the drainage fluxes of basic cations (Ca²⁺, Mg²⁺, K⁺ and Na⁺). Most of the atmospheric inputs of inorganic N were retained (average of 74% of wet precipitation and 85% total deposition) in the watershed. There were differences among years (56 to 228 mol ha^?1 yr^?1) in drainage water losses of N with greatest losses occurring during a warm, wet period (1989–1991).     

Influence of atmospheric deposition on lake mass balances in the Turkey Lakes Watershed,Central Ontario

Ion mass budgets were measured for 2 water yr (June–May, 1981–83) for a high and a ).ow elevation lake and their associated catchments. The lakes are located in the Turkey Lakes Watershed (TLW) in central Ontario, Canada, which is an undeveloped basin located on the Canadian Shield, 50 km north of Sault Ste. Marie. The ionic budgets of the lakes show that atmospheric deposition directly to the lakes' surfaces is the principal input pathway for H⁺ and NH₄, whereas basic cations, SO₄, NO₃, and probably alkalinity are supplied primarily by inflow from the surrounding terrestrial basin and/or upstream lake. The lakes strongly retain H⁺ (i.e. output ? input), weakly retain the N species, and are in balance (i.e. output = input) for other ions except Ca and alkalinity which show an excess output compared to measured + estimated inputs. We hypothesize that an input of groundwater and/or seepage accounts for most of the Ca and alkalinity imbalance although the existence of within-lake alkalinity generation is probable also.     

Response to a smelter closure in Cascade mountain lakes

A statistically significant decrease in sulfate was observed in high elevation Cascade lakes during 1983 through 1988. The total decrease averaged 2.2 μeq L^?1 in two slow-flush lakes and 4.2 μeq L^?1 in three fast-flush lakes for 1983–1985 vs 1986–1988, respectively. Coincident with these changes in sulfate concentrations were a sharp decrease of SO₂ emissions from the ASARCO smelter (100 km SE of the lakes), from 87 to 70 kt yr^?1 during 1983–1984 to 12 in 1985, the year of its closure, and a gradual change in SO₂ emissions from Mt. St. Helens, from 39 to 27 during 1983–1984 to 5 in 1988. The sharpest decreases occurred in non-marine sulfate in fast-flush lakes from 1984 to 1985 (about 2 μeq L^?1) and in slow-flush lakes from 1985 to 1986 (1 μeq L^?1, which point to the ASARCO closure as the sole cause. However, some of the more gradual decline in non-marine sulfate observed during 1983 through the 1988 sampling periods may have been due to a slow washout of sulfate enriched ash from the 1980 Mt. St. Helens' eruption. Sulfate concentrations in precipitation also declined significantly by about 2 μeq L^?1, but changes in volume-weighted sulfate content were not significant. Lake alkalinity did not show a consistent increase in response to decreased sulfate. This was probably due to either watershed neutralization of acidic deposition or the greater variability in alkalinity measurements caused by small changes in acidic deposition making it difficult to detect changes.     

Watershed vs in-lake alkalinity generation: A comparison of rates using input-output studies

As a means of assessing the relative contributions of watershed (terrestrial) and in-lake processes to overall lake/watershed alkalinity budgets, alkalinity production rates for watersheds and low alkalinity lakes were compiled from the literature and compared. Based on net alkalinity production data, derived using wet or bulk deposition data, mean and median alkalinity production for 20 watersheds in North America and Europe were 89 and 69 meq m^?2 yr^?1 (range 20 to 235 meq m^?2 yr^?1). For a subset of 10 watersheds with dry deposition data, terrestrial alkalinity production neutralized an additional 35 meq m^?2 yr^?1 of acidic deposition. For 11 lakes, mean and median in-lake alkalinity generation were 99 and 88 meq m^?2 yr^?1 (range 22 to 240 meq m^?2 yr^?1). Analysis of data indicates that for the low alkalinity systems described here, areal alkalinity production rates for watersheds and lakes are approximately equal. This relationship suggests that watershed area to lake area ratio can be used as a convenient estimator of the relative importance of watershed and in-lake sources of alkalinity for drainage lake systems. For precipitation-dominated seepage lakes and other systems where hydrology limits soil-water contact, hydrologic flow paths and residence times can be of overriding importance in determining alkalinity sources. For regions dominated by drainage lakes with high watershed area to lake area ratios (such as the Northeastern U.S.), however, alkalinity budgets are dominated by watershed processes. Omission of in-lake alkalinity consideration for most lakes in such regions would have little impact on computed alkalinity budgets or on predicted response to changes in acidic deposition loadings.     

Lake and watershed neutralization strategies

The Experimental Watershed Liming Study (EWLS) evaluated the application of CaCO₃, to a forested watershed to mitigate the acidification of surface water. During October 1989, 6.9 Mg CaCC₃/ha was applied by helicopter to two subcatchments of about 50% (102.5 ha) of the Woods Lake watershed area. The EWLS team investigated the response to treatment of soils (chemistry and microbial processes), vegetation, wetland, stream and lake waters, and phytoplankton and fish, and applied the Integrated Lake Watershed Acidification (ILWAS) model in predicting a watershed treatment duration of up to 50 years. Observations showed a gradual change in pH, acid neutralizing capacity (ANC) and Ca²⁺ in the water column; direct lake additions of CaCO₃ (three different times) were characterized by abrupt changes following base addition and subsequent rapid reacidification. Moreover, the watershed treatment eliminated the snowmelt acidification of the near-shore region of the lake observed during direct lake treatments. Positive ANC water in the tributary and near-shore area improved conditions for fish reproduction and for a viable fish population. Budgets for 12-month periods before and after the watershed treatment showed that the lake shifted from a source of ANC to a sink due to retention of elevated inputs of Ca²⁺ from the watershed CaCO₃ application.     

A protocol for determining lake acidification pathways

The chemistry of 282 sampled low pH (<6.0) lakes in the U.S. E.P.A. Eastern Lake Survey (ELS) was evaluated in an attempt to assess why these systems have low pH. Evaluations were made using a decision protocol for classifying lakes according to several hypothesized acidifying mechanisms: acidic deposition, presence of wetlands and organic soils, acid mine drainage, watershed S sources, salt driven acidification, and changes in land use. The algorithm evaluates lakes in three steps: (1) initial exclusion criteria exclude from consideration lakes with pH greater than 6.0 or subject to strong confounding influences (e.g., road salt); (2) a general classification discriminates between lakes according to anion dominance; and (3) a secondary classification of lakes within each anion dominant class determines the most likely acidification pathway, using preliminary quantitative criteria designed to discriminate among competing hypotheses. Results computed for sampled lakes were scaled-up to produce regional population estimates, using the statistical framework of the ELS. Acidic deposition appears to be the most likely cause of low pH conditions in about two-thirds of the non-excluded lakes in the ELS low pH target population. Organic acidity arising from wetlands or land use changes appears to be primarily responsible for the low pH status of one quarter of these lakes. Watershed S sources and acid mine drainage appear to be of negligible importance, though further information on dry deposition rates and/or watershed soils is required to confirm this.     

Temporal chemical variability in acid sensitive high elevation lakes

Chemical indicators suggest that slight, but discernable acidification occurs during smowmelt in some highly sensitive Cascade Mountain Lakes (mean alkalinity 20 ueq L^?1). Although some SO₄ in the lakes (mean 13 μeq L^?1 ) comes from local geologic sources, several considerations suggest that some also comes from atmospheric deposition and anthropogenic sources. If sampling is stratified, the relatively low lake-to-lake and year-to-year variability in chemical constituents demonstrates that these highly sensitive lakes represent excellent indicators of acidification. The contention that precipitation pH >4.6 is needed for protection of sensitive lakes is supported. However, the slight but temporary acidification effect currently detectable during snowmelt, suggests that for adequate protection of these highly sensitive lakes, precipitation pH should be >4.7 to 4.8.     

Relations between lake acidification and sulfate deposition in Northern Minnesota,Wisconsin, and Michigan

From a level of 1 kg ha^?1yr^?1 in north central Minnesota, emission-related wet SO₄ deposition increases across northern Wisconsin and northern Michigan to about 18 kg ha^?1yr^?1 in south central Michigan. Samples taken from 82 clearwater (low color) lakes across this region in the summer of 1984 showed a pattern of acidification in proportion to deposition. We found a linear increase in the difference between alkalinity and Ca+Mg and in lake SO₄ concentration with increasing deposition. We developed a simple equation to predict the emission-related SO₄ deposition levels that will cause the alkalinity of sensitive clear-water lakes to go to zero.     

Episodic acidification during snowmelt of high elevation lakes in the Sierra Nevada mountains of California

Atmospheric loads to dilute lakes in the Sierra Nevada mountains of California are very low, and fall almost entirely as snow. When acidic anions preferentially elute from melting snow, these low loads may nontheless be enough to acidify low ANC lakes. Two of the ten lakes included in the Sierra Episodes Study are discussed here: High Lake, the only lake in the study to become acidic during snowmelt; and Treasure Lake, typical of the remainder of the lakes. All lakes exhibited increases in NO₃ ^? concentrations during early snowmelt; these were accompanied by increases in base cations, primarily Ca²⁺. In the first few days of snowmelt, NO₃ ^? concentrations at High Lake increased more rapidly than concentrations of base cations, resulting in ANC values below zero. Export of both NO₃ ^? and SO₄ ^2? from the watersheds exceeded the inputs from the snowpack, suggesting that other sources (e.g., watershed minerals, stored inputs from the previous summer, transformations of other inputs) of these anions are important.     

Alkalinity production in epilimnetic sediments: Acidic and non-acidic lakes

Interstitial water profiles in epilimnetic sediments of lakes with varying water column alkalinities were collected to assess the origin and importance of sedimentary alkalinity in freshwater lakes. Release of Ca²⁺ and NH₄ ⁺, and consumption of SO₄ ^? are the most important contributors to alkalinity production m sediments of non-acidic lakes. In acidic lakes, Fe²⁺ and Mn²⁺ replace Ca²⁺ as the dominant cation contributors to alkalinity production. The sedimentary alkalinity flux is an important component of the acid neutralizing capacity of freshwater lakes. However, the presence of large alkalinity gradients in sediment porewaters does not necessarily indicate a large source of alkalinity for the lake, as a significant portion of the alkalinity iu associated with the formation of Fe²⁺, Mn²⁺ and NH₄ ⁺ Oxidation of Fe²⁺ and Mn²⁺ at the anoxic-oxic interface and biological removal of NH₄ ⁺ in the overlying water column results in consumption of the co-diffusing alkalinity.     

Atmospheric Deposition in a Rural Area in India - Net and Potential Acidity

Atmospheric deposition in India is generally described as alkaline and well buffered against an increase of acidifying components. Such conclusion has been based on measurements usually performed in urban or suburban areas. Our data on deposition in NE India (in the countryside N of Bhubaneswar) obtained with wet-only and bulk collectors show that: 1) the weighted mean concentrations (and wet deposition) of H⁺ and HCO₃ ^? are almost equal, with dustfall contributing a negligible amount of HCO₃ ^?, 2) the deposition of potential acidity, defined as the acidity that would give the same acidity contribution to the receiving surface as the actual deposition provided that all ammonium was converted to nitrate in the soil, could be as high as 40 mmol H⁺ m^?2 a^?1 corresponding to a pH of 4.3 in precipitation. The low buffer capacity against acidification and high potential acidity were discovered from long- term measurements in a vegetation covered rural area. Similar measurements in a nearby suburban area gave a much higher input of HCO₃ ^? both in wet deposition and dustfall. Further increase of the emission of acidifying components in the region will increase the acid deposition.     

Acidity neutralization mechanism in a forested watershed in Central Japan

The contributions of cation exchange and mineral weathering to the neutralization of acidity in the Jingahata watershed in central Japan were estimated through a laboratory weathering experiment and runoff chemistry measurements. The laboratory experiment was conducted in a stirred-flow reactor for a whole soil sample collected from the C horizon in the watershed. The concentration ratios of base cations (Ca²⁺, Mg²⁺, K⁺ and Na⁺) to Si (BC/Si) released during the steady-state stage of the laboratory experiment were in good agreement with the ratios of the net flux of base cations to the flux of Si in the streamwater (BC _{N ET}/Si _L).This result suggests that the acidity in the watershed is neutralized primarily by mineral weathering without causing a net loss of base cations from exchange sites. The alkalinity/acidity balance estimated for the watershed shows that the total weathering rate of base cations is approximately 3.26 keq ha^?1 yr^?1. Weathering of plagioclase (An₄₁) contributes 83% of the total weathering rate. The dominant acidity source is CO₂ released within the soil horizons, accounting for roughly 85% of the total acidity flux (3.20 keq ha^?1 yr^?1). This high internal production of acidity suppresses the relative importance of atmospheric acidity inputs (0.3 keq ha^?1 yr^?1).     

Soil acidification during more than 100 years under permanent grassland and woodland at Rothamsted

Abstract Soil samples have been taken periodically from unlimed plots of the 130-year-old Park Grass Experiment and from the 100-year-old Geescroft Wilderness at Rothamsted. Changes in the pH of the samples show how acidification has progressed. The soils are now at, or are approaching, equilibrium pH values which depend on the acidifying inputs and on the buffering capacities of the soils. We have calculated the contributions to soil acidification of natural sources of acidity in the soil, atmospheric deposition, crop growth and nutrient removal, and, where applicable, additions of fertilizers. The relative importance of each source of acidification has changed as the soils have become more acid. Acid rain (wet deposited acidity) is a negligible source, but total atmospheric deposition may comprise up to 30% of acidifying inputs at near neutral soil pH values and more as soil pH decreases. Excepting fertilizers, the greatest causes of soil acidification at or near neutral pH values are the natural inputs of H⁺ from the dissolution of CO₂ and subsequent dissociation of carbonic acid, and the mineralization of organic matter. Under grassland, single superphosphate and small amounts of sodium and magnesium sulphates have had no effect on soil pH, whilst potassium sulphate increased soil acidity slightly. All of these effects are greatly outweighed under grassland, however, by those of nitrogen fertilizers. Against a background of acidification from atmospheric, crop and natural inputs, nitrogen applied as ammonium sulphate decreased soil pH up to a maximum of 1.2 units at a rate in direct proportion to the amount added, and nitrogen applied as sodium nitrate increased soil pH by between 0.5 and 1 unit.     

Acidification by nitric acid — Future considerations

HNO₃ is more efficient in acidifying lakes than has been generally believed. This is because as nitrate loading to lakes increases, the efficiency of in-lake nitrate removal decreases markedly. Efficiencies decrease because algal N requirements are exceeded and because denitrification, which becomes an important removal process, is not as efficient as algal removal. Thus, nitrate and the accompanying H⁺ accumulate and HNO₃ becomes an important factor in acidification. Data from an experimentally acidified system suggest that midsummer surface-water nitrate concentrations in excess of only 1 µmol L^?1 indicate that algal requirements have been exceeded. While 1 µmol L^?1 NO₃ ^? is not a significant quantity in terms of affecting the acidity of the water, it is useful as an indicator to identify lakes where algal requirements have been exceeded and where further increases in HNO₃ loading could lead to lake acidification.     

Monitoring Acid Waters in the UK: 1988–1998 Trends

Since 1988, a network of lakes and streams has been monitored in areas of the UK sensitive to surface water acidification. Analysis of 10 years data has focused on the identification and quantification of time-trends in chemical parameters, to establish whether declines in emission of acidifying pollutants have resulted in recovery of acidified surface waters. A national decline in S deposition in the UK since 1988 has not generally been accompanied by a significant improvement in freshwater chemistry. At the three sites where xSO₄ concentrations have declined, NO₃ has increased and there has been no increase in pH or alkalinity. Upward trends in pH and alkalinity observed at several other sites are not associated with downward trends in acidic anions. Temporal variation in xSO₄, NO₃, acidity, DOC and other important meaures of surface water quality can all be linked to decadal-scale variation in climate, and this has important implications for the detection of recovery-related trends.     

Element Budgets in a Forested Watershed in Southern China: Estimation of a Proton Budget

We evaluated the element budgets in a forested watershed in Jiulianshan, southern China. The element input in bulk precipitation was characterized by high depositions of H⁺, NH₄ ⁺, Ca²⁺, and SO₄ ^2?, i.e., 400, 351, 299, and 876 eq/ha/yr, respectively. The outputs of H⁺, NH₄ ⁺, and SO₄ ^2? from the watershed were very low, while those of Ca²⁺ and Mg²⁺ were high, 712 and 960 eq/ha/yr, respectively. The element budgets suggested that i) the net retentions of H⁺, NH₄ ⁺, and SO₄ ^2? in this watershed were high, and ii) the net release of Mg²⁺ from this watershed was high mainly due to weathering. The net release of Ca²⁺ was not so high because of the high atmospheric deposition, while atmospheric deposition of Mg²⁺ was not so high (130 eq/ha/yr). Decrease of acid neutralizing capacity in the soil, i.e., net soil acidification, was caused mainly by the net release of Mg²⁺. Moreover, the net retention of SO₄ ^2? also contributed to soil acidification.     

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 SO₄ ^2?. Lake-to-lake variability in acid neutralizing capacity (ANC) was largely due to differences in the supply of basic cations (Ca²⁺, Mg²⁺, K⁺, Na⁺; C_B) 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 SO₄ ^2?, consistent with decreases in SO₂ emissions and SO₄ ^2? in precipitation in the eastern U.S. Reductions in SO₄ ^2? 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.     

Ten years of precipitation chemistry in Haifa,Israel

Rain event samples have been collected in Haifa, Israel, for nine hydrological years 1981 to 1990. Precipitation amount, pH, SO₄ ⁼, NO₃ ^?, Cl^?, NH₄ ⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺ and alkalinity of rainwater samples were recorded. The sampling and analysis program was based on WMO recommendations for background networks. The sampling was performed manually, and the analysis was based on wet chemistry for ions and atomic absorptions for metals. Data of 187 rain samples showed that the average pH was 5.3±1.1∶ 26% of the rain events were below pH of 5.6 and 23% above pH of 7.0. Some simple chemical mass-balance considerations indicate that natural sources, sea salt and soil carbonates are the main contributors to rain chemistry. However, the presence of low pH events observed over the years suggests that the impact of anthropogenic emissions may overwhelm the buffering capacity of the alkaline aerosol.     

Development of methodology for predicting reacidification of calcium carbonate treated lakes

As part of the EPRI funded Lake Acidification Mitigation Project(LAMP), an ongoing study to evaluate the feasibility of acid lake liming, two models (WAM and ALaRM) have been tested for two calibrated Adirondack Mountain study lakes(Woods and Cranberry). A priori predictions of reacidification rates based on Ca carbonate application and historical hydrological data are presented. WAM (Watershed Acidification Model) is a deterministic model that is capable of simulating the movement of water and chemical constituents through a watershed. WAM generates indata in the form of hydrologic and alkalinity to ALaRM (Acid Lake Reacidification Model), a general mass balance model developed for the temporal prediction of the principal chemical species in both the water column and sediment pore water of small lakes and ponds. A matrix of runs to determine model sensitivity to input loadings, kinetic coefficients, and sediment dosage levels indicate that reacidification of the water column is most sensitive to variation in hydrologic loading followed by variation in sediment dosage levels. Baseline estimates (initial water column alkalinity between 400 to 500 μeqL^?1) indicate that reacidification to near zero alkalinity occurs after a time period equivalent to approximately two to three average hydraulic retention times.