Jeffrey S. Kahl

Regional trends in aquatic recovery from acidification in North America and Europe

Rates of acidic deposition from the atmosphere (‘acid rain’) have decreased throughout the 1980s and 1990s across large portions of North America and Europe. Many recent studies have attributed observed reversals in surface-water acidification at national and regional scales to the declining deposition. To test whether emissions regulations have led to widespread recovery in surface-water chemistry, we analysed regional trends between 1980 and 1995 in indicators of acidification (sulphate, nitrate and base-cation concentrations, and measured (Gran) alkalinity) for 205 lakes and streams in eight regions of North America and Europe. Dramatic differences in trend direction and strength for the two decades are apparent. In concordance with general temporal trends in acidic deposition, lake and stream sulphate concentrations decreased in all regions with the exception of Great Britain; all but one of these regions exhibited stronger downward trends in the 1990s than in the 1980s. In contrast, regional declines in lake and stream nitrate concentrations were rare and, when detected, were very small. Recovery in alkalinity, expected wherever strong regional declines in sulphate concentrations have occurred, was observed in all regions of Europe, especially in the 1990s, but in only one region (of five) in North America. We attribute the lack of recovery in three regions (south/central Ontario, the Adirondack/Catskill mountains and midwestern North America) to strong regional declines in base-cation concentrations that exceed the decreases in sulphate concentrations.

Comparison of Hg and Pb fluxes to hummocks and hollows of ombrotrophic big heath bog and to nearby Sargent Mt. Pond, Maine, USA

Two hummock cores (separated by 1 m), two hollow cores (separated by 1 m and both within 5 m of the hummock) from ombrotrophic Big Heath, and a single core from Sargent Mountain Pond (12 km north-northeast of the bog), Mt. Desert Island, Acadia National Park, Maine, USA were collected in 1983 and dated using 210Pb and analyzed for a suite of major and trace metals. The hummock cores correspond closely in terms of dating profiles, concentrations of Hg and Pb, and thus trends and values for accumulation rates. The hollow cores agree generally with each other but give more subdued peaks in concentration and lower integrated anthropogenic burdens of Hg and Pb and 50% lower unsupported 210Pb than the hummock cores. Σ210Pbuns. (Bq/cm2), ΣHganth. (ng/cm2), and ΣPbanth. (µg/cm2) for the two hummock cores were 0.744 and 0.773, ≈ 130 and 130 (ng/cm2), and ≈ 159 and 138 (µg/cm2), respectively. The values for Sargent Mountain Pond were 0.411, 269, and 72, respectively. Hummock cores agree closely with the lake sediment core with respect to timing of maximum accumulation rates which occurred in the 1970s;Background atmospheric deposition rates of Hg and Pb to coastal Maine appear to have been about 2.5 to 3 ng/cm2/yr and <0.2 µg/cm2/yr, respectively. Atmospheric deposition of Hg and Pb increased to as much as 20 ng/cm2/yr and 2 µg/cm2/yr, respectively, by the 1970s and has decreased since then. Probably more than 50% of the Hg and Pb are deposited in dry and occult deposition.

Mechanisms of episodic acidification in low-order streams in Maine, USA.

Five factors contribute to episodic depressions in pH and ANC during hydrologic events in low-order streams in Maine: (1) increases of up to 50 microeq litre(-1) NO3; (2) increases of up to 75 microeq litre(-1) organic acidity; (3) increases of as much as 0.3 in the anion fraction of SO4; (4) as much as 100 microeq litre(-1) acidity generated by the salt-effect in soils; and (5) typically < or = 40% dilution by increased discharge. In conjunction with increased discharge, factors 1, 2 or 4 appear necessary to depress pH to less than 5.0. The chemistry of individual precipitation events is irrelevant to the generation of acidic episodes, except those caused by high loading of neutral salts in coastal regions. Increases in discharge, but not necessarily in dilution of solutes, in combination with the chronically high SO4 from atmospheric deposition, provide the antecedent chemical conditions for episodic acidification. Differences in antecedent moisture conditions determine the processes that control output of either ANC or acidifying agents to aquatic systems.

The Bear Brook Watershed, Maine (BBWM), USA

The Bear Brook Watershed Manipulation project in Maine is a paired calibrated watershed study funded by the U. S. EPA. The research program is evaluating whole ecosystem response to elevated inputs of acidifying chemicals. The project consists of a 2.5 year calibration period (1987-1989), nine years of chemical additions of (NH4)2SO4 (15N- and 34S-enriched for several years) to West Bear watershed (1989-1998), followed by a recovery period. The other watershed, East Bear, serves as a reference. Dosing is in six equal treatments/yr of 1800 eq SO4 and NH4/ha/yr, a 200% increase over 1988 loading (wet plus dry) for SO4 and 300% for N (wet NO3 + NH4). The experimental and reference watersheds are forested with mixed hard- and softwoods, and have thin acidic soils, areas of 10.2 and 10.7 ha, and relief of 210 m. Thin till of variable composition is underlain by metasedimentary pelitic rocks and calc-silicate gneiss intruded by granite dikes and sills. For the period 1987-1995, precipitation averaged 1.4 m/yr, had a mean pH of 4.5, with SO4, NO3, and NH4 concentrations of 26, 14, and 7 μeq/L, respectively. The nearly perrenial streams draining each watershed have discharges ranging from 0 (East Bear stops flowing for one to two months per year) to 150 L/sec. Prior to manipulation, East Bear and West Bear had a volume weighted annual mean pH of approximately 5.4, alkalinity = 0 to 4 μeq/L, total base cations = 184 μeq/L (sea-salt corrected = 118 μeq/L), and SO4 = 100 to 111 μeq/L. Nitrate ranged from 0 to 30 μeq/L with an annual mean of 6 to 25 μeq/L; dissolved organic carbon (DOC) ranged from 1 to 7 mg/L but was typically less than 3. Episodic acidification occurred at high discharge and was caused by dilution of cations, slightly increased DOC, significantly higher NO3, and the sea-salt effect. Depressions in pH were accompanied by increases in inorganic Al. The West Bear catchment responded to the chemical additions with increased export of base cations, Al, SO4, NO3, and decreased pH, ANC, and DOC. Silica remained relatively constant. Neutralization of the acidifying chemicals occurred dominantly by cation desorption and mobilization of Al.

CAN SITE‐SPECIFIC TRENDS BE EXTRAPOLATED TO A REGION? AN ACIDIFICATION EXAMPLE FOR THE NORTHEAST

In the absence of true regional data on changes in the acid/base status of lakes in the northeastern United States, we explore the possibility of using site-specific trends information from a judgment sample of lakes to assess the efficacy of the Clean Air Act Amendments. A meta-analytical technique is used to combine trends results from 44 Long-Term Monitoring (LTM) lakes in the Northeast for the period 1982–1994, with the goal of producing estimates of overall trends in the region. The lakes are subdivided into subpopulations (High ANC, Intermediate Till Drainage, Thin Till Drainage and Perched Seepage lakes) on the basis of their expected response to changes in acidic deposition, and they appear to represent the most acid-sensitive of these lake classes well. While the overall tendencies in the trends are as expected (e.g., most of the recovery is observed in the most sensitive subpopulations), there is significant trend heterogeneity among the lakes within most of the subpopulations; this heterogeneity prohibits the summarizing of trends at the regional level (i.e., for all of the Northeast). This heterogeneity is explained by differences in the responses of lakes in two subregions (Adirondacks vs. New England), and we present trends results separately for each subpopulation within these two subregions. All subpopulations in both subregions showed decreasing trends in sulfate concentrations, probably a reflection of decreasing trends in sulfur deposition in the region. Few trends in nitrate concentrations were observed. Recovery (as evidenced by increasing trends in acid-neutralizing capacity) was evident in Thin Till and Intermediate Till Drainage lakes in New England, but not in the Adirondacks. Most groups of lakes exhibited downward trends in base cations (Σ[Ca2+ + Mg2+ + Na+ + K+]); the magnitudes of these trends were always greater in Adirondack lakes than in similar New England lakes. This suggests that the depletion of soil cation pools in the Adirondacks may be responsible for some of the differences in recovery between Adirondack and New England lakes. While export of base cations may be the key difference producing different trends results in the two subregions, the site-specific nature of the trends, and their possible lack of regional representation, should be considered in interpreting the overall results.

Interpretation of210Pb profiles and verification of the CRS dating model in PIRLA project lake sediment cores

This paper reports results and analysis of210Pb-activity measurements in 51 lake-sediment cores from 32 lakes in the four PIRLA (Paleoecological Investigations of Recent Lake Acidification) project regions (Adirondack Mountains [New York], Northern New England, Northern Florida, and the Northern Great Lakes States). General application of the Constant Rate of Supply (Constant Flux) model for210Pb dating is valid for lakes in the PIRLA study, although application of the model is equivocal in a few lakes.210Pb inventories and profiles are replicable among closely spaced cores within a lake. Specific210Pb activity in surface sediments is negatively correlated with bulk sediment accumulation rate in seepage lakes, but not in drainage lakes. Drainage lakes with lower pH have lower unsupported210Pb inventories in sediments, but the relationship does not occur in seepage lakes.210Pb profiles in only seven of the cores, all from either the Adirondacks or the northern Great Lakes states, exhibit exponential decay curves. Deviations from an exponential profile include a flattening of the profile in the top few cm or excursions of one or a few measurements away from an exponential curve.210Pb dates typically agree with other chronostratigraphic markers, most of which are subject to greater uncertainty. Several hypotheses, including sediment mixing, hydrologic regime, sediment focusing, and acidification, are proposed to explain variation of210Pb distribution among lakes and regions. Hydrologic factors exert control on unsupported210Pb inventories in PIRLA lakes, and there is a strong focusing effect in drainage lakes but a weak focusing effect in seepage lakes.

Underestimation of dry deposition by throughfall in mixed northern hardwood forests

Abstract Throughfall and catchment mass balance approaches were used to estimate dry deposition inputs of SO 4 and Cl to the Bear Brook in Maine for the period November 1988–November 1992. For throughfall, dry deposition of SO 4 and Cl averaged 320 and 261 eq ha −1 year −1 , respectively, or 0.9 and 1.5 times the input in precipitation, respectively. The equivalent values for the catchment gave dry deposition of SO 4 and Cl as 616 and 380 eq ha −1 year −1 , respectively, or 1.7 and 2.2 times the input in precipitation, respectively. The lower estimates obtained by the canopy mass balance method are primarily attributed to an underestimation of the importance of conifers in intercepting dry and occult deposition, particularly in mixed hardwood-softwood canopies such as the Bear Brook. Although net throughfall is commonly used to estimate dry and occult deposition in forested catchments, dry deposition may be underestimated, particularly in uneven-aged, mixed hardwood-softwood forests characterized by occasional superdominant conifers. The uneven canopy architecture of such forests increases its aerodynamic roughness and thus its efficacy in capturing dry deposition. Mixed-species forests, resulting from both natural and anthropogenic disturbance, are a common component of the landscape throughout much of New England.

Stable sulfur isotopes of sulfate in precipitation and stream solutions in a northern hardwood watershed

Stable S isotopes of SO{sup 2{minus}}{sub 4} in precipitation, throughfall, and stream water solutions in a northern hardwood watershed (Bear Brook Watershed, Maine) were examined to determine sources of stream SO{sup 2{minus}}{sub 4} and to identify watershed processes that may affect atmospherically deposited SO{sup 2{minus}}{sub 4} prior to reaching the streams. Similarity among temporal patterns in {delta}{sup 34}S of precipitation, throughfall, stream SO{sup 2{minus}}{sub 4} indicate that the {delta}{sup 34}S values of stream SO{sup 2{minus}}{sub 4} are controlled principally by atmospheric deposition. Lack of correlation between {delta}{sup 34}S of stream SO{sup 2{minus}}{sub 4} and stream (SO{sup 2{minus}}{sub 4}{sub total}), elevational position, and stream discharge supports the interpretation that fractionation of stable S isotopes within the watershed appears to have, at most, a minor influence on {delta}{sup 34}S of stream SO{sup 2{minus}}{sub 4}.

The History of Atmospheric Deposition of Cd, Hg, and Pb in North America: Evidence from Lake and Peat Bog Sediments

Lake and ombrotrophic peat bog sediments record increases in the concentrations and accumulation rates of Cd, Hg, and Pb for most of temperate North America for the last 100 years. These increases are largely related to the burning of coal, smelting of nonferrous metals, the transportation industry, and the industrial production of chlorine.

Response of the West Bear Brook Watershed, Maine, USA, to the addition of (NH4)2SO4: 3-year results

Abstract Addition of 1800 eq (NH 4 ) 2 SO 4 ha −1 year −1 for 3 years to a catchment that originally produced low acid neutralizing capacity (ANC) runoff has resulted in significant episodic acidification during high discharge and slight chronic acidification during base flow. These relationships imply that the response to chemical amendments is occurring largely in upper soil horizons and shallow flow paths. The principal responses in stream water chemistry include: elevated volume-weighted concentrations of NO 3 , SO 4 , base cations, Al, and H, coupled with decreased ANC. The export of dissolved organic carbon may have decreased slightly. Export of SiO 2 has not changed significantly; thus the increase in base cations is interpreted to be largely from desorption from soil. Effects of the manipulation increase from Year 1 to Year 2 to Year 3. Percentage retention of the added SO 4 is declining; retention of NH 4 remains at 100% while export of NO 3 is increasing with time, implying nitrogen saturation and an important change in N dynamics. The export of base cations and Al continues to increase.