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.     

Precipitation and streamwater chemistry from undisturbed watersheds in the cascade mountains of Oregon

Long-term records of precipitation and streamwater chemistry are rare; such records from forested watersheds relatively free of acidic deposition are even rarer. Precipitation and streamwater chemistry have been measured on two undisturbed forested watersheds at the H. J. Andrews Experimental Forest located on the western slopes of the Cascade Mountains of Oregon. Data from one watershed spans the period 1973–1985, the other 1969–1985. The mean annual pH of precipitation was 5.5 with a range of 4.7 to 6.0. HCO₃ ^? was the dominant anion; Ca²⁺ and Na⁺ were the dominant cations. The mean annual pH of streamwater was 7.3, and was dominated by HCO₃ ^? and Ca²⁺. These data contrast sharply with data from other calibrated watersheds in the north Cascade Mountains of Washington and British Columbia, and with data from New Hampshire and North Carolina where pH of precipitation averages 4.14 and 4.43, respectively, with SO₄ ^2? the dominant anion.     

Changes in water quality in the Laflamme Lake Watershed Area,Canada

The Laflamme Lake Watershed Area is located in a sensitive region on the Canadian Shield and is subjected to wet atmospheric loading between 17 and 25 kg ha^?1 yr^?1. From 1981 to 1988, the level and fluctuations of the atmospheric deposition of acidifying substances has led to various responses in the water chemistry of headwater lakes in the area. The general trend in atmospheric inputs is a gradual increase of acidifying substances from 1981 to 1985 followed by a 2 yr decrease then a return to previous values. In the two lakes with almost no alkalinity acidification has occured throughout the 1983 to 1988 period. In the four lakes with slightly higher alkalinity values, a reversal in acidification is seen when atmospheric loading decreased in 1986. Along with the interannual trends, seasonal variability to acidification occurs with sensitivity of surface waters being highest during spring melt. Sensitivity to acidification can also be altered by watershed processes and in the Laflamme Lake Watershed, soil processes are effective in altering the acidity of precipitation before it reached the lake. In this watershed, wet atmospheric inputs of H⁺ and NO₃ ^? are larger than surface water outputs while the reverse occurs for Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^? and SO₄ ^2?.     

Litterfall Mercury in Two Forested Watersheds at Acadia National Park, Maine, USA

Litterfall can be an important flux of mercury (Hg) to soils in forested landscapes, yet typically the only available data to evaluate Hg deposition is from precipitation Hg monitoring. Litterfall was collected at 39 sampling sites in two small research watersheds, in 2003 and 2004, and analyzed for total Hg. Four vegetation classes were designated in this study as hardwoods, softwoods, mixed and scrub. The mean litter Hg concentration in softwoods (58.8 ± 3.3 ng Hg g^?1 was significantly greater than in mixed (41.7 ± 2.8 ng Hg g^?1 and scrub (40.6 ± 2.7 ng Hg g^?1, and significantly lower than in hardwoods (31.6 ± 2.6 ng Hg g^?1. In contrast, the mean weighted litter Hg flux was not significantly different among vegetation classes. The lack of a significant difference in litter Hg flux between hardwoods and softwoods was attributable to the large autumnal hardwood litter Hg flux being balanced by the higher softwood litter Hg concentrations, along with the higher chronic litterfall flux throughout the winter and spring in softwoods. The estimated annual deposition of Hg via litterfall in Hadlock Brook watershed (10.1 μg m^?2 and Cadillac Brook watershed (10.0 μg m^?2 was greater than precipitation Hg deposition and similar to or greater than the magnitude of Hg deposition via throughfall. These results demonstrate that litterfall Hg flux to forested landscapes can be at least as important as precipitation Hg inputs.     

Acid precipitation and surface-water vulnerability on the western slope of the high colorado rockies

Precipitation and surface waters in a high-elevation watershed on the western slope of the Colorado Rockies were studied over a three-year period. The volume-weighted average pH for summer events was 4.61, for winter events was 5.11 and for the entire period was 4.91. Within the watershed, both low- and medium-alkalinity surface waters exist. Weekly alkalinity measurements ranged from 10 to 70 μeq L^?1 in the former and 100 to 900 μeq L^?1 in the latter.     

Primary production in a eutrophic acid lake

Total P concentrations, chlorophyll concentrations, and phytoplankton production were investigated bi-weekly in Tibbs Run Lake, Monongalia County, West Virginia, from March 1977 to March 1978. Mean H⁺ concentration in the lake was 25.1 μeq 1^?1 (pH 4.6). The acidic condition of the lake is attributed to inputs of acid via precipitation (mean H⁺ concentration of the bulk precipitation was 79 μeq 1^?1, pH 4.1), and the low buffering capacity of the watershed (bedrock composition of sandstone). Effect of the watershed is shown by the net retention of imput of P (ca. 26%) and H⁺ (ca. 68%). Total P loading to the lake was 0.495 g P m^?2 yr^?1. The single inflow accounted for 95% of the total loading while bulk precipitation accounted for the remainder. Mean summer chlorophyll concentration was 22.2 mg m^?2. Phytoplankton production expressed volumetrically as aP-vol-x value was 9.78 mg C m^?3 h^?1. Regression analysis indicated that H⁺ do not affect chlorophyll concentrations or phytoplankton production but rather that P limits algal biomass. Trophic status of Tibbs Run Lake based on a P budget model, chlorophyll concentration, and volumetric production all indicate that the lake is meso-eutrophic.     

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.     

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.     

Polluted precipitation and the geochronology of mercury deposition in lake sediment of northern Minnesota

Elevated Hg levels in game fish from wilderness lakes in northern Minnesota led to the present study of sediment cores from two lakes to ascertain the source and history of Hg deposition. Natural background levels of Hg were found to range from 0.03 to 0.06 μg g^?1, with cultural levels as high as 0.16 μg g^?1. Reconstructed geochronologies reveal a dramatic two-fold increase in Hg flux, from 0.008 to 0.017 μg cm^?2 yr^?1, occuring after the year 1880, suggesting an anthropogenic influence. No industrial or geologic source of Hg is found in the study watersheds. The entire historical increase in Hg flux can be accounted for by atmospheric loading provided that 1/5 of all the Hg presently supplied to the watershed via precipitation is ultimately deposited in lake sediment. Hg levels in fish are not correlated with Hg levels in lake sediment, although there is a link to acid-sensitivity of lake water, amount of acid-neutralizing geologic material exposed in the watershed, and watershed area/lake volume ratio. Thermal stratification of lake water and a complexation-adsorption mechanism are proposed to account for variations in Hg levels observed in sediment collected from different sites.     

In-lake alkalinity generation by sulfate reduction: A paleolimnological assessment

The generation of alkalinity by SO₄ reduction and net storage of reduced S in lake sediments has been estimated from an analysis of sediment cores from 16 lakes in ME, VT, NY, MI, MN, and WY. The cores have been dated by ²¹⁰Pb. The rate of pre-1850 (background) storage of S in lake sediments suggests that alkalinity contribution to lake water from this process ranged from 0.2 to 9.3 geq L^?1, with an average of 4 geq L^?1, Background values are similar for all lakes and remain low in the WY lakes up to the present. Maximum alkalinity contributions recorded in sediment, from upper mid-west and eastern lakes, dated between 1850 and 1985 are between 0.4 and 33 geq L^?1, with a lake mean maximum of 9.9 geq L^?1, Significant increases in recent S storage only occur in eastern lakes. Average values for net S accumulation in the sediment of most lakes for post-1850 sediment are typically less than half of maximum values.     

Occurrence of salmonid parr and eel in relation to water quality in small streams on the west coast of Sweden

In the summer of 1983 twelve streams on the west coast of Sweden were electrofished, and water samples were collected. In three of the streams water sampling was continued throughout the winter of 1983–84. Although summer pH showed little variation between stations there were large differences in fish abundance. The abundances of salmonid parr and eel were correlated with the alkalinity, and were significantly greater at stations with summer alkalinity above 0.25 meq·1^1?1. The winter water sampling programme showed drastic and rapid drops in the alkalinity during the snow-melt. Electrofishing data compiled from 7 streams during 1956–84 showed decreasing parr abundances in small streams (catchment area <1000 km²) with low (<0.25 meq·1^?1) alkalinity. It is suggested that the negative effect on fish is mainly due to toxic forms of A1 and to a lesser extent on the direct toxicity of low pH levels.     

Variation in trace metal exports from small Canadian shield watersheds

Annual exports of Cu, Pb, and Cd were estimated for eleven headwater and two lake outflow streams in the District of Muskoka, Ontario, Canada. COCl₂-APDC coprecipitation coupled with anodic stripping voltammetry was used to determine metal concentrations. Concentrations of all three metals were similar to those reported from other temperate forested ecosystems, being usually less than 1 ug L^?1 with Cd undetectable (<25 ng L^?1) in many samples. There was limited evidence for a spring peak in metal concentrations associated with the snowpack melt. Annual export (mass per unit area per year) of each metal was calculated; variability between streams was small. Exports of Pb appear to be related to the organic content of the water. Copper export was correlated with watershed area, not with organic acids, implying that a different transport mechanism and possibly a different source are important for this metal. The exports of Pb and Cu were much lower in all streams than the estimated annual atmospheric metal depositions reported for this area in the literature. Terrestrial retention was higher than 95% for all catchments, while lake retention was slightly lower in some cases. Despite the small fractional loss of trace metal from the watersheds, terrestrial inputs can comprise a significant portion of the total metal load to typical lakes in this region because of size of the catchments relative to the lake areas.     

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.     

Acidification of Lake Annie,Highlands Co., FL

Lake Annie, a clear-water seepage lake in south-central Florida remote from significant pollution sources, was sampled by the USGS from 1966 until 1978, and by Battoe and others from 1978 until present. Since 1966, pH and bicarbonate alkalinity have decreased (pH from above 6.0 to about 5.1; bicarbonate from 5.0 to less than 0.5 mg L^?1), while specific conductivity and sulfate concentration have increased (cond. from 30 to 55 μS; sulfate from 3.0 to 8.0 mg L^?1). The chemical changes are apparently due to acidification from atmospheric deposition. There are no proximate sources of acidity and the stability of lake level indicates there has been no significant concentration or dilution of solutes. It is suggested that Lake Annie's acidification was a threshold phenomenon wherein, following depletion of the watershed's buffering capacity, acidification of the lake was rapid.     

Acid-base chemistry of high-elevation streams in the great smoky mountains

Longitudinal and temporal variations in water chemistry were measured in several low-order, high-elevation streams in the Great Smoky Mountains to evaluate the processes responsible for the acid-base chemistry. The streams ranged in average base flow ANC from ?30 to 28 μeq L^?1 and in pH from 4.54 to 6.40. Low-ANC streams had lower base cation concentrations and higher acid anion concentrations than did the high-ANC streams. NO₃ ^? and SO₄ ^2? were the dominant acid anions. NO₃ ^? was derived from a combination of high leaching of nitrogen from old-growth forests and from high rates of atmospheric deposition. Streamwater SO₄ ^2? was attributed to atmospheric deposition and an internal bedrock source of sulfur (pyrite). Although dissolved Al concentrations increased with decreasing pH in the study streams, the concentrations of inorganic monomeric Al did not follow the pattern expected from equilibrium with aluminum trihydroxide or aluminum silicate phases. During storm events, pH and ANC declined by as much as 0.5 units and 15 μeq L^?1, respectively, at the downstream sites. The causes of the episodic acidification were increases in SO₄ ^2? and DOC.     

Mercury Transport in a High-Elevation Watershed in Rocky Mountain National Park, Colorado

Mercury (Hg) was measured in stream water and precipitation in the Loch Vale watershed in Rocky Mountain National Park, Colorado, during 2001–2002 to investigate processes controlling Hg transport in high-elevation ecosystems. Total Hg concentrations in precipitation ranged from 2.6 to 36.2 ng/L and showed a strong seasonal pattern with concentrations that were 3 to 4 times higher during summer months. Annual bulk deposition of Hg was 8.3 to 12.4 μ g/m² and was similar to deposition rates in the Midwestern and Northeastern U.S. Total Hg concentrations in streams ranged from 0.8 to 13.5 ng/L and were highest in mid-May on the rising limb of the snowmelt hydrograph. Stream-water Hg was positively correlated with dissolved organic carbon suggesting organically complexed Hg was flushed into streams from near-surface soil horizons during the early stages of snowmelt. Methylmercury (MeHg) in stream water peaked at 0.048 ng/L just prior to peak snowmelt but was at or below detection (< 0.040 ng/L) for the remainder of the snowmelt season. Annual export of total Hg in Loch Vale streams ranged from 1.2 to 2.3 μ g/m², which was less than 20% of wet deposition, indicating the terrestrial environment is a net sink of atmospheric Hg. Concentrations of MeHg in stream water and corresponding watershed fluxes were low, indicating low methylation rates or high demethylation rates or both.     

Chemical and biological trends associated with acidic atmospheric deposition in the Rhode River watershed and estuary

The Rhode River estuarine/watershed system is a tributary of Chesapeake Bay located on the inner Atlantic Coastal Plain. Its soils are fine sandy loams. Bulk precipitation pH in the spring season declined from 4.95 in 1974 to 3.82 in 1981 and was 4.03 in 1985. The changes in pH of a forested primary stream were more related to changes in bulk precipitation pH than were the changes in pH of agricultural streams, reflecting the importance of other major terrestrial sources of acidity on agricultural systems. Surges in acidity and dissolved total Al concentration in primary (first order) streams reached extremes of pH 3.2 and 300 μg Al L^?1. Higher order streams were observed to have surges in acidity with pH minima below 5.0. Surges in acidity ocurred during accelerated groundwater percolation following storm events and did not coincide with surface runoff or snowmelt. One of the reasons why groundwater is more acidic than surface runoff is that the vegetation exchanges H30 ⁺ for alkaline cations in the soil and translocates these ions to the vegetational canopy. When it rains, subsequently, H₃0⁺ in the precipitation displace some of these alkaline cations from the canopy. The end result is that overland flow during storms is enriched in alkaline cations, while groundwater is enriched in H₃0⁺. Although the source of dissolved Al is dissolution of clay minerals with atomic ratios of Al to silicate of 1:l, 1:2, or 1:3, this ratio in stream water rapidly declined to 1:1200 due to loss of Al. On average, forest drainage was the most acidic, the highest in dissolved Al, and the lowest in Ca. Surges in acidity were most severe from pastureland, and next most severe from cropland. Total fluoride concentrations were high relative to Al from all three land uses. Rhode River spawning runs of Perca flavescens declined drastically from the early 1970s to essentially zero since 1981. Larval bioassays of acidity indicate negligible toxicity to Hyla crucifer, significant toxicity to Perca flavescens and drastic effects on Morone saxatilis at pH 5.0.     

Estimation of the Maximum Critical Load for Sulfur in South Korea

The maximum critical load of sulfur and its exceedance by the sulfur deposition of 1994–1997 were mapped for South Korea with a spatial resolution of 11 × 14 km using the steady-state mass balance method. The Korean soil and geological maps were used as basis for the estimations of the critical alkalinity leaching and the weathering rate of base cations. The normalized difference vegetation index data obtained from the Advanced Very High Resolution Radiometer (AVHRR) together with the observed primary productivity of plants were used for the estimation of the critical uptake of base cations. Wet deposition of the non-sea-salt base cations was derived from measured base cation concentrations in precipitation, precipitation rate and air concentration of total suspended particulate while dry deposition of base cations was estimated using the inferential technique using scavenging ratios. The predominant ranges of base cation weathering, uptake and deposition were estimated to be of 200 – 600 eq ha^?1 yr^?1, 200 – 400 eq ha^?1 yr^?1 and 400 – 600 eq ha^?1 yr^?1, respectively. Critical alkalinity leaching was mainly in the range of 1000 – 2000 eq ha^?1 yr^?1 due to relatively high value of precipitation runoff. Exceedance of sulfur critical load was found at 40 % of the ecosystems considered mainly in the southeastern part of Korea, and about 60 % of Korea ecosystems were sustainable against sulfur acidity loadings.     

Seasonal,annual and long-term variability in the water chemistry of a remote high mountain lake: Acid rain versus natural changes

Seasonal fluctuations as well as long-term trends in water chemistry were studied in Schwarzsee ob Sölden (Tyrol, Austria), an oligotrophic softwater lake situated at 2796 m a.s.l. The catchement is composed of granite, plagioclase and micaschists containing considerable amounts of sulphur, with little soil cover. The lake is ice covered for about nine months, during this time the deepest layers (>16m) become anoxic. During summer overturn, alkalinity (ALK) is lowest (?8 μeq l^?1) in the whole water column, whereas pH reaches its minimum (4.88) at the surface during snowmelt. A decrease of pH from 5.8 to 5.4 during winter is caused by CO₂ oversaturation, but deep water ALK increases to up to 130 μeq l^?1 due to in-lake ALK generation by reductive processes and base cation (BC) release. The seasonal pattern of ALK in SOS is driven by in-lake processes in winter, the snowmelting in spring and watershed processes and precipitation during summer. Since 1989 summer sulfate concentrations in SOS, originating mainly from the catchment, show a tendency to increase presumably caused by enhanced weathering. In contrast, SO₄ ^2? concentrations in other high mountain lakes which are dominated by atmospheric depositions show a decreasing trend. SOS is a good example for the complexity of interactions between catchment and in-lake processes which act at different time scales and depend on climate changes and atmospheric inputs.     

Empirical models for lake acidification in the upper Great Lakes Region

A large data base on inland lakes in the Upper Great Lakes Region (UGLR) was used to evaluate assumptions and relationships of empirical acidification models. Improved methods to calculate background alkalinity and background SO₄ ^2? are reported; SO₄ ^2? enrichment factors indicate that terrestrial SO₄ ^2? sources and watershed or lake sinks must be considered for site-specific background SO₄ ^2? estimates. Significant relationships were found between lake acidification estimated as change in SO₄ ^2? and precipitation acidity but not between changes in lake alkalinity and precipitation acidity in this lightly impacted region.