Environ. Sci. Technol. 1991, 25, 2024-2030
Chemistry of a Near-Shore Lake Region during Spring Snowmelt Chad P. Gubala" and Charles T. Driscoll Department of Civil and Environmental Engineering, 220 Hinds Hall, Syracuse University, Syracuse New York 13244-1 190
Robert M. Newton Department of Geology, Smith College, Northampton, Massachusetts 01603
Carl L. Schofleld Department of Natural Resources, Fernow Hall, Cornell University, Ithaca, New York 14853
The thermal and chemical characteristics of a spawning area within a base-neutralized lake were monitored through the spring snowmelt of 1989. While the greater part of the lake remained relatively neutral, severe pH (G.0) and acid neutralizing capacity (ANC; €0 kequiv L-l) depressions were observed in the upper (1-1.5 m) portion of the water column. Thermal stratification of the near-shore area and a combination of groundwater and surface runoff events controlled the hydrologic response and the changes in water chemistry that occurred during the acid pulse. Dilution of basic cations (SBC) combined with N03-eluted from the snowpack and/or soils resulted in low ANC, low pH runoff affecting the near-shore area. Increases in monomeric A1 and H+ were coincident with the ANC depression, creating a harmful environment for fish fry emerging from the spawning zone.
Introduction The end of the winter (late March to early April) in the Adirondack region of New York State often coincides with snowmelt and marked declines in lake water and streamwater acid neutralizing capacity (ANC) (I, 2). The degree of ANC depression and associated biotic effects depends largely upon the surficial geology and morphometry of lake/watershed systems (2-4). Most lakes in the Adirondacks are ice-covered and exhibit thermal stratification during peak spring discharge resulting in noticeable pH and ANC depression during snowmelt (2). Episodic acidification and the degree of ANC depression of surface waters during spring are controlled in part by hydrologic flow paths (4). In many parts of the Adirondack region of New York State surficial deposits are shallow and neutralization of acidic snowmelt is incomplete during transport through the terrestrial environment. Small, high-elevation Adirondack lakes with relatively large watersheds (watershed to lake surface area ratio of >6) may be particularly sensitive to episodic acidification since they tend to have larger snowpacks, shallower deposits of glacial till, and faster flushing rates ( 4 , 5 ) . As the hydrology returns to "low-flow'' conditions following snowmelt, acidic precipitation may be neutralized more completely as the hydrologic residence time within the watershed increases and runoff contacts deeper, base-rich geologic material. The biological consequences of springtime ANC and pH depressions in Adirondack lakes are profound (6). In particular, the thermal and chemical environments of lake spawning areas are critical to the survival of early life stages of fish. The declines in pH, ANC, and Ca2+and increases in monomeric A1 that coincide with spring snowmelt are potentially lethal to young fish inhabiting the near-shore zone. Several laboratory and field studies (7-9) have noted the potential significance of episodic ~
* Present address: ManTech Environmental Technology, Inc., 200 SW 35th St., Corvallis, OR 97333.
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Environ. Sci. Technol., Vol. 25, No. 12, 1991
Table I. Lake and Watershed Characteristics of Woods Lake basin area, km2 lake surface area, km2 watershed area, km2 maximum basin relief, m forest cover, % av thickness of surficial deposits, m lake volume, lo5 m3 maximum lake depth, m mean lake depth, m lake surface altitude, m lake area/watershed area av lake water residence time, days
2.07 0.23 1.84 122.0 98.1 1.9
8.14 11.6 3.5 606
0.13 174
acidification in near-shore areas for survival of early life history stages of brook trout (Saluelinus fontinalis) and lake trout (Salvelinus namaycush). However, the actual consequences of these events on egg and fry survival rates of naturally reproducing populations have not yet been firmly established. Logistical constraints have limited the intensive collection of samples from near-shore regions of Adirondack lakes during snowmelt episodes. Observations from lake inlets, water columns, and outlets have been made (I), but these may not be indicative of the chemical conditions of important spawning areas that are affected by a combination of hydrologic sources (e.g., groundwater, streamwater, snowmelt water). In response to this problem, a device was constructed and employed to explicitly monitor the chemical and thermal conditions of a near-shore spawning area within an experimental lake in the Adirondacks (10). The water quality data collected with this near-shore sampling device are examined in this study and compared to inlet, outlet, and in-lake observations to assess the relative contribution of various hydrologic inputs (i.e., groundwater/porewater, surface runoff') in controlling the chemical conditions of the near-shore area. Moreover, because temporal patterns in pelagic and/or outlet waters are often used to assess the magnitude of episodic acidifiction, the results from this study are also used to illustrate the limitations of interpreting results from these sampling areas in the absence of in-lake data.
Site Description Woods Lake (42'52' N, 71'58' W), located in the west-central region of the Adirondack Park of New York State, is typical of chronically acidic lake/watershed systems in that area (refs 3, 4, and 11;Figure la-c, Table I). The lake is small and shallow with a short lake water residence time (ref 12; 174 days). It is surrounded by a watershed characterized by shallow deposits of glacial till and acidic soils (13). The 110-140 cm of acidic precipitation that enters the Woods Lake watershed each year (14) is only partially neutralized before entering the lake (refs 4, 15, and 16; Table I). Acidification of inlet and lake
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(d) (C) Flgure 1. Woods Lake (a) location. (b) watershed, (c) bathymelry, andI (d) sampling site location maps. waters is particularly severe during spring snowmelt (March-April), when approximately 25% of the annual hydrologic input enters the lake. During this period, hydrologic flow paths are altered by the routing of water through upper acidic soil horizons resulting in limited neutralization of drainage water (4, 17,18).
Methods Woods Lake was treated by addition of CaCO, in 1985 and again in 1987 to mitigate acidic conditions (19). Since 1985, the lake generally had a positive ANC, with the exception of the upper few meters of the water column during the periods of spring snowmelt (20). A description of these manipulations and subsequent results are detailed elsewhere (12,1%29). While the ANC of Woods Lake had been artificially increased by CaCO, addition, the findings of this paper may be directly relevant to Adirondack lakes with naturally low mean summertime ANC values (25-100 requiv L-') that are particularly susceptible to episodic acidification during spring snowmelt (2).
The data used in this analysis were derived primarily from the Lake Acidification Mitigation Program (LAMP) and the Experimental Watershed Liming Study (EWLS). Inlet, deep-water column, outlet, and near-shore lake water sampling was conducted as part of the EWLS pretreatment program at various intervals through a period from June of 1988 to May of 1989 (Figure Id). Near-shore sampling was accomplished by means of an episodic event sampler (EES) (IO). A set of protocols has been used for the chemical analysis of all aqueous samples and is detailed elsewhere (27). One parameter of particular interest in this study is AI. Inorganic species of aqueous AI are thought to be toxic to fish (30). Samples collected were analyzed for inorganic monomeric A1 and organic monomeric A1 by the fractionation procedure developed by Driscoll (31). In acidic waters of Woods Lake, inorganic monomeric AI comprises most of the solution AI (27,281. Therefore, the AI monitoring data are generally reported as monomeric Al. However, to determine the equivalence of A1 (AI"*) in Environ. Sci. Technol., Vol. 25, No. 12. 1991 2025
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Figure 3. Inlet stream discharge and relative groundwater stage at Woods Lake during spring 1989.
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Flgure 2. Pelagic (a) pH, (b) ANC, and (c) temperature isopleths at Woods Lake during sprlng 1989.
solution, values of inorganic monomeric Al, pH, F, SO:-, and temperature were used as input to a chemical equilibrium model to calibrate the speciation of A1 (32).
Results Prior to the application of CaC03, marked declines in pH or ANC were not evident at a water column (pelagic) sampling station in Woods Lake (12-m total depth), as the lake was already acidic (20). However after base treatment, pH and ANC depressions were evident to a depth of 1-2 m in the lake during snowmelt periods (ref 20; Figure 2a,b). Snowmelt acidification was restricted to the surface of the lake because acidic meltwater enters under ice cover at a temperature of less the 1 "C and has a lower density than that of the lower waters (3 "C; Figure 2c). As a result, this water migrates along the ice/water interface before exiting by the lake outlet. After ice-out and spring turnover, the acidity of the upper waters was lessened through mixing with the remainder of the lake. Finely resolved sampling of 2- and 3-m-depth near-shore regions during the snowmelt period of 1989 yielded a more detailed depiction of episodic acidification in a biotically important region of the lake. Several interconnected phenomena are apparent in these data. A knowledge of the hydrologic patterns during snowmelt is necessary to understand the temporal trends of the chemistry of the near-shore area. Groundwater stage began to increase in pulses in mid-March, a few days prior to the peak streamflow (Figure 3). This pattern is common for snowmelt hydrology within the Adirondacks, as relatively shallow deposits of glacial till (
