THE CHEMICAL RESPONSE OF ACIDIC LAKES TO CALCIUM CARBONATE TREATMENT
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ABSTRACT The effects of atmospheric deposition on acid-sensitive watersheds have become increasingly apparent. Lake/watershed systems that cannot completely neutralize strong acid inputs are characterized by low pH values and elevated concentrations of trace metals. Populations of fish and other aquatic biota are endangered by this phenomenon. One approach used to mitigate the effects of surface water acidification is direct application of calcium carbonate (CaCO3). Through the Lake Acidification Mitigation Project (LAMP), we investigated the chemical response of acidic lakes to base treatment. Immediately following base application, there was a marked increase in pH, acid neutralizing capacity (ANC), calcium and dissolved inorganic carbon (DIC) associated with the dissolution of calcium carbonate in the treated lakes. The large increase in pH was attributed to the low dissolved inorganic carbon concentrations in the water column prior to liming and limited pH buffering capacity. During the four week period following base application the intrusion of atmospheric carbon dioxide facilitated additional dissolution of the remaining suspended calcium carbonate. This dissolution was accompanied by a gradual decrease in pH (to below 8) and increases in acid neutralizing capacity, dissolved calcium and dissolved inorganic carbon concentrations within the lakes. Concentrations of trace metals in the upper waters decreased about one order of magnitude due to reduced solubility at circumneutral pH values. The rate of reacidification was directly related to the hydrologic input to the lake. Elevated discharge during the fall coinciding with a completely mixed water column greatly facilitated reacidification. Although discharge was very high during spring snowmelt, ice cover and inverse thermal stratification restricted intrusion of acidic meltwater to the ice-water interface. The rapid rate of reacidification was largely attributed to the shallow depth and short hydraulic retention of these lakes.Keywords:
Acid neutralizing capacity
Total inorganic carbon
Snowmelt
Snowpack accumulates water and acid anions from atmospheric deposition. During the snowmelt, the rapid release of acid anions can cause an episodic pH depression in surface water. The potential for the snowmelt to acidify the surface water of the Sierra Nevada was investigated at the Eastern Brook Lane watershed. The acidification potential of snowpack is determined by the anion release rate, which is difficult to measure directly. Limited measurements of snowpack were combined with a mathematical model to calculate the solute flux in snowmelt runoff. In the springs of 1984–1986, the snowpack accumulated an average of 40 cm of water. It stored 80 equivalents per hectare of acidic anions. The melting rate of the snowpack was 0.5 cm of water equivalent per day and the anion release rate was 1.4 equivalents per hectare per day. This anion release rate was one‐tenth that of the Woods Lake watershed in the Adirondacks, New York.
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Small watersheds in the Canadian Prairies are characterized by seasonally disconnected hydrologic networks whereby stream channels are hydrologically connected during snowmelt but have disconnected reaches throughout the remainder of the year. Snowmelt is the most significant hydrological event in the Canadian Prairies, yet few studies have investigated the role of snowmelt in the nutrient budget of prairie streams. We quantified hydrologic and nutrient dynamics during snowmelt for ten agricultural subwatersheds distributed along a gradient of human activity in the Red River Valley, Canada, to evaluate the timing of nitrogen (N) and phosphorus (P) export. Elevated concentrations of total P (TP) and total N (TN) were observed during the snowmelt peak, with maximum concentrations reaching 3.23 mg TP L(-1) and 18.50 mg TN L(-1). Dissolved P and N dominated the total nutrient pool throughout snowmelt, likely due to reduced erosion and sediment transport resulting from the combination of the flat topography, frozen soil and stream banks, and gradual snow cover melt. Significant correlations were observed between snowmelt N load (r=0.91; p<0.05) and both agricultural land cover and fertilizer usage, with a weaker correlation between snowmelt P load (r=0.81; p<0.05) and agricultural area. Our results showed that snowmelt plays a key role in nutrient export to prairie aquatic ecosystems and this may have serious impacts on downstream ecosystems. Land use management practices need to consider the snowmelt period to control nutrient loads to Lake Winnipeg and other waterbodies in the Great Plains.
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Sediment–water interface
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Researchers compiled data on 168 low-ANC lakes in the Sierra Nevada, California. They then formulated an Episodic Event Model (EEM) that was used to simulate lake ANC during snowmelt episodes. During snowmelt the surface water ANC is near zero due to dilution from snowmelt. During this period acidification can occur should acidic rain enter the watershed. Using the EEM to simulate a 20-day runoff event at snowmelt, the model results indicate that about 80% of the lakes considered would reach an ANC less than 40 micro eq/l. There are insufficient spatial or temporal data on the chemistry or amount of deposition to high-elevation areas in the Sierra. Also, the model assumes no interaction between the watershed and incoming deposition.
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The objectives of this study are to adopt a snowmelt model for coupling a rainfall-runoff model and to study snowmelt effects for long-term runoff analysis on the northeast mountaneous area in Korea. The NWS temperature-index snowmelt model was selected and tested on the 1,059+,6 km Naerinchen basin. It can be observed that the time variations of the computed areal extents of snow cover from the model are well agreement with those of the observe station snowfall records on the Inje meteorological station. It is also evident that the computed soil water contents and river flows indicate quite different behaviors with or without snowmelt model. It is concluded that the snowmelt model works well and the snowmelt effects for multi-decadal river flow computations are important on the study area.
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본 연구는 준 분포형 장기유출모형인 SWAT모형을 이용하여 융설모의에 따른 유출 및 수문성분의 영향을 분석하고자 하는데 그 목적이 있다. 대상유역으로는 충주댐 유역을 선정하였으며 융설 매개변수를 산정하였다. 충주댐지점에서의 관측유량과 융설모의 전후의 모의유량을 비교한 결과 융설모의를 수행한 유출거동이 관측치와 유사하게 모의되었으며, 특히 3, 4월에 융설 영향이 큰 것으로 나타났다. 상류유역 2개 지점에 대해서도 유사한 결과를 보였다. 또한, 융설 모의시 표고밴드 설정에 따른 영향을 분석하였으며, 융설 고려에 따른 각 수문성분들이 시간적, 양적으로 다른 거동을 보이는 것을 제시하였다. 다만, 융설 모의시 융설 발생 임계온도 전후의 온도변화에 따라 유출량이 민감하게 반응하는 문제가 도출되었으며, 이는 향후 연구를 통해 개선되어야 할 것으로 판단된다. The objective of this study is to evaluate the snowmelt effects on the hydrological components, especially on the runoff, by using the soil water assessment tool(SWAT) which is a continuous semi-distributed long term rainfall-runoff model. The model was applied to the basin located in the upstream of the Chungju Dam. Some parameters in the snowmelt algorithm were estimated for the Chungju basin in order to reflect the snowmelt effects. The snowmelt effects were assessed by comparing the simulated runoff with the observed runoff data at the outlet of the basin. It was found out that the simulated runoff with considering the snowmelt component matches more satisfactorily to the observed one than without considering snowmelt effect. The simulation results revealed that the snowmelt effects were noticeable on March and April. Similar results were obtained at other two upstream gauging points. The effect of the elevation bands which distribute temperature and precipitation with elevation was analyzed. This study also showed that the snowmelt effect significantly affects the temporal distribution as well as quantity of the hydrological components. The simulated runoff was very sensitive to the change of temperature near the threshold temperature which the snowmelt can occur. However, the reason was not accounted for this paper, Therefore, further analyses related to this feature are needed.
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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 concentrations during early snowmelt; these were accompanied by increases in base cations, primarily Ca 2§ 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 and SO42 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.
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