Böhlke John Karl, Michel Robert L
U.S. Geological Survey, 431 National Center, Reston, VA 20192, USA.
Sci Total Environ. 2009 Jul 1;407(14):4363-77. doi: 10.1016/j.scitotenv.2009.02.007. Epub 2009 Mar 4.
Watershed mass balances for solutes of atmospheric origin may be complicated by the residence times of water and solutes at various time scales. In two small forested headwater catchments in the Appalachian Mountains of Virginia, USA, mean annual export rates of SO(4)(=) differ by a factor of 2, and seasonal variations in SO(4)(=) concentrations in atmospheric deposition and stream water are out of phase. These features were investigated by comparing (3)H, (35)S, delta(34)S, delta(2)H, delta(18)O, delta(3)He, CFC-12, SF(6), and chemical analyses of open deposition, throughfall, stream water, and spring water. The concentrations of SO(4)(=) and radioactive (35)S were about twice as high in throughfall as in open deposition, but the weighted composite values of (35)S/S (11.1 and 12.1x10(-15)) and delta(34)S (+3.8 and +4.1 per thousand) were similar. In both streams (Shelter Run, Mill Run), (3)H concentrations and delta(34)S values during high flow were similar to those of modern deposition, delta(2)H and delta(18)O values exhibited damped seasonal variations, and (35)S/S ratios (0-3x10(-15)) were low throughout the year, indicating inter-seasonal to inter-annual storage and release of atmospheric SO(4)(=) in both watersheds. In the Mill Run watershed, (3)H concentrations in stream base flow (10-13 TU) were consistent with relatively young groundwater discharge, most delta(34)S values were approximately the same as the modern atmospheric deposition values, and the annual export rate of SO(4)(=) was equal to or slightly greater than the modern deposition rate. In the Shelter Run watershed, (3)H concentrations in stream base flow (1-3 TU) indicate that much of the discharging ground water had been deposited prior to the onset of atmospheric nuclear bomb testing in the 1950s, base flow delta(34)S values (+1.6 per thousand) were significantly lower than the modern deposition values, and the annual export rate of SO(4)(=) was less than the modern deposition rate. Concentrations of (3)H and (35)S in Shelter Run base flow, and of (3)H, (3)He, CFC-12, SF(6), and (35)S in a spring discharging to Shelter Run, all were consistent with a bimodal distribution of discharging ground-water ages with approximately 5-20% less than a few years old and 75-95% more than 40 years old. These results provide evidence for 3 important time-scales of SO(4)(=) transport through the watersheds: (1) short-term (weekly to monthly) storage and release of dry deposition in the forest canopy between precipitation events; (2) mid-term (seasonal to interannual) cycles in net storage in the near-surface environment, and (3) long-term (decadal to centennial) storage in deep ground water that appears to be related to relatively low SO(4)(=) concentrations in spring discharge that dominates Shelter Run base flow. It is possible that the relatively low concentrations and low delta(34)S values of SO(4)(=) in spring discharge and Shelter Run base flow may reflect those of atmospheric deposition before the middle of the 20th century. In addition to storage in soils and biota, variations in ground-water residence times at a wide range of time scales may have important effects on monitoring, modeling, and predicting watershed responses to changing atmospheric deposition in small watersheds.
大气源溶质的流域质量平衡可能会因水和溶质在不同时间尺度上的停留时间而变得复杂。在美国弗吉尼亚州阿巴拉契亚山脉的两个小型森林源头集水区，硫酸根（SO₄²⁻）的年平均输出速率相差两倍，大气沉降和溪水中硫酸根（SO₄²⁻）浓度的季节变化并不同步。通过比较³H、³⁵S、δ³⁴S、δ²H、δ¹⁸O、³He、CFC - 12、SF₆以及对开放沉降物、穿透水、溪水和泉水的化学分析来研究这些特征。穿透水中硫酸根（SO₄²⁻）和放射性³⁵S的浓度大约是开放沉降物中的两倍，但³⁵S/S的加权综合值（11.1和12.1×10⁻¹⁵）以及δ³⁴S（+3.8和+4.1‰）相似。在两条溪流（避难所溪、磨坊溪）中，高流量期间的³H浓度和δ³⁴S值与现代沉降物相似，δ²H和δ¹⁸O值呈现出衰减的季节变化，并且³⁵S/S比值（0 - 3×10⁻¹⁵）全年都很低，这表明两个流域中大气硫酸根（SO₄²⁻）存在季节到年际的储存和释放。在磨坊溪流域，溪流基流中的³H浓度（10 - 13 TU）与相对年轻的地下水排放一致，大多数δ³⁴S值与现代大气沉降值大致相同，硫酸根（SO₄²⁻）的年输出速率等于或略大于现代沉降速率。在避难所溪流域，溪流基流中的³H浓度（1 - 3 TU）表明，大部分排放的地下水是在20世纪50年代大气核弹试验开始之前就已沉积，基流δ³⁴S值（+1.6‰）显著低于现代沉降值，硫酸根（SO₄²⁻）的年输出速率小于现代沉降速率。避难所溪基流中的³H和³⁵S浓度，以及排入避难所溪的一口泉水中的³H、³He、CFC - 12、SF₆和³⁵S浓度，都与排放地下水年龄的双峰分布一致，其中约5 - 20%的年龄小于几年，75 - 95%的年龄大于40年。这些结果为硫酸根（SO₄²⁻）通过流域的三个重要时间尺度提供了证据：（1）降水事件之间森林冠层中干沉降的短期（每周至每月）储存和释放；（2）近地表环境中净储存的中期（季节到年际）循环；（3）深层地下水中的长期（十年至百年）储存，这似乎与主导避难所溪基流的泉水排放中相对较低的硫酸根（SO₄²⁻）浓度有关。泉水排放和避难所溪基流中硫酸根（SO₄²⁻）相对较低的浓度和较低的δ³⁴S值可能反映了20世纪中叶之前大气沉降的情况。除了在土壤和生物群中的储存外，广泛时间尺度上地下水停留时间的变化可能对小流域中监测、模拟和预测流域对不断变化的大气沉降的响应具有重要影响。
