Reductions in acidic deposition following the 1990 Amendments to the Clean Air Act have led to improvements in the acid/base status of many freshwater bodies in the eastern United States, but such recovery has not been as widespread in Virginia mountain streams. In the current analysis, water chemistry trends are determined for 63 streams within Shenandoah National Park, Virginia, and surrounding national forests, where water samples have been collected on a quarterly basis from 1987 to 2019. Over this timeframe, a majority of the most acid-sensitive streams – those with watersheds underlain by siliciclastic bedrock – have experienced reductions in acid neutralizing capacity (ANC). This outcome, which differs from the modest recovery predicted by earlier modeling studies, is likely due to further depletion of soil base cations and perhaps increases in organic acids. Meanwhile, stream water pH has increased for all watershed bedrock types, averaging between 0.05 and 0.10 pH units per decade. High-flow sampling was conducted at three sites from 1992 to 2019, revealing an assessment of episodic acidification that is more encouraging than that of chronic acidification. For two of these sites, pH during high-flow conditions increased much more rapidly than pH during low-flow conditions. Steep reductions in sulfur deposition over the three decades since the 1990 Amendments have led to a “flushing” of sulfate from shallow soils, which has contributed to the improved acid/base status during episodes of high flow. The findings suggest that continued low acidic deposition rates will lead to further improvements in stream water quality, with the timescales of recovery dependent upon bedrock geology of the watersheds.

在1990年《清洁空气法》修正案之后，酸性沉积物的减少已导致美国东部许多淡水体的酸/碱状态得到改善，但这种回收在弗吉尼亚州的山区河流中并未得到广泛的应用。在当前的分析中，确定了弗吉尼亚州雪兰多厄国家公园内和周边国家森林中63条溪流的水化学趋势，从1987年至2019年每季度收集一次水样品。在此时间范围内，大多数酸敏感的水流-那些在硅质碎屑基岩衬托下流域的水流-酸中和能力（ANC）降低了。这一结果与早期建模研究预测的适度恢复有所不同，这可能是由于土壤碱性阳离子的进一步消耗，以及有机酸的增加。同时，所有分水岭基岩类型的溪水pH值均增加，每十年平均pH值介于0.05和0.10之间。从1992年到2019年，在三个地点进行了高流量采样，揭示了对情景酸化的评估比对慢性酸化的评估更令人鼓舞。对于这些站点中的两个站点，高流量条件下的pH值增长比低流量条件下的pH增长快得多。自1990年修正案以来的三十年中，硫沉积的急剧减少已导致硫酸盐从浅层土壤中“冲出”，这有助于改善高流量时期的酸/碱状态。