Hóa học acid-base của nước bề mặt thường trưng bày thay đổi đáng kể trong nội bộ và liên hàng năm. Biến đổi theo mùa trong nồng độ các thông số hoá học thường thay đổi nhiều hơn số tiền của quá trình axit hóa có thể xảy ra trong phản ứng để lắng đọng có tính axit. Biến đổi như vậy làm cho lượng của quá trình axit hóa và phản ứng phục hồi khó khăn, và cũng có thể làm phức tạp nỗ lực để đánh giá độ nhạy cảm với axit hóa chỉ dựa trên chỉ số. | 6 Episodic Acidification Background and Characteristics of Sensitive Systems The acid-base chemistry of surface waters typically exhibits substantial intra- and inter-annual variability. Seasonal variability in the concentration of key chemical parameters often varies by more than the amount of acidification that might occur in response to acidic deposition. Such variability makes quantification of acidification and recovery responses difficult and also complicates attempts to evaluate sensitivity to acidification based solely on index chemistry. The latter term is applied to chemical characterization data that correspond with periods when the chemistry is expected to be relatively stable. These are typically summer or fall for lakes and spring baseflow for streams. Lakes and streams exhibit short-term episodic decreases in ANC and often also pH usually in response to hydrological events such as snowmelt or rainfall. Periods of episodic acidification may last for hours to weeks and sometimes result in depletion of ANC to negative values with concurrent increases in potentially toxic inorganic Al in solution. Precipitation inputs to a watershed typically pass through the soil profile prior to reaching stream channels. The typical soil profile in acid-sensitive watersheds has lowest pH approximately 4 in upper organic soil horizons increasing down the profile to pH greater than 6 at depth Norton et al. in press . Drainage water chemistry is generally somewhat reflective of conditions in the lower soil horizons and therefore generally has pH greater than 6. During high discharge snowmelt or rainfall events however flow routing favors water flowpaths through upper horizons. During such events drainage water chemistry therefore typically reflects the lower pH higher organic content and lower ANC of these upper soil horizons. This is one of the major reasons why many surface waters are lower in pH and ANC during hydrological
