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Monitoring Surface Water Chemistry | Current TIME/LTM Regions | Map of Sites | Regional Trends

Temporally Integrated Monitoring of Ecosystems (TIME)

TIME was developed as a special study within Environmental Monitoring and Assessment Program (EMAP)-Surface Waters to track, in more detail, the trends in the acid relevant chemistry of particular classes of acid sensitive lakes in the northeast and streams in the mid-Appalachians. The TIME monitoring program is statistically based, thereby allowing population estimates to be developed. TIME relies on the base EMAP sample plus increased density in certain regions of the northeast of particular concern. Because they are probability surveys, EPA can provide an estimate of uncertainty or confidence in the estimates generated. These surveys are conducted during July-early September and the estimates they generate are specific to that time period. TIME consists of a synoptic survey of acid sensitive lakes in the northeast and a synoptic survey of acid sensitive streams in the mid-Atlantic. These surveys are probability surveys of 80 sites annually that allow statements of uncertainty to be associated with our estimates of status. TIME consists of probability surveys designed to answer questions like: "how many acidic lakes are there? What is the trend in the number or proportion of lakes which are acidic?". These can be answered for specific types of lakes, e.g., seepage vs. drainage lakes, or for specific regions, e.g., Adirondacks vs. New England uplands. Over sufficiently long time periods (12-20 years), we can also document trends in individual lakes or streams in the survey.

Long Term Monitoring Project (LTM)

The primary objective of LTM is to detect long-term trends in acid/base status of lakes and streams across a gradient of acidic deposition. The LTM network consists of a subset lakes and streams that are particularly sensitive to acidity with most site records extending back to the early 1980s. Sites are sampled 3 to 15 times per year with a higher proportion of sampling conducted in the spring which coincides with higher rates of runoff (e.g., snow melt) and higher stream flows. This information is used to characterize how the most sensitive of aquatic systems in each region are responding to changing deposition, as well as giving information on seasonal chemistry and episodic acidification. Aquatic effects of acidic deposition often exhibit seasonality (e.g., causing harm in the winter or spring when the more sensitive life stages of some fish species are present). In most regions, a small number of higher ANC (e.g., Gran ANC > 100 ueq/L) sites are also sampled, and help separate temporal changes due to acidic deposition from those attributable to other disturbances (e.g., climate change, landuse change) (Stoddard 2003).

To evaluate the acidity of surface waters in sensitive ecosystems the following regions have long-term data: The Adirondack Mountains, New England, the Northern Appalachian Plateau, the Ridge/Blue Ridge provinces, and the Upper Midwest (sampling has been discontinued). Trends are available from these sites of the chemical constituents as measured in surface water.

The primary chemical indicators of acidity in surface waters are: measured ions of sulfate and nitrate, acid neutralizing capacity (ANC), base cations (mainly calcium and magnesium), and dissolved inorganic aluminum. ANC is a measure of the sensitivity and the degree of surface water acidification or recovery that occurs over time. In other words, the process of decreasing ANC over time is called acidification and reflects the ability of water in the lake or stream to neutralize strong acids. Streams having spring time ANC values less than 50 microequivalents per liter are generally considered potentially 'sensitive' to acidification. Those having higher ANC are generally considered less sensitive or insensitive. When ANC is low, and especially when it is negative, stream water pH is also low (less than about 5 to 6), and there may be adverse impacts on fish and other aquatic life forms. The capacity of a watershed to resist decreases in ANC, and associated decreases in stream water pH is determined by the relative amounts of base cations in the water.


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