Water Quality Monitoring and Atmospheric Deposition: How are they Linked?

Douglas A. Burns
U.S. Geological Survey
425 Jordan Rd.
Troy, NY 12180

The atmospheric deposition of air pollutants triggers numerous interactions with ecosystems including uptake as a nutrient, toxicity, and changes in species richness and biodiversity. One way in which scientists document the effects of air pollutant deposition is through monitoring surface water quality. This presentation focuses on water quality monitoring to document the effects of atmospheric sulfur (S), nitrogen (N), and mercury (Hg) deposition on aquatic ecosystems. An underlying assumption of such monitoring is that surface water chemistry is a surrogate for the effects of air pollutants on aquatic ecosystems. Biological monitoring is more costly and time consuming than water quality monitoring, and is therefore less common; however, biological recovery of aquatic ecosystems from decreased loads of acid deposition often does not parallel water quality recovery because of ecological factors such as competition and dispersal. The extent to which water chemistry reflects air pollutant deposition varies widely among solutes and among different regions of the US. For example, surface water sulfate concentrations in the Northeast generally reflect sulfate deposition, whereas the same is not true in the Southeast where soils readily adsorb sulfate. Spatial patterns of N deposition are often reflected by surface water nitrate concentrations, but temporal deposition patterns commonly show little synchronicity with these concentrations because of high demand for N as a nutrient. Mercury deposition is generally even less clearly related to surface water Hg concentrations than is S or N because watershed factors (for example, wetlands) generally affect mobility to a greater extent than does recent atmospheric loads. Soils generally play a key role as either source or sink, and predictive models greatly benefit from soil monitoring data and an understanding of key soil processes. Several monitoring programs focus on the link between air pollutant deposition and water quality including the LTM/TIME program of the U.S. Environmental Protection Agency, the Hydrologic Benchmark program of the U.S. Geological Survey, monitoring in several National Parks (i.e. - Rocky Mountain and Shenandoah), several experimental forests operated by the U.S. Forest Service (i.e. - Hubbard Brook and Coweeta), and several LTER sites funded by the National Science Foundation (i.e. - Andrews Forest and Niwot Ridge). Little monitoring of Hg in waters exists in the US with no operating national network. Because watersheds serve as chemical stores and can greatly alter air pollutants during transit, water quality monitoring can provide greater understanding and improved models of the ecosystem effects of air pollutants.

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