1 Introduction -- 1.1 1990 NAPAP Reports and Integrated Assessment -- 1.2 Scope -- 1.3 Goals and Objectives -- 1.4 Outline of State of Science Update -- 2 Background and Approach -- 2.1 Overview -- 2.1.1 Atmospheric Inputs -- 2.

1.2 Sensitivity to Acidification -- 2.2 Chemical Response Variables of Concern -- 2.2.1 Sulfur -- 2.2.2 Nitrogen -- 2.2.

3 Acid Neutralizing Capacity -- 2.2.4 pH -- 2.2.5 Base Cations -- 2.2.6 Aluminum -- 2.2.

7 Biological Effects -- 2.3 Monitoring -- 2.4 Historical Water Quality Assessment Techniques -- 2.4.1 Historical Measurements -- 2.4.2 Paleolimnological Reconstructions -- 2.4.

3 Empirical Relationships and Ion Ratios -- 2.5 Models -- 2.5.1 Empirical Models -- 2.5.2 Dynamic Models -- 3 Chronic Acidification -- 3.1 Characteristics of Sensitive Systems -- 3.2 Causes of Acidification -- 3.

2.1 Sulfur -- 3.2.2 Organic Acidity -- 3.2.3 Nitrogen -- 3.2.4 Base Cation Depletion -- 3.

2.5 Land Use -- 3.2.6 Climate -- 3.2.7 Fire -- 3.2.8 Hydrology -- 3.

3 Effects of Acidification -- 3.3.1 Aluminum -- 3.3.2 Effects on Aquatic Biota -- 3.3.3 Effects on Amphibians -- 4 Extent and Magnitude of Surface Water Acidification -- 4.1 Northeast -- 4.

1.1 Monitoring Studies -- 4.1.2 Paleolimnological Studies -- 4.1.3 Experimental Manipulation -- 4.1.4 Model Simulations -- 4.

2 Appalachian Mountains -- 4.2.1 Monitoring Studies -- 4.2.2 Model Simulations -- 4.3 Florida -- 4.3.1 Monitoring Studies -- 4.

3.2 Paleolimnological Studies -- 4.3.3 Model Simulations -- 4.4 Upper Midwest -- 4.4.1 Monitoring Studies -- 4.4.

2 Paleolimnological Studies -- 4.4.3 Experimental Manipulation -- 4.4.4 Model Simulations -- 4.5 West -- 4.5.1 Monitoring Studies -- 4.

5.2 Paleolimnological Studies -- 4.5.3 Model Simulations -- 5 Chemical Dose-Response Relationships and Critical Loads -- 5.1 Quantification of Chemical Dose-Response Relationships -- 5.1.1 Measured Changes in Acid-Base Chemistry -- 5.1.

2 Space-for-Time Substitution -- 5.1.3 Paleolimnological Inferences of Dose-Response -- 5.1.4 Model Estimates of Dose-Response -- 5.2 Critical Loads -- 5.2.1 Background -- 5.

2.2 Progress in Europe -- 5.2.3 Progress in the U.S. and Canada -- 5.2.4 Establishment of Standards for Sulfur and Nitrogen -- 6 Episodic Acidification -- 6.

1 Background and Characteristics of Sensitive Systems -- 6.2 Causes -- 6.2.1 Natural Processes -- 6.2.2 Anthropogenic Effects -- 6.3 Extent and Magnitude -- 6.4 Biological Impacts -- 7 Nitrogen Dynamics -- 7.

1 Nitrogen Cycle -- 7.2 Environmental Effects -- 7.3 Nitrogen in Surface Waters -- 8 Experimental Manipulation Studies -- 8.1 Whole-System Nitrogen and /or Sulfur Enrichment Experimental Manipulations -- 8.1.1 Gardsjon, Sweden -- 8.1.2 Sogndal, Norway -- 8.

1.3 Lake Skjervatjern, Norway -- 8.1.4 Aber, Wales -- 8.1.5 Klosterhede, Denmark -- 8.1.6 Bear Brook, ME -- 8.

2 Whole-System Nitrogen Exclusion (Roof) Studies -- 8.2.1 Gardsjon, Sweden -- 8.2.2 Ysselsteyn and Speuld, Netherlands -- 8.2.3 Klosterhede, Denmark -- 8.2.

4 Soiling, Germany -- 8.2.5 Risdalsheia, Norway -- 8.3 Climatic Interactions -- 8.4 Results and Implications -- 9 Predictive Capabilities -- 9.1 Model of Acidification of Groundwater in Catchments (MAGIC) -- 9.1.1 Background and General Structure as Used -- for the NAPAP 1990 Integrated Assessment -- 9.

1.2 Recent Modifications to the MAGIC Model -- 9.1.2.1 Regional Aggregation and Background Sulfate -- 9.1.2.2 Organic Acids -- 9.

1.2.3 Aluminum -- 9.1.2.4 Nitrogen -- 9.1.3 Cumulative Impacts of Changes to the MAGIC Model -- 9.

1.4 MAGIC Model Testing and Confirmation Studies -- 9.1.4.1 Lake Skjervatjern (HUMEX) -- 9.1.4.2 Risdalsheia (RAIN) -- 9.

1.4.3 Bear Brook (WMP) -- 9.1.5 Evaluation of MAGIC Projections -- 9.2 Nitrogen Models -- 10 Case Study: Adirondack Park, NY -- 10.1 Background and Available Data -- 10.1.

1 ELS-I -- 10.1.2 ALSC -- 10.1.3 ELS-II -- 10.1.4 DDRP -- 10.1.

5 PIRLA -- 10.1.6 ALTM -- 10.1.7 ERP -- 10.2 Watershed History -- 10.3 Lake-Water Chemistry -- 10.4 Organic Acidity -- 10.

5 Role of Nitrogen in Acidification Processes -- 10.6 Role of Landscape and Disturbance -- in Acidification Processes -- 10.7 Overall Assessment -- 11 Case Study: Class I Areas in the Mountainous West -- 11.1 Background -- 11.2 Sierra Nevada -- 11.2.1 Atmospheric Deposition -- 11.2.

2 Surface Water Chemistry -- 11.2.3 Seasonality and Episodic Processes -- 11.2.4 Weathering and Cation Exchange -- 11.3 Rocky Mountains -- 11.3.1 Glacier National Park -- 11.

3.2 Yellowstone National Park -- 11.3.3 Grand Teton National Park -- 11.3.4 Rocky Mountain National Park -- 12 Conclusions and Future Research Needs -- Definitions -- References -- Index.