Critical Loads for Atmospheric Deposition: Assessing, Evaluating and

Protecting Natural Resources

Tamara Blett Air Resources Division – National Park Service

CANS Workshop

Feb 2006

National Park Service

DEVELOPMENT (what is the CL?)

IMPLEMENTATION (how can the CL be used to protect parks & wilderness areas?

Development/Use of Critical Loads

RegulatorsScientists

Land Managers, (NPS/FS), EPA

Definitions…

Critical load/Target load

• Critical load: “The quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge.” (Nilsson and Grennfelt 1988)

• Target load: The level of exposure to one or more pollutants that results in an acceptable level of resource protection; may be based on political, economic, or temporal considerations.

Critical loads and target loads can be developed for any pollutants.

“…conserve [natural and cultural resources]…as will leave them unimpaired for the enjoyment of future generations.” (NPS Organic Act)

“Wilderness areas…shall be administered…in such a manner as will leave them unimpaired for future use and enjoyment as wilderness…” (Wilderness Act of 1964)

“…preserve, protect and enhance the air quality in national parks, national wilderness areas, national monuments, national seashores…” (Clean Air Act as amended in 1977)

“In cases of doubt the land manager should err on the side of protecting the air quality-related values for future generations.” (Senate Report No. 95-127, 95th Congress, 1977)

Preserve and Protect

[Rocky Mountain NP should be managed for] “…preservation of the natural conditions and the scenic beauties thereof.” (1915 park enabling legislation)

Defining the critical load

1) ”The maximum amount of pollution into an ecosystem that does not cause significant damage to system resources, survival, structure or function”

2) “The quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge.” (Nilsson and Grennfelt 1988)

This first CL definition used in some European approaches… request that we use the *second definition for sensitive U.S. systems and this modeling effort

Indicators and Thresholds….

N Load (kg/ ha /yr)

Changes in soil & water chemistry

Effects on aquatic animals (episodic acidification)

Lethal effects on fish, other aquatic animals (chronic acidification)

Surface water N saturation

Rocky Mountain National Park: Continuum of Impacts to Ecological Health

Changes in tree chemistry

Change in alpine plant species

Change in aquatic plant species composition

Forest decline (acidification effects on trees)

Ecosystem endpoints for CL: Conceptual Diagram

Critical loads are defined for specific indicators and endpoints

Scientists conduct empirical studies to identify resources sensitive to deposition

Scientists derive critical loads from empirical studies and modeling analyses.

Federal manager is guided by agency policy in selecting sensitive resources and indicators of change; defines ”harmful” changes to sensitive resources based on policy goals.

Decisions about interim or sustainable levels of N and S deposition on federal lands are made by federal manager, with consultation with air regulators and others if target loads will be used for emissions control strategies.

CRITICAL LOAD DEVELOPMENT

SCIENCE FEDERAL MANAGER

Science/Policy Integration Recommendation- March 2004 FLM CL Workshop

4 box model is useful conceptual framework for considering S and N Critical Loads

Acidif. Nutrient

Aquatic

Terrestrial

“Sensitive Elements” and “Effects”

• “Specified sensitive elements”: ecosystems sensitive to nitrogen and/or sulfur deposition, e.g.,– Poorly buffered lakes, streams, soils– Ecosystems that evolved under low nutrient

conditions and/or with short growing season (e.g. deserts or alpine areas)

• “Harmful effects”: changes in the natural functioning of an ecosystem, e.g.,– Loss of acid-neutralizing capacity affecting biota

(growth, viability, condition, etc)– Unwanted enrichment by nitrogen resulting in

changes in natural vegetative community