Shenandoah National Park: Critical Load/Target Load Case Study

Shenandoah National Park:

Critical Load/Target Load Case Study

WESTAR November 2005

Tamara Blett- National Park Service

Photo credit: University of Virginia

Focus on:1.Use of ecosystem and deposition data in critical loads development for aquatic ecosystems

2. Selection of critical loads and target loads from modeling outputs

3. Why land health goals are needed before each critical load or target load is developed



No such thing as a “generic” critical load for a park or wilderness area…. A CL is the loading at which SOMETHING happens in an ecosystem.

Defining which SOMETHING is relevant to the FLM area is a critical part of CL development …

Lo

ad (

kg/

ha

/yr)

Changes in soil

productivity

Aquatic plant species

shifts from fertilization

Episodic acidification

Chronic acidification

Forest dieback



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.”

WESTAR FRAMEWORK WORKGROUP QUESTION:

What are the AQRVs that should be addressed and what are the criteria for addressing them?

1. N & S acidification effects to aquatic ecosystems (via water chemistry thresholds)

2. N & S acidification effects on forest ecosystems (via soil chemistry thresholds)

3. N fertilization effects on aquatic and terrestrial plant and animal species (via species shifts)

Simulation modeling

Field data and experiments

3 types of critical loads development efforts:

Which AQRVs to use in CL development?

• FLMs have the responsibility to identify our AQRVs and develop strategies to protect them

• FLMs will protect most sensitive resources from unacceptable change

• FLMs will base critical loads on best available science and revise and update as appropriate

Current Deposition Loadings:

• Shenandoah National Park has long experienced high deposition loadings.

• Sulfur deposition (rather than N) is the primary cause of acidification of park soils and waters.


Use of Ecosystem and Deposition Data in CL Development

Stream chemistry sampling tells us which aquatic plants and animals can grow, survive, reproduce… and which cannot

Photo credit: University of Virginia

•Pre-1900, all Shenandoah streams had ANC>50 ueq/l (levels that sustain healthy aquatic biota)

•In a 1992 survey, about half of the most sensitive streams had ANC<0, with lethal effects on brook trout probable •The rest had ANC between 0-20, with sub-lethal or lethal effects possible.

•Modeling suggests that most sensitive streams in the park have lost up to 4 fish species

Park streams are highly impacted by deposition:

Aquatic ecosystemsDesired goals: healthy biota (invertebrates, fish, aquatic plants, etc.)

Measurement endpoints: ANC, pH, base cations, Al

– ANC of 50-100 ueq/l considered acid-sensitive but capable of supporting healthy biota

– ANC < 50 ueq/l sensitive to acidification; potential effects to biota

– ANC < 20 ueq/l: episodically acidic, sub-lethal or lethal effects on biota

– ANC < 0 ueq/l: chronically acidic, lethal effects on many biota

FL

M s

elec

ts d

esir

ed e

nd

po

ints

an

d e

ffec

tsWESTAR FRAMEWORK WORKGROUP QUESTION: Are there simple endpoints (like visibility thresholds) that can be used to assess effects?

loss of acid sensitive species*

4.0

4.5

5.0

5.5

6.0

6.5

7.0

-40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0

Daily stream ANC (ueq/L)

Da

ily

str

ea

m p

H

Loss of additional fish / invertebrates*

*Baker et al. 1990

small decrease in species richness*

metals solublefree ionic form

Toxic!

Extremely Acid Sensitive*Acidified* Acid Sensitive*

Less soil buffering capacity More soil buffering capacity

Healthy!

Deposition accumulates in soils over time:

•Soils soak up pollutants like a sponge over time

•Excess sulfur accumulates in soils, acidic conditions develop, which then strip beneficial nutrients from the soils and weaken plants

•Acidic compounds “use up” the buffering ability of soils and surface waters.

Development of Critical load or 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)

Significant harmful effects from deposition have been documented in Shenandoah streams BUT they are above the critical load… and we don’t know the loading at which they first began……

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.

Now the more relevant question is: what loading is desired to achieve to meet ecosystem goals?

Lo

ad (

kg/

ha

/yr)

Changes in soil & water chemistry

Healthy aquatic biota

Episodic acidification

Chronic acidification

Critical loads are defined for specific indicators and effects.

>50ueq/L

<20 ueq/L

<0 ueq/L

Surface Water Effects Thresholds Used to Develop Critical & Target Loads

Water quality dataused in modeling CL/TL

14 streams in Shenandoah have sufficient water chemistry data for calibrating the MAGIC* model.

Streams were routinely sampled for 12 yrs as part of the SHEN Watershed Study and the Virginia Trout Stream Sensitivity Study.

Of the 14 streams, only 5 were on very sensitive (siliciclastic) bedrock; the remainder were relatively insensitive.

*Modeling for Acidification of Groundwater In Catchments

Interim Target

Current

Deposition reduction

Glide path

Dep

osit

ion

(k

g/h

a/yr

)

Time

•Set interim target

•Establish glide path

•Re-evaluate periodically

Re-evaluate; adjust

Natural

Developing Interim Target Loads

• FLM sets management goal: “restore healthy ecosystems (ANC=50) to Shenandoah NP”

• Scientists may/may not be able to determine the actual CL (the deposition loading at which Shenandoah ecosystems became unhealthy in the past)

• Target loads to achieve ANC of 50 in 50 years ranges from 0-4 kgS/ha/yr (depending on individual stream).

• Models (MAGIC and others) used to determine how deposition reductions could improve stream chemistry conditions

• Deposition in Shenandoah is now ~10 kg S/ha/yr. • States (w/FLM involvement) develop Interim Target

Loads to set improvement goals over time (similar to a glide path for visibility improvement).

Current (1991) Future (2041)0

10

20

30

40

50

60

% o

f br

ook

trou

t str

eam

s

Current vs. 40% Reduction

Not acidic(Suitable for brook trout)

Chronically Acidic(Unsuitable for brook trout)

Shenandoah researchers project that streams will continue to acidify in the future due to S and N deposition, even with projected reductions in SO2

emissions.

So…. May need longer time frames for recovery when developing target loads

AND

May need to consider more stringent emissions reductions if ecosystem goals are to be met to protect parks for “future generations”

Eastern US sites are worse off… there is still time to avoid major ecosystem damage in the west if deposition levels are reduced.

Conclusions

1.Need good ecosystem data as model inputs to develop critical loads/target loads modeling estimates

2. Critical loads may be difficult to determine where already exceeded (unless we know when they were first exceeded and what the deposition was in that time and place)

3. Target loads can be established via modeling for areas where CL have been exceeded

4. Defining ecosystem health and recovery goals is crucial before using modeled critical load or target load outputs.

Shenandoah National Park: Critical Load/Target Load Case Study

Documents