Imperviousness and Population Increases in the Chesapeake Bay

Watershed

Presented by Michael S. RolbandP.E., P.W.S., P.W.D., LEED® AP

Wetland Studies and Solutions, Inc.5300 Wellington Branch Drive . Suite 100

Gainesville . Virginia 20155www.wetlandstudies.com

May 5, 2010

Natural & Cultural Resource consulting firm

75 Staff: Archeology; Engineering; Environmental Science & Ecology; Environmental Technology; Compliance; GIS; Regulatory; Surveying; Wildlife Biology

Wetland Studies and Solutions, Inc.

Wetland

3Wetland©

The Great Debate: Imperviousness Growth and Population Growth

"From 1990 to 2000, impervious surfaces increased by 41% - a rate 5 times greater than the 8% rate of population growth during that time."

Source Time Period Impervious Surface Growth

Population Growth

Ratio of Impervious Surface Growth to Population Growth

EPA Sound Bite 1990-2000 41.0% 8.0% 5.1 : 1

EPA Response Memo 1990-2007 34%[1] 18% 1.9 : 1

Phase 4.3 Model 1985-2008 26.3% 26.5% 1.0 : 1

Phase 5.2 Model 1985-2008 38.4% 26.5% 1.5 : 1

Phase 5.3 Model 1985-2007 19.2% 26.5% 0.7 : 1

Source Time Period Impervious Surface Growth

Population Growth

Ratio of Impervious Surface Growth to Population Growth

EPA Sound Bite

1990-2000

41.0% 8.0% 5.1 : 1

EPA Response Memo 18.0%[1] 10.3% 1.7 : 1

Phase 4.3 Model 11.9% 10.3% 1.2 : 1

Phase 5.2 Model 14.2% 10.3% 1.4 : 1

Phase 5.3 Model 8.4% 10.3% 0.8 : 1

[1] Estimated from EPA’s response memo language (versus numbers).

Calculating the Population Change

WSSI calculated the population increase from 1990 to 2000 using U.S. Census data to be 10.3%

On March 9, 2010, the EPA agreed with WSSI’s calculations of 10.3% (instead of the previously reported 8%)

4

“The most important variable is not the rate at which impervious surface is growing, but rather the additional amount of impervious surfaces added each year.”

Does Rate Matter?

Yes! Rate is defined as:

an amount of something measured per unit of something else.

Without rate, we cannot forecast future pollutant loads.

5Wetland©

EPA Response Memo: April 19, 2010

YearImpervious surfaces added

6Wetland©

Pollutant loads from impervious surfaces are directly related to impervious area.

The impervious areas must be accurate for the pollutant loads to be accurate.

Does Rate Matter? And Should the Model Provide a Good Estimate?

Forecasted Pollutant Load in 2025

TP (lb/yr)

(I.A. x 2.1)1

TN (lb/yr)

(I.A. x 11.8)1

Sound Bite 3,073,622 17,270,828

Phase 4.3 3,010,907 16,918,430

Phase 5.2 2,224,093 12,497,283

Phase 5.3 1,541,793 8,663,406

1 Factors were obtained from: U.S. EPA, 2009. Chesapeake Bay Phase 5 Community Watershed Model In Preparation EPA XXX-X-XX-008 Chesapeake Bay Program Office, Annapolis MD. January 2009.

7Wetland©

EPA Response to Congressman Connolly: April 19, 2010

“We do not agree that evidence presented by the third-party invalidates the methodology used to derive the statistic of 41% increase in impervious surface between 1990 and 2000.”

Yet it is not used by the Chesapeake Bay Model.

8Wetland©

The Case Against a 41% Increase Between 1990 and 2000

9Wetland©

Peer-reviewed source of the 41% claim:

Jantz, Patrick, Scott Goetz, and Claire Jantz. “Urbanization and the Loss of Resource Lands in the Chesapeake Bay Watershed.” Environmental Management 36 (2005): 808-825.

Another peer-reviewed article:

Roberts, Allen D., Stephen D. Prince, Claire A. Jantz, and Scott J. Goetz. “Effects of Projected Future Urban Land Cover on Nitrogen and Phosphorus Runoff to Chesapeake Bay.” Ecological Engineering 35 (2009): 1758-1772.0

This article states that, if development trends continue, there will be:

“an estimated 19% and 20% reduction in overall TN and TP to the Chesapeake Bay. Although substantial increases in development-induced, point source N and P loadings were apparent watershed-wide, the estimated conversion of agricultural lands leading to declines in delivered fertilizer loadings to stream was the primary reason for the overall reductions in TN and TP delivery to the Bay that were simulated to occur from 2000 to 2030.”

“The Study is Peer Reviewed...”

Note: This does not address stream erosion and TSS issues that stormwater management quantity controls and E and S measures are necessary to control.

10Wetland©

WSSI agrees that impervious surfaces are probably increasing faster than population and that we should better manage runoff from urban land uses.

Only one data set, presented herein, concludes that impervious surfaces are growing 5 times as fast as population. WSSI has shown that this data set contains serious flaws.

EPA’s 3 most recent models and new “residential housing” metric indicate that impervious surfaces are growing between 0.7 to 1.9 times as fast as population.

EPA continues to cite the original statistic because “our per capita impervious footprint is getting larger, as is the relative impact of urban sources on water quality.” This fact is not negated by citing the statistics derived from their model.

The modeled information and the sound bite must match. Otherwise, one or both of them is incorrect.

Conclusion