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Urbanization, Hydrology, and Nitrogen Export. Ryan Albert 2/26/08. Outline. General Background Accotink and Pohick Study Introduction Estimating Urban Land Use Water Quality Analysis Discussion. Nonpoint Sources of Pollution.

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Presentation Transcript
  • General Background
  • Accotink and Pohick Study
    • Introduction
    • Estimating Urban Land Use
    • Water Quality Analysis
  • Discussion
nonpoint sources of pollution
Nonpoint Sources of Pollution
  • Nonpoint sources of pollution are those which are more diffuse and cannot be traced to a single outlet
  • Most nonpoint sources are moved into streams via runoff from rainfall events
  • Examples include:
    • Runoff from an agricultural field
    • Runoff from an industrial site
    • Runoff from an urban or suburban area (urban stormwater)
impact of urban stormwater
Impact of Urban Stormwater
  • Urban Stormwater is a major cause of impairment to aquatic ecosystems
  • Causes increase in loadings of nutrients and toxics from land cover to water bodies and changes in Hydrology.
  • Cumulative impact is significant
typical pollutant loadings from runoff by urban land use lbs acre yr
Typical Pollutant Loadings from Runoff by Urban Land Use (lbs/acre-yr)

NURP Study: Homer et al, 1994

importance to the potomac watershed
Importance to the Potomac Watershed
  • Urban land use is increasing as population rises and household density decreases.
  • Fast paced urbanization has resulted in significant changes for many of the watershed’s formerly forested watersheds.

Image from Jantz et al. (2004)

gunston cove
Gunston Cove
  • As discussed by Chris Jones
    • Nutrient concentrations have decreased,
    • Sechi depth has increased, and
    • Submerged aquatic vegetation have partially reestablished.
  • Successes attributable to decreases in traditional point source loadings
selected objectives of this study
Selected objectives of this study
  • 1. Quantify historic urbanization (past 30 years) in the Accotink and Pohick watersheds
  • 2. Characterize impact of urbanization, specifically on flow and nitrogen.
  • 3. Quantify potential future urbanization scenarios
selected objectives of this study1
Selected objectives of this study
  • 4. Characterize potential changes caused by urbanization on water quality and hydrology. Identify changes needed to keep WQ loadings at current levels (or reduce loadings).
  • 5. Discuss relative threat to each ecosystem. Further analyze how these specific threats integrate with county watershed managers’ goals?
estimating historic land use change
Based on using Remote Sensing Data (NLCD 1992 and MRLC 2000) and Fairfax County Annual Demographic Household Data

Uses a density of households per unit of urban area ratio to determine urban area for years in which remote sensing data are not available.

Estimating Historic Land Use Change
future land use jantz model output
Future Land UseJantz Model Output
  • Jantz Model output based on Cellular Automata model known as SLEUTH
  • Three different policy assumption scenarios: current conditions (base), smart growth, and sustainable development
  • 45 meter resolution: 100 Monte Carlo iterations run
  • Data shared in probability of urbanizing map format.
  • Model output for full Baltimore/DC area: Adapted to Accotink and Pohick
future land use
Future Land Use
  • Five Scenarios used:
    • One unadjusted from Jantz et al. (2004) used as high estimate
    • Three adjusted from Jantz et al. (2004) as mid-range. Adjustments made based on comparison to NLCD/MRLC data and calibration data for Jantz Model.
    • One based on historic development ratios and county planner projections.
      • Known underestimate: build out approaches do not take into account changes in zoning or political landscape.
urban land use projections


Urban Land Use Projections
  • Projections for Accotink vary from 70.0% urban to 87.3% urban (compared to 61.9% for 2004).
  • Projections for Pohick vary from 56.7% urban to 77.2% urban (compared to 49.5% for 2004).
Based on Previous GMU sample sites.

Site at USGS gauge station added

Sampling Sites

nitrogen projections
Nitrogen Projections
  • Calculated historic loadings using a variety of methods, including a statistical model (LOADEST) and a simple approach (Export coefficients).
  • Compared historic estimated loadings
  • Export coefficient method used in combination with land use projections.
    • 4 loading scenarios: high, expected, Chesapeake Bay Program Loadings, and low.
    • 3 scenarios of Noman Coles Point Source Loadings (High, Expected, Low).
export coefficient nutrient loadings
Export Coefficient:Nutrient Loadings
  • Estimated LOADEST 1992 nitrate-N loadings (left) and 2005 loadings (right) (for Pohick sites 1 and 2 and Accotink 13 (listed as 3) compared to loadings for the four export coefficient scenarios with observed 1992 and 2004 land use.
nitrate n loading reductions in the expected scenario
Nitrate-N loading reductions in the Expected Scenario
  • Removal of 40 percent nitrate-N from new development and 10 to 15% of nitrate from existing development required to decrease loadings
tn loading reductions in the high scenario
TN loading reductions in the High Scenario
  • Removal of 40 percent TN from new development and 10 to 15% of nitrate from existing development required to decrease loadings
  • Observed values for the upper Accotink watershed (watershed above site 20) analyzed using USGS gauge station data.
  • Hydrologic Simulation Program Fortran (HSPF) hydrology model calibrated for 1991-1993 using USGS gauge data: validated using 1990 and 1994 data.
    • Used to simulate past flow conditions for all sites.
    • Used to project future flow conditions for all sites.
hspf hydrology validation
HSPF Hydrology Validation
  • 1990 and 1994 within 10% annual flow.
  • Observed vs. Modeled flow showed high correlation
changes to hydrology 1992 2030
Changes to Hydrology: 1992-2030
  • Hydrograph for Pohick 1 comparing 1992 land use conditions (red solid) to projected ADJ-JCC land use conditions in 2030 (green dashed) modified from Jantz et al. (2004) using 1991-1993 climatic conditions.
changes to hydrology 1975 2030
Changes to Hydrology: 1975-2030
  • Hydrograph for Pohick 1 comparing 1975 land use conditions (red solid) to projected land use ADJ-CC conditions in 2030 (green dashed) modified from Jantz et al. (2004) using 1974-1976 climatic conditions.
  • Are some issues more significant than others?
    • Nitrogen Loadings to the Bay
    • Hydrological changes
  • Are there viable solutions to mitigating these issues? Are they mutually exclusive?
    • Green Infrastructure – Low Impact Development
    • Smart Growth
    • Conservation Development
fairfax county watershed managers goals
Fairfax County Watershed Managers’ Goals
  • Prevent further degradation of Accotink
  • Prevent further degradation for most of Pohick, restore portions of Pohick
  • Reduce loadings of nitrogen to the Chesapeake Bay from urban runoff
fairfax county goals are they achievable
Fairfax County Goals: are they achievable?
  • Public sector efforts:
    • Increased funding ($.01 per $100 assessed value for real estate in 2007)
    • Demonstration projects
    • Enhanced monitoring and maintenance
  • However, more must be done/required on private development.
    • RPAs (buffer) protection is significant, and may help reduce some nitrogen.
    • Dry detention Basin Approach is insufficient to meet nitrogen or hydrology goals.
  • Thanks to dissertation committee members: Drs. R. Chris Jones, Dawn Parker, Ron Stewart, and Jon Cooper
  • Elizabeth Fine, Matt Cline, Craig Wilderman, Laura Vacherlon, Iyad Usama Abu Rdeineh, and Ruba Almukarker for helping with sampling and analyzing data
  • EPA colleagues: Dr. Don Brady, Dr. John Powers, Jenny Molloy, and Rachel Herbert