Watershed Modeling

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Watershed Modeling

• Class 3

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Assessment Pick an Image
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Aquatic Biological Impacts of Urban Land Use

• Typical, but complex, effects
• Chemical, Physical, Biological
• Biological altered interactions organism
  death
• Regulatory attempt to avoid adverse impacts
• Biological damage still can occur when chemical
  water quality criteria have been met
• Variable exposure patterns, toxicity
• Chemical transformations after release
  interactions
• Diluted pollution still hurts
• Cumulative impact
• Pollutant accumulation (examples beyond sediment)

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Environmental Concerns, Impacts their Causes

• Multiple uses
• Groundwater
• Aquatic habitat
• Water supply
• Wildlife habitat
• Recreation
• Agricultural
• Negative impacts
• Causes
• Altered discharge
• Suspended solids
• Urban pollution

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Environmental Concerns Impacts Contd

• Urbanization impacts are multifaceted
• Tree removal reduces interception
• Loss of duff top soil removes soil reservoir
• Regrading eliminates natural depressions
• New conveyance systems (pipes)

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Hydrologic Related Physical Impacts

• Typical hydrographs before/after urbanization
• 2 to 5 fold increase is common (Leopold, 1968)
• Channel flooding is common
• Faster response is common
• Lower flows in dry periods is common

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Ecological Consequences of Hydrologic Changes

• Erosion of stream channel
• Rapid incision of the channel
• Sediment deposits in pools eggs
• Suspended sediment abrades gills
• Loss of riparian veg. shade
• Loss of woody debris food

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Urban Runoff in Lake Eutrophication

• Degrades ecosystem in several ways
• Filamentous algae are poorer food than diatoms
• Algae clog water intakes
• Algae reduce clarity
• Algae impact swimming
• Algae death lead to anoxia

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Continuous vs. Intermittent (3/24hr) Zinc Exposure

• Same concentration total load 122mg vs. 90 mg
• Same species tolerate toxicity differently
• Gradual decline for continuous (11.3 days)
• Acclimation defense mechanisms may develop
• Load vs. Delivery

Extinction by 2nd treatment
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Metals Toxicity in Fish (Davies, 1986)

• Increases in pH, alkalinity, and hardness
  decreases metal toxicity
• Runoff discharge into well buffered waters with
  most metals in solid state creates less toxic
  reactions than dissolved metal discharge into
  soft waters

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Metals Toxicity in Fish (2/11)

• Metal toxicity generally increases as temp.
  increases because of increased metal activity
  metabolism
• Runoff discharge in the winter and runoff into
  shaded or groundwater-fed waters in the summer
  creates less toxic reactions than into warmer
  waters.

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Metals Toxicity in Fish (3/11)

• Smaller, younger organisms are more sensitive to
  metals, and fish are generally more sensitive
  than macroinvertebrates.
• Runoff discharge during rearing periods creates
  more toxic reactions than during adult stages.

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Metals Toxicity in Fish (4/11)

• If exposed during the embryonic stage in soft
  water, fish can acclimate to metals and are less
  sensitive to higher exposures later.
• Relatively low continuous exposures throughout
  life can somewhat insulate fish from periodic
  elevated exposures in runoff.

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Metals Toxicity in Fish (5/11)

• Metal toxicity increases as exposure period
  lengthens.
• Fish can better tolerate a high exposure for a
  short interval than continuous delivery. This
  says nothing about repeated, periodic exposures,
  as with urban runoff.

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Metals Toxicity in Fish (6/11)

• The apparent mechanism of acute (1 to 4 days)
  metal toxicity in fish is gill irritation by the
  ionic metal, causing mucus secretion internal
  destruction of the gill, resulting in suffocation.

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Metals Toxicity in Fish (7/11)

• Free metals include the most toxic ionic forms,
  as well as other labile forms. Most or all metals
  present can be free in soft waters, but can be a
  small fraction of total metals in hard waters.
• In hard waters, fish can withstand much higher
  total metals, but toxicity to free metals is the
  same in both hard and soft water.

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Metals Toxicity in Fish (8/11)

• Chronic toxicity exhibits trends similar to acute
  toxicity with respect to free and total metals,
  but at much lower levels.

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Metals Toxicity in Fish (9/11)

• Complexation, which reduces toxic free metals, is
  not instant (2 days for Cd), and mortality
  increases in tests with unaged water.
• Urban runoff dynamics do not allow time for
  complexation, and therefore buffering is a
  smaller benefit with intermittent than with
  continuous releases.

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Metals Toxicity in Fish (10/11)

• Toxic concentrations are frequently measured in
  natural water samples, but all fish still live.
• This contraction stems partly from acclimation
  and partly from comparing a standard meant for
  continuous exposure with a condition that is only
  intermittent, as well as using acid to preserve
  samples (which frees metals)

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Metals Toxicity in Fish (11/11)

• The use of dissolved metals measurement can only
  approximate the toxic forms because some are
  solubilized over time.
• Realistic water quality criteria for waters
  affected by urban runoff require considering
  duration and frequency of exposures, chemical
  forms, and species.

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Thermal Impacts of Urban Runoff (Washington
State)

• Air temp. strongest control on stream temp.
• Avg. stream temp. increased linearly with
  impervious area percentage
• Criteria violations occurred w/ 12 impervious
  area
• Structural treatment practices w/ surface
  discharge had violations of temp. criteria under
  baseflow and stormflow

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Thermal Impacts of Urban Runoff (2/2)

• Order of practices in raising receiving water
  temp. causing violations, from least
• Infiltration basin lt extended-detention wetlands
  lt extended-detention dry ponds lt wet ponds
• Thermal conditions could cause algal succession
  from cold water (diatoms) to water-water
  filamentous greenblue algae
• Implications on shade pool outlet design

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Fish Habitat Impacts Habitat Protection

• Life Cycle Characteristics
• Habitat Requirements function of life stage