Groundwater Pollution Contaminant Transport

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Groundwater Pollution- Contaminant Transport

• 12.8 Supplementary Info.

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Outline

• Facts
• Contaminants that stick to soil
• Pure chemicals
• Chemicals dissolved in water

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Facts

• Up to 25 of the useable groundwater in the US is
  contaminated.
• According to the EPA, the gasoline additive MTBE
  is found in 5-10 of drinking water in areas
  using reformulated gasoline.
• Wells in 38 states contain pesticide levels high
  enough to threaten human health.

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Facts

• Every major aquifer in New Jersey is
  contaminated.
• In Florida, where 92 of the population drinks
  groundwater, more than 1,000 wells have been
  closed because of contamination and over 90 of
  the remaining wells have detectable levels of
  industrial or agricultural chemicals.

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Facts

• In 2003, the EPA reported 436,494 confirmed
  releases of dangerous volatile organic compounds
  leaking from underground fuel storage tanks
  (USTs) in the US.
• REMEMBER Groundwater makes up less than 1 of
  all water on Earth.

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Pollution

• Can be in various forms depending on the chemical
  properties and if it is mixed w. other chemicals.
• Forms solids, pure chemical, vapor (gas),
  solution (contaminant dissolved in water)

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Sources
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Sorption

• Adsorption- chemical sticking to soil particles.
• Sorbing contaminants- chemicals that have a
  tendency to stick on the sediments

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Strongly sorbing contaminants

• Strongly sorbing compounds will be soil or
  sediment pollutants
• Will tend to remain near surface in zone of
  aeration
• will not produce extensive gw contamination
• Will be deposited stream or lake bottoms
• Include PCBs (polychlorinated biphenyls)
• Include many metals

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If they dont stick to the soil

• Nonsorbing contaminants will move with water
• Low to medium sorbing contaminants will move with
  water but at a slower velocity- this is called
  retardation.

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Pure chemicals

• called NonAqueous Phase Liquids (NAPLs)
• How far a NAPl will penetrate is controlled by
• Permeability of the soil
• Whether it floats or sinks (density)

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Light NAPLs

• Densities less than water (so they float), Ex.
  Gasoline
• If spilled on the surface, it will move downward
  through the unsaturated zone leaving behind small
  amounts of the NAPL in the pore spaces.
• When it reaches the water table, it will float on
  top.

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• Good news These do not get too far past the
  water table.
• Bad news chemical components in the mixture CAN
  dissolve in the groundwater and be carried
  downgradient producing a plume. (MTBE and
  benzene are soluble components of gasoline)

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Dense NAPLs

• Have densities greater than water (sink) Ex.
  Industrial degreasers, dry cleaning agents-
  Trichloroethylene (TCE) A Civil Action
• If spilled on the surface, will move downward
  through unsaturated zone leaving behind small
  amounts of NAPL in the pore space
• If enough volume of DNAPL is spilled and
  permeability permits, DNAPL will penetrate past
  the water table and continue moving downward into
  the saturated zone due to gravity effects.
• Chemical constituents of DNAPL will dissolve into
  groundwater and produce a plume downgradient.

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Dissolved Chemical Transport- Darcys Law

• Dissolved chemicals (solutes) will travel with
  flowing water.
• Groundwater flow calculated from Darcys Law
• V K I / F
• V velocity, K hydraulic conductivity, I
  gradient, F porosity
• Hydraulic conductivity- the capability of a
  medium to transmit water
• K sand 10-3, K clay 10-9 cm/s
• Use effective (not total) porosity. Sand
  10-20,Clay 1-5
• Example K 100 ft/day, I 0.001, porosity
  0.25 (change to decimal)
• v 100 ft/day X 0.001 / 0.25 0.4 ft/day

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Travel Time- how long until it gets to your well

• Travel time length of time for a dissolved
  chemical to travel a certain distance
• Example Source area is 100 ft upgradient of
  stream
• If v 0.4 ft /day,
• Travel time
• 100 ft / 0.4 ft per day 250 days

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Retardation

• If a contaminant is retarded in its transport due
  to sorption, the retardation factor, R, can be
  estimated or measured
• Example MTBE R 1.0, benzene R 2.0
• The velocity of the retarded contaminant is
• Velocity of MTBE v/1.0 0.4 ft/day
• Velocity of benzene v/2.0 0.2 ft/day
• In previous example,
• MTBE would arrive at stream in 250 days
• Benzene would take 2x as long or 500 days

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Who contaminated the well?

• What information do we need?
• Remember Darcys Law, v K I / F
• Need knowledge of the geology of the region
• Unfractured bedrock, clay layers impermeable
• Sands, gravels permeable
• Can guess from previous studies what hydraulic
  conductivity (permeability) of the
  sediments/rocks are

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Who contaminated the well?

• Can measure K from monitoring water flow and
  hydraulic head (potentiometric) conditions in the
  laboratory (permeameters) or in the field
  (hydraulic tests, most common is called a pump or
  pumping test)
• Need knowledge of the water levels
  (potentiometric levels) in the aquifer
• Install monitoring wells in numerous occasions
• Make water level contour map

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WATER LEVEL CONTOUR MAP

• Groundwater flow direction is from high water
  level to low water level.
• Draw groundwater flow lines at right angles
  (perpendicular) to groundwater contour lines on a
  gw topo map.
• This will tell you the direction of groundwater
  flow and contaminant transport.

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TRAVEL TIME

• Once you have the direction of transport, the
  next question is usually How long will it (or
  did it) take to get from point A to point B?
• Remember Darcys Law v K i / F
• We have i change in water level over a distance
• We have information on the aquifer material from
  our geologic studies (K, porosity)
• Calculate groundwater velocity from the above
  equation
• Measure travel distance from map and calculate
  travel time using your velocity

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HETEROGENEITY

• Homogeneous conditions occur when your aquifer is
  made of a uniform material
• What if geologic conditions change along the
  travel path?
• Heterogeneous conditions occur when your aquifer
  is made of many different geologic materials
  (more common)
• Example Source----10 ft sand------10 ft
  silt-----well

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Solution to heterogeneity

• You can apply Darcys Law in segments
• V in sand 10 ft/day, travel time through 10 ft
  of sand is 1 day
• V in silt 1 ft/day, travel time through 10 ft
  of silt is 10 days
• Total travel time 1 day 10 days 11 days

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REAL LIFE

• Hydraulic gradients may not be constant (equal
  spacing between contours) particularly when wells
  are pumping in the aquifer
• Extremely complex geology can make simple
  applications of Darcys Law difficult
• Computer models are used to apply Darcys Law
  under these complex scenarios

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• Lets try a problem!