Transcript Subsurface Stratigraphy

Factors that Influence the
Occurrence of Nitrate in the
Upper Willamette Valley
Basic Groundwater Hydrology
Understanding Nitrate and its
Distribution
Groundwater Basics
• Where does ground-water
come from?
• How does ground-water
occur?
• How does ground-water
move and how fast does it
travel?
• How susceptible is
groundwater to
contamination?
Important Groundwater Facts
 Groundwater originates from precipitation sinking into
the ground from the surface to the water table.
 Groundwater occurs in the open spaces between silt,
sand and gravel particles or in natural fractures in rocks
(not underground rivers or lakes).
 Groundwater moves from where the water table is high
to where it is low.
 How fast it moves depends on the slope of the water
table and the aquifer permeability.
 Groundwater is susceptible to contamination from
surface activities.
Origin of Groundwater
Types of Aquifers
• Unconfined
–
–
–
–
Shallow
Local Recharge
Water Table
Susceptible
• Confined
–
–
–
–
–
Deeper
Confining Layer
Distant Recharge
Water under pressure
Less susceptible
Local vs. Distant Recharge
The Nature of Aquifers and
Groundwater Movement
Groundwater Movement
Seasonal Water Levels: Summer
Groundwater and Surface Water
Reactions
Streams and Groundwater
Pumping wells
Close to Streams
Drawing in surface
Water?
How much
pumped
How long
pumped
Potential for Groundwater
Contamination
• Soil and subsurface
materials as filtering
mechanism
– Particulates (Pt)
– Dissolved (Ds)
• Bacteria (Bc)
• Chemical processes affect
dissolved constituents:
Organic (Or) vs. Nitrate
(NO3)
Or
Pt
Bc
Ds
NO3
Aquifer Sensitivity
• Nature of vadose zone
• Permeability of
material is highly
variable
• Approximate travel
time across 20 vertical
feet under saturated
conditions
• Aquifer character,
depth to aquifer, old
wells, rainfall,etc.
Gravel
< 1 hr
Sand
6d
Fractured Bedrock
60 d
Silt
300 d
Limestone
750 d
Sandstone
12 yr
Hard Basalt
175 yr
Clay
1700 yr
Groundwater Contamination
Why is the Groundwater in the
Valley of Concern?
• Aquifer is considered sensitive to contamination
• Diverse land use practices
• Soils in many areas described as well-drained and
have rapid time-of-travel across them
• High rainfall (infiltration)
• Shallow, unconfined aquifer supplies many area
wells
Generalized Stratigraphy of the Southern
Willamette Basin
Willamette Silt
0 to 20 feet
Willamette Aquifer
20 to 220 feet
Willamette
Confining Unit
Up to 300 feet
Bedrock: Sed or
Vol
Nitrate and Factors that Influence
its Occurrence
What is Nitrate?
• One of several nitrogen-bearing compounds that may
occur in groundwater under varying conditions
– Ammonium (NH4+)
– Nitrite (NO2-)
– Nitrate (NO3=)
• Potential Sources of Nitrate
– Natural Sources, e.g., decomposition of organic material (generally
leads to less than 1 mg/L (ppm) in groundwater)
– Septic system effluent, animal waste, land application of foodprocessing waste water
– Commercial fertilizer, munitions, landfills, urban runoff
Drinking Water Standards
• EPA-established maximum contaminant level (MCL) in
public supply water: 10 mg/L (ppm) reported as
nitrogen or 45 mg/L reported as nitrate
• Concern associated with nitrate
– Methemaglobinemia “blue-baby syndrome”
– Infants, pregnant women, nursing mothers
• Concentration below which the risk of disease is
considered acceptable. No known case of
methemaglobinemia at concentrations of 10 ppm or less
• Other concerns?
Impact of Nitrate on Pets and
Other Domestic Animals
• A function of total nitrate in diet (feed + H2O)
• Cattle, sheep, horses, dogs and cats more
sensitive to nitrate than pigs and poultry
• Recommendations:
– < 10 mg/L: safe for all animals
– 10 –20: safe unless feed has high (>1000 ppm) nitrate
levels
– >40: animals at risk
Nitrate Variability
• Nitrate concentration varies seasonally
– Higher during one season than the other
• Nitrate concentration from one well to
another
– Neighbor’s well has different concentration
• Reasons related to the nature of
groundwater and how the well is
constructed
– Where groundwater comes from, how it occurs, and
how it moves?
– How is the well sealed and where is the well screened?
Seasonal Compositional
Variations
• Seasonal Differences
within a single well
• Recharge related
– Infiltration of nitrate from a
nitrate source (A)
– Dilution of contaminated
aquifer (B)
• Significant Variations
– Up to 100% have been
observed, e.g., <10 to >20
mg/L
A
B
Seasonal Pumping Variations
• In winter months,
dashed lines,
groundwater flow, and
therefore the capture
zone of a domestic well
may differ from that
during irrigation
season when largecapacity wells alter the
flow pattern.
Between Wells Variability
• Recharge and/or
source related
• Wells screened in
different aquifers
• Wells screened at
different depths in
the same aquifer
Well Construction: Components
Water
• Bore Hole
• Concrete Slab: protects
against inflow of surface
water
• Casing Seal: Protects
against inflow of shallow
water
• Casing: holds hole open
• Screens/Perforations:
allow access of water
Slab
Casing Seal
Casing/Liner
Bore Hole
Screen/Perforation
Pump
Improper Well Construction
• Well A: Properly
sealed into confining
layer
• Well B: Screened in
both shallow and deep
aquifer
• Well C: Producing
from deep aquifer but
not sealed correctly
A
B
C
Summary
• Aquifer is shallow and unconfined in most
places
• Groundwater originates as infiltrating
precipitation
• Nitrate may show seasonal variations in a given
well
• Nitrate variability between wells related to
recharge, different screened levels in aquifer or
different aquifer
• Proper well construction important in limiting
the distribution of nitrate.