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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.
Types of Aquifers
• Unconfined– Shallow– Local Recharge– Water Table– Susceptible
• Confined– Deeper– Confining Layer– Distant Recharge– Water under pressure– Less susceptible
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)
Pt
Ds
Bc
Or
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 6 d
Fractured Bedrock 60 d
Silt 300 d
Limestone 750 d
Sandstone 12 yr
Hard Basalt 175 yr
Clay 1700 yr
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 SouthernWillamette Basin
Willamette Silt
Willamette Aquifer
WillametteConfining Unit
Bedrock: Sed orVol
0 to 20 feet
20 to 220 feet
Up to 300 feet
20 to 220 feet
Up to 300 feet
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 food-
processing 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
A B• 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
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 large-capacity 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
• 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
Pump
Water
Slab
Casing Seal
Casing/Liner
Bore Hole
Screen/Perforation
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.