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 è Groundwater originates from precipitation sinking into the ground from the surface to the water table. è Groundwater occurs in the open spaces between silt, sand and gravel particles or in natural fractures in rocks (not underground rivers or lakes). è Groundwater moves from where the water table is high to where it is low. è How fast it moves depends on the slope of the water table and the aquifer permeability. è 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) 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 Sand6 d Fractured Bedrock60 d Silt300 d Limestone750 d Sandstone12 yr Hard Basalt175 yr Clay1700 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

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 (NH 4 + ) –Nitrite (NO 2 - ) –Nitrate (NO 3 = ) 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 + H 2 O) 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., 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 ABC

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.