A Primer on Water Quality: Agricultural Contaminants - Background Information

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 Agricultural contaminants
Agricultural contaminants includes sediment; agricultural chemicals such as inorganic fertilizers, herbicides and insecticides; organic matter such as animal wastes and decaying plant material; irrigation residues like salts and trace metals; and microorganisms. Plant nutrients such as nitrogen (N) and phosphorus (P), present in organic and inorganic fertilizers, are considered contaminants when they enter a water supply.

Sediment is both a pollutant and a carrier of pollutants (Robbins 1979). Soil erosion can result in high sediment loads to surface water. Sediments become suspended in water and cause turbidity. Turbid water can impair water treatment processes while excessive sediment deposition can degrade habitat for aquatic plants, fish and other aquatic organisms, ultimately impacting the entire food chain. In addition, excessive deposition of sediments can fill in valuable wetland areas and reduce the capacity of water storage facilities. Some contaminants can attach to and be carried with suspended sediments (e.g. arsenic, lead, mercury and phosphorus) to surface water.


  • Phosphorus
    Sources of phosphorus and nitrogen include animal manures, commercial inorganic fertilizers, phosphate-containing detergents and natural levels found in soils. Phosphates tend to bind tightly to soil particles and therefore, are much less likely to leach into ground water and more likely to contaminate surface water (Council for Agriculture Science and Technology (CAST) 1992).

    Elevated concentrations of nutrients such as phosphorus and nitrogen accelerate aquatic weed and algal growth in surface waters. Excessive weed and algal growth deteriorates water quality and is termed eutrophication. Large blue-green algal blooms can produce toxins which are responsible for wildlife and domestic animal (e.g. cattle, dogs) deaths (Kotak et al. 1994). When large algal blooms and aquatic plants die off and decompose, a lot of oxygen is used up by microorganisms and bacteria which break down the decomposing organic matter. Plant respiration and decomposition can lead to severe reductions in dissolved oxygen in the water. This depletion of oxygen increases the biochemical oxygen demand. Fish and other aquatic organisms can die from the lack of dissolved oxygen. In addition, anoxic (no oxygen) conditions at the sediment-water interface in deep lakes can facilitate the release of phosphorus from the sediments to overlying lake water. This release of phosphorus increases algal growth once the phosphorus-rich bottom water circulates to the surface where algae can grow.

  • Nitrogen
    Nitrate-nitrogen is water soluble and can move readily through the soil profile into ground water. Agricultural sources of nitrate include manure, nitrogen-containing inorganic fertilizers and legume rotations. Other sources of nitrate in the environment include septic systems and naturally decaying organic matter (CAST 1992). High nitrate concentrations can cause methaemoglobinaemia (blue baby syndrome) in young infants as nitrate is converted to nitrite which compromises the ability of the blood to reversibly interact with oxygen, thus depriving tissues of oxygen (CAST 1992). High ammonia concentrations can cause similar oxygen depriving effects in fish. High nitrate concentrations in water used for livestock can result in weight loss and poor feed conversion (CAST 1992).
Pesticides - herbicides and insecticides
Herbicides and insecticides are transported to surface waters in runoff either in solution or attached to sediment, or are leached through the soil profile to ground water. Herbicide and insecticide movement is complex because of specific chemical properties such as: 1) solubility - how easily a chemical dissolves in water to become a solution; 2) persistence - how long it takes a chemical to break down; and 3) adsorption - the ability of a chemical to attach itself to soil particles. Persistent herbicides and insecticides can extend beyond target weeds and insects when introduced into aquatic environments. These chemicals can accumulate up the food chain whereby top predators (e.g. fish-eating birds) can consume toxic dosages.

Microorganisms, such as bacteria, viruses and protozoans (e.g. Cryptosporidium and Giardia), cause serious health risks to humans if the contaminated water is not treated prior to consumption. Both surface and ground water can become contaminated by microorganisms through direct land application of sewage effluent, septic systems and improper manure storage and handling. Health risks associated with microorganisms include respiratory, gastrointestinal, eye, ear, skin and allergy illnesses and can be potentially fatal to immunocompromised patients such as cancer or autoimmune deficiency syndrome (AIDS) patients (Edwards 1993).

Salts and trace elements
Salinity refers to the total concentration of soluble salts present in water (CAST 1992). Naturally occurring salts include calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K) salts with chloride (Cl), sulfate (SO4) and bicarbonate (HCO3). Trace elements such as selenium (Se), boron (Bo), and molybdenum (Mo) are natural constituents of soils or underlying geologic materials. When trace elements are present in ground or surface waters above critical concentrations, growth of certain crops can be limited. Also, animals can be harmed by consuming plants with high concentrations of trace elements (CAST 1992).

Pathways of agricultural pollutants
Contaminants generated by agriculture can vary according to the type of agricultural activities. Agricultural contaminants are carried to surface and ground water through all phases of the water or hydrologic cycle. They are generated by either point or non-point sources. A point source of pollution is effluent discharged from one location such as a pipe, tank, pit or ditch. Non-point source pollution originates from no well-defined source, such as runoff from a field or from streets in a city, making identification and assessment of sources difficult. Contaminants can be carried in runoff, precipitation, wind deposition, drainage, infiltration, ground water flow and ground water recharge to surface water.

A pollutant's characteristics determines how it moves in the environment and impacts surface waters. Pollutants move into surface waters through runoff flowing over compacted, saturated or frozen soil (overland flow) and through water moving laterally under the surface of the soil (subsurface flow). Pollutants can also move downward through the soil profile and infiltrate to ground water. Ground water is contained in interconnected pores located below the water table. Ground water can recharge lakes, rivers, streams, ponds, wetlands, or the soil surface.

Water quality criteria, guidelines, objectives and standards
Various terms such as criteria, guidelines, objectives and standards are used to assess water quality issues for specific water uses. The Canadian Water Quality Guidelines (CCME 2014) defines the terms as follows:

  • Criteria: scientific data evaluated to derive the recommended limits for water uses.
  • Water quality guideline: a numerical concentration or narrative statement recommended to support and maintain a designated water use.
  • Water quality objective: a numerical concentration or narrative statement which has been established to support and protect the designated uses of water at a specified site.
  • Water quality standard: an objective that is recognized in enforceable environmental control laws of a level of government.
Water quality guidelines are designed for specific uses of water including drinking water, recreational activities and aesthetics, freshwater aquatic life, agricultural uses such as irrigation and livestock watering, and industrial water supplies. Water quality guidelines vary for the different designated water uses and can vary among jurisdictions (states, provinces and countries).

Other Documents in the Series

  A Primer on Water Quality
A Primer on Water Quality: Agricultural Impacts on Water Quality
A Primer on Water Quality: Agricultural Contaminants - Background Information - Current Document
A Primer on Water Quality: Impact of Crop Production Practices on Water Quality
A Primer on Water Quality: Impact of Livestock Production Practices on Water Quality
A Primer on Water Quality: Pollutant Pathways
A Primer on Water Quality: Pollutant Processes in Rivers and Lakes
A Primer on Water Quality: References
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For more information about the content of this document, contact Barry Olson.
This document is maintained by Rupal Mehta.
This information published to the web on March 4, 2002.
Last Reviewed/Revised on June 11, 2018.