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Water Quality. Water Quality Variables. Water quality refers to the basic chemical and physical characteristics of water that determine its suitability for life or for human uses Some basic water quality variables include: Temperature Conductivity pH Alkalinity Dissolved oxygen Hardness

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water quality variables
Water Quality Variables
  • Water quality refers to the basic chemical and physical characteristics of water that determine its suitability for life or for human uses
  • Some basic water quality variables include:
    • Temperature
    • Conductivity
    • pH
    • Alkalinity
    • Dissolved oxygen
    • Hardness
    • Turbidity


water quality temperature
Water Quality: Temperature
  • Temperature is a basic water quality variable
  • Temperature determines the suitability of water for various forms of aquatic life
  • For example, trout and salmon require cool temperature for survival and reproduction whereas bass and sunfish do better at warmer temperatures
  • Temperature in water bodies generally follows mean daily air temperature


water quality temperature1
Water Quality: Temperature
  • Temperature in water bodies generally follows mean daily air temperature
  • Temperature does not vary as much diurnally as air temperature
  • Temperature may be moderated by ground water (cooler in summer, warmer in winter)
  • Temperature may be increased by removal of riparian vegetation
  • Temperature may be increased by lake surface inflows


water quality temperature2
Water Quality: Temperature
  • Development of watersheds generally results in increased summer temperatures
    • Pavement is heated by the sun and warms water flowing on it
    • Riparian vegetation is removed and the sun heats streams, lakes and wetlands directly



10.4 Influence of temperature on growth rates



Daily Growth Rate (mg/mg/day)










Temperature C

Modified from Benke 1993

water quality conductivity
Water Quality: Conductivity
  • Conductivity measures the ability of water to conduct an electrical current
  • Conductivity is a good way to determine the ionic strength of water because the ability of water to conduct a current is proportional to the number of ions in the water


water quality conductivity1
Water Quality: Conductivity
  • Freshwater generally has low conductivity measured in microSiemens (uS)
  • Estuarine and marine systems have much higher conductivity measured in milliSiemens (mS) which can easily be converted to salinity
  • Ground water typically has higher levels of conductivity than surface water


water quality conductivity2
Water Quality: Conductivity
  • Another difference between freshwater and salt water is that freshwater is a dilute solution of calcium bicarbonate whereas salt water is a more concentrated solution of sodium chloride
  • Development of watersheds generally increases conductivity due to the increase in chemicals applied to soils and paved surfaces which wash into streams, lakes and wetlands
  • Road salts can result in large increases in NaCl in freshwaters which are detrimental to freshwater life.


water quality dissolved oxygen
Water Quality: Dissolved Oxygen
  • Oxygen is required for most all living things
  • In air, oxygen is very abundant (20%, 20 parts per hundred)
  • In water, oxygen is more scarce
    • Water bubbled with air can only reach about 10 parts per million (mg/L)



water quality dissolved oxygen1
Water Quality: Dissolved Oxygen
  • Oxygen solubility in water is a function of temperature
  • As temperature increases, the amount of oxygen that the water can hold decreases
  • Since most aquatic organisms require at least 5 mg/L of oxygen, at warmer temperature oxygen can become a limiting factor


water quality dissolved oxygen2
Water Quality: Dissolved Oxygen
  • Decreases in dissolved oxygen are generally due to excess respiration in the water
  • Excess respiration is caused by input of organic matter such as sewage, garbage, and to some extent leaf litter
  • As oxygen decreases, water quality declines
  • Oxygen levels above 100% indicate rapid photosynthesis which may eventually lead to other water quality problems


In most unpolluted streams dissolved oxygen is near saturation and is, therefore, of minor biological concern.

Current serves to deliver oxygen rich water to respiratory structures.

Fish can actively move water over gills, however, most invertebrates cannot and are dependent on current to deliver oxygen.

These organisms are referred to as respiratory conformers: their respiratory rate closely follows the ambient oxygen concentration.



oxygen related adaptations
Fish actively swim and draw water over their gills

Macroinvertebrate larvae with conspicuous gills are respiratory conformers

Midge larvae with hemoglobin are adapted to live in poorly oxygenated stream sediments




Oxygen related adaptations


water quality ph
Water Quality: pH
  • pH is a measure of the acidic or basic characteristics of water
  • Specifically, pH = -log [H+]
  • So a pH of 7 means the [H+]=10-7
  • A pH of 7 is considered neutral, below 7 is acidic and above 7 is basic


water quality ph1
Water Quality: pH
  • Organisms generally prefer a pH of 5-9 with some being even more picky
  • A near neutral pH will allow the greatest diversity of life


water quality ph2
Water Quality: pH
  • Precipitation with a pH of 4-4.5 is called “acid rain”
  • This is due to air pollution by NOx and SOx from vehicles and power plants
  • Even where there no pollution, rain has a pH of about 5.4 because of dissolution of CO2 from the aie
  • Many species are vulnerable to acid rain


water quality alkalinity
Water Quality: Alkalinity
  • The ability of a water body to resist acidification is called alkalinity
  • Alkalinity is generally attributable to the amount of carbonate (CO3-2) and bicarbonate (HCO3-) in the water
  • Alkalinity is generally measured in units of mg/L as CaCO3


water quality alkalinity1
Water Quality: Alkalinity
  • CO3-2 can absorb one H+ to become HCO3-
  • HCO3- can absorb another H+ to become H2CO3
  • By absorbing H+ ions, these two ions can decrease the [H+]


water quality alkalinity2
Water Quality: Alkalinity
  • Streams, lakes and wetlands with moderate to high alkalinity are not subject to acidification whereas low alkalinity waters are very vulnerable
  • Alkaline soils can buffer acid precipitation (limestone)
  • Development tends to increase alkalinity somewhat due to the mobilization and washing of carbonates and bicarbonates
  • Alkalinity in streams is influenced by rocks and soils, salts, certain plant activities, and certain industrial wastewater discharges.


where does alkalinity come from in water
Where does alkalinity come from in water?
  • Detergents and soap-based products are alkaline)
  • Limestone geology contains large quantities of calcium carbonate (CaCO3 )
  • Granite and volcanic bedrock is deficient in alkaline materials
  • Addition of lime as a soil amendment can runoff into surfacewaters and increase alkalinity.
  • Low nutrient)(oligotrophic) lakes tend to have lower alkalinity
  • Eutrophic (high nutrient) lakes tend to have higher
  • alkalinity.
  • Naturally occurring form of nitrogen
  • Important plant nutrient;
  • Increased nitrate in streams can lead to algal blooms
  • Elevated nitrate in drinking water can also cause human health problems.
  • Elevated amounts of nitrate in streams are a sign that inputs from human sources have increased or that that plants in the system are under stress.
  • Elevated stream nitrate might come from land clearing, the use of fertilizer in the watershed, or from rain and snowfall (in the form of acid rain).
ammonia nitrogen
Ammonia - Nitrogen

Equilibrium Reaction - Ammonia

NH4+ + OH -  NH3+ H2O

ammonium ammonia

Increase in pH

Increase in temperature

Ammonia is converted to nitrite and nitrate by nitrifying bacteria

  • Water quality
    • Temperature
    • pH
    • Alkalinity
    • Nitrate and ammonia
  • Think about role of ground water, surface water and precipitation in water quality