Properties of Water & Water Quality R. Christian Jones George Mason University June 2008 Tidal Potomac Teacher Training Workshop Sponsored by Virginia.

Properties of Water &Water Quality

R. Christian JonesGeorge Mason University

June 2008Tidal Potomac Teacher Training Workshop

Sponsored by Virginia Environmental Endowment and Chespeake Bay Restoration Fund

Properties of Water• Molecular Structure• Water is H2O, two atoms

of hydrogen attached to one atom of oxygen

• Because the O atom has a greater mass the electrons spend more time near it than near the H atoms

• Thus, a charge asymmetry is created such that the O end of the molecule has a negative charge and the H end has a positive charge, this a called a dipole.

Properties of Water

• This charge asymmetry results in weak bonding of adjacent molecules

• The net result is that water has some unusual properties

• It is a liquid at “room” temperature when it should be a gas

• This because of the binding of adjacent water molecules

Properties of Water

• One interesting property of water is that its solid form (ice) is less dense than its liquid form (water)

• This is due to the fact that when ice forms the water molecules are arranged in a crystalline lattice with fixed distances whereas in liquid water the less extensive bonding results in tighter packing

Properties of Water

• Water molecules in motion: http://www.edinformatics.com/math_science/info_water.htm

Properties of Water• Density of water is thus a

function of temperature• Ice has low density which

becomes much higher when it melts to become liquid water

• As water warms it becomes more dense to 4°C, the temperature of maximum density of water

• As temperature increases above 4°C water becomes progressively less dense

• The rate at which it becomes less dense increases with increasing temperature

Properties of Water

• Water is considered the “universal solvent” because of its abilities to dissolve a wide range of chemicals

• This ability is related to its dipolar nature

• The polar parts of the water molecule can bind to negative and positive ions like chloride (Cl-) and sodium (Na+)

Properties of Water

• High Heat Capacity (calories required to heat 1 g of water by 1°C)

• High Heat of Melting and Evaporation (calories required to melt of evaporate 1 g of water)

• These two properties allow water to maintain a more constant temperature between day and night and change more slowly with the seasons

Properties of Water

• Transparency to light– Water is quite

transparent to light– Transparency is a

function of wavelength

– Blue light (460nm) penetrates best (Still 70% at 70 m)

– Red light (720nm) is absorbed very rapidly (only 1% at 5 m)

Properties of Water

• As water clarity decreases, wavelength of maximum penetration increases

• Water clarity decreases due to particles and dissolved colored substances in the water

• Green light penetrates best in less clear water

Numbers by curves are coefficients of attenuation which increase as water clarity decreases

Properties of Water

• High Viscosity (frictional resistance to a moving body) (high compared to air, low compared to honey)

• Surface Tension (surface “film” that allows some insects to skate on or hang from the air/water interface)

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– Dissolved oxygen– pH– Alkalinity– Hardness– Transparency – Secchi depth

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: 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 groundwater (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: 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

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: 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

• Humans and other terrestrial animals require fresh water for survival as do plants and animals normally found in freshwater

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

• 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)

O2

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 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

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: 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: pH

• Precipitation in our area has a pH of 4-4.5 and 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 air

• Many species are vulnerable to acid rain

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: 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: Alkalinty

• Streams, lakes and wetlands with moderate to high alkalinity are not subject to acidification whereas low alkalinity waters are very vulnerable

• Development tends to increase alkalinity somewhat due to the mobilization and washin of carbonates and bicarbonates

Water Quality: Hardness

• Hardness is a measure of the amount of calcium (Ca+2) and magnesium (Mg+2) ions in the water

• Hardness determines how hard or easy it is to lather soap

• The units are, like alkalinity, mg/L as CaCO3

• Hardness also affects the toxicity of heavy metals

• Hardness also tends to increase somewhat with increasing watershed development

Ca++

Mg++

Water Quality: Transparency• Water clarity is an important

element of water quality• Water clarity is generally

obscured by fine particles (e.g. algal cells, fine detritus, clays) suspended in the water. The effect of these particles is generally much greater than the natural absorbance of water

• Water clarity is important for primary production (plants and algae need light for photosynthesis)

• It also affects our aesthetic view of a water body

Water Quality: Transparency

• Transparency can be easily measured with a Secchi disc

• The Secchi depth (zSD) is that water depth to which the disc is visible

• The depth at the bottom of the photic zone (zPZ) is about 3 x the zSD

• zPZ corresponds with about 1% of surface light

Water Quality: Transparency

• At this 1% light level photosynthesis in a plant roughly equals its respiration cost

• Above zPZ plants can grow, below it they will not develop or will die

zPZ sets the lower boundary of the “littoral zone”

References

• http://waterontheweb.org/index.html

• http://perec.cos.gmu.edu

Basic WQ procedures

• Temperature, Conductivity, and Dissolved Oxygen using YSI Model 85– Turn on unit by pressing <ON/OFF>– Remove probe from holder and lower to depth of

measurement– Unit will continuously display temperature. Use MODE

button to toggle between parameters in this order:• DO (% saturation), DO (mg/L), Conductivity (uS), Specific

Conductance (uS, corrected for Temp), Salinity (parts per thousand), [rc1 (recall memory), Erase (erase memory)]

– It’s best to bob the probe slightly when measuring DO

Basic WQ procedures

• pH using the Hach Water Ecology Kit– Fill two glass sample tubes to the 5 mL mark with

water sample– Add 6 drops of Wide Range pH indicator solution to

one of the tubes– Insert this tube into the right opening of the color

comparator– Insert the tube of untreated water into the left opening

of the color comparator– Hold the comparator up to the light and rotate the

color disc until the two solutions are matched in color– Read the pH through the scale window

Basic WQ procedures

• Total Alkalinity using Hach Water Ecology Kit– Fill the mixing bottle to the 15 mL mark with sample

water– Add the contents of 1 BromCresol Green-Methyl Red

powder envelope to sample and swirl to mix. Solution should be greenish.

– Add sulfuric acid standard solution (0.03N) dropwise counting the drops until color changes from green to pink throughout

– Multiply number of drops by 7 to obtain Total Alkalinity in mg/L as CaCO3

Basic WQ procedures

• Total Hardness using Water Ecology Kit– Fill the mixing bottle to the 15 mL mark with sample

water– Add 6 drops Buffer Solution (Hardness 1) and swirl to

mix– Add 3 drops ManVer Hardness Indicator (Hardness 2)

and swirl to mix. Sample should be pink.– Add Titrant reagent (Hardness 3) dropwise sirling the

mixing bottle while the drops are counted until the solution changes from pink to blue.

– Multiply number of drops by 7 to obtain Total Hardness in mg/L as CaCO3

Basic WQ procedures• Nitrate using Water Ecology Kit

– Fill one of the viewing tubes to the mark with DI water and shake vigorously. Empty and repeat to clean tube

– Rinse the plastic dropper with DI water.– Fill plastic dropper to the 1 mL mark with sample and add to color

viewing tube. Fill to the mark with DI water.– Add contents of one NitraVer 6 packet to the sample. Stopper the tube

and shake for 3 minutes. Allow the sample to settle for 30 seconds (Cd metal will settle).

– Pour contents of the prepared sample into a second tube carefully allowing metal particles to remain.

– Add contents of one Nitrite reagent packet to the sample. Stopper and shake for 30 seconds.

– Allow 10 minutes for color development, then insert prepared sample into right opening in color comparator.

– Rinse Cd metal from first tube (into designated waste container). Then fill first tube to the mark with sample water. Place this tube in left opening.

– Hold color comparator to light source and rotate disc until colors of two tubes match. Read the mg/L nitrate through the scale window and multiply by 2.2 to get nitrate nitrogen in mg/L.

Secchi Disc Procedure• Water clarity influences a variety of stream characteristics. A

measure of water clarity is important as an indicator of many different processes at work in the system. The simplest method of measuring water clarity is a secchi disk. While a secchi disk is typically used to measure water transparency in large, deep water-bodies, it can be used to measure water clarity in streams. The exception to this is in shallow, fast running streams. A secchi disk is a 20cm disk with black and white quadrants. For stream work, the disk is attached to a wooden calibrated rod. The secchi depth measurements described below are read from calibrated marks on the rod.Field instructions:1. Lower the disk into the water until it disappears from view and record this depth.2. Drop the disc several cm more.3. Slowly pull the disk back up and record the depth at which it reappears.4. Calculate the secchi depth by averaging these two measured depths.