Dissolved Oxygen Percent Saturation, or DO % Saturation is the actual concentration of DO in the water divided by the equlibrium saturation value times 100%:

DO % Saturation = 100% x (DO concentration in mg/l)/(DO equilibrium saturation in mg/l)

As discussed below, the DO equilibrium saturation value can be determined if temperature, salinity and atmospheric pressure are known.

DO Saturation and Temperature

Our common experience with dissolving solids leads us to believe that hot liquids can hold more dissolved material than cold liquids can:

You can dissolved more sugar in hot tea than in iced tea: conversion of solids to liquids is enhanced as more heat energy is supplied

But dissolved gases behave differently than dissolved solids. Qualitatively, think of dissolved gas molecules as being "trapped" within surrounding molecules of liquid water. Water molecules move more sluggishly when cold, giving the dissolved gas fewer opportunities to slip though gaps between them. As the water molecules jostle more and faster with increasing temperature, it is more difficult to keep the dissolved gas "trapped" in between.

Our experience with carbonated beverages supports this idea: unsealing a warm can or bottle of soda will produce more vigorous bubbling than unsealing a cold can. Dissolved gas is held less readily in warm soda, so it escapes more rapidly.

DO Saturation and Salinity

The solubility of substances is not only affected by temperature, but also by the presence of other dissolved substances. The so-called common-ion effect in chemistry...

1. milligrams per liter (mg/l), which is mass per unit volume, or
2. parts per million (ppm), which is the number of molecules per million molecules.

For fresh water without excessive turbidity, 1 liter has a mass very close to 1 kilogram, or 1 million milligrams. Under these conditions, 1 mg/l is essentially the same as 1 ppm.

Dissolved oxygen is typically measured in one of three ways:

• Colorimetrically (modified Winkler titration)
• Electrochemically (polarographically)
• By luminescence

Electrochemical (polarographic) measurement of dissolved oxygen requires a more-expensive meter attached to a DO probe. The DO probe is immersed in water, and dissolved oxygen diffuses through a plastic or teflon membrane to encounter two electrodes separated by potassium chloride solution. DO reacts with water at the gold cathode, a reaction which is balanced at the silver anode by the formation of silver chloride and attendant release of electrons. The resulting current flow between the electrodes is measured by the electronics of the meter and is proportional to the DO present. Because DO is consumed by the reaction, there must be sufficient flow of water across the membrane surface to maintain equilibrium. The instrument must also be calibrated frequently.

Luminescent measurement of dissolved oxygen is the latest (and most expensive) technology, which requires less-frequent calibration than the polarographic method. A thin film of luminescent material (luminophor) is stimulated by a blue light-emitting diode (LED). As the luminophor returns to its non-stimulated state, it emits red light which is measured by the photo diode (the red LED is used for internal reference/standardization). Dissolved oxygen quenches the luminophor response: the more DO, the faster the return to the non-stimulated state and the shorter the delay before red light is emitted back. The meter electronics measure the amount of delay, and convert that value into the concentration of dissolved oxygen.

Dissolved oxygen has two main sources:

1. Most comes from the atmosphere, and enters the water through molecular diffusion or from turbulent mixing;
2. Some DO comes from photosynthesis by aquatic plants (algae or macrophytes)

Conductivity meters operate by immersing two electrodes in water and imposing positive and negative voltages on them. Electrically neutral molecules (e.g., the water molecules themselves) are not strongly attracted to either electrode and thus do not contribute much to an electric current.

In contrast, positively and negatively charged ions are attracted to the electrode of opposite sign and move toward them, generating an electric current within the water that is measured by the meter. The more dissolved ions in the water, the greater will be the current generated.

Conductivity (G) is calculated from the measured current (I) and voltage (E) using Ohm's Law: G = I/E.

Units of measurement
Electrical conductivity (G) is the reciprocal of electrical resistance (R): G = 1/R. Conductivity units were once called mhos (the resistance unit ohms spelled backward). The Systeme Internationale (SI) ("metric system") subsequently decreed that all units had to named after famous scientists (hence we no longer have degrees Centigrade but rather degrees Celsius). The basic units of conductivity became Siemens, named after Werner von Siemens, a German electrical engineer who in the 19th century helped develop the telegraph industry.

Conductivity also must take into account the distance over which the current travels (consider the electrode spacing in the operation of the meter). It is therefore reported and Siemens per m (S/m) in standard SI notation.

In detail, conductivity in most natural waters are quite low, so is typically recorded in milliSiemens per centimeter (mS/cm) or microSiemens per centimeter (mS/cm), respectively a thousandth and a millionth Siemens per cm. For reference, normal sea water has a conductivity of about 42 mS/cm (~42,000 mS/cm).

"Full-featured" conductivity meters, which typically cost hundreds of dollars, usually allow the measurement of conductivity over a range of concentrations, and allow the user to select units of measure conductivity, specific conductance, salinity and/or total dissolved solids.

"Conductivity pens" are cheaper, typically $50-$100, but only measure over a fixed range and in one unit.

Pollutants

-road salt

-landfill/septic leachate

-impervious surface runoff

-agrictultural runoff

-first flush (graph)

Landfill: When landfills get old, or are not properly built, landfill and septic leachate can get into the groundwater and carried into the watershed increasing Conductivity.

Spreading Fertilizer: Image from: http://www.christinahandleystock.com /stock_photos/i_60428_141.jpg
Fertilizers from farms and lawns contain phosphate and nitrate which react with the soil and rain water and are carried to watersheds through runoff or groundwater and increase the conductivity in the watershed.

Precipitation-> distilled water->low Electrical Conductivity

-When water evaporates it leaves the dissolved ions behind and therefore has a low conductivity as rainfall.

Groundwater-> reactions with minerals->higher Electrical Conductivity

-Soil contains minerals which react with the groundwater resulting in a higher conductivity. The more turbid the water is, conductivity is typically higher. (salt water vs. soil water experiment)

Type of bedrock

-Water flowing over limestone which dissolves easily, will have a higher conductivity than water flowing over quartz which does not dissolve easily nor does it put alot of ions into the water when it does. Rocks containing calcite will also dissolve ions into the water resulting in a high conductivity.

Thermistors are "electronic thermometers" that measure the electrical resistance of an imposed current and use the relationship between resistance and temperature to convert that measurement to a temperature value. Many thermistors have the advantage of providing digital read-outs, reducing estimation error and, when combined with some kind of datalogger, allowing temperatures to be recorded at specified intervals over time.

Continuous Temperature Record from Nequasset Stream, Maine: Note the diurnal (daily) temperature fluctuations

Temperature probe (thermistor): image from www.campbellsci.com

Temperature logger (thermistor): image from www.onset.com

Turbidity is measured in one of two fundamental ways:

1. By observing the depth of water through which objects can be viewed (includes Secchi disks and turbidity tubes);
2. By using a turbidity meter (turbidimeter) that shines light into a water sample and measures electronically the scatter of that light by solid particles.