TDS, Specific Conductivity, and Gasses in Water Stream Ecology January 29, 2009

Primarily, Total Dissolved Solids (TDS) is made up of the following ions: –Ca 2+, Mg 2+, Na +, K +, Cl -, SO 4 2-, HCO 3 -, SiO 2 TDS is not synonymous with specific conductivity.

Conductivity is a measure of the ability of an aqueous solution to carry an electric current. This ability depends on the concentration of ions and temperature of the water (the warmer the water, the higher the conductivity). Organic compounds like oil, phenol, alcohol, and sugar do not conduct electrical current well.

Conductance, G, is defined as the reciprocal of resistance, R: G = 1/R Where the unit of R is ohm and the unit of G is ohm -1 or mho. G can also be reported as a siemen (1 siemen = 1 mho)

Conductivity is usually reported in microsiemens (μS/cm) or micromhos (μmho/cm) which is 1/1,000,000 of a siemen or mho. Conductance is measured using two electrodes 1 cm apart.

Since the charge on ions in solution facilitates the conductance of electrical current, the conductivity of a solution is proportional to its ion concentration.

TDS and Conductivity The method most commonly used to estimate TDS levels relies on its’ relationship with conductivity. In most waters, the correlation between TDS and conductivity is; –TDS/conductivity = 0.55 – 0.7 A TDS meter does not directly measure dissolved solids but rather, it directly measures conductivity and has a built-in calibration curve that estimates TDS levels.

Weigh an empty beaker (tare weight). Filter a known volume of sample water into the beaker (using a 0.2 micron filter). Allow the water in the beaker to evaporate. Re-weigh the beaker. The increase in weight is proportionate to the amount of total dissolved solids in the sample. Measuring TDS

Fresh waters - sufficiently dilute to be potable (TDS < 1,000 mg L -1 ). Brackish waters - too saline to be potable, but significantly less saline than seawater (TDS = 1, ,000 mg L -1 ). Saline waters - similar to or more saline than seawater (TDS ~ 35,000 mg L -1 ). Brines - significantly more saline than seawater.

Graphical Displays of Water Composition Usually reported in milliequivalents (one-thousandths of an equivalent). An equivalent is the amount of a substance (mg or mM) that can enter into a reaction, or replace, one mole of H+.

Stiff Diagrams Ion concentrations in meq L -1 are plotted on the horizontal axis. Cations are plotted to the left, anions to the right, of a vertical axis. The data are plotted in four rows and the points are connected to form a polygon.

Piper Diagrams Cations are plotted on the Ca-Mg-(Na + K) triangle as percentages. Anions are plotted on the HCO 3 - -SO Cl - triangle as percentages.

Displaying general water chemistry results graphically is a quick method of categorizing waters based on major ionic form. They are not very detailed and can not easily be accommodated for waters where other cations/anions may significantly contribute to solute load.

Alkalinity and Bicarbonate Buffering Alkalinity is a measure of the buffering capacity of a solution to resist a change in pH. Because pH is a measure of the number of hydrogen ions in a solution, alkalinity represents the ability of a solution to absorb or take up hydrogen ions.

Alkalinity results from the dissolution of calcium carbonate (CaCO 3 ) from limestone bedrock which is eroded during the natural processes of weathering. The carbon dioxide (CO 2 ) released from the calcium carbonate into the streamwater undergoes several equilibrium reactions.

If hydrogen ions are added to the solution, they combine with available bicarbonate or carbonate ions causing the reactions to shift to the left and eventually liberate carbon dioxide and water molecules. Addition of carbonate (or bicarbonate) to the solution causes the hydrogen ions to be occupied and shifts the reaction back to the right.

Total alkalinity in streams is reported as mg/L AS CaCO 3. Alkalinity is measured by titrating the amount of sulfuric acid needed to bring the water sample to a pH of 4.2.

The dominant form of inorganic carbon in water is pH dependent

Diel Fluctuations in pH Photosynthesis of aquatic plants (primarily benthic algae in streams) causes pH levels to rise during the day due to incorporation of CO 2 into cells. 6CO 2 + 6H 2 O + Light Energy = C 6 H 12 O 6 + 6O 2

In the absence of light, respiration occurs. C 6 H 12 O 6 + 6O 2 = 6CO 2 + 6H 2 O + Energy

Dissolved Oxygen and Gasses in Water

Of all physico-chemical properties of water, none are more important for biota than dissolved oxygen dynamics. DO influences nearly all chemical and biological processes within water bodies.

Solubility/Temperature Rules If the dissolution process absorbs energy, then solubility will be increased with increasing temperature. If the dissolution process releases energy, then solubility will decrease with increasing temperature.

Dissolving gasses in water releases heat. Solubility of oxygen (or any gas) is decreased at higher temperatures. How does this effect stream organisms in Arizona differently than it might those in, for example, Minnesota?

The DO concentration at any given time reflects a balance between various sources and sinks

At a temperature of 0 o C and normal atmospheric pressure at sea level (760 mmHg) and in “pure” water, the solubility of oxygen in water is 14.6 mg/L.

Henry’s Law The concentration of dissolved gas in solution depends on the partial pressure. The partial pressure determines the number of “collisions” of gas molecules with the surface of the water. An increase in partial pressure results in more collisions and therefore, more gas goes into solution (oxygen).

ρ = K c c ρ = partial pressure of the gas in mmHg c concentration of gas in mmoles, mL, or mg/L at a constant temp. K c the solubility factor, different for each gas

There is a direct, linear relationship between the partial pressure and the concentration of gas in solution. For example, if the partial pressure is increased by 1/4, the concentration of gas in solution is increased by 1/4 and so on. This is because the number of collisions of gas molecules on the surface of the water is directly proportional to increases or decreases in partial pressure.

What affects atmospheric pressure and therefore, the solubility of oxygen in water?

In most situations, knowing % saturation of dissolved oxygen is more important, biologically, than knowing mg/L or ppm. % Saturation = (DO mg per L/ Saturation Level) x 100

Factors Affecting Dissolved Oxygen Sources –Turbulence, atmospheric, photosynthesis Sinks –Suspended sediment, increased heat, respiration (algae and bacteria), excess nutrients, organic waste.

Stream Impairment due to Excessive Organic Loading Domestic sewage contains about 300 to 400 mg/L of organic compounds, 60% of which is readily degradable by bacteria commonly found in nature. Readily degradable implies that most of the material will be decomposed within about a week in a stream.

As population centers grow, effluent dependence and dominance of streams will increase

Roger Road WWTP Tucson Santa Cruz River

Linear profile at the Santa Cruz River below RRWWTP, 06/2003

Diel profile at the Santa Cruz River below RRWWTP, 06/2003

Linear Profile from Jacks Canyon, 06/2003

Jacks Canyon diel profile, 06/2003

Measuring Oxygen Demand The biochemical oxygen demand (BOD) is the most commonly used parameter in the analysis of oxygen resources in water. The BOD is the amount of oxygen consumed over time, usually 5 to 20 days, as the organic matter is oxidized both microbially and chemically.

Chemical Oxygen Demand COD is used as a measure of the oxygen equivalent of the organic matter content susceptible to oxidation by a strong chemical oxidant.

Although COD is comparable to BOD, it actually measures chemically oxidizable matter. The COD test is not a direct substitute for the BOD test; however, a ratio usually can be correlated between the two tests. This requires COD versus BOD testing over a specified period of time.

Measuring Dissolved Oxygen

Dissolved Oxygen Meters

The material of the membrane is specially selected to permit oxygen to pass through. Oxygen is consumed by the cathode which will create a partial pressure across the membrane causing oxygen to diffuse into the electrolyte solution. Thus, a DO meter actually measures the pressure of oxygen in water.