عدد المساهمات : 3484
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: ازالة ملوحة مياه البحار(كيفية تقييم ملوحة مياه البحار لتحديد طرق ازالتها)2 الخميس ديسمبر 02, 2010 11:28 am|| |
UNITS OF MEASURE
The most common unit of measure of trace elements, gases, ions and pesticides is parts per million (p.p.m.).
A part per million is the same as a milligram/liter (mg/l).
Smaller concentrations are sometimes reported as parts per billion (p.p.b.) which is the same as a microgram/liter(ug/l).
Larger concentrations are sometimes reported as parts per thousand (g/l).
Sometimes, and particularly with ionized components of salts, a measure called equivalents per million (e.p.m.) is used.
Equivalents per million is the same as the expression milliequivalents/liter (meq/l).
To convert e.p.m. to mg/l one can multiply by the following factors for each ion: calcium, 20; magnesium, 12.2; potassium, 40; sodium, 23; carbonate, 30; bicarbonate, 61; sulfate, 48 and chloride, 35.5.
Rarely the measure grains/gallon is used. One grain/ gallon equals 17.1 parts per million. Other peculiar units of measure are described under appropriate subheadings below.
MEASUREMENTSCOMMONLY REPORTED IN WATERTESTING
The pH isa measure of hydrogen ions in the water.
The pH scale spans a number range of 0 to 14 with the number 7 being neutral.
Measurements above 7 are basic and below 7 are acidic.
The farther a measurement is from 7 the more basic or acidic is the water.
Acid and alkaline (basic) death points for fish are approximately pH 4 and 11.
Growth and reproduction can be affected between pH 4 and 6 and pH 9 and 10 for some fishes.
pH of some ponds may change during the course of a day and is often between 9 and 10 for short periods of afternoons.
Fish can usually tolerate such rises that result when carbon dioxide, an acidic substance, is used up by plants in photosynthesis.
Water samples taken from such ponds are usually delayed for some time prior to laboratory testing and during the delay the pH can equalize and not show an extreme reading.
The most common pH problem for pond fish is when water is constantly acidic.
The nature of the bottom and watershed soils is usually responsible.
Such water has a stable and low pH that is only correctable with liming.
Salinity is the measure of the total concentration of all dissolved ions in water.
Sodium chloride is the principal ionic compound in sea water but most inland ponds contain substantial concentrations of other ionic compounds (salts) such as compounds of sulfate and carbonate.
Because sodium chloride is so important in measurement of marine waters, marine water laboratories sometimes base by proportion the value of salinity on a measure of the chloride of this salt (chlorinity).
Salinity also can be and often is measured according to the density the salts produce in the water, the refraction they cause to light by electrical conductance.
The result in all cases is reported in parts per thousand (ppt.) salinity.
Standard sea water is 35 ppt and inland surface waters of arid West Texas may range up to 10 ppt.
Well waters sometimes accumulate high amounts of dissolved ions due to ionization of compounds of underground minerals or as a result of leaching from the high salt content of arid land surfaces.
Most of the information which is available on aquatic animal tolerance to dissolved ionic material has been done as a result of exposure to various dilutions of sea water.
Examples for freshwater fish with highest tolerable levels in milligrams/liter for good survival and growth
Specific Conductance or Conductivity
As mentioned above, dissolved ionic substances can be measured by electrical conductance.
On laboratory reports this may be shown as specific conductivity.
Conductivity is reported as micromhos/cm or ECX106.
From such an electrical measure, tables can be used to derive tons/acre-ft., parts per million, grains/gallon, etc.
Natural surface waters would be expected to have conductivities that measure from 50 to 1500.
Where well waters from saline ground water strata are used, the conductivity could run higher.
Problems with most freshwater fish species should only be encountered at measurements of over 15,000.
Another measure reporting the presence of dissolved ionic constituents is total dissolved solids.
This measurement is made by weighing the residue of an evaporated sample after it has passed through filter paper.
If the sample is not filtered the reported value will be that of total solids (TS) instead of total dissolved solids(TDS).
Because ionic compounds dominate the content of dissolved substance in most water samples, TDS reflect closely the numerical quantity derived in the measure of salinity.
TDS is reported in mg/liter of whole sample. In both surface and ground waters of inland areas where TDS exceed 2000 mg/liter the principal anions are sulfate and chloride.
The measure of these ions in mg/liter when added together will usually approximate ½ the measure of TDS.
Although not the same as salinity where sodium chloride may greatly predominate as with seawater, TDS must sometimes serve as the only measure on which to make decisions regarding dissolved salts.
As mentioned above for salinity, 2000 mg/liter (2 parts per thousand) is known to adversely affect sensitive species or younger stages of some species.
Catfish are known to handle 6000 to 11,000 mg/liter salinity quite well depending on acclimation. improvement.
Sediment (mud) is removed and analyzed to determine the amount of ground limestone needed to bring the pH of soils to an acceptable level and hold it there for 2 to 5 years.
For aquaculture use the test is designed to determine the amount of lime necessary to raise the pH of the bottom mud to 5.8 and raise the water hardness to acceptable levels.
The lime requirement of the bottom mud must be satisfied before lasting effects can be expected in the water column.
The lime requirement is reported in pounds/acre and may amount to one to several tons of lime per acre. Liming efforts may be futile where sodium presence is great or muds are extremely acid.
Suspended Solidsand Turbidity
Suspended solids (unfiltered residue) will measure less than 2000 mg/l in muddy pond waters.
Many times this amount is needed to directly affect fingerlings and adult fishes.
Muddiness can affect natural food production at 250 mg/liter suspended solids by shutting out sunlight and can interfere with reproduction of some fish at less than 500 mg/liter.
Sometimes turbidity is used as a measure of suspended solids and is given in turbidity units. Turbidity units are derived by light transparency.
Turbidity in fertile surface waters is largely due to organic material, particularly algae.
Expected measurements would be: clear, 2 units; and algae-rich, 200 units.
Trace elements are those ionic constituents of water that dissolve to a small extent even though the concentration in the soil of bottom and watershed may be considerably greater.
Some elements are routinely included in reports of irrigation water and soil tests because of the impact on plants.
Boron is most common in this category.
More generalized water reports will likely include certain metals which could have toxic effects and some laboratories offer metal analyses as special order items.
Aquatic organisms of all types are sensitive to metal poisoning when the concentration of these reaches a certain level in the water column.
Certain fish groups tend to be more sensitive than others to particular metals.
Copper for example is more toxic to rainbow trout than to channel catfish. Exact levels of tolerable metals in solution which are considered safe for aquatic Authors of some reference books have searched out the lowest recorded level of a substance which for one reason or another was considered safe and then reduced that measure still ten fold to set a standard for a clean bill of health.
The toxic nature of metals is very much influenced by the hardness of the water.
A metal may poison fish in very soft water at a rate that would have to be increased ten fold to produce the same effect in hard water.
Because the concentration of a metal required to produce toxicity may differ according to the overall water chemistry a clear statement to the toxicity of the
various metals is difficult to find.
The recommendation on how to determine actual toxicity in a certain body of water then is often merely to conduct a bioassay.
Availability of trace elements to do toxic damage is much affected by water hardness, dissolved organics and suspended clays.
Toxic action is also influenced by the form (i.e., free ion, bound in organic compound) in which the element is present.
Trace elements can be toxic and some are essential for health of aquatic life.
If a report form shows dashes or zero in the appropriate space the element is probably in the water but present at concentrations too low for analytical detection.
The following trace elements are normally present in unpolluted surface waters at concentrations of less than one mg/liter: aluminum, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, iron, lead, manganese, mercury, molybdenum, nickel, selenium, silver, and zinc.
Except for aluminum, arsenic, barium, and iron, the elements of the previous list should be considered potentially harmful when present in concentrations above 0.1 mg/1.
Dissolved gases should be given consideration in determining the basic suitability of water for fish survival.
Dissolved gases include oxygen, carbon dioxide, nitrogen, ammonia, hydrogen sulfide, chlorine and methane.
Dissolved gases are usually not found on water analysis report forms because the manner by which samples are collected and shipped can cause gas measurements to be much unlike the actual on-site water condition.
Ammonia is the most stable of the group and if a sample is processed within a day after collection it should measure fairly accurate.
Other measures are best taken at the water site using appropriate meters or chemical test procedures. Dissolved oxygen gas is especially important because of vital need of the fish life and the facts that it can vary greatly in natural surface water and is characteristically absent in groundwaters.
Most aquatic animals need more than a 1 mg/l concentration for survival and, depending on culture circumstances more than 3 to 5 mg/l to avoid stress.
Concentrations considered typical for surface water are influenced by temperature but usually exceed 7 to 8 mg/l.
Carbon dioxide is present in surface water at less than5 mg/l concentrations but may exceed 60 mg/l in many well waters and 10 mg/l where fish are maintained in large numbers.
Some aquatic animals, including fish, can endure stress and survive at up to 60 mg/l but where oxygen is lowered into its stress-causing range this carbon dioxide limitation is reduced to 20 mg/l.
Hydrogen sulfide is present in some well waters but is so easily oxidizable that exposure to oxygen readily converts it to harmless form.
Its toxicity depends on temperature, pH, and dissolved oxygen.
Any measurable amount after providing reasonable aeration could be considered to have potential to harm fishes life.
Ammonia is present in slight amounts in some well an pond waters.
As fishes become more intensively cultured or confined, ammonia can reach harmful levels.
Any amount is considered undesirable but stress and some death loss occurs at more than 2 mg/l and at more than 7 mg/l fish loss can be expected to increase sharply.
Chlorine is rarely not listed on report forms but is usually present at approximately 1 mg/liter in municipal water supplies as a result of chlorination.
Fish will succumb quite easily at these levels and in some cases ponds have been filled with chlorinated water causing a fish loss.
Nitrogen and methane are normally not considered to play a critical role. Nitrogen could be considered an exception when it contributes to an abnormal total dissolved gas concentration.
Total gasses may be driven high in waters that are plunged deeply at dam outfalls or
experience pump cavitation. As total gas concentrations exceed 115% of normal amount, fish are affected by bubble formation in the blood.
Total gases is a measure sometimes used in aquatic analyses but is not often seen in laboratory reports.