Importance of Water and Its Unique Properties
Regarding the importance of water, Water is one of the most important and abundant compounds of the ecosystem. All living organisms on the earth need water for their survival and growth.
Earth is the only planet having about 70 % of its surface covered by water. But due to increased human population, industrialization, use of fertilizers in the agriculture and man-made activity, the water bodies within the planet earth are highly polluted with different harmful contaminants.
Therefore, it is necessary that the quality of drinking water should be checked at regular time interval, as human population had suffered greatly from various water borne diseases due to the use of contaminated drinking water.
Water samples are collected and analyzed to ascertain characteristics of a body or mass of water.
The sample is usually only an infinitesimal part of the total volume and is therefore representative of the total mass only to the degree that uniformity of chemical composition exists within the total mass.
In their natural state, surface and ground water are subjected to forces that promote mixing and homogeneity.
The fact that such tendencies exist, however, is not sufficient cause for assuming that a body of water is so well mixed that no attention to sampling technique is required. Often, because of local conditions, the body of water may not have uniform composition.
The composition of water is subject also to change with the passage of time. The chemical quality of surface or ground water is the resultant of the geologic, hydrologic, biologic, and cultural environment of the water and varies from time to time as well as from place to place.
Generally, changes in the quality of surface water are more pronounced and rapid than in ground water. However, marked changes in ground-water quality can, and often do, accompany such shifts in hydrologic equilibrium as variations in recharge or discharge rate, salt-water encroachment, or induced infiltration of surface water.
Water Sampling
Sampling of water for Hydro-geochemical analysis can either be from surface water or groundwater. The type of investigation, purpose of the study, and the anticipated variation in chemical quality determine to a large degree, the location of the surface- or ground-water sampling site and the frequency of sample collection.
Samples of surface and ground water should be collected at intervals such that no important change in quality could pass unnoticed between sampling times. This requisite immediately gives rise to two additional questions:
What magnitude of change is important, and what are the physical and economic factors that must be considered in obtaining the record? By necessity the sampling schedule adopted is usually a compromise between the accuracy and detail desired in the water-quality record and the funds and personnel available for the work.
Surface Water Sampling
Sampling of surface water is usually carried out to determine the discharge of dissolved constituents past a point, to describe the changes in water quality with respect to time, to collect data that will aid in predicting water quality in the future or in estimating the nature and magnitude of past events, to study the effect of geologic, hydrologic and cultural changes on water quality, or a combination of these purposes.
Groundwater Sampling and Analysis
Groundwater taken from any given place contains a wide variety of dissolved constituents. We have a variety of means for analyzing water, both in the field and in the laboratory, which characterize the concentrations of dissolved solutes in the water.
Ground water is analogous to a surface-water reservoir in that most usable ground water is in motion, although the rate of movement may be very slow and the areal extent very wide.
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A well can be considered as a sampling point in a large body of slowly moving water, which differs in chemical composition vertically as well as aerially. Most of the forces which cause mixing in bodies of surface water are absent or much weaker in ground- water reservoirs.
Turbulence is virtually nonexistent. The major forces that tend to mix ground water are probably the differences in velocities as the water moves through material of heterogeneous permeability, pressure differentials and, to a lesser extent, ionic diffusion.
The degree of movement induced by pump age and natural discharge affects the quality.
The diversified nature and solubility of the rocks with which the water comes in contact and variations in rate and chemical composition of recharge from precipitation and from the surrounding area tend to make the water heterogeneous.
Sampling programs are planned to determine the mineral content of ground water through the aquifer, although a completely comprehensive answer is not always practical.
Efficient collection of water-quality data and intelligent selection of the ground-water sampling site generally require more judicious consideration than the selection of a surface-water sampling site because the elements influencing water quality are not as easily observed.
Because of the diversified purposes of ground-water investigations, it is impractical to attempt specific direction for the selection of sampling sites. Nevertheless, some general suggestions can be given.
If changes in ground-water quality are not considered in the investigation, there are perhaps two equally satisfactory approaches to the problem of adequate and economical coverage of ground-water quality of an area; both employ comprehensive and partial analyses.
One approach utilizes the determination of key constituents (one or several) in a large number of samples collected over the entire area.
By this means an aerial water-quality pattern is developed that is then of value in selecting the sites for collection of samples for comprehensive analysis. In some investigations, the identity of the key constituents may be unknown at the beginning of the investigation.
Then, the reverse approach may be required, and comprehensive analyses may be made early in the study, and these data augmented by partial and additional comprehensive analyses of samples collected at other sites to complete the water-quality picture.
Either method requires the greatest of care in the selection of sampling points from available sites.
When analyzing groundwater, it is common practice to measure temperature, pH, alkalinity, total dissolved solids (TDS), and specific conductance.
Water pH: is defined as the inverse log of hydrogen ion activity in the water (activity is basically like concentration). For example, a pH of 7 means that the activity (or concentration) of hydrogen in the water is equal to 1 x 10-7 mol/L. A pH of 5.6 means that hydrogen activity is equal to 1 x 10-5.6 mol/L.
Alkalinity: refers to the ability of the water to neutralize an acid. This is directly related to specific dissolved species in the water; in most groundwater systems, the predominant acid-neutralizing species is the bicarbonate ion.
Therefore, alkalinity is usually used as a direct measurement of the concentration of bicarbonate in groundwater.
Alkalinity is usually measured in the field, when the samples are taken, because exposure to air can cause some of the bicarbonate in solution to turn into carbon dioxide and leave the water.
The total dissolved solidsrepresent the total concentration of dissolved constituents in the water; usually measured by evaporating a liter of the sample and measuring the weight of the remaining solids. TDS is measured in mg/L.
Shallow groundwater (within 200 meters of the surface) generally ranges from 100 mg/L to 10,000 mg/L TDS (seawater is 35,000 mg/L). The standard limit for drinking water is generally considered to be less than 1000 mg/L, with less than 500 preferable (although people can tolerate up to 2000 mg/L).
Specific conductanceis a measure of the ability of the sample to conduct electricity. This acts as an approximation of the TDS, since the electrical conductivity of water is a function of the amount of dissolved material in the water.
In addition to these basic chemical parameters, we also analyze for the concentration of individual constituents in the water. Most of the dissolved constituents in groundwater are inorganic cations and anions derived from the dissolution of minerals.
In any natural groundwater sample about 95-99% of the dissolved constituents (by weight) will consist of the following: Bicarbonate, Calcium, Chloride, Magnesium, Silicon, Sodium, Sulfate, and Carbonic acid.
These are usually referred to as the major constituents or primary constituents, and generally have concentrations greater than 5 mg/L.
These constituents come directly from the dissolution of mineral phases in the subsurface, and a set of laboratory analyses will generally test for them.
Minor constituents, which generally occur in concentrations ranging from 0.1 –10 mg/L, include: Boron, Carbonate, Fluoride, Iron, Nitrate, Potassium, and Strontium.
In addition to these, there are a host of trace constituents that occur in concentrations less than 0.1 mg/L. Some of these are heavy metals that are toxic in small quantities.
These constituents are generally not analyzed for. If we analyze a water sample for the major and minor constituents, we will identify just about all of the mass dissolved in the sample.
There are ways that we can check our analytical results, to make sure that we are measuring correctly.
If we add up the concentrations of the individual species, as determined by various lab procedures, we can compare that sum to the total dissolved solids concentration as determined by evaporation.
If the two are different, then most likely there is a problem with the lab analyses Water is electrically neutral; for every positive ion in solution, there is an equal number of negative charges to offset the charge.
We can do a charge balance to see if the positive charges equal the negative. This is done by converting the concentrations to meq/L, then adding up the meq of the cations and subtracting from that the meq of the anions.
Ideally, they should be within 5% of each other (owing for analytical uncertainty and the trace elements that were missed in the analysis.
Sample Containers
Factors that are pertinent in selecting containers used to collect and store water samples are; Resistance to solution and breakage, Efficiency of closure, Size, Shape, Availability,, and Cost.
Preferences for one type of container over another varies, and selection is guided largely by experience, supposition of the possible effect of the container on the water sample, and use of containers in the individual laboratories.
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No adequate study of all factors has been made. Hard rubber, polyethylene and perhaps other plastics, and some types of borosilicate glass are believed to be suitable on the basis of experience and the reports of others in water chemistry.
A limited investigation, conducted by the American Geological Survey Agency, of the relative merits of four common types of bottles showed that storage in Pyrex and polyethylene did not significantly alter the silica, sodium, total alkalinity, chloride, and boron content, or the specific conductance, pH, or hardness of the water during a storage period of about 5 months, although some investigators have avoided Pyrex because of suspected contamination from the boron in the glass.
Before use, all new bottles must be thoroughly cleansed, filled with water, and allowed to soak several days.
The soaking removes much of the water-soluble material from the container surface. Because of their design and construction material, some bottles are more satisfactory for transporting water than others.
The bottle must be resistant to impact and to internal pressure, which is increased by expansion of the water or by release of dissolved gases at elevated temperatures.
Well-sealed fragile bottles of liquid when shipped by air may not withstand the large differential pressures or freezing temperatures in the rarefied atmosphere. In respect to fragility alone, polyethylene bottles are more satisfactory than glass.
However, samples in polyethylene bottles must be protected from compression else the liquids may be squeezed out around the threads of the cap.
Samples subjected to freezing temperatures are very likely to be lost through breakage of glass bottles but are retained by polyethylene. However, this advantage may sometimes be more apparent than real.
The chemical analysis of a previously frozen sample is always suspect because the original chemical character may not be completely reconstructed after the sample thaws. Although the analysis of the previously frozen sample is usually of some value,
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