measuring water levels,dissolved oxygen and ph levels

8/6/2019 Measuring Water Levels,Dissolved Oxygen and pH Levels

1/18


2/18

No, you don't often hear your local news broadcastersay "Folks, today's pH value of Dryville Creek is 6.3!" But pH is quite animportant measurement of water. Maybe for a science project in school youtook the pH of water samples in a chemistry class ... and here at the U.S.Geological Survey we take a pH measurement whenever water is studied.

Not only does the pH of a stream affect organisms living in the water, achanging pH in a stream can be an indicator of increasing pollution or someother environmental factor.

pH: Definition and measurement units

pH is a measure of how acidic/basic water is. The range goes from 0 - 14,with 7 being neutral. pHs of less than 7 indicate acidity, whereas a pH ofgreater than 7 indicates a base. pH is really a measure of the relative amount

of free hydrogen and hydroxyl ions in the water. Water that has more freehydrogen ions is acidic, whereas water that has more free hydroxyl ions isbasic. Since pH can be affected by chemicals in the water, pH is animportant indicator of water that is changing chemically. pH is reported in"logarithmic units," like the Richter scale, which measures earthquakes. Eachnumber represents a 10-fold change in the acidity/basicness of the water.Water with a pH of five is ten times more acidic than water having a pH ofsix.

Importance of pH

The pH of water determines the solubility (amount that can be dissolved inthe water) and biological availability (amount that can be utilized by aquaticlife) of chemical constituents such as nutrients (phosphorus, nitrogen, andcarbon) and heavy metals (lead, copper, cadmium, etc.). For example, inaddition to affecting how much and what form of phosphorus is mostabundant in the water, pH also determines whether aquatic life can use it. Inthe case of heavy metals, the degree to which they are soluble determinestheir toxicity. Metals tend to be more toxic at lower pH because they aremore soluble. (Source:A Citizen's Guide to Understanding and MonitoringLakes and Streams)

Diagram of pH

As this diagram shows, pH ranges from 0 to 14, with 7 being neutral. pHsless than 7 are acidic while pHs greater than 7 are alkaline (basic). Normalrainfall has a pH of about 5.6slightly acidic due to carbon dioxide gas fromthe atmosphere. You can see that acid rain can be very acidic, and it canaffect the environment in a negative way.


3/18


4/18

Water Science PicturespH meter measures acidity

Measuring pH

The U.S. Geological Survey analyzes hundreds of thousands of watersamples every year. Many measurements are made right at the field site,and many more are made on water samples back at the lab. pH is animportant water measurement, which is often measured both at the samplingsite and in the lab. There are large and small models of pH meters. Portablemodels are available to take out in the field and larger models, such as this

one, are used in the lab.

To use this pH meter, the water sample is placed in the cup and the glassprobe at the end of the retractable arm is placed in the water. Inside the thinglass bulb at the end of the probe there are two electrodes that measurevoltage. One electrode is contained in a liquid that has a f ixed acidity, or pH.The other electrode responds to the acidity of the water sample. A voltmeterin the probe measures the difference between the voltages of the twoelectrodes. The meter then translates the voltage difference into pH and

displays it on the little screen on the main box.

Before taking a pH measurement, the meter must be "calibrated." The probeis immersed in a solution that has a known and stable pH (a "buffersolution"). The knobs on the box are used to adjust the displayed pH value tothe known pH of the solution, thus calibrating the meter.

Taking pH at home or school

One of the most popular school science projects is to take the pH of waterfrom different sources. Your school might have electronic pH meters forschool projects, as these meters have come down in prices a lot. The mostbasic way of testing if a solution is acidic or basic is to use litmus paper,which uses a color change to give you this information. There are also paperpH test strips available that can give you a better estimate of pH of solutions.With these, the strips show a certain color to indicate certain pH values.These strips can be used to in school science classes and fairs, to test thewater in an aquarium, or to test one's urine, even.


5/18


6/18

Turbidity

Turbidity is the amount of particulatematter that is suspended in water. Turbidity measures the scattering effectthat suspended solids have on light: the higher the intensity of scattered light,the higher the turbidity. Material that causes water to be turbid include:

y clay

y silt

y finely divided organic and inorganic matter

y soluble colored organic compounds

y plankton

y microscopic organisms

Turbidity makes the water cloudy or opaque.The picture to the left shows highly turbid water from a tributary (whereconstruction was probably taking place) flowing into the less turbid water ofthe Chattahoochee River in Georgia. Turbidity is measured by shining a lightthrough the water and is reported in nephelometric turbidity units (NTU).During periods of low flow (base flow), many rivers are a clear green color,and turbidities are low, usually less than 10 NTU. During a rainstorm,particles from the surrounding land are washed into the river making thewater a muddy brown color, indicating water that has higher turbidity values.

Also, during high flows, water velocities are faster and water volumes arehigher, which can more easily stir up and suspend material from the streambed, causing higher turbidities.

Turbidity can bemeasured in the laboratory and also on-site in the river. A handheld turbiditymeter (left-side picture) measures turbidity of a water sample. The meter iscalibrated using standard samples from the meter manufacturer. The picturewith the three glass vials shows turbidity standards of 5, 50, and 500 NTUs.Once the meter is calibrated to correctly read these standards, the turbidity ofa water sample can be taken.

State-of-the-artturbidity meters (left-side picture) are beginning to be installed in rivers toprovide an instantaneous turbidity reading. The right-side picture shows acloseup of the meter. The large tube is the turbidity sensor; it reads turbidityin the river by shining a light into the water and reading how much light isreflected back to the sensor. The smaller tube contains a conductivity sensorto measure electrical conductance of the water, which is strongly influenced

by dissolved solids (the two holes) and a temperature gauge (the metal rod).


7/18

Dissolved oxygen

You can't tell by looking at water that thereis oxygen in it (unless you remember that chemical makeup of a watermolecule is hydrogen and oxygen). But, if you look at a closed bottle of a softdrink, you don't see the carbon dioxide dissolved in that - until you shake itup and open the top. The oxygen dissolved in lakes, rivers, and oceans iscrucial for the organisms and creatures living in it. As the amount ofdissolved oxygen drops below normal levels in water bodies, the waterquality is harmed and creatures begin to die off. Indeed, a water body can

"die", a process called eutrophication.

Although watermolecules contain an oxygen atom, this oxygen is not what is needed by

aquatic organisms living in our natural waters. A small amount of oxygen, upto about ten molecules of oxygen per million of water, is actually dissolved in

water. This dissolved oxygen is breathed by fish and zooplankton and isneeded by them to survive.

Rapidly moving water, such as in a mountain stream or large river, tends tocontain a lot of dissolved oxygen, while stagnant water contains little.Bacteria in water can consume oxygen as organic matter decays. Thus,excess organic material in our lakes and rivers can cause an oxygen-deficient situation to occur. Aquatic life can have a hard time in stagnantwater that has a lot of rotting, organic material in it, especially in summer,when dissolved-oxygen levels are at a seasonal low.

Hardness

The amount of dissolved calcium and magnesium in water determines its"hardness." Water hardness varies throughout the United States. If you livein an area where the water is "soft," then you may never have even heard ofwater hardness. But, if you live in Florida, New Mexico, Arizona, Utah,Wyoming, Nebraska, South Dakota, Iowa, Wisconsin, or Indiana, where thewater is relatively hard, you may notice that it is difficult to get a lather upwhen washing your hands or clothes. And, industries in your area might haveto spend money to soften their water, as hard water can damage equipment.

Hard water can even shorten the life of fabrics and clothes! Does this meanthat students who live in areas with hard water keep up with the latestfashions since their clothes wear out faster?


8/18

Every household and every factory uses water, and none of it ispure. One class of impurity that is of special interest is "hardness". Thisrefers to the presence of dissolved ions, mainly of calcium Ca

2+and

magnesium Mg2+

which are acquired through contact with rocks andsediments in the environment. The positive electrical charges of these ionsare balanced by the presence of anions (negative ions), of which bicarbonate

HCO3

and carbonate CO32

are most important. These ions have theirorigins in limestone sediments and also from carbon dioxide which is presentin all waters exposed to the atmosphere and especially in groundwaters.

Origin of water"hardness"

Carbon dioxide reacts withwater to form carbonic

acid (1) which at ordinaryenvironmental pH existsmostly as bicarbonateion (2). Microscopic marineorganisms take this up ascarbonate (4) to form calciteskeletons which, overmillions of years, have builtup extensive limestonedeposits. Groundwaters,made slightly acidic byCO2 (both that absorbedfrom the air and from therespiration of soil bacteria)dissolve the limestone (3),thereby acquiring calciumand bicarbonate ions andbecoming "hard". If theHCO3

concentration is

sufficiently great, thecombination ofprocesses (2) and (4) causes calcium carbonate ("limescale") to precipitate out onsurfaces such as the insidesof pipes. (Calciumbicarbonate itself does notform a solid, but always

precipitates as CaCO3.)


9/18

These "hardness ions" cause two major kinds of problems. First, the metalcations react with soaps, causing themto form an unsightly precipitate thefamiliar "bathtub ring".

More seriously, the calcium andmagnesium carbonates tend toprecipitate out as adherent solids onthe surfaces of pipes and especially onthe hot heat exchanger surfaces ofboilers. The resulting scale buildup canimpede water flow in pipes. In boilers,

the deposits act as thermal insulation that impedes the flow of heat into thewater; this not only reduces heating efficiency, but allows the metal tooverheat, which in pressurized systems can lead to catastrophic failure.

Types of water hardness

Temporaryhardness

This refers to hardness whose effects can be removed by boiling the water inan open container. Such waters have usually percolated though limestoneformations and contain bicarbonate HCO3

along with small amounts of

carbonate CO32

as the principal negative ions. Boiling the water promotes

the reaction

2 HCO3

CO32

+ CO2

by driving off the carbon dioxide gas. The CO32

reacts with Ca2+

orMg

2+ions, to form insoluble calcium and magnesium carbonates which

precipitate out. By tying up the metal ions in this way, the amounts availableto form soap scum are greatly reduced.

Permanent hardness

Waters than contain other anions such as chloride or sulfate cannot beremediated by boiling, and are said to be "permanently" hard. The onlypractical treatment is to remove all the ions, normally by the methoddescribed below.

Conventional water softening

Most conventional water-softening devices depend on a process knownas ion-exchange in which "hardness" ions trade places with sodium andchloride ions that are loosely bound to an ion-exchange resin ora zeolite (many zeolite minerals occur in nature, but specialized ones are

often made artificially.)

The illustration depicts a negatively-charged zeolite to which [positive]sodium ions are attached. Calcium or magnesium ions in the water displacesodium ions, which are released into the water. In a similar way, positively-


10/18


11/18


12/18


13/18


14/18


15/18


16/18


17/18


18/18

3. Place the probe in the sample, allow the meter toequilibrate, and read the DO concentration directly off thescale. NOTE: The probe may need to be gently stirred to aidwater movement across the membrane.

Field DO probes are easily ruined through deterioration of themembrane, trapping of air bubbles under the membrane, andcontamination of the sensing element. It often is difficult to assesswhether or not a probe is functioning properly. Because of this, themeter must be calibrated before and after each series ofmeasurements. When you calibrate the instrument, you compareDO concentrations measured by the probe to those measured usingthe Azide-Winkler method described above and then correct allsamples for any measurement error. The meter manufacturers

calibration procedure should be followed exactly. If the error ishigh or erratic, all sample results should be discarded.

QA/QC Considerations

Even though the Winkler dissolved oxygen method is the methodagainst which the others are calibrated, there are still tests thatcan be made to ensure that the Winklers themselves are accurate.To test the method, you need to have samples with a knownoxygen concentration so you can compare your results to what youknow is the real answer. These are called calibration samples orstandards. A 100 percent saturation solution can be prepared bybubbling air into distilled water. If low DOs are expected, a zero DOsolution can be made by adding excess sodium sulfite and a traceof cobalt chloride to a sample. In a professional lab, a calibrationstandard would be analyzed with each bath of samples run.

Randomly select 5 to 10 percent of the samples for duplicatelaboratory analysis. If you are interested in field variability, select 5to 10 percent of the samples for field duplication (e.g., collect twosamples from the same station).

If you are using a probe and meter or field kit for measurement, 5to 10 percent of your samples should be checked against theWinkler DO method.

The next section discusses how to measure pH.

measuring water levels,dissolved oxygen and ph levels

Documents