The Leaching Effects of Rainwater Passing Through Terrestrial Environments

QUESTIONS

Identify the factors that affect the concentrations of a range of ions in solution in natural bodies of water such as rivers and oceans

There are many factors that affect the concentration of a range of ions in solutions in natural bodies of water such as rivers and oceans. These include:

The leaching effects of rainwater passing through terrestrial environments

When rainwater containing dissolved carbon dioxide falls on the earth’s surface and percolates through the rocks and soils, it dissolves ions. The most soluble ions, such as sodium, potassium, and chloride dissolve most readily followed by calcium and magnesium. These ions are carried down to the seas and oceans by rivers. Billions of tones of various mineral salts each year are added to the oceans by rainwater leaching off terrestrial environments.

The pH of the rain

Water from acid rain is better able to leach certain cations such as calcium, magnesium and iron(III) from the soil it passes over or through, and so the concentration of these ions increases.

The nature and amount of human activity in the catchments

The clearing of land generally leads to an increase in water running across the land and into rivers/streams. This increases sediment load. Also sections of agricultural land with lots of fertilizer use (crop growing) often leads to it being carried into the rivers/streams thus increasing nitrate and phosphate concentrations in the water.

Describe and assess the design and composition of microscopic membrane filters and explain how they purify contaminated water

Microscopic membrane filters are filters that contain microscopic pores on them, which block harmful substances in the water. The use of an appropriate sized filter can avoid the need to chemically treat the water.

The membrane is made from synthetic polymers, which are dissolved in a mixture of solvents and water-soluble powders of a particular size. The mixture is then spread out and left for the solvent to dry. The polymer membrane containing particles of water-soluble powder is then placed in water. The remaining solvent and the powder particles dissolve, leaving microscopic sized pores where the water-soluble particles were.

The microscopic membrane purifies contaminated water by allowing the water to flow across it but not through it so as to reduce blockages. This method of micro-filtration removes bacteria, protozoa’s, colloids, colouration and some viruses. The size of the pore determines which sized particle or organism may pass through the membrane. The smaller the size of the pore the more expensive it is to produce the membrane.

The Advantages and disadvantages of using microscopic membrane filters

ADVANTAGES:

1)They filter out extremely small particles that are bigger than the size of the pores.

2)They can withstand pressure differences of up to about 2-5 atmospheres so it is harder to break.

3)They can be cleaned and reused.

DISADVANTAGES:

1)The cost of producing the membrane is high.

2)Costly to install

3)Cannot remove substances that are actually dissolved in the water, such as nitrates and heavy metal ions.

Gather, process and present information on the range and chemistry of tests used to

-A. Identify heavy metal pollution of water.

-B. Monitor possible eutrophication of waters

(A)

Heavy metal pollution of water is due to high and unacceptable levels of the ions such as mercury, lead, zinc and copper.

Techniques to identify the presence of heavy metals include precipitation and flame testing. This is because of the substance unique flame colour due to its electron energy level.

Quantitively you can identify heavy metal pollution using volumetric and galvametric analysis as well as AAS as these can quite accurately determine the amount of that heavy metal in the water.

(B)

Eutrophication is the increase in nutrient content of a body of water (i.e. Nitrates and phosphates from fertilizer overuse and sewage), resulting in excessive growth of plants, which when they die leads to:

Water being oxygen depleted
Smelly ponds covered with green scum
Algae growth at the surface reducing light from normal water plants

Tests to determine the extent of eutrophication of a waterway involve measurement of dissolved oxygen and nutrients such as nitrates and phosphates, by the Winkler method or use of a dissolved oxygen meter. Monitoring of these waters is crucial in order for safe waters. They reduce the amount of oxygen in the water thus killing aquatic life, further pollution the waterway. So using the tests mentioned with proper monitoring and frequent testing is required to prevent the cause of highly polluted waterways

4.Gather process and present information on the features of the local Sydney water supply (for a specific Sydney dam) in terms of:

-A. Catchment area.

-B. Possible sources of contamination in this catchment

-C. Chemical tests available to determine levels and types of contaminants

-D. Physical and chemical processes used to purify water

-E. Chemical additives in the water supply and the reasons for presences of these additives.

(A)

Sydney’s Water Supply – The Warragamba Dam

Catchment area – The dam was completed in 1960 and holds 2 057 000 megalitres of water, 4 times the volume of Sydney Harbour bridge. The main rivers flowing into the Warragamba are the Cox and Wollongdilly. The area behind the dam is called Lake Burragorang.

(B)

Possible sources of contamination include fertilizers and pesticides in run-offs. Also the inorganic compounds leading into waterways due to rainwater percolating into the rocks and soils of nearby rivers which flow into the dam. Another possible

source of contamination is pipe corrosion, which occurs after a long period of time where the water flows.

(C)

There are many common methods used to test the chemicals that are present in the dam. Turbidity and dissolved oxygen, total hardness, pH, alkalinity, conductivity and true colour are some of the things they determine to test the quality of water and if any contaminants are present. A poor result in one test may be due to contaminants in which appropriate measures are taken to reduce and remove the contaminants. AAS can be used when a contaminant has been detected and needs to be determined the concentration of it in the water. Electrolysis is also used to determine the total dissolved solids and compare it with a standard of potable water and observe for particularly high concentrations of solids

(D)

The first stage involves the removal of particulate matter through filtration. The

filtration process involves the addition of one or more coagulants to the water.

The primary coagulant used by Sydney Water is Ferric Chloride.

The second stage involves the inactivation of microbiological organisms

(disinfection) and the provision of a disinfection residual to protect the water

from potential recontamination as it travels from the water filtration plant

through the distribution system to consumers taps. This is normally achieved

by the addition of a strong oxidant.

The third stage involves adding lime and carbon dioxide to the water filtration plants where the water is very soft to adjust and buffer the pH of the treated water. The carbon dioxide reacts with the lime to form calcium bicarbonate which buffers the water (increases the resistance to changes in pH), increases hardness and

reduces the general corrosivity of the water.

(E)

Fluoride is added to help prevent tooth decay. Lime corrects pH and chlorine disinfects the water, helping to protect it from contamination during transport and storage. Chlorine is added to Sydney's water system to provide a residual disinfection capability in the distribution system. Chlorine decays with time. Higher levels are therefore necessary at the initial chlorination point to ensure that a minimum residual reaches remote parts of the system.

Microbiological

/ Physical / chemical

Total Coliforms CFU/100ml

/ Turbidity
Units /

Colour

Units

/ Total Chlorine
Mg/L / Fluoride
Mg/L / Iron
Ug/L / Managanese
Ug/L
0 / <5 / <15 / <5 / <1.5 / <20 / 6

The amount present of certain chemical/physical/microbiological chemicals in the water of the Warragamba Dam.

Describe and assess the effectiveness of methods used to purify and sanitise mass water supplies.

There are many methods that are used to purify and sanitize mass water supplies. Most are variations of the following process:

The first step involves the collection of water in dams and pumping it into a treatment site, where large solids that can be removed by screens, such as pieces of plastics. It is then aerated to increase the dissolved oxygen, which makes the other chemicals easier to remove and improve the taste.

The second step is flocculation. Chemicals are added such as iron(III) chloride or aluminum hydroxide and a cationic polymer to make fine suspended particles clump together so they can be more easily filtered.

The third step is sedimentation. This is the settling of flocs. The treated water is allowed to settle in a settling tank so the flocs settle at the bottom as sludge, which is removed. At this point 95% of the suspended impurities have been removed.

The fourth step is filtration. A sand and gravel filter is used to remove the coagulated solids. Over 300 000 small nozzles, in the bottom of each 2 metre deep bed of sand, filters 24 cubic metres of water per square metre of sand per hour. Clear water results from this filtration. However, if the water is still coloured, charcoal is used to remove the colours.

The last step is chemical treatment, also known as sanitization. This is where fluorine and chlorine is added. Chlorine (often as sodium hypochlorite) is used to bleach colouring matter in the water, to reduce the amount of coagulant chemicals needed to disinfect and to kill the bacteria. Fluorine is added (to a concentration of 1ppm) to strengthen tooth enamel and prevent dental decay.

Assessment of the effectiveness and sanitization method should involve:

Microbiological testing of water samples throughout the distribution system, in particular before and after the sanitization process
Public health surveys

Advantages and disadvantages of this method of purifying and sanitizing mass water supplies:

ADVANTAGES:

1) Can remove at least 98% of all impurities to make the water potable

2) Relatively inexpensive to purify and sanitise

DISADVANTAGES:

1) Constant monitoring is required incase of contaminated by harmful substances like giarida.

2) Cost is increased if microscopic membrane filters are required as they are costly to produce and install.

3) Constant cleaning of filter equipment is required, such as cleaning filters by backwashing with water and air to remove solid particles then centrifuging and drying them for use as compost.

Bibliography

Chemistry Context 2 by –Irwin Farrelly and Vitlin Garnett

Published 2002 by Pearson education.

Chemistry Pathways 2 – Geoffry Thickett

Published 2000 by Macmillan Education Australia

Surfing Chemistry – Chemical monitoring and management – Marilyn Schell and Margaret Hogan

Published 2002 by Science Press

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