rainwaterharvesting

Water Quality Concerns
Minimum and Desirable constituents to be determined for common uses of water

	Domestic	Public supply	Irrigation	General Industrial Supply
Suggested minimum analysis	Iron Hardness Detergents Fluorides	Silica Iron Manganese Magnesium Bicarbonate Sulphate Chloride, Fluoride,Nitrate, Dissolved solids pH, Colour,Turbidity, Phenols Detergents Heavy metals	Calcium Magnesium Sodium Bicarbonate Boron Spec. Conductance	Iron, Manganese, Sodium Bicarbonate Hardness Dis.solids ph Temperature
Desirable additional analysis	Manganese Bicarbonate Sulphate Chloride Nitrate Diss.solids, pH		Lithium Sulphate Chloride Diss.solids	Silica Chloride Colour, Turbidity Sulphide

As per World Health Organisation (WHO) and Indian Standards, the permissible limit of various constitutents in the potable water is as follows:

STANDARDS AND CRITERIA FOR DRINKING WATER IN MG/I

Substance of Property	World Health Organisation		Public Health Committee of Govt. of India
. .	Maximum acceptance	Maximum Allowable	Permissive	Excessive
Physical	.	.	.	.
Turbidity (Units)	5	25	5	25
Colour (Platinum Cobalt Scale)	5	50	5	25
Taste and Odour	Unobjection-able	--	Unobjection-able	--
Chemical	.	.	.	.
pH range	7- 8.5	--	7 - 8.5	--
Total Solids	500	1500	500	1500
Total hardness as CaCo3	--	--	300	600
Calcium	75	200	75	200
Magnesium	50	150	50	150
Iron	0.30	1.00	0.30	1.00
Manganese	0.10	1.50	0.10	0.50
Copper	1.00	1.50	1.00	3.00
Zinc	5	15	5	15
Chloride	200	600	250	1000
Sulphate	200	400	250	400
Magnesium + Sodium Sulphate	500	1000	---	---
Phenolic substances (as phenol)	0.001	0.002	0.001	0.002
Flouride	--	1.5	1.0	2.0
Nitrate Nitrogen	--	45	20	50
Nitrite Nitrogen	--	--	--	--
Carbon Chloroform extract (CCE-Organic Pollutants)	0.20	0.50	--	--
Alkyl benzoyal sulphonates (ABS Sulphactants (ms/l)	0.50	1.00	--	--
Toxic: Cadmium	--	0.01	--	--
Chromium (as Hexavelent)	--	0.05	--	0.01
Cynaide	--	0.20	--	0.01
Arsenic	--	0.05	--	0.20
Lead	--	0.05	--	0.10
Selenium	--	0.01	--	0.05

The rain water is the purest form of water. Therefore, the rain water collected from neat and clean surfaces like terrace can be directly used. It is advisable to filter and boil the water so collected as when it runs on the surface source dust particles may come with it.

Soil Lithology and Rainwater Harvesting
The quantum of the rain water which can be harvested are recharged into the underground aquifer depends on the nature of the soil. The design of the RWH structures depends on the soil nature. If the soil is having good permeability, water will be recharged fast but if the permeability of the soil is poor, the water recharge will be very slow. For example, the permeability of sandy soil is good and of clay is poor. Therefore, it can be seen that the water drains/seeps underground very fast while in clay it is very slow.

Therefore the RWH structure is to be designed in such a way that through recharge pits/bores the water is taken upto sandy strata so that the rate of percolation is fast.

The map below indicates various soil zones in Chennai city and the desired depth of the RWH structures.

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Suitable Aquifer zones for Rain Water Harvesting in Chennai City

Details of favourable aquifer zones
Sl.No	Colour	Selected areas	Water level during summer in 'm'	Depth of the aquifer in 'm'	Saturated thickness in 'm'	Recharge zone in 'm'
1	Blue	Secretariat,Port,Chepauk,Triplicane,Santhome Seashore,Karpagam Garden, M.R.C. Nagar	3	10	7	3
.	.	Egmore,BesantNagar SasthriNagar,Karpagam Avenue,Thiruvanmiyur	4	10	6	4
.	.	Ekkattuthangal	6	10	6	4
2	Orange	ParrysCorner,Mint,Sevenwells,Kothaval Chavadi, VallalarNagar,ParkTown,Sowcarpet,Mylapore, Mandaiveli,Alwarpet,RajaAnnamalapuram, Pattinapakkam	6	15	9	6
3	Yellow	Nungamakkam(Lakearea), T.Nagar,West CITNagar, Teynampet, Gopalapuram, Royapettah, Thousand Lights, KK Nagar	9	15	6	9
.	.	Adayar,GandhiNagar,IndiraNagar,Kasthuribha Nagar	8	15	7	8
4	Light Red	Ayanavaram, ICFQuarters,Kilpauk,Kilpauk Garden,Shenoy Nagar (East)	7	20	13	7
.	.	Anna Nagar, Anna Nagar (West), Shenoy Nagar (West)	12	30	18	12
.	.	Koyambedu	14	30	16	14
5	Green	Perambur	7	30	23	7
..	.	Alandur,Guindy, Chinnamalai, Tharamani, St.Thomas Mount	Hard Rock	.	.	.

The following table shows the porosity, specific yield and permeability characteristics of soil formation.
Representative Values for Selected Soil Formation

Sl.No	Description	Porosity percent	Specific Yield percent	Permeability gallon per day/sq.ft.
1.	Clay	45 - 55	1 - 10	0.011 - 2.0
2.	Sand	35 - 40	10 - 30	100 - 3000
3.	Gravel	30 - 40	15 - 30	1000 - 15000
4.	Sand and gravel	20 - 35	15 - 25	200 - 5000
5.	Sandstone	10 - 20	5 - 10	0.1 - 50
6.	Shale	1 - 10	0.5 - 5	0.00001 - 0.1

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