WATER QUALITY IN KENTUCKY, SUMMARY SHEET:
BUILDING A CISTERN FOR HOME WATER SUPPLY
by Joe Taraba; Elwyn Holmes; Tom Ilvento; Linda Heaton
In many parts of the state groundwater
may be inadequate, hard to find or contaminated. When an adequate supply
of high quality well or spring water is found nearby, this is the best
system to develop. However, you must know the history of a spring before
spending money on developing it into your primary water supply because
many Kentucky springs are seasonal.
Sometimes a rural water district is
located in the area, but too often they are already operating at capacity.
The only choices left are to build a cistern or develop a pond water system.
The cistern is usually lower in first cost and maintenance. However, a
properly designed cistern and its essential supply components are not inexpensive.
Cistern components include an adequate
roof area, roof gutters, downspouts, downspout diverters, roof washer,
water pump and distribution pipe (Figure 1).
A sand filter and chlorine treatment are wise investments in helping to
guarantee safer water.
Determining Water Needs
When planning a cistern, first determine
the family's water needs; a conservative family will use about 50 gallons
per person per day. A cistern should hold about a 45-day supply. A family
of five would need a 1 l,250-gallon cistern (5 people x 50 gallons x 45
days - 11,250 gallons).
Annual rainfall averages 40 inches
in northern Kentucky and 50 inches along the Tennessee border. The family
needs 91250 gallons a year (250 gallons x 365 days = 91,250 gallons). (See
2 for annual water needs in relation to Kentucky rainfall.)
A northern Kentucky family would need
5,300 square feet of roof area, and a southern-area family would need 4,400
square feet. The water supply should then be adequate except during a severe
drought. The roof area needed above is much greater than that found on
most homes with only 1,200 to 2,000 square feet.
If the cistern or collection roof is
too small, the family will have to depend on hauled water during periods
of low rainfall. Therefore, a large barn may be needed to supplement the
Sizing and placement of the roof gutters
are very important to maximize the amount of rainfall caught. Also gutters
need to be anchored securely to the roof as ice and snow can tear them
loose. Gutter guards made of 1/4-inch wire mesh help keep out leaves and
trash. However, regular cleaning is still required. The downspout needs
a diverter valve and waste pipe to drain roof water away from the cistern
when it is full.
A roof washer (Figure
3) is a very good investment as it diverts contaminants washed off
the roof before allowing the rainwater to enter the cistern. Between rains
drifting foreign materials such as sulfates and nitrates discharged into
the atmosphere from power plants, highway vehicles and other sources can
contribute to roof sediment. Bird droppings and chimney smoke from wood
fires can also contaminate the roof. It has been estimated that the first
0.01 inch of rainfall is adequate to wash foreign material off a roof.
As an example, 15 gallons of waste water would be adequate for 2,400 square
feet of roof.
A sand filter (Figure
4) comprised of sand and gravel will remove some contaminants and --
if the gravel is limestone rock will help neutralize the acidic rain water.
A survey in 1986 showed that Kentucky rain water had a pH of 4.3. Because
a sand filter acts slowly, a large surface area is required. As an example,
a 4-foot-by-4-foot filter will be adequate for 1,500 square feet of roof.
Any filter must be backwashed periodically,
or it will slowly clog, resulting in ideal conditions for bacterial growth
in the filter. Also a mat can form from bacterial activity that will eventually
plug the sand.
The top one to two inches of sand become
clogged with sediment and should periodically be removed and replaced with
new washed, screened beach or quarried sand.
Such a filter is very heavy (a cubic
foot of rock and sand can weigh up to 150 pounds) and should not be placed
on the cistern roof unless the roof was designed for such a load.
Cisterns can be made from reinforced
concrete, reinforced concrete block, metal or fiberglass. Because of the
expected permanence of a cistern, reinforced concrete is usually the best
investment. Concrete blocks have the potential for leakage at their many
joints, metal has a tendency to rust and fiberglass has strength problems
when buried. A cistern built of concrete also helps neutralize water acidity.
A concrete cistern needs a quality
concrete made with a stiff mix of one part cement, two parts clean sand
and three parts clean gravel. A round shingle gravel that does not exceed
3/4-inch diameter helps make a water-tight concrete. Setting the forms
and reinforcing rods correctly is essential to wall strength. A continuous
pour of air entrained concrete and proper tamping will prevent honeycombs
and potential leaks. Any excess water in the concrete mix will leave the
concrete porous. Place a screened drain pipe in the wall one foot below
the roof for overflow.
The top can be poured separately with
an opening for the roof water, a manhole for inspecting and cleaning and
a raised lip and heavy lid for safety. The cistern floor should have a
light slope (1" for 12') toward a small sump under the manhole. This allows
easy water removal during cistern cleanup.
Groundwater pressure when the cistern
is empty can be relieved with drainage tile around the base if it is possible
for gravity to drain the tile. To assure water tightness, plaster the cistern
wall on both sides. Then treat the inside with a quality masonry sealant,
and treat the outside with a waterproof sealant below grade.
If possible, the cistern should be
buried at least to the level of the overflow pipe for the following reasons:
1.to give ample room to place a roof
washer on top of the cistern,
2.to make landscaping easier and
3.to prevent cistern water from freezing.
Preparing a Concrete Cistern for Use
Before a cistern is used, it should
be cleaned and disinfected. After cleaning out any dirt and other debris
accumulated during construction, scrub the interior with a solution of
1/4 cup of 5% chlorine bleach mixed in 10 gallons of water. CAUTION: Make
sure that there is ample ventilation for the workers inside the cistern.
After this treatment, hose down the interior until the chlorine odor disappears
and then drain. A cistern needs to be cleaned at least every five years.
This might be needed more often where blowing dust, leaves and fireplace
or stove ash fall on the roof. Inspecting and cleaning the gutters, downspout,
roof washer and filter will help to keep a cistern cleaner.
•Remember that cistern water is only
as clean as the cistern itself and the water allowed to enter.
•For more detailed information on cisterns,
contact your local Cooperative Extension office for a copy of "Cisterns
for Kentucky," IP-4. A cistern blueprint (#KY 11.8553-16) can also be obtained
through that office.
This material is based on work supported
by the U.S. Department of Agriculture, Extension Service, under special
project number 89-FWQI-9156.