You have an interesting question. I think I can get you started
in the right direction.
The first thing to ask is why would the different types of water
affect the grass differently? What is it about the water that
will alter the plant growth?
The truth is, there are a good factors to consider.
1) pH - the acidity or alkalinity of the water. Plants
typically like between 5.5 and 6.8 depending on the species.
Does the pH vary among the types of water? (pH test strips can
be found in the aquarium supplies at a pet store or ask your
science teacher. They are a fairly common item.)
2) Nutrient content - Plants need to collect amounts of 13
mineral elements from the soil and water in order to survive. As
the plants grow, they deplete the soil supply of these nutrients.
The river and city water might contain enough of these elements
to re-supply the soil and keep the plant happy. The nutrient
types and amounts are going to vary based on the geology of the
aquifer the water came from (limestone, bedrock, etc.). Water
tests are available to check for nutrients levels and
contamination (I will get to this in a moment) in your water. It
has been a long time since I looked into them, but if I remember
correctly, they can be a bit expensive.
3) Amount of water - Another thing the water can do is wash away
the minerals the plant needs. This is called leaching. A
certain amount of leaching is needed to prevent the buildup of
certain minerals, but too much can carry away all of the nitrogen
and potassium from the soil.
4) Contamination - Many city water systems use small amounts of
chlorine and fluorine in the water. These elements in any
significant quantity are very harmful to plants. Another concern
is heavy metals such as lead. They accumulate in the cells
leading to plant injury.
5) Bacteria and fungi - With the water and plants you are using,
I don't think this will be a problem, but plant disease organisms
such as Pythium, which causes damping off disease, can be spread
by reusing contaminated water.
Another question for you: What differences are you looking for?
Each of the factors above will affect the growth and appearance
of the grass in a different way. The two key things you might
see are changes in growth rate and altered leaf color.
To sum it up, the differences in plant growth, if any, are going
to be caused by differences in the properties of the water. Keep
in mind just because differences do not occur right away or even
in a couple of weeks does not mean they will not show up. If the
contamination or nutrient deficiency is very mild, it might take
quite a while to build up to the place where it becomes
noticeable.
I wish you luck with the experiment. Thanks for the
question!
Eric J. Biddinger
Grad Student - Department of Horticulture
Penn State University ejb190@psu.edu