In my science project I poured nitrate contaminated water
through different types of soil. For example, water measuring 10.3 ppm
nitrate (tested with spectrophotometer) was poured through 4'0" sand. The
water collected after it filtered through the sand tested at 16.5 ppm
nitrate. I thought the water may have picked up nitrate from the soil
sample so I had the sand analyzed at a lab. It's nitrate content came
back at only .185 ppm. Why did the water 's nitrate content increase so
much? This happened each time I did my experiment. Is it possible that my
water sample could have contained nitrite and picked up an oxygen atom in
the sand, converting it to nitrate? Please help!
You have done an excellent experiment. You are clearly a very capable
scientist already. I cannot answer your question but I can suggest a
The concentration of nitrate in the soil (0.185ppm) is not necessarily what
you are interested in. What you need to know is how much nitrate is in the
sand. In 1kg of your sand you would find 0.185mg of nitrate.
Nitrate is extremely soluble. As the water soaks through the sand it will
dissolve all available nitrate on the surface of the sand. Even though the
concentration of nitrate is very low in the sand, you used several
kilograms of sand (containing 0.185mg in each kg) and if all the nitrate is
available (ie on the surface of the sand grains) there may be enough
nitrate to raise the concentration of the water to 16ppm.
It would be interesting to test the concentration of nitrate in sand both
before and after the water has soaked through it. You could then weigh the
sand and calculate the difference between the total amount (not
concentration) of nitrate before and after. Then you could calculate the
amount of nitrate which has been added to the water. The two figures
should be the same.
I hop ethis is helpful to you.
Click here to return to the Environmental and Earth Science Archives
Update: June 2012