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Name: Kelly
Status: Student
Age: 13
Location: N/A
Country: N/A
Date: N/A 


Question:
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!



Replies:
Hi Kelly

You have done an excellent experiment. You are clearly a very capable scientist already. I cannot answer your question but I can suggest a possibility.

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.

Cam



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