Plants can be affected by so many different things from disease, over/under fertilization, weather, humidity, air flow, etc., and you haven't provided enough information for anyone to give any useful feedback.
What part of the world were the tomatoes planted? Different areas have different problems. Country, region, city...
What "chemicals" were used in the water? Was it bleach? That would definitely kill the plants :)
What was previously grown in that area? Was it previously uncultivated? Were there tomatoes? Beets? Corn? Grass? Tomatoes in particular can benefit from proper crop rotation.
What's the soil like? Send samples to a lab. In the US a local university extension office can provide all sorts of tests on the soil to let you know what needs to be improved and how. Make sure you indicate what you plan to grow when you submit samples to get the right kind of recommendations.
What actually killed the plants? Was it disease? Was it insects? Was it improper feeding or watering? Was it from lack of sunlight? Did you try and grow the tomatoes on a football field and they were trampled to death? What symptoms did they exhibit? Lots of extension offices can also analyze the plants and determine what's wrong with them and can identify the pathogens that are affecting it. They'll be able to provide guidance as to what you can do to help.
I don't know who you're "local expert" is but I would go through legitimate university extension offices and labs (not their Master Gardeners) to get a good prognosis. Especially if I were trying to raise tomatoes to sell which sounds like what your uncle is doing based on your language.
