"a substantial impact" is correct and usable in written English. It is usually used to describe something that has a marked or significant influence or effect. You can use it in any situation where you would like to emphasize the extent to which something influences an outcome. For example, "The introduction of the new policy had a substantial impact on the company's success.".
"But in some areas it's already having a substantial impact".
The report found a substantial impact on the Texas economy.
Morgan added that Creasy had also suffered a "substantial impact" as a result of the abuse.
Three of these would have a substantial impact on architecture in New York.
The answer will have a substantial impact on the tone of this review.
If so, these twin rulings from influential courts will have a substantial impact.
Despite this, the changed political and fiscal landscape is also having a substantial impact.
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Claudia Letizia
Head Translator and Proofreader @ organictranslations.eu