I am a researcher in EIA matters in the University of Alicante's Dept. of Ecology and am involved in DSS. I would be happy if I can help you in any way.
Even though MatLab is a pretty useful tool (in fact, I used it to build up a fuzzy logic-based inference system to help in environmental impacts' assessment) I think it is neither necessary nor the right and only choice if you want to get a DSS for EIA; in fact, EIA is just a decision-making process regarding environmental issues; hence, some other tools will be needed.
From my point of view, a mathematically sound impact assessment method (not the worldwide mostly used "crisp numbers" method, please) is required; and then, a similarly mathematically strong decision-making method (I suggest ANP, but any other method of your choice can be used) should be applied (bearing in mind that it is about a MC environment).