How would filling in
a wetland to create a field affect the surrounding environment?

The filling in a wetland can have bad effects, as burring the wetland vegetation, increasing the elevation and eliminating periodical inundation of the wetland.

Explanation:

Filling in a wetland can be done with various types. One of them is a communal waste. Also, the construction materials are disposed of in wetland areas.  

This can produce dangerous chemical elements and release toxins. Filling in the wetland can cause the loss of these areas and the fertile lands used for agriculture.