My neighbor recently removed a good number of trees and stumps from his property. Now the water runoff from their property, which was previously absorbed by the ground, is flowing into a pond on conservation land. I am concerned about the impact on the environment and I am wondering if anything can be done about this.

Unless the work that your neighbor did causes direct damage to you, such as a higher water table which causes your basement to flood or water runoff onto your land, or causes a nuisance such as an increase in bugs and mosquitoes in the summer, there is nothing you can do about it directly since your property is not directly impacted by the action of your neighbor. However when you are talking about conservation land, particularly about wetlands, there are stringent regulations dealing with what work can be done on one's property if it has an impact in the wetlands. Generally there are buffer zones so that you cannot do anything with your property if it is within 50 to 100 feet of the wetlands. Wetlands is a term of art which includes vegetation which is indigenous to wetlands. Generally when a developer wants to develop property which may contain wetlands, they hire a botanist to flag the wetland areas so that the setback areas can be determined. The municipal organization which is generally in charge of conservation land is the Conservation Commission. That Commission is very often staffed by local residents who volunteer for that job, like the Zoning Board of Appeals or the Planning Board. However, very often, there is a paid employee of the city or town who is the conservation officer of the town who would have authority to investigate violations of the bylaws of the town dealing with the wetlands. If there is a determination that someone has impacted the wetlands in violation of the local ordinances, the Commission can order the owner to restore the area to the condition that it was in prior to the disturbance created by the removal of the trees and stumps. I suggest that you take pictures of the pond and of the runoff and then seek out the conservation officer or commission in your town and see what their position is with respect to the work your neighbor did.