concurring in part and dissenting in part. While I agree with the majority’s treatment of the mootness and estoppel issues, I dissent from its affirmance of the Environmental Board’s conclusions regarding significant impacts.
The Board established a two-part test to invoke Act 250 jurisdiction. Once the Board finds a change in development, the second prong of the test is that the Board must find that the change has caused a significant impact under one or more of the ten criteria. In re H.A. Manosh Corp., 147 Vt. 367, 370, 518 A.2d 18, 20 (1986). There, the Board examined for and found actual and potential impacts. In this case, the Board has departed from this test and relied solely on potential impacts. It emphasized in its opinion that it did not determine whether there were actual impacts, but only whether there were potential impacts. Its findings indicate that the increased extraction rate had been in existence for some twelve years and that the change from sporadic to daily use had probably existed for that same period, although the findings are not totally clear on this point. The failure to determine whether impacts actually exist because of the changes during this period is incomprehensible. Although we give substantial weight to an agency’s interpretation of its own rules, this deferential level of review does not equate with passivity in determining the propriety of these interpretations. In re Vitale, 151 Vt. 580, 583, 563 A.2d 613, 615 (1989).
The Board found that there were three changes at issue: the use of a crusher at the pit, a significant increase in the yearly extraction rate, and a change after 1970 from sporadic use to daily use. It concluded that the crusher did not have the potential for significant impacts but that the increase in the rate of *526extraction and the frequency of use did.* The potential impacts relied upon were an increase in noise, which might impact on air pollution and aesthetics, and an increase in trips, which has the potential for significant impacts on traffic safety and congestion, air pollution and aesthetics. The findings are sparse or nonexistent on these potential impacts. With respect to noise, they indicate that “the pit is now approximately 150 feet from the closest edge of the Burdick tract” and as a result pit operations have become more audible to persons residing on that tract. This finding might support a conclusion of actual impact, but it hardly supports a conclusion of potential impact. The findings do not indicate whether the pit operation will come closer to the Burdick property with a resulting increase in noise or will move, away with a decrease. The increase in the number of trips (from ten or twelve to sixteen) is characterized by the Board as slight, and it is not explained how a slight increase in number has the potential for a significant impact. The Board made no attempt to determine whether the increase over the twelve-year period had had any adverse impact on the criteria with which it was concerned.
An examination for potential impacts makes sense in a case where the change in the development is immediately challenged and actual significant impacts cannot be ascertained. But where, as here, the changes occurred years ago, it is absurd not to determine whether they have substantially impacted any of the criteria. At the very least, the impacts from what has occurred and what is actually occurring should be examined in deciding whether potential impacts may result. The Board studiously avoided doing this. The order should be reversed and the matter remanded with a direction to consider the actual impacts of the changes found.
Interestingly, in its proposed findings and conclusions the Board concluded that the change in operation had not increased the noise and traffic and that the gravel pit was exempt from the Act 250 permit requirement. It concluded otherwise in its final order with no change in the findings of fact.