Agency Memorandum

State of Vermont
Agency of Natural Resources
Department of Environmental Conservation

Agency Memorandum

To: Brian Fitzgerald
Thru: Paul Wishinski
From: Dan Riley
Subject: Addressing some uncertainties in our acceptance of the pre-application modeling Protocol for Southern Vermont Gas ^ Combined Cycle Power plants.
Date: March 22, 1999

In the telephone memorandum from you to the A2 file dated March 3, 1999, regarding Southern Vermont Gas, uncertainty exists concerning the need for preconstruction monitoring of meteorological data. This memorandum addresses that question and offers additional advice concerning new modeling guidance from the IWAQM Phase 2 Summary Report for modeling long-range transport impacts.

Addressing the first question, as you know, for the ambient air quality standards in class II domain we have typically waived the requirement for onsite meteorological data. However, this question must be evaluated with respect to possible complex terrain impacts, and subsequent need of a model such as CTDMPLUS, which does require on-site measurements. Additionally, there may be a dual purpose for on-site met. Measurements in examination of impacts on the Class I areas, but only if a refined analysis is necessary. If the refined analysis were necessary, other meteorological data fields, such as from a prognostic model such as RAMS could also be valuable for running the Calpuff modeling system as outlined in the IWAQM Phase 2 Summary Report. In either case, the primary benefit of additional meteorological data would be definition of the windfield and stability classification in the deep, isolated valley the sources are proposed to be sited. If on-site meteorology were collected with the intent of dual usage, measurement must occur at multiple vertical levels to establish stability and wind profiles.

Since there may be no need for on-site meteorology for a refined Class I impact analysis if the proposed source passes the Calpuff screening analysis, the applicant should perform this as a first step. Some concern has arisen regarding sufficient conservatism of the analysis. However, as long as the source to receptor wind direction is not the prevailing wind direction, as possibly demonstrated with a wind rose, the screen analysis should be conservative with respect to the refined analysis, except for deposition impacts. CALPUFF does not adequately account for the aqueous phase oxidation of sulfur-dioxide to sulfate, which can become dominant in the presence of fog or clouds. For deposition impacts on Lye Brook it may be advisable to use the IWAQM phase I procedures, starting with the screening analysis, since it is considerably more conservative than the phase II analysis.