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t Mr. Greg Basso -2- 16 October 1991 Drainage control in the HLA proposal is based on the 100-year, 24-hour storm event. Discharge Specification B.17 requires that Class II waste management units have precipitation and drainage control systems designed and constructed to accommodate the anticipated volume of precipitation and peak flows from surface runoff for a 1,000-year, 24-hour storm event. HLA also calculated annual surface water discharge from the land treatment unit to the Class II surface impoundment based on average annual precipitation conditions plus one 100-year, 24-hour storm event. Discharge Specification B.16 requires this calculation to be based on the 100-year wet season plus the 1,000-year, 24-hour storm event. Under these precipitation conditions, at least two feet of freeboard must be preserved in the Class II impoundment. In addition, the HLA analysis used raw evaporation data without applying a pan coefficient correction factor. We believe certain design features should be modified to improve water quality protection. The PVC FML should have at least a two percent slope across the treatment unit and one percent slope toward the sump. The collection pipe should slope at least one percent toward the sump. A geofabric should also be used to prevent soil migration from the clay liner into the lower drainage layer. A geofabric may also be needed to prevent clogging of the slotted HDPE pipe in the leachate collection trench. Please note the Section B design drawing is not clear due to the projection of the sump structure onto the midline of a treatment unit. The Section B drawing should be revised. The design drawings do not adequately indicate how liquid on the FML is directed to the sump. We are concerned, for example, that the elevation of 31.5 feet shown in Section C on the FML at the midpoint of the northern unit is lower than the elevation of 32 feet shown in Section A on the FML beneath the blank HDPE header. This situation would appear to prohibit fluid flow on the FML toward the sump. We recommend that a response plan be prepared which explains how much leakage through the clay liner and onto the FML is permissible and what the site operator will do if this volume is exceeded. The currently proposed monitoring does not appear to satisfy the requirements of Article 5 in Chapter 15. HLA should specify the types and locations of monitoring devices planned for the land treatment unit and explain why the monitoring points were selected. The facility's ground water monitoring network should be expanded to include the land treatment unit