In all districts, the Board of Standards and Appeals may permit the extraction of sand, gravel, or clay listed under Use Group I from a zoning lot which is limited in size to a maximum of 50 acres and which is located not less than 1,000 feet from the nearest boundary of any zoning lot 10 acres or more in area used for such extraction, provided that the Board finds that such use is so located as not to impair the essential character or the future use or development of the surrounding area, and provided that the following conditions are met:

1. that the applicant submits a site plan showing the proposed extent and depth of the area to be excavated, together with the certification of the Department of Buildings that the proposed method of operations and the final elevation of the pit will not undermine or cause settlement to nearby streets, sewers, buildings or other structures, or installations;
2. that the applicant submits a plan for the rehabilitation of the zoning lot to be undertaken after the completion of the excavation operations which is satisfactory to the Board, and posts a bond, in an amount to be determined by the Board, for the performance of such rehabilitation;
3. that the entire perimeter of the zoning lot, except for necessary truck roads, is fenced, including locked gates, so as to prevent children from gaining access to the excavated areas;
4. that one accessory off-street parking space is provided for every 2,000 square feet of lot area or for every three employees, whichever shall require a lesser number of spaces;
5. that the performance standards for M1 Districts shall apply to such operations in all districts other than M2 or M3 Districts, where the applicable performance standards shall govern;
6. that all drilling, blasting, or excavation operations shall be limited to Mondays through Fridays between the hours of 8:00 a.m. and 5:00 p.m.;
7. that the emission of process dust either from the area of operations or from the excavated materials themselves shall be minimized by frequent watering or by such other means as the Board shall direct;
8. that the warning notices respecting unlawful entry shall be posted on the fence, and that a watchman shall be stationed on the premises to police the entire area after normal working hours and on weekends and holidays; and
9. that excavation operations shall be undertaken in such manner as to avoid the creation of undrained pockets and the formation of stagnant pools. When topographical conditions make such compliance impossible, all pools of water resulting from surface drainage shall be sprayed in accordance with the requirements of the Department of Health to eliminate breeding places for mosquitoes or other insects.

The Board may prescribe additional appropriate conditions and safeguards to protect the public health, safety and general welfare during the period between the cessation of operations and the final rehabilitation of the site in accordance with approved plans.