In return for such agreement by the landowner, including owner of water rights, resident, or tenant, the Secretary of Agriculture is authorized to furnish financial and other assistance to such landowner, including owner of water rights, resident, or tenant, in such amounts and subject to such conditions as the Secretary of Agriculture determines are appropriate in the public interest for carrying out the land use and conservation treatment set forth in the agreement. Grants made under this section, depending on the income-producing potential of the land after reclaiming, shall provide up to 80 per centum of the cost of carrying out such land uses and conservation treatment on not more than one hundred and twenty acres of land occupied by such owner, including water rights owners, resident, or tenant, or on not more than one hundred and twenty acres of land which has been purchased jointly by such landowners, including water rights owners, residents, or tenants, under an agreement for the enhancement of water quality or quantity or on land which has been acquired by an appropriate State or local agency for the purpose of implementing such agreement; except the Secretary may reduce the matching cost share where he determines that (1) the main benefits to be derived from the project are related to improving offsite water quality, offsite esthetic values, or other offsite benefits, and (2) the matching share requirement would place a burden on the landowner which would probably prevent him from participating in the program: Provided, however, That the Secretary of Agriculture may allow for land use and conservation treatment on such lands occupied by any such owner in excess of such one hundred and twenty acre limitation up to three hundred and twenty acres, but in such event the amount of the grant to such landowner to carry out such reclamation on such lands shall be reduced proportionately. Notwithstanding any other provision of this section with regard to acreage limitations, the Secretary of Agriculture may carry out reclamation treatment projects to control erosion and improve water quality on all lands within a hydrologic unit, consisting of not more than 25,000 acres, if the Secretary determines that treatment of such lands as a hydrologic unit will achieve greater reduction in the adverse effects of past surface mining practices than would be achieved if reclamation was done on individual parcels of land.