Rural Septic Tank Access Act This bill amends the Consolidated Farm and Rural Development Act to reauthorize and modify the Department of Agriculture (USDA) Household Water Well System Grant Program. (Under current law, the program awards grants for private nonprofit organizations to provide loans to eligible individuals for the construction, refurbishing, and servicing of individual household water well systems that are or will be owned by the eligible individuals in rural areas.) The bill modifies the program to: prohibit the household income of an eligible individual from exceeding 60% (currently 100%) of the median nonmetropolitan household income, require grant recipients to provide subgrants to eligible individuals instead of loans, authorize subgrants for household decentralized wastewater systems, and limit the amount of each subgrant provided to an individual to $20,000 for each water well system or decentralized wastewater system. In the event of well water contamination, USDA must allow a subgrant to be used to install water treatment measures as needed beyond the point of entry, with or without the installation of new well equipment.