The commissioner of the department or his designee responsible for issuance of SPDES permits hereunder, is hereby prohibited from receiving, or during two years prior to approval of an application for a permit from having received, a significant portion of his income directly or indirectly from permit holders or applicants for a permit. For the purposes of this section (a) "significant portion of his income" shall mean ten percent of gross personal income for a calendar year, except that it shall mean fifty percent of gross personal income for a calendar year if the recipient is over sixty years of age and is receiving such portion pursuant to retirement, pension, or similar arrangement; (b) "income" includes retirement benefits, consultant fees, and stock dividends; and (c) income is not received "directly or indirectly from permit holders or applicants for a permit" where it is derived from mutual-fund payments, or from other diversified investments over which the recipient does not know the identity of the primary sources of income.