N.C. Gen. Stat. § 156-127

The collection of assessments adjusted under this Article and of interest accrued under G.S. 156-126 shall be set aside in a fund and shall be applied as follows: One third of such collections may be used solely for operating and administrative expenses of the district, but the remaining two thirds thereof shall be reserved as additional security for the payment of the refunding bonds, or for the purchase and retirement of such refunding bonds, at prices not exceeding par and accrued interest.