A method of reducing rainwater and/or groundwater inflow and infiltration into a wastewater treatment collection grid. The method preferably involves the steps of (a) dividing the grid into a plurality of major subsystems, (b) determining flow depth levels in each major subsystem under dry and wet conditions, (c) using these wet weather and dry weather flow depth measurements to determine a volume flow ratio for wet versus dry conditions, (d) using these flow ratios to identify the particular major subsystem(s) in which the greatest amount of inflow or infiltration is occurring, and then (e) further dividing the highest ranking major subsystem(s) into smaller subsystems in which the same dry and wet weather level measurement and ranking analysis is preferably conducted to further isolate problem locations for surveillance, maintenance, and/or repair.