5 Assessmentand Priority Ranking of Outfalls
The MS4 Permit requires an assessment and priority ranking of outfalls in terms of their potential to
have ilicit discharges related public health significance. The ranking helps determine the priority order
for performing IDDE investigations and meeting permit milestones.
5.1 Outfall Catchment Delineations
The catchments for each of the MS4 outfalls will be delineated to define contributing areas for
investigation of potential sources of illicit discharges. Refined catchment delineations will be completed
as part of the Phase II mapping to reflect information collected during catchment investigations
5.2 Outfall nventory and Initial
Ranking
Nashua has completed an initial outfall and interconnection inventory and priority ranking to assess
illicit discharge potential based on existing information. The inventory will be updated annually to
include data collected in connection with dry weather screening and other relevant inspections and
included in each annual report.
Outfalls were classified into one of the following categories:
1. Excluded outfalls:
¢ Outfalls with no potential for illicit discharges including roadway drainage in undeveloped
areas with no dwellings and no sanitary sewers; drainage for athletic fields, parks or
undeveloped green space and associated parking without services; cross-country drainage
alignments (that neither cross nor are in proximity to sanitary sewer alignments) through
undeveloped land.
2. Problem Outfalls: Outfalls with known or suspected contributions of illicit discharges based
on existing information shall be designated as Problem Outtfalls. This shall include any outfalls
where previous screening indicates likely sewer input. Problem Outfalls need not be screened
pursuant to Dry Weather Outfall and Interconnection Screening and Sampling. Likely sewer
input indicators are any of the following:
e Olfactory or visual evidence of sewage,
e Ammonia 2 0.5 mg/L, surfactants 2 0.25 mg/L, and bacteria levels greater than the
water quality criteria applicable to the receiving water, or
e Ammonia 2 0.5 mg/L, surfactants 2 0.25 mg/L, and detectable levels of chlorine.
3. High Priority Outfalls: Outfalls that have not been classified as Problem Outfalls and that are:
e Discharging to an area of concern to public health due to proximity of public beaches,
recreational areas, drinking water supplies or shellfish beds
Hic it Discharge Detection and Eimination Plan
