Dry-Weather Flow in a Phase II MS4
A Phase II MS4 serves a 45,000-person city and operates an IDDE program under its NPDES permit. During a dry-weather outfall screening after 72 hours without rain, staff find a steady 15 gpm flow from a 36-inch storm sewer into a creek. Field readings are ammonia 4.2 mg/L, surfactants 0.8 mg/L, chlorine 0.02 mg/L, pH 7.4, and specific conductance 620 µS/cm. The outfall drains a commercial corridor with restaurants, a car wash, an industrial laundry, and older septic systems. No dye reaches the outfall from a nearby fire hydrant flushing test, and the chlorine reading is near the detection limit. The creek is listed as impaired for E. coli. The city's IDDE ordinance prohibits non-stormwater discharges except for specified exemptions; the car wash has a discharge permit to the sanitary sewer. Staff have one day to decide whether to issue an immediate cease-and-desist or conduct more tracing.
1. What does the chemistry suggest about the source, and what additional tracing would you do before enforcement?
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The combination of elevated ammonia and surfactants with very low chlorine and no dye from the hydrant test argues against a chlorinated drinking-water line leak. It is more consistent with sewage, septic effluent, greywater, or a commercial discharge such as laundry or restaurant wastewater. The car wash is a candidate only if its sanitary connection is crossed or bypassing, because its permit sends process water to the sanitary sewer. Before issuing a cease-and-desist, staff should trace upstream by manhole sampling to isolate the reach, review sewer and septic records, and use CCTV or dye testing of private laterals. Additional parameters such as E. coli, optical brighteners, potassium, and caffeine can help distinguish sewage from other sources. Because the creek is impaired for E. coli, staff should also evaluate whether the discharge presents an immediate health risk that justifies an emergency response while tracing continues.
2. If the source is a broken private sewer lateral, what authority and steps does the MS4 have?
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The MS4's IDDE ordinance is the primary authority: it can prohibit the non-stormwater discharge, issue a notice of violation, require the property owner to stop the discharge and repair the lateral, and recover costs for investigation or cleanup. The city should coordinate with the local health department and sanitary sewer authority because a broken lateral may also be a public health issue. Enforcement should include a compliance schedule, reinspection, and documentation of the discharge volume and downstream impact. If the owner does not act, the ordinance should provide for escalating enforcement, including termination of the connection or court action. The city should not treat the matter as only a storm sewer problem; the sanitary sewer agency may have separate authority and should be brought in early.
3. How should the city document the screening and follow-up to satisfy its MS4 permit?
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The city should keep an IDDE record for the outfall that includes the date, time since last rain, flow rate, visual observations, field parameters, laboratory results, and the names of staff. It should document the upstream tracing, manhole sampling, dye or CCTV results, and the basis for identifying or excluding each candidate source. Once a source is found, the file should include the enforcement notice, repair schedule, reinspection results, and any cost recovery. If the discharge continues, the city should record its evaluation of interim measures and public health coordination. These records support the annual MS4 report and demonstrate that the program is not merely reactive. Good documentation also protects the city if enforcement is challenged.