Zinc Benchmark Exceedances, a Field Duplicate, and a 60-Day Notice
Ridgeline Fabrication runs a 110-employee sheet-metal shop (SIC 3441) covered by the industrial stormwater general permit. Its outfall discharges to a ditch that reaches Miller Creek. The site has a galvanized roof, an outdoor scrap rack, and a pressure-washing pad whose wash water drains to the storm sewer. The total-zinc benchmark in the permit is 0.12 mg/L.
Quarterly sampling: Q1 = 0.31 mg/L, Q2 = 0.27 mg/L, Q3 = 0.09 mg/L. For Q2 the laboratory also ran a field duplicate from the same grab sample and reported 0.11 mg/L. The plant manager asks you to report the duplicate as the Q2 result because it "shows the first number was a fluke," and notes that funding for a corrective-action project is not approved until the next fiscal year. The same week, the facility receives a 60-day notice of intent to sue under CWA §505 from a downstream property owner alleging unpermitted discharges of metals and pressure-wash wastewater. The SWPPP was last updated in 2019 and does not mention the wash pad.
1. Are the Q1 and Q2 zinc results permit violations, and what does the permit require now?
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Two separate questions must be answered: enforceability and required response.
Enforceability. Under the industrial general permit framework, benchmark values are indicators, not effluent limitations. A benchmark exceedance by itself is not a permit violation, and the enforceable duties are procedural: monitor, report, review the SWPPP, and take corrective action. So the Q1/Q2 numbers are not automatically "violations," but failing to respond to them would be. Before saying "not a violation" to anyone, check the permit and any state certification for numeric limits, and check whether Miller Creek has a TMDL with a wasteland allocation that has been translated into a limit for this outfall.
Required response. A single exceedance triggers a SWPPP review: source identification, potential pollutant sources, and BMP adequacy. Two consecutive exceedances (Q1 and Q2) trigger corrective action in most states' adoption of the general permit, typically within 30 days or before the next qualifying storm event, and must be documented in the SWPPP. The evident sources here are the galvanized roof and outdoor scrap (atmospheric deposition plus contact runoff) and the unaddressed wash pad. Defensible steps include covering or containing scrap, routing wash water to the sanitary sewer under an industrial user permit or eliminating the discharge, adding media filtration or catch-basin inserts, and increasing inspection frequency. Record the corrective action in the SWPPP and reference it in the next discharge monitoring report.
2. The plant manager wants the duplicate reported instead of the 0.31 mg/L result. How do you handle the data, and what goes on the report?
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No. A field duplicate is a quality-control sample, not a replacement result. Both numbers are data; the primary sample is the compliance sample, and the duplicate tells you about precision.
The relative percent difference here is roughly 190% ((0.31 - 0.11) / mean), far outside any reasonable acceptance criterion at a 0.12 mg/L benchmark. That is a signal that something went wrong (non-homogeneous sample, container contamination, laboratory error) or that zinc concentrations are genuinely variable, not a license to choose the favorable number.
Correct handling:
- Report the primary result on the discharge monitoring report and retain both results in the monitoring records, with the QC result flagged as such.
- Open a QA investigation: review the laboratory's method blank, matrix spike, and calibration; review field practice (new gloves, acid-preserved bottles, whether the "duplicate" was a true split or a second grab).
- Re-sample at the next qualifying storm, preferably with two duplicates, and document the outcome.
- Do not retroactively substitute the duplicate.
Substituting a favorable QC result to lower a reported value is falsification of a Clean Water Act monitoring report. It exposes the facility to 18 U.S.C. §1001 and exposes you to decertification, and it is exactly the kind of thing a state data audit looks for.
3. How do you respond to the 60-day notice of intent to sue?
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Treat it as a document-preservation and investigation trigger, not just a legal problem.
Preserve. Issue a litigation hold covering all SWPPP versions, sampling logs, laboratory data, calibration records, maintenance records, and wash-pad plumbing drawings.
Evaluate the two allegations separately. The metals allegation is weak if zinc is only a benchmark and the facility is covered by the general permit, but the notice may also allege contribution to a water quality standards exceedance. The wash-water allegation may be strong: pressure-washing wastewater is an industrial discharge, and if it enters the storm sewer without permit coverage, that is a genuine unpermitted discharge and also a failure to identify all potential sources in the SWPPP.
Fix and document. Immediately contain and reroute the wash water, plug the drain if necessary, and amend the SWPPP. The 60-day window is a chance to cure, and documented corrective action matters both to the court and to the regulator.
Coordinate. Notify counsel and, if the permit requires it, report the unpermitted discharge to the state within the required time. Make no written admissions to the notice-giver and do not discuss the case with the downstream owner without counsel.