Flow-dominated wet-month total suspended solids loading across Great Lakes wastewater permit units

Abstract

Monthly regulatory records can show whether wet periods coincide with greater solids discharge, but mixed reporting bases can obscure the flow and concentration components of the mass signal. We linked U.S. Environmental Protection Agency discharge monitoring reports with monthly precipitation for 675 National Pollutant Discharge Elimination System permit units associated with publicly owned treatment works in the U.S. Great Lakes basin from January 2010 through September 2024. Reporting-aligned records shared the same permit, outfall, month, monitoring location, and average statistical basis. The primary sample contained 40,585 permit-months from 377 permit units. Permit-unit and year-month fixed-effects models estimated monthly within-permit associations. An interquartile increase in precipitation was associated with 21.7% higher effluent flow (95% confidence interval, 18.7-24.8), 3.4% higher flow-weighted total suspended solids concentration (1.3-5.6), and 25.9% higher reconstructed load (21.6-30.4). Here, flow-dominated means that the fitted flow coefficient was larger than the concentration coefficient; it did not identify a physical mechanism. This ordering persisted across time periods, warm-month thresholds, weather products, and aligned EPA mass-rate records. In 31,837 exactly aligned permit-months, the direct-to-reconstructed load ratio had a median of 0.999 and a 5th-95th percentile range of 0.633-1.400. Under the primary audit, the precipitation contrast corresponded to a model-standardized difference of 311 metric tonnes per representative month; a weather-grid bootstrap gave a 95% interval of 178-476 tonnes. The top 10% of permit units supplied 76.5% of the summed positive standardized difference, largely because of baseline throughput. These monthly associations can guide follow-up monitoring but do not rank treatment performance, compliance, or receiving-water risk.