Low-flow dilution of authorized wastewater discharges in Peru: a national assessment linking permits, river hydrology, and water quality

Whether receiving rivers can dilute authorized effluents during low flow is rarely assessed nationally. We evaluated all 2,578 wastewater discharge permits of the National Water Authority of Peru (2009–2026), consolidated into 1,352 outfalls, against river low flows and 122,428 laboratory results from 1,652 monitoring sites (2014–2025). Outfalls and sites were linked by flow-direction tracing on MERIT Hydro (93.8% of CAMELS-PE nested-gauge links recovered). The flow exceeded 95% of the time (Q95) was regionalized from 73 unregulated CAMELS-PE catchments (cross-validated Nash–Sutcliffe efficiency of log flows, 0.57; nested selection, 0.49) to compute a dilution factor (DF, Q95 over the flow equivalent to the authorized annual volume) with 80% prediction intervals. Exceedances of water-quality standards were related to time-varying authorized pressure with site and year fixed effects and placebo tests. 28.3% of outfalls had robustly insufficient dilution (upper-bound DF < 10), rising to 49.4% in mining and 63.4% on streams draining under 10 km²; 153 outfalls were authorized above the natural Q95 of their receiving stream. Most insufficient outfalls lie in catchments smaller than any gauged one (an extrapolation); 88% remained insufficient under a conservative regional empirical bound. Within sites, activation of upstream permits was associated with a 6.7 percentage-point higher probability of exceeding metal and pH standards (95% CI 3.5–9.8) per log unit of effluent fraction, driven by iron and lead; for this sub-index, downstream-outfall and future-permit placebos showed no association. Organic and microbiological exceedances were associated mainly with upstream population. Permit assessment should require receiving-stream low-flow statistics.