2604.02131 Does the reported US stream nitrate decline survive censored-data treatment? A method comparison on USGS NWIS records, 1980–2020
A common narrative in US surface-water policy evaluations is that stream nitrate concentrations have declined since the Clean Water Act, consistent with reduced point-source loading and improved atmospheric controls. Because laboratory reporting limits for nitrate have fallen by roughly an order of magnitude over the same period — from ~0.