Long-term decreasing sulfate concentrations in rivers across the northeastern United States indicate a strong source of atmospheric sulfur in the early 20th century. The sulfate levels and declines are too great and early to be explained by acid rain mitigation in the late 20th century. Anthracite coal was heavily mined in eastern Pennsylvania between 1860 and 1960 and burned across several U.S. states. Even though it is a relatively low-sulfur coal, the amount of anthracite mined between 1860 and 1960 was great-roughly five billion tons. Nearly all of this coal was burned in the populous region of the northeastern United States which contributed to rain acidity levels in the 1920s that are estimated here to have been four times that during the 1970s. Subsurface sorption and transit-time-distribution equations were applied to 38 watersheds in the region with discharging river sulfate data as early as the 1920s. Mining records from the U.S. Geological Survey allows for an accurate input signal. Results show a good fit to quantities of sulfur mined and the concentrations and trends in rivers, with median subsurface residence times between 13 and 137 years. In many watersheds, soil sorption/desorption or long groundwater-lag times could equally explain the delayed response. On Long Island, the deep porous sandy aquifer has delayed sulfate response to the extent some concentrations have only peaked in the last decade. The average delayed sulfate delivery of several decades implies that similar ongoing and future retention of other diffuse-source contaminants could occur in other watersheds.
