Excess Nitrate Export in Mountaintop Removal Coal Mining Watersheds.

Throughout the Central Appalachian ecoregion, mountaintop removal coal mining (MTM) is the predominant form of land use change. The streams draining MTM impacted watersheds have been reported to contain high stream nitrate (NO3−) concentrations, yet the source and persistence of elevated NO3− remains unknown. Here we compiled data from multiple sources to conduct a regional evaluation of the impact of mining on stream NO3−, examine NO3− persistence after mining cessation, and identify potential N sources. Using water quality data from six large watersheds, we observe elevated NO3− in watersheds with the highest active mining density. At four small MTM watersheds with repeat sampling, we found that high levels of NO3− concentrations declined after mining cessation but remain elevated above reference after several decades. At MTM watersheds, we found annual mass flux of NO3− was 3.68 to 26.4 kg N ha−1 year−1, which is 1 to 2 orders of magnitude higher than a nearby forested reference watershed. Stream water NO3− isotopic ratios at these sites did not match previously suggested NO3− sources such as explosives used during mining and fertilizer applied during reclamation but were highly enriched in both δ15N and δ18O compared to the reference watershed suggesting high rates of NO3− retention. Explosive residue could account for the bulk of watershed NO3− export during active mining phases but other mining related N sources including fertilizer and rock‐derived N, the construction of valley fills, and alterations in watershed NO3‐ cycling likely contribute to the persistence of elevated NO3− export observed in this study. Plain Language Summary: Mountaintop removal coal mining, a form of surface coal mining common in Central Appalachia, causes deep transformations of previously forested mountain landscapes. Previous studies demonstrated that mining adversely impacts downstream water quality including causing high levels of dissolved nitrate, a form of nitrogen. Excessive stream water nitrate can damage stream food webs, trigger algal blooms, and pose a threat to human health. This work identifies elevated nitrate as a widespread problem caused by mountaintop removal coal mining. We sought to identify the sources of this nitrate and how long the problem persists after mining stops. We found evidence that nitrate levels in Central Appalachian rivers increased with the amount of active mining in their watersheds. In heavily mined watersheds, we found the level of nitrate exported was 1 to 2 orders of magnitude higher than at an adjacent forested watershed. Nitrate levels remained higher in mined watersheds even several decades after mining ended. Our work identifies explosives as a likely source of stream water nitrate during active mining. In addition, changes to the physical structure of the landscape and the increased chemical breakdown of newly exposed rock may also contribute to persistent high nitrate levels in affected streams. Key Points: Nitrate fluxes from mined watersheds were 1‐2 orders of magnitude higher than an adjacent forested watershed, impacting regional water qualityNitrate export declined after the cessation of active mining but persisted above reference levels for decades post‐miningThe persistence of mining‐associated N export suggests N sources are linked to deeper flow paths within constructed valley fills [ABSTRACT FROM AUTHOR]