Recent Changes in the Geochemistry of the Reclaimed Porcupine Creek Alluvial Valley Floor, North Antelope/Rochelle Mine Campbell County, Wyoming

Abstract

Two sections of the Porcupine Creek alluvial valley floor have been reconstructed at Powder River Coal Company's North Antelope Rochelle Mine using specialized reclamation methods. The lower section of Porcupine Creek (AVF Reach 1) was re-constructed in 1985, but flow was bypassed by a diversion until early-2001. In addition, the lower creek was reconstructed using old channel design criteria that utilized a guide channel and limited the amount of pooled water. The lower reclaimed Porcupine Creek was constructed from native alluvium as required by the Wyoming Department of Environmental Quality / Land Quality Division (WDEQ/LQD) at the time. TDS and selenium concentrations in the alluvial monitoring wells in the lower reclaimed channel have been higher than in premining alluvial waters. The water quality reflected the new mobility of the constituents after mixing and oxidation of the highly mineralized alluvial material occurred during mining and aquifer construction. The alluvial water quality slowly improved between 1985 and 2001, but the future postmining water quality remained a concern. Following construction of pools and counter-weirs on AVF Reach 1 in early 2002 as part of a wetland establishment project, alluvial water levels have risen significantly. Boron concentrations spiked upward in the alluvial waters as water flushed through the vadose zone, but are now lower. Selenium concentrations are now much lower and geochemical conditions appear to be more reducing. The results of recent monitoring highlight the important geochemical processes ongoing in the aquifer and show that the construction of the pools and wetlands has improved the alluvial water quality in the AVF Reach 1. Further study of AVF Reach 1 is warranted as water quality in the reclaimed alluvium stabilizes. An upstream section of Porcupine Creek (AVF Reach 2) was reclaimed in 1999 primarily using selected overburden material as alluvium. TDS and selenium concentrations are much lower in AVF Reach 2.