Fig. 3

Heat map reflecting abundance of the 20-most abundant bacterial classes across the eight study sites located in the Fayetteville shale of north-central Arkansas. Columns represent study sites (X-axis) and rows are bacterial classes. The heat map was generated by the program QIIME [40] with intensities of colors (=heat) reflecting abundances as depicted by the scale to the right of the map. Study sites within ‘minimally impacted catchment zones’ (MICZ) are on the left (1-A through 1-D), whereas sites within ‘potentially impacted catchment zones’ (=PICZ) are on the right (2-A through 2-D). PICZ-sites have significantly greater density of unconventional natural gas (UNG) well sites