Fig. 4

a Results of a biplot analysis where the first two principal components depict relationships among the eight sites in the Fayetteville shale of north-central Arkansas versus principal component loadings for a suite of ten environmental variables in three defined categories (i.e., stream morphology, land use, and water chemistry) using library “prcomp” in R [41]. Sites in red text are within a ‘potentially impacted catchment zone’ (=PICZ) that signifies a significantly greater density of unconventional natural gas (UNG) well sites, whereas those in blue text are found within a ‘minimally impacted catchment zone’ (MICZ). Variables in the biplot are represented as vectors, and the angle at their origin reflects pairwise correlations (i.e., the more acute the angle, the greater the correlation). TotPh = Total Phosphorus, TotN = Total Nitrogen, Cond = Conductance, Strahler = Stream order. b Results of a biplot analysis in which the first two principal components reflect relationships among the eight sites in the Fayetteville shale of north-central Arkansas versus principal component loadings for the 20-most abundant bacterial classes, where densities are represented as arcsin-transformed percentages