Coupled Dynamics of Fecal Indicator Bacteria in Sandy Sediments and the Water Column: a 3-Year High-Frequency Study at a Pennsylvania Creek

Elevated concentrations of the fecal indicator bacteria (FIB) Escherichia coli and enterococci in recreation and irrigation waters indicate a human health risk. Little is known about the role of sandy bottom sediments as a factor for the microbial quality of water. The objective of this work was to compare the dynamics of FIB concentrations and total numbers in water columns and sandy sediments along a 30-km reach of the Conococheague Creek in Southern Pennsylvania. Sampling locations were located near forested headwater subwatershed outlet, in the mixed agricultural use area, and in the mixed urban-agricultural land use area. Water and sediment samples were taken weekly for 3 years. Concentrations of both FIB followed similar persistent intra-annual sine-like annual trends with amplitudes of 3 to 3.5 orders of magnitude in the water column and sediment. Concentrations generally increased from the forested site to the mixed urban-agricultural site. Pearson correlation coefficients between logarithms of concentrations in water and in sediment were between 0.622 and 0.756 for E. coli and between 0.320 and 0. 621 for enterococci. FIB areal densities were computed as total CFU per unit area for water columns and for 1-cm top sediment. The ratios of E. coli CFU areal densities in water and sediment increased with the stage by 4 to 12% per 1 cm increase of the stream stage. Such a trend was also seen for enterococci at the forested site. Overall, populations of both FIB were substantially larger in water than in sediment at agricultural and mixed sites, urban-agricultural development sites. In such conditions, a first approximation of monitoring and modeling microbial water quality results can apparently be obtained without including the sediment compartment in observations and computations.