Foreshore sand as a source of Escherichia coli in nearshore water of a Lake Michigan beach

Abstract

Swimming advisories due to excessive Escherichia coli concentrations are common at 63rd Street Beach, Chicago, Ill. An intensive study was undertaken to characterize the source and fate of E. coli in beach water and sand at the beach. From April through September 2000, water and sand samples were collected daily or twice daily at two depths on three consecutive days per week (water samples, n = 1,747; sand samples, n = 858); hydrometeorological conditions and bird and bather distributions were also recorded. E. coli concentrations in sand and water were significantly correlated, with the highest concentration being found in foreshore sand, followed by those in submerged sediment and water of increasing depth. Gull contributions to E. coli densities in sand and water were most apparent on the day following gull activity in a given area. E. coli recolonized newly placed foreshore sand within 2 weeks. Analysis of variance, correlation, cluster analyses, concentration gradients, temporal-spatial distribution, demographic patterns, and DNA fingerprinting suggest that E. coli may be able to sustain population density in temperate beach sand during summer months without external inputs. This research presents evidence that foreshore beach sand (i) plays a major role in bacterial lake water quality, (ii) is an important non-point source of E. coli to lake water rather than a net sink, (iii) may be environmentally, and perhaps hygienically, problematic, and (iv) is possibly capable of supporting an autochthonous, high density of indicator bacteria for sustained periods, independent of lake, human, or animal input.

FIG. 1.

63rd Street Beach, with five sampling transects identified. Sampling sites were 1 m onshore and in water 45 and 90 cm deep. The shaded area indicates beach sand; contour lines indicate selected depths. N, north revetment sampling sight; O, offshore sampling site.

FIG. 2.

Hierarchical cluster analysis of distance for E. coli concentrations in sand and water and selected factors associated with E. coli concentrations. Connected vertical lines designate joined cases. The dendrogram rescales the actual distances to numbers between 0 and 25, preserving the ratio of the distances between steps (SPSS, Inc.).

FIG. 3.

Daily mean log E. coli counts in sand and water over the sampling period. The lower portion shows the 3-day moving average. Circles, foreshore sand; triangles, 45-cm-deep water. The star indicates when sand was replaced along 400 m of the shore to a depth of 10 to 15 cm and inland 10 m.

FIG. 4.

E. coli counts in sand and water at 63rd Street Beach by distance from shore. Error bars indicate ± 1 standard error of the mean.