Bacteria in beach sands: an emerging challenge in protecting coastal water quality and bather health

Abstract

To protect bather health at recreational beaches, fecal indicator bacterial standards are used to monitor water quality, and waters exceeding the standards are subsequently closed to bathers. However beachgoers are also in contact with beach sands, the sanitary quality of which is not included within beach monitoring programs. In fact, sands and sediments provide habitat where fecal bacterial populations may persist, and in some cases grow, in the coastal zone. Specific pathogens are less well studied in beach sands and sediments, but there is a body of evidence that they too may persist in these environments. This paper reviews the current state of knowledge regarding the abundance and distribution of fecal indicator bacteria and pathogens in beach sands of diverse climatological regions, and at beaches subjected to varied levels of anthropogenic impact. In all regions fecal indicator bacteria are nearly ubiquitous in beach sands, and similar relationships emerge among fecal indicator abundance in dry sand, submerged sands, and water. Taken together, these studies contextualize a potential public health issue and identify research questions that must be addressed in order to support future policy decisions.

Figure 1. Pathways of fecal indicator bacteria (FIB) into and out of beach sands

(A) Runoff: Precipitation causes diffuse land-based runoff that concentrates FIB from urban areas (roadways, parking lots, gutters, lawns, pets), agriculture (overflow of animal waste), or feces from wildlife in the watershed into stormwater. Stormwater flows through local waterways or runs directly over beach sands into the intertidal zone. (B) Aging Infrastructure: In urban areas with combined sewer overflows, heavy precipitation delivers a mix of urban runoff and raw sewage to beach sands and/or coastal waters, depending on outfall location and tidal stage. Leaky sewer infrastructure, failed septic systems and buried drainage pipes in the coastal zone may also be sources of FIB to beach sands. (C) Swash zone: Periodic tidal rewetting enables FIB deposited in dry sands to persist or regrow, and waves may deliver FIB from the water column into the upper intertidal sands. (D) Exchange: Resuspension of sand into water by tidal or wind-driven waves may redistribute bacteria from sand to water; humans are then exposed to these bacteria when bathing. Likewise, deposition of particulate matter may introduce or return bacteria to the sand. Accretion of sands could bury FIB-rich sands at the beach, and erosion could alternately expose or relocate contaminated sands along the beach. (E) Water: Residence time of water at the beach may quickly remove or alternately retain bacteria near shore; thus, local hydrography and wind direction contribute to rates of removal or retention. (F) Fecal events: Animals (birds, dogs, wildlife, humans) on the beach may directly introduce FIB to sands, which can subsequently be redistributed over a greater area of beach by pedestrian traffic or weather events. (G) Additional Refugia: Wrack, harboring robust bacterial populations seeded from land-based runoff or surfzone water, may shed FIB to sand or water during high tides.