The biogeochemical environment of benthic sediments is affected by seawater transport across the sediment-water interface and within the sediment pore spaces. Water circulation through the near surface sediments is produced by passive pumping of relict biostructures such as worm tubes and burrows. This passive ventilation results from hydrodynamic interactions between structures that protrude from the sediment surface and flowing water in the overlying boundary layer. Flow over the end of a cylindrical worm tube projecting above the sediment surface into the benthic boundary layer results in lowered pressure in the tube and in sediments around the bottom end of the tube. This induces flow of surface water through the adjacent sediment, and back to the surface through tube. A physical mechanism for this passive circulation is presented. A mathematical model of interstitial water circulation produced by passive hydrodynamic effects has been solved analytically for steady flow in homogeneous isotropic sediments. modflow, a finite difference groundwater flow model, is used to numerically determine the hydraulic gradients induced and the resulting flow through sediments near a structure under various conditions. The magnitude of passive irrigation fluxes depends on the size and position of relict structures, velocity of overlying surface water, hydraulic conductivity of the sediments, and the spatial distribution of tubes. For a typical sandy sediment, the hydraulic head in sediments near the bottom of a tube can be reduced by up to several centimeters, and the zone of reduced head can extend over several hundred square centimeters around the tube. Surface water is drawn into the sediment throughout this area with a mean influx velocity of 10 −3 cm/s or greater, yielding discharge rates on the order of tens to hundreds of milliliters per hour. These passive fluxes are of the same magnitude as active pumping rates, significantly greater than molecular diffusive fluxes, and may represent a major driving mechanism for cycling water through surface sediments.