Porewater pressure gradients generated by activities of the lugworm Arenicola marina were measured in the laboratory and in unrestrained field situations to determine the spatial extent of influence of individuals on porewater advection. These subsurface deposit-feeders live head-down in sediments in J-shaped burrows and defecate on the surface. Individual animals generated pressures of up to 18 cm H2O in the region of the feeding pocket, and the pressures decayed radially from the source with an exponent of -1.3. Pressure transients from defecation are detectable 50 cm from the source in unrestrained field populations. The porewater flows resulting from the observed pressure gradients in the field were modeled using Darcy's law and a dipole model of the pressure field. The daily activities of lugworms were separated into categories of feeding, burrow maintenance, defecation, and burrowing, and the integrated effects of these activites were used to generate the porewater flow patterns. In a sandy mud, individuals pump 1 L d-1 of water into the sediment in the region of their feeding pocket. As a result of this pumping, porewater is completely replaced once per day in a spheroidal region 6.8 cm in diameter, centered on the feeding pocket, and at least 10% of the porewater is replaced per day in a column of sediment 24 cm in diameter centered on the feeding pocket. In field populations with 50 animals per m2, the flow fields generated by activities of neighboring animals all overlap, so the entire feeding zone between the depths of 15 and 20 cm is continuously flushed by worm activites on spatial scales of thousands of hectares. The large overlapping influences of neighboring worms on porewater fluxes through the sediment-water interface indicates that packed cylinder models of porewater transport need to be used with great care in these bioadvective systems. The fluxes from these field measurements also indicate that our models to date grossly underestimate the scale and intermittency of porewater flux and therefore its impact on biogeochemical cycling.