Irrigation by the lugworm Arenicola marina has a major impact on solute transport and distributions in nearshore environments due to the subsurface injection of overlying water at depth (typically 15-20 cm) and the resulting advective water flow within the sediment back toward the sediment surface. A mechanistic one-dimensional diffusion and advection model which emphasizes the injection and advective transport of solutes within the sediment not described in previous irrigation models was developed to describe irrigation by A. marina. The model was verified by comparing its results to measured pore-water tracer (Br) profiles and it is concluded that the model adequately described the experimental tracer profiles, including the characteristic subsurface peaks often observed in sediments inhabited by lugworms. There are three model output parameters, P (t), κ and β, which contain biological information about irrigation. P (t) is the pumping rate of A. marina [ml/h]. κ describes the ratio between the area of the advective transport zone (i.e. zone affected by the return advective transport of irrigation water) and the total area of the sediment core [dimensionless]. Finally, β is a measure of the increased effectively diffusive transport due to the presence of A. marina [cm2/h]. Pumping rates were estimated to 8.8-21.3 mL H2O/g/h which are within the range of reported values for A. marina. Model calculations of κ were between 0.1 and 0.53, equivalent to areas of 5.5 to 29 cm2, indicating that water was more or less locally transported to the sediment surface. In all simulations β was greater than 1 indicating that diffusion-like transport was enhanced by lugworms, affecting solute transport.