Sulfidic seeps with methane ebullition were observed at the low-water line of intertidal sand flats at a number of locations in the Wadden Sea. Bioturbating fauna was absent in the seep areas but abundant in the more central areas of the tidal flat. At one site, the vertical methane and sulfate distribution in pore water was determined along transects from the low-water line toward the interior of the sand flat. The resulting two-dimensional distributions showed a plume of methane-rich and sulfate-depleted pore water reaching from a depth below 1.2 m beneath the sand surface up to the sediment surface at the low-water line. The δ¹³C of methane released at the seeps was -68.6%₀, indicating a biological origin. The ¹⁰C signature of methane was clearly elevated by anthropogenic radiocarbon, which shows that the methane was formed less than 50 yr ago. The observations indicate an internal circulation, where water enters the sand flats in the central area and exits at the low-water line. Pore-water flow patterns in the sand flat during the tidal cycle were calculated from the surface topography and from the pressure distribution at the flat surface across the tidal cycle. The calculated flow patterns explain the measured methane and sulfate distributions and predict a residence time of the seepage water of about 30 yr. Intertidal sand flats act as one-way valves, passing water from the central surface through the interior of the flat to an outflow zone at and below the low-water line with a velocity of millimeters to centimeters per day. The flow causes permeable tidal flats to emit methane to the surface water and atmosphere in substantial amounts.