GAS SEEP INDUCED INTERSTITIAL WATER CIRCULATION - OBSERVATIONS AND ENVIRONMENTAL IMPLICATIONS

An interstitial water circulation, generated by gas Row through a perm eable sediment, was observed at an intertidal site on the Kattegat coa st of Denmark. Concentrations of methane dissolved in the interstitial water of the near-surface sediment decreased sharply only centimetres away from gas seeps venting almost pure methane (approximate to 99% m ethane). Water was driven out of the sediment by the rising bubbles of gas at the seep and was replaced by an equivalent draw-down of overly ing, oxygenated water into the surrounding sediment. This process stee pened the chemical gradients close to the gas Row channel, with the ef fects progressively diminishing with increasing distance from the seep . The position of the redox potential discontinuity (RPD) moved by as much as 7 cm deeper into the sediment close to the seep: this effect w as less marked, but still detectable, 50 cm away. The degree of displa cement from the ''normal'' sediment profiles depended on the magnitude of the interstitial Row rate. The distribution of pore water pH and s ulphate:sodium ratios were also dependent on the Row rate of the circu lating water. The concentrations of sulphide, thiosulphate and sulphit e in the interstitial water from the top 10 cm of sediment, were high at a seep, decreased to a minimum at 20-30 cm distance, then increased again at 40-50 cm distance. Laboratory experiments confirmed that gas bubbling through a fluid filled permeable matrix generated a Row, out of the sediment at the gas exit and into the sediment over the periph eral surfaces surrounding the outlet. Experimentally determined rates of dispersion, for gas Bow rates of 3-20 mi min(-1),for a 40 g l(-1) s odium chloride solution, were 62.5 x 10(-9) to 540 x 10(-9) m(2) s(-1) , 40-400 times the molecular diffusion coefficient. Linear interstitia l fluid velocities of 3-12 mm min(-1), were recorded at 14-3 cm from t he seep axis respectively, with a gas how rate of 5 mi min(-1). Two-di mensional modelling of the experimental system confirmed the Row patte rn determined visually with dye. Implications of this process with reg ard to the recycling rates of elements generally, and of nutrient and waste materials, in particular, are discussed.