Scm. Ohara et al., GAS SEEP INDUCED INTERSTITIAL WATER CIRCULATION - OBSERVATIONS AND ENVIRONMENTAL IMPLICATIONS, Continental shelf research, 15(8), 1995, pp. 931-948
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.