FIELD-EVALUATION OF SEEPAGE METERS IN THE COASTAL MARINE-ENVIRONMENT

The response of seepage meters was evaluated in a nearshore marine env ironment where water motion effects are more pronounced than in lake s ettings, where these meters have been used traditionally. Temporal and spatial variations of seepage, as well as potential artifacts, were e valuated using empty and 1000-ml pre-filled bag measurements. Time-ser ies measurements confirmed earlier observations that anomalously high fluxes occur during the early stages (less than or equal to 10 min) of collection. As deployment rimes increased (30-60 min), measured flow rates stabilized at a level thought to represent the actual seepage fl ux. Pre-filling the plastic measurement bags effectively alleviated th is anomalous, short-term influx. Reliable seepage measurements require d deployment times sufficient to allow a net volume of at least 150 mi into the collection bag. Control experiments, designed by placing see page meters inside sand-filled plastic swimming pools, served as indic ators of external effects on these measurements, i.e. they served as s eepage meter blanks. When winds were under 15 knots, little evidence w as found that water motion caused artifacts in the seepage measurement s. Tidal cycle influences on seepage rates were negligible in the pres ent study area, but long-term temporal variations (weeks to months) pr oved substantial. Observed long-term changes in groundwater flux into the Gulf of Mexico correlated with water table elevation at a nearby m onitoring well. (C) 1997 Academic Press Limited.