Spatial and temporal distributions of submarine groundwater discharge rates obtained from various types of seepage meters at a site in the Northeastern Gulf of Mexico

Direct measurements of submarine groundwater discharge (SGD) were taken by three different ( continuous heat, heat pulse, and ultrasonic) types of automated seepage meters as well as standard Lee-type manually operated meters. SGD flux comparisons and the spatial and temporal variations in groundwater flow were analyzed. Seepage rates measured by the different meters agree relatively well with each other ( more than 80% agreement). Comparisons of flux rates as a function of distance offshore using exponential approximations show that more than five measurement locations ( 200 m offshore) are needed for a precise integrated estimation of SGD offshore within an accuracy of +/- 10%. The dominant period of seepage variations is estimated to be about 12 hours, which closely matches the semidiurnal tides in this area. Our analysis also shows that short duration measurement periods may cause significant underestimates or overestimates of the daily averaged groundwater flow rates ( +/- 25% - +/- 60% difference when the measurement duration is less than 12 hours). Thus, continuous measurements of SGD using automated seepage meters with high time resolution should enable us to evaluate temporal and spatial variations of dissolved material transports via groundwater pathways. Such inputs may affect biogeochemical phenomena in the coastal zone.