Development and evaluation of an ultrasonic ground water seepage meter

Submarine ground water discharge can influence significantly the near-shore transport and flux of chemicals into the oceans. Quantification of the sou rces and rates of such discharge requires a ground water seepage meter that provides continuous measurements at high resolution over an extended perio d of time. An ultrasonic flowmeter has been adapted for such measurements i n the submarine environment. Connected to a steel collection funnel, the me ter houses two piezoelectric transducers mounted at opposite ends of a cyli ndrical flow tube. By monitoring the perturbations of fluid flow on the pro pagation of sound waves inside the flow tube, the ultrasonic meter can meas ure both forward and reverse fluid flows in real time. Laboratory and field calibrations show that the ultrasonic meter can resolve ground water disch arges on the order of 0.1 mum/sec, and it is sufficiently robust for deploy ment in the field for several days. Data from West Neck Bay, Shelter Island , New York, elucidate the temporal and spatial heterogeneity of submarine g round water discharge and its interplay with tidal loading. A negative corr elation between the discharge and tidal elevation was generally observed. A methodology was also developed whereby data for the sound velocity as a fu nction of temperature can be used to infer the salinity and source of the s ubmarine discharge. Independent measurements of electrical conductance were performed to validate this methodology.