An automated dye-dilution based seepage meter for the time-series measurement of submarine groundwater discharge

We designed an automated seepage meter that can detect and quantify both groundwater outflow and sea-water infiltration. Based on a dye-dilution technique, this instrument provides high-resolution time-series data for submarine groundwater discharge to the coastal zone. The dye-dilution method involves two repeatable steps: (1) the timed injection of a water-soluble dye into a "dye-mixing chamber" mounted in series with a seepage chamber and (2) the subsequent timed measurements of the absorbance of the dyed solution. The rate at which the dyed solution is diluted by the inflow or outflow of water is directly proportional to the flow rate moving through the surface area of the seepage housing. In addition to describing the instrument's components and the operating principle, we provide examples of laboratory flow calibrations and field deployments. As indicated by two sets of time-series studies, this instrument has performed reliably in field tests at Waquoit Bay (Cape Cod, Massachusetts) and Shelter Island (Long Island, New York). The instrument has yielded hydrologically consistent flow rates and has revealed major and subtle connections between tidal stage and the rate and direction of submarine groundwater discharge.