Automated Seepage Meter Anthropogenic sources of nitrogen have adversely impacted the water quality of coastal ponds on Cape Cod and other coastal regions of the nation. A major source of "new" nitrogen to these coastal waters and estuaries is groundwater, which intercepts septic tank fields along its flow path to the coastline. The temporal and spatial variability of the groundwater-derived nitrogen flux to coastal waters is understudied and, therefore, poorly understood.

This proposal is a one year "follow-on" to a currently funded CICEET project with the same title.  Under the current grant, a new automated and autonomous method for measuring the rate of submarine groundwater discharge (SGD) using conventional seepage meters was developed.  We have moved from bench top testing to a limited amount of field testing in Waquoit Bay where the new instrument recorded tidally-dependent variability in the magnitude and direction of the SGD.  This new method involves injecting dye into a mixing chamber attached to the seepage meter and following the rate of dye dilution by groundwater with an in situ spectrometer.  The use of a commercial in situ nitrate analyzer to inject the dye and make the absorbance measurements also allows one to measure the concentration of nitrate in the ambient water.  The overall goal is to transform the current instrument from a prototype to a more refined version that can measure high frequency rates of SGD and associated nutrients to coastal embayments over periods of days to weeks.  This goal will be addressed through the continued development and testing of a combined automated seepage meter/DIN (nitrate and ammonia) analyzer.  This will include using a newly acquired EcoLAB instrument that can measure both nitrate and ammonia in surface water and upper pore waters.

The first part of the proposal year will be used to integrate the existing technologies with field testing at our primary study site (Waquoit Bay NEER). The second part of the year will entail instrument deployment at an alternate NEER location, possibly within the Peconic Estuary on Long Island, NY. We are participating in an SGD intercomparison experiment in this area in the spring of 2002.

 Many thanks to NOAA and CICEET for their support of this project

Traditional seepage meters are notorious for providing ambiguous data in low-flow settings. We have therefore designed an automated seepage meter that can detect and quantify groundwater in many environmental settings.  Based on a dye-dilution technique, this instrument provides high-resolution time-series data for 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 inflowing or out flowing water is directly proportional to the flow rate moving through the surface area of the seepage housing. As indicated by two sets of time-series studies, this instrument has performed reliably in field tests at Waquoit Bay (Cape Cod, Massachusetts, USA) and Shelter Island (Long Island, New York, USA). The instrument has yielded hydrologically consistent flow rates and has revealed major and subtle connections between tidal stage and the rate and direction of groundwater discharge. This new seepage meter will be deployed for a short time (approximately one to two weeks) at the nested groundwater monitoring wells constructed last year and at the new wells constructed as part of this work plan. This will allow us to "rate" the head differences measured in the wells to the flow measured by the meter.  Once the rating is completed for each site, we are optimistic that the continuously measured heads in the wells will allow us to calculate continuous seepage rates.  The meter will also be deployed for short times at other locations and at other times to determine the spatial and temporal variability in seepage rates.

Many thanks to the USGS for their support of this project.

back to top