USGS Groundwater Information: Branch of Geophysics
F.D. Day-Lewis, H.N. Karam, C.F. Harvey, and J.W. Lane, Jr.
Fresh submarine ground-water discharge (SGD) represents an important pathway for nutrients and contaminants to reach coastal ecosystems, yet the spatial and temporal distributions of SGD remain poorly understood. New fiber-optic distributed temperature sensor (DTS) technology represents a promising approach for high-resolution mapping of SGD. DTS instruments transmit laser light through fiber-optic cables and analyze one of two forms of backscatter (Raman or Brillouin) to produce temperature estimates at distributed points along the cable. Commercially available systems based on Raman scatter offer spatial, temporal, and thermal resolutions of 1 m, 1 minute, and 0.1 degree Celsius, respectively.
We present results from a field demonstration of a DTS system at the Waquoit Bay National Estuarine Research Reserve (WBNERR), East Falmouth, Massachusetts. A 1.3-km fiber-optic cable was deployed on a grid extending 80 m parallel and 60 m perpendicular to shore. The cable was weighted and submerged into the bay sediment to a depth of several cm near shore; further offshore, the cable sank into mud under its own weight. During the study period in May-June 2006, the bay was about 5-10 degrees Celsius warmer than fresh ground water at the site; hence cold anomalies in the DTS data are indicative of fresh SGD. DTS temperature measurements were interpolated to produce time-lapse maps of bay-floor temperature at about 1 minute intervals for a 2-week period. The DTS results were interpreted in the context of additional information: local meteorological data, including precipitation and air temperature; bay temperature; bay salinity; and ground-water levels in coastal wells. The DTS data indicate: (1) the development of a cold region at low tide extending from shore to 5 m offshore; (2) a positive correlation between bay-floor temperature and tidal level, with the strength of correlation decreasing with distance offshore; and (3) a positive correlation between bay-floor temperature and bay water temperature, with increasing strength of correlation with distance offshore. From the DTS data, we infer a zone of tide-driven, near-shore, fresh SGD, which is consistent with direct measurements of seepage and pore-fluid salinity. Our results demonstrate that DTS measurements can provide high-resolution information to help infer the extent and timing of SGD.
Final copy as submitted to American Geophysical Union 2006 Fall Meeting for publication as: Day-Lewis, F.D., Karam, H.N., Harvey, C.F., and Lane, J.W., Jr., 2006, Monitoring submarine ground-water discharge using a distributed temperature sensor, Waquoit Bay, Massachusetts [abs.]: EOS Transactions, American Geophysical Union, v. 87, no. 52, Fall Meeting Supplement, Abstract NS24A-02 Invited.