EXAMINING SUBMARINE GROUND-WATER DISCHARGE INTO SIMPSONBAY, ALASKA WITH ELECTRICAL RESISTIVITY AND RADON

摘要

Estimates of submarine ground-water discharge (SGD) have remained one of the most elusivecomponents of a coastal water and constituent budget. Nonetheless, environmental impacts of SGD areoften widespread, and have been tied to coastal eutrophication, algal blooms, and near-shore bacterialtransport. New geochemical and geophysical techniques have recently been developed to better quantifyrates and scales of SGD. Simpson Bay, in eastern Prince William Sound, Alaska, was chosen as a SGDendmember field site where hydrogeologic conditions should maximize coastal ground-water discharge.In Simpson Bay, the tidal range is large (> 5 m), there is significant recharge (annual precipitation canexceed 400 cm), and the ubiquitous glacially-derived coastal sediment can contain hydrologicallytransmissive units. The natural geochemical tracer ~(222)Rnwas used to quantify total (saline + fresh watercomponent) SGD at one site within the bay, while simultaneous, multi-electrode, land-based resistivitysurveys were used to examine the dynamics of the fresh water / salt water interface and SGD mixingscales and patterns. A water-column, multi-day time series in Simpson Bay recorded a range in 222Rn activities(2,000 – 14,000 dpm m~(-3))that fluctuated in an inverse manner with the tide. In contrast, ~(222)Rnchangedlittle in near-shore ground-water (50,000 – 65,000 dpm m~(-3))collected from a shallow piezometer sitelocated just seaward of the high-tide line. Such a strong tracer gradient can be used to construct anadvective transport model – in this case of submarine ground-water discharge. A ~(222)Rn mass balanceproduced a record, mean advective SGD rate of 320±590 cm d-1. In addition to the radon measurements,land-based resistivity profiles were conducted at the same study site. This system consisted of an AGISuperSting 8 channel receiver attached to an external switch box that controlled the current flow to 56electrodes, spaced 2 m apart. Four resistivity images collected during a low- to rising-tide indicate thatSGD occurred in a band that extended more than 100 m from the high-tide line, that the position of thefresh water / salt water interface was modulated directly by the large tidal range, and that the offshoreflow of ground water is expectedly constrained to geological units of higher transmissivity. The near-shore lithology was also mapped using an ultra high-resolution CHIRP (2-16 kHz) system, and appearedto reveal zones of enhanced, near-shore submarine ground-water discharge. In Simpson Bay, SGD isexpectedly large and thought to contribute significantly to the bay's overall water and constituentbudgets.