Isotope and tracer techniques

Abstract

Submarine groundwater discharge is recognized as an important source of water, nutrients, and other dissolved constituents to the coastal ocean (e.g., Johannes 1980; Church 1996). It occurs both as diffuse seepage and as focused flow at vents or cold seeps, depending on its hydrogeological setting. Globally, the total freshwater flux from SGD is estimated to be on the order of 10% of the freshwater flux from rivers (Zektser and Loaiciga 1993; Burnett et al. 2003a), although in some settings submarine discharge can exceed river discharge, particularly during times of low flow (e.g., Moore 1996). In areas where the flux of groundwater is focused into vents or regions of strong seepage, diverse ecosystems can be found (Paull et al. 1984; Bussmann et al. 1999; Rutkowski et al. 1999). Given the spatial scales of the discharge zones, location and quantification of the groundwater flow to the coastal ocean can be challenging. A common approach for quantifying these discharge rates is to use geochemical tracers that are naturally enriched in groundwater relative to seawater and have well-understood chemistries within the marine environment. Most tracer studies of SGD have used naturally occurring radionuclides from the uranium and thorium decay series (e.g., Cable et al. 1996a; Moore 1996; Moore and Shaw 1998; Hussain et al. 1999). The flux of groundwater is determined by closing the tracer's mass balance within the marine environment, which requires knowledge of the end member (groundwater and seawater) concentrations, the flushing rate of the coastal water, surface water fluxes, and the removal pathways of the tracer (e.g., radioactive decay, gas exchange with the atmosphere, etc). In some settings, deliberate tracer experiments may be the best approach, especially when hydraulic connections between source areas and the coastal ocean need to be established.