Branscomb mud volcano Briceland gas well Petrolia oil field Salmon Creek gas seep Mattole River gas seep Eureka C a l i f o r n i a

Each submarine groundwater discharge technique has individual strengths and weaknesses. Seepage meters provide a direct measurement of localized flow. They can also easily provide 'clean' seep water samples. However, seep meters may be susceptible to possible artifacts caused by interaction of tides and waves, although such limitations have not been thoroughly tested. The radioisotopes are less difficult to sample in the field than using seepage meters, but their measurement requires sophisticated laboratory equipment that is not widely available. One important characteristic of the radioisotope techniques is that they provide an integrated value of seepage rates across the entire lagoon. They are thus complementary to the seepage meter technique. Chloride concentrations indicate that only a minor component (1 -5%) of seep water originates from meteoric ground water. This implies that 95 -99% of the interstitial water has to be recycled lagoon seawater. The isotopic concentration of strontium (87Sr/86Sr) was nearly identical in the seep water and lagoon water, yet was measurably lower than that in modern seawater. The 87Sr/86Sr ratios were also systematically lower during the rainy season, reflecting the greater influx of seep water into lagoon water and short groundwater residence times. Nutrient concentrations were 3-5 times elevated in the seep water over the lagoon water, and suggest that sediment/water interface exchange processes, such as submarine groundwater discharge, are critical components of coastal nutrient budgets (Johannes, 1980; Krest et al., 2000). References

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