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To derive estimates of ground-water seepage into Indian River Lagoon, the following suite of tracers, chemical constituents and sampling devices were measured or utilized: nutrients, Cl-, conductivity, pH, temperature, dissolved oxygen, 87Sr/86Sr, d18O, 223,224,226Ra, 222Rn, seep meters, multi-samplers, and benthic flux chambers (Martin et al., 2000). Seepage rates were spatially and temporally heterogeneous, yet similar to rates previously measured in Indian River Lagoon using identical techniques. The seepage rates ranged from 3 - 100 ml m-2 min-1 during May (dry season) to 22 - 144 ml m-2 min-1 during August (rainy season). The average value for all meters increased from 40 to 63 ml m-2 min-1 from the dry to the rainy season, implying that there may be a connection between rainfall and increased seepage rates. The heterogeneous nature of these rates is likely caused by fluctuations in sediment permeabilities and other geologic characteristics.
Radon-222 and Ra isotopes have previously provided regionally integrated estimates of seepage flux in varied coastal environments (Cable et al., 1996; Moore, 1999; Swarzenski et al., in press). Benthic fluxes of Ra to the Indian River Lagoon were calculated using three independent methods that rely on the activities of short-lived Ra isotopes: 1) lagoon budget, 2) benthic flux chambers and 3) pore-water modeling. The first two methods yield direct measurements of flux across the sediment/water interface, whereas the third technique generates an indirect flux estimate on the basis of pore-water Ra profiles. Calculations of the benthic flux of Ra range up to almost 500 dpm m-2 day-1. Using 226Ra pore-water activities, a maximum upward subsurface water flow of about 5 - 17 cm day-1 is required to sustain these fluxes. These values are similar to the values measured directly with the seepage meters.
By using 222Rn and 226Ra as mass balance tracers of seepage flux to the northern Indian River Lagoon, it is possible to obtain measurements of seepage that are independent of the short-lived Ra isotopes. Assumptions required for this mass balance approach are that negligible effects were observed from surface water exchange to the lagoon, tides, and diffusion from the sediments. Analogous to the short-lived Ra isotopes, seepage fluxes measured on the basis of excess 226Ra activities are similar in magnitude to those estimated using seepage meters.
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).
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