Time/Date 2002
RADIUM ISOTOPES AS TRACERS OF COASTAL MIXING AND SUBMARINE GROUNDWATER DISCHARGE
3. Interpretation of Radium Isotope Data 1. Separating sources of SGD
3.4. Evaluating estuary residence times
FIG.5. The concentrations of total dissolved nitrogen and phosphorus display strong
correlations with 226Ra in samples from monitoring wells (excluding K) off the South Carolina coast [11].
Assuming that the water supplying excess 226Ra to the coastal ocean is similar to these well waters, we use the nutrient–Ra correlation to estimate the nutrient flux (Fig.5). For example the total dissolved phosphate (TDP) to 226Ra ratio is 0.93 µM·dpm–1. To support a 226Ra flux of 1.5 × 1011 dpm·d–1, the accompanying TDP flux is 1.4 × 105 moles·d–1. The total dissolved nitrogen (TDN)/226Ra ratio is 26.4 µM·dpm–1; thus the TDN flux would be 4.0 × 106 moles·d–1.
3.4. Evaluating estuary residence times
Moore [21] developed a model to estimate the ages of water masses in the coastal ocean. This model assumed that radium was only added to the water near the shoreline; after the water left the coast and lost contact with the bottom, radium additions ceased. This model is not applicable to estuaries and salt marshes where radium additions from sediments and groundwater are continuous. To use radium isotopes to estimate ages in these systems, Moore et al. [18] used a different approach. A similar approach was developed by Hwang et al. [17].
Assume the system under study is in steady state, that is radium additions are balanced by losses.
Additions include radium fluxes from sediment, river, and groundwater; losses are due to mixing and, in the case of 223Ra and 224Ra, radioactive decay. Write an equation for the 224Ra balance:
F 224Ra = I 224Ra (λ224 + 1/τ) (12)
where F 224Ra is the total flux of 224Ra to the system, I 224Ra is the inventory of 224Ra in the system, λ224 is the decay constant for 224Ra, and τ is the apparent age of water in the system. We can write a similar equation for 228Ra; but, because the half-life is 5.7 years, the effect of decay can be ignored.
F 228Ra = I 228Ra (1/τ) (13)
Now divide eq.12 by eq.13.
F (224Ra/228Ra) = I 224Ra (λ224 + 1/τ)/ I 228Ra (1/τ) (14) This equation can be rearranged and solved for τ:
τ = [F(224Ra/228Ra) – I(224Ra/228Ra)]/I(224Ra/228Ra)λ224 (15) In this case F (224Ra/228Ra) is the (224Ra/228Ra) activity ratio (AR) of the flux into the system, I (224Ra/228Ra) is the (224Ra/228Ra) AR in the system, and λ224 is the decay constant of 224Ra (0.19 day–1).
Here we use the 224Ra/228Ra AR because the time scale is appropriate to the 224Ra half-life. In cases where ages are expected to be on the order of weeks, a similar equation based on 223Ra (λ = 0.0608 day–1) would be more appropriate.
This model is quite sensitive to the value selected for the 224Ra/228Ra AR of water entering the system.
This can be determined by finding areas of discharge via infrared imaging or other techniques and measuring shallow groundwaters at these sites. Another approach is to collect as much data as possible from shallow permeable sediments and plot 224Ra vs 228Ra to determine the average activity ratio.
4. Summary
There are good reasons why radium is closely linked to SGD studies. Because radium is highly enriched in salty coastal groundwater relative to the ocean, small inputs of SGD can be recognized as a strong radium signal. In many cases this signal can be separated into different SGD components using the two long-lived Ra isotopes. The short-lived Ra isotopes are useful in evaluating the residence time or mixing rate of estuaries and coastal waters. Such estimates coupled with distributions of the long-lived Ra isotopes enables a direct estimate of radium fluxes. At steady state these fluxes must be balanced by fresh inputs of Ra to the system. If other sources of Ra can be evaluated, the Ra flux that must be sustained by SGD can be evaluated. By measuring the Ra content of water in the coastal aquifers, the amount of SGD necessary to supply the Ra is determined. SGD fluxes of other components are determined by measuring their concentrations in the coastal aquifer or by determining relationships between the component and Ra and multiplying the component/Ra ratio by the Ra flux.
REFERENCES
[1] LI, Y.-H., MATHIEU, G., BISCAYE, P., SIMPSON, H.J., The flux of 226-Ra from estuarine and continental shelf sediments, Earth Planet. Sci. Lett. 37 (1977) 237–241.
[2] MOORE, W.S., Large groundwater inputs to coastal waters revealed by 226Ra enrichments, Nature 380 (1996) 612–614.
[3] BURNETT, W.C., COWART, J.B., DEETAE, S., Radium in the Suwannee River and estuary:
spring and river input to the Gulf of Mexico, Biogeochemistry 10 (1990) 237–255.
[4] RAMA, MOORE, W.S., Using the radium quartet for evaluating groundwater input and water exchange in salt marshes, Geochim. Cosmochim. Acta 60 (1996) 4645–4652.
[5] MOORE, W.S., The effects of groundwater input at the mouth of the Ganges–Brahmaputra Rivers on barium and radium fluxes to the Bay of Bengal, Earth Planet. Sci. Lett. 150 (1997) 141–150.
[6] MOORE, W.S., SHAW, T.J., Chemical signals from submarine fluid advection onto the continental shelf, J. Geophys. Res. 103 (1998) 21543–21552.
[7] MOORE, W.S., The subterranean estuary: a reaction zone of ground water and sea water, Mar.
Chem. 65 (1999) 111–126.
[8] KREST, J.M., MOORE, W.S., GARDNER, L.R., Marsh nutrient export supplied by groundwater discharge: evidence from radium measurements, Glob. Biogeochem. Cycles 14 (2000) 167–176.
[9] CHARETTE, M.A., BUESSELER, K.O., ANDREWS, J.E., Utility of radium isotopes for evaluating the input and transport of groundwater-derived nitrogen to a Cape Cod estuary, Limnol. Oceanogr. 46 (2001) 465–470.
[10] KELLY, R.P., MORAN, S.B., Seasonal changes in groundwater input to a well-mixed estuary estimated using radium isotopes and implications for coastal nutrient budgets, Limnol.
Oceanogr. 47 (2002) 1796–1807.
[11] MOORE, W.S., KREST, J., TAYLOR, G., ROGGENSTEIN, E., JOYE, S., LEE, R., Thermal evidence of water exchange through a coastal aquifer: Implications for nutrient fluxes, Geophys.
Res. Lett. 29 (2002) doi: 10.1029/2002GL014923.
[12] CROTWELL, A.M., MOORE,W.S., Nutrient and Radium Fluxes from Submarine Groundwater Discharge to Port Royal Sound, South Carolina, Aquat. Geochem. 9 (2003) 191–208.
[13] MOORE, W.S., Sources and fluxes of submarine groundwater discharge delineated by radium isotopes, Biogeochemistry 66 (2003) 75–93.
[14] KIM, G., LEE, K.-K., PARK, K.-S., HWANG, D.-W., YANG, H.-S., Large submarine groundwater discharge (SGD) from a volcanic island, Geophys. Res. Lett. 30 (2003) doi:10.1029/2003GL018378.
[15] KREST, J.M., HARVEY, J.W., Using natural distributions of short-lived radium isotopes to quantify groundwater discharge and recharge, Limnol. Oceanogr. 48 (2003) 290–298.
[16] MOORE, W.S., WILSON, A.M., Advective flow through the upper continental shelf driven by storms, buoyancy, and submarine groundwater discharge, Earth Planet.Sci. Lett. 235 (2005) 564–576.
[17] HWANG D.W., KIM, G., LEE, Y.-W., YANG, H.-S., Estimating submarine inputs of groundwater and nutrients to a coastal bay using radium isotopes, Mar. Chem. 96 ( 2005) 61–71.
[18] MOORE, W.S., BLANTON, J.O., JOYE, S., Estimates of Flushing Times, Submarine Groundwater Discharge, and Nutrient Fluxes to Okatee River, South Carolina, J. Geophys. Res.
(2006) in press.
[19] MOORE, W.S., Radium Isotopes as Tracers of Submarine Groundwater Discharge in Sicily, Cont. Shelf Res. 26 (2006) 852–861.
[20] MOORE, W.S., Determining coastal mixing rates using radium isotopes, Cont. Shelf. Res. 20 (2000) 1993–2007.
[21] MOORE, W.S., Ages of continental shelf waters determined from 223Ra and 224Ra, J. Geophys.
Res. 105 (2000) 22117–22122.
[22] MOORE, W.S., Sampling Radium-228 in the Deep Ocean, Deep Sea Res. 23 (1976) 647–651.
[23] MOORE, W.S., ARNOLD, R., Measurement of 223Ra and 224Ra in coastal waters using a delayed coincidence counter, J. Geophys. Res. 101 (1996) 1321–1329.
[24] GIFFIN, C., KAUFMAN, A., BROECKER, W.S., Delayed coincidence counter for the assay of actinon and thoron, J. Geophys. Res. 68 (1963) 1749–1757.
[25] MOORE, W.S., Radium Isotope Measurements Using Germanium Detectors, Nucl. Inst.
Methods 223 (1984) 407–411.
[26] MOORE, W.S., KJERFVE, B., TODD, J.F., Identification of rain–freshened plumes in the coastal ocean using Ra isotopes and Si, J. Geophys. Res. 103 (1998) 7709–7717.
MODELLING METHOD FOR SGD PHENOMENON AND SGD MEASUREMENTS IN THE GULF OF TRIESTE
PART A: MODELLING METHOD FOR SIMULATION OF SGD