Henrieta Dulaiova

Estimating the dynamics of groundwater input into the coastal zone via continuous radon-222 measurements.

Submarine groundwater discharge (SGD) into the coastal zone has received increased attention in the last few years as it is now recognized that this process represents an important pathway for material transport. Assessing these material fluxes is difficult, as there is no simple means to gauge the water flux. To meet this challenge, we have explored the use of a continuous radon monitor to measure radon concentrations in coastal zone waters over time periods from hours to days. Changes in the radon inventories over time can be converted to fluxes after one makes allowances for tidal effects, losses to the atmosphere, and mixing with offshore waters. If one assumes that advective flow of radon-enriched groundwater (pore waters) represent the main input of 222Rn in the coastal zone, the calculated radon fluxes may be converted to water fluxes by dividing by the estimated or measured 222Rn pore water activity. We have also used short-lived radium isotopes (223Ra and 224Ra) to assess mixing between near-shore and offshore waters in the manner pioneered by. During an experiment in the coastal Gulf of Mexico, we showed that the mixing loss derived from the 223Ra gradient agreed very favorably to the estimated range based on the calculated radon fluxes. This allowed an independent constraint on the mixing loss of radon-an important parameter in the mass balance approach. Groundwater discharge was also estimated independently by the radium isotopic approach and was within a factor of two of that determined by the continuous radon measurements and an automated seepage meter deployed at the same site.

Shelf-derived iron inputs drive biological productivity in the southern Drake Passage

water entrainment while Fe/ 228 Ra ratios were used to calculate the Fe flux. In the summer of 2006 we found rapid mixing and significant lateral iron export, namely, a dissolved iron flux of 1.1 � 10 5 mol d � 1 and total acid leachable iron flux of 1.1 � 10 6 mol d � 1 all of which is transported in the mixed layer from the shelf region offshore. This dissolved iron flux is significant, especially considering that the bloom observed in the offshore region (0.5–2 mg chl a m � 3 ) had an iron demand of 1.1 to 4 � 10 5 mol Fe. Net vertical export fluxes of particulate Fe derived from 234 Th/ 238 U disequilibrium and Fe/ 234 Th ratios accounted for only about 25% of the dissolved iron flux. On the other hand, vertical upward mixing of iron rich deeper waters provided only 7% of the lateral dissolved iron flux. We found that similarly to other studies in iron-fertilized regions of the Southern Ocean, lateral fluxes overwhelm vertical inputs and vertical export from the water column and support significant phytoplankton blooms in the offshore regions of the Drake Passage.

Coupled radon, methane and nitrate sensors for large-scale assessment of groundwater discharge and non-point source pollution to coastal waters

We constructed a survey system of radon/methane/nitrate/salinity to find sites of submarine groundwater discharge (SGD) and groundwater nitrate input. We deployed the system in Waquoit Bay and Boston Harbor, MA where we derived SGD rates using a mass balance of radon with methane serving as a fine resolution qualitative indicator of groundwater. In Waquoit Bay we identified several locations of enhanced groundwater discharge, out of which two (Childs and Quashnet Rivers) were studied in more detail. The Childs River was characterized by high nitrate input via groundwater discharge, while the Quashnet River SGD was notable but not a significant source of nitrate. Our radon survey of Boston Harbor revealed several sites with significant SGD, out of these Inner Harbor and parts of Dorchester Bay and Quincy Bay had groundwater fluxes accompanied by significant water column nitrogen concentrations. The survey system has proven effective in revealing areas of SGD and non-point source pollution.

Seepage rate variability in Florida Bay driven by Atlantic tidal height

Atlantic tidal fluctuations drive pressure head variations in shallow offshore wells drilled into the limestone subsurface on both the Florida Bay and Atlantic sides of Key Largo, Florida, USA. We tested the hypothesis that these pressure head variations influence groundwater flow and that flux rate variability is associated with tidal variability. We used an automated Rn monitor to make continuous measurements of 222Rn, a natural tracer of groundwater discharge, in Florida Bay waters. We also deployed three types of seepage meters, including an automated heat pulse meter to collect a continuous record of seepage from the sediments. Drum type seepage meters inserted into soft sediments and fiberglass meters cemented to the rocky bay floor were utilized with pre-filled 4-l bag collectors, and monitored on an hourly basis. Maximum Rn inventories in Florida Bay waters were associated with high tide on the Atlantic side of the island. Modeling of the Rn variation indicated variable groundwater discharge rates with maximum flux occurring at high Atlantic tide. Seepage meter results in Florida Bay were consistent with 222Rn modeling. Florida Bay seepage meter rates showed positive correlation with Atlantic tide, meter 1, r = 0.63, n = 12, p < 0.025 and meter 2, r = 0.67, n = 12, p < 0.025. A seepage meter offshore of the Atlantic side of Key Largo exhibited rates that were inversely correlated with Atlantic tide (r = 0.87, n = 9, p < 0.005) showing negative rates when the tide was high, and positive rates when the tide was low. Overall, our results are consistent with the hypothesis of Reich et al. (2002), that pressure head variations driven by Atlantic tide influence groundwater seepage rate variability in Florida Bay off Key Largo. Effectively, as proposed by Reich et al. (2002), Key Largo functions as a semi-permeable dam separating Florida Bay and the Atlantic Ocean.

Groundwater Discharge as an Important Land-Sea Pathway into Manila Bay, Philippines

Abstract A multidisciplinary approach was taken to assess the potential importance of groundwater seepage to nutrient inputs into Manila Bay, Philippines. Three lines of seepage meters were installed in transects along the coast at Mariveles, Bataan Province, during the period 8–10 January 2005. The overall average seepage flux was 5.1 ± 5.4 cm d−1 (n = 73) with a range of 0–26 cm d−1 and a calculated integrated shoreline flux of 12.4 m3 m−1 d−1. Additional methodologies employed included automatic seepage meters, resistivity measurements, sampling for nutrient analyses in both seepage meters and ambient seawater, and use of natural radon as a groundwater tracer. Seepage meter and tracer results provided consistent results of estimates of submarine groundwater discharge into Manila Bay. Many lines of evidence suggest that seepage fluxes are not steady state but are modulated by the tides. Resistivity profiles show that the saline-freshwater interface moves on a tidal timescale, consistent with the observed drop in salinity of the seepage waters as low tide approaches. Our results show that dissolved inorganic nitrogen (DIN) fluxes via submarine groundwater discharge are comparable in magnitude to DIN fluxes from each of the two major rivers that drain into Manila Bay.

A Geochemical and Geophysical Assessment of Coastal Groundwater Discharge at Select Sites in Maui and O’ahu, Hawai’i

This chapter summarizes fieldwork conducted to derive new estimates of coastal groundwater discharge and associated nutrient loadings at select coastal sites in Hawai’i, USA. Locations for this work were typically identified based on pronounced, recent ecosystem degradation that may at least partially be attributable to sustained coastal groundwater discharge. Our suite of tools used to evaluate groundwater discharge included select U/Th series radionuclides, a broad spectrum of geochemical analytes, multi-channel electrical resistivity, and in situ oceanographic observations.

Journal of Environmental Radioactivity special issue: radium and radon isotopes as environmental tracers.

Radium isotopes (Ra) and radon (Rn) are intermediate nuclides in the U–Th decay series, and have been applied with great success to a wide range of environmental dating and tracing studies. In recent years, advances in measurement instrumentation (RaDeCC, Rad-7, low-background gamma spectrometry) and demonstration of their utility in studying water mixing and submarine groundwater discharge (SGD) have greatly increased interest in Ra and Rn isotopes in the environment. A workshop on the measurement and application of short-lived Ra isotopes held in Monaco in 2006 drew broad international attendance, and covered a range of topics from technical issues of isotope measurement to coastal and open ocean studies of isotope cycling and mass balance (Charette and Scholten, 2008). The success of this first workshop led to the organization of a second, held in Venice, Italy, during 7–11 April 2008. The meeting was attended by 52 scientists representing 14 countries, and covered an even more diverse range of topics related to the use of Ra and Rn isotopes in environmental studies. The scope of presentations included technical aspects of isotope measurements, the use of Ra isotopes as chronometers, and process studies in atmospheric, oceanic, and groundwater systems (Moore, 2008a, b). A number of works presented at the workshop have been elaborated in articles compiled in this special issue. Several working groups also were convened during the workshop to discuss pertinent, emerging topics: analytical standards for short-lived Ra isotopes, identification of appropriate Ra–Rn endmembers for calculating SGD, application of Ra and Rn to freshwater systems, and Ra isotopes in coastal margin processes and the Mediterranean. The standards group recommended methods and tracer solutions for preparing 223Ra and 224Ra standards, and a plan was formulated for the organization of an intercalibration exercise. The SGD endmember group concluded that systems should be characterized initially to identify likely SGD pathways, and that the relationship between Ra and salinity (as well as Fe, Mn, redox, and pH) should be assessed in the subterranean mixing zone between fresh and salt water (the “subterranean estuary”; Moore, 1999). This relationship should be used in conjunction with seepage meter and sediment equilibration samples to guide the endmember choice. The freshwater systems workgroup discussed the site-specific success of Ra isotope application in fresh waters, and N. Dimova presented preliminary experiments on the use of 220Rn as a very short-lived tracer. The group also concluded that Rn would remain only a qualitative tracer of groundwater input to rivers until the emanation (degassing) term could be better constrained. The margins group identified a critical need to quantify the shelf sediment input term for use of Ra isotopes as tracers