Jaye E. Cable

Estimating groundwater discharge into the northeastern Gulf of Mexico using radon-222

Abstract Submarine groundwater discharge (SGD) may provide important chemical constituents to the ocean, but the dispersed nature of this process makes locating and quantifying its input extremely difficult. Since groundwater contains 3–4 orders of magnitude greater radon than seawater, 222Rn may be a useful tracer of this process if all other sources of radon to bottom waters can be evaluated. We report development of a SGD tracing tool based on radon inventories in a coastal area of the northeastern Gulf of Mexico. We evaluated factors that influence the concentration of radon in the water column (i.e., production-decay, horizontal transport, and loss across the pycnocline) using a linked benthic exchange-horizontal transport model. Total 222Rn benthic fluxes (≥2420 dpm m−2 day−1) measured with in situ chambers are of the magnitude required to support measured sub-pycnocline 222Rn inventories, while estimates of molecular diffusion show that this input is relatively small (≤230 dpm m−2 day−1). Using this model approach, together with measurements of the radon inventory, we estimated a regional subsurface fluid flow ranging from 180 to 710 m3 sec−1 into the 620 km2 study area. This discharge, equivalent to an upward advective velocity of approximately 2–10 cm day−1 dispersed over this entire study area, is equivalent to approximately 20 first magnitude springs.

Magnitude and variations of groundwater seepage along a Florida marine shoreline

Direct groundwater inputs are receiving increasingattention as a potential source of nutrients and otherdissolved constituents to the coastal ocean. Seepageinto St. George Sound, Florida was measuredextensively from 1992 to 1994 using seepage meters. Spatial and temporal variations were documented alonga 7-km stretch of coastline and up to 1 km from shore. Measurements were made at 3 transects perpendicular toshore and 1 transect parallel to shore. The generalresults indicated that seepage decreased with distancefrom shore (2 of 3 transects), and substantialtemporal and spatial variability was observed inseepage flow from nearshore sediments. In addition,trends in mean monthly integrated seepage rates weresimilar to precipitation patterns measured at a nearbycoastal weather station. Based on these measurements, weestimate that the magnitude of groundwater seepage intothe study area is substantial, representing from 0.23 to4.4 m3 ⋅ sec-1of flow through the sediments, approximately equivalentto a first magnitude spring. Although it is unknown howrepresentative this region is with respect to globalgroundwater discharge, it demonstrates thatgroundwater flow can be as important as riverine andspring discharge in some cases. Our subsurfacedischarge rates suggest groundwater is an importanthydrologic source term for this region and may beimportant to the coastal biogeochemistry as well.

Importance of Storm Events in Controlling Ecosystem Structure and Function in a Florida Gulf Coast Estuary

Abstract From 8/95 to 2/01, we investigated the ecological effects of intra- and inter-annual variability in freshwater flow through Taylor Creek in southeastern Everglades National Park. Continuous monitoring and intensive sampling studies overlapped with an array of pulsed weather events that impacted physical, chemical, and biological attributes of this region. We quantified the effects of three events representing a range of characteristics (duration, amount of precipitation, storm intensity, wind direction) on the hydraulic connectivity, nutrient and sediment dynamics, and vegetation structure of the SE Everglades estuarine ecotone. These events included a strong winter storm in November 1996, Tropical Storm Harvey in September 1999, and Hurricane Irene in October 1999. Continuous hydrologic and daily water sample data were used to examine the effects of these events on the physical forcing and quality of water in Taylor Creek. A high resolution, flow-through sampling and mapping approach was used to characterize water quality in the adjacent bay. To understand the effects of these events on vegetation communities, we measured mangrove litter production and estimated seagrass cover in the bay at monthly intervals. We also quantified sediment deposition associated with Hurricane Irenes flood surge along the Buttonwood Ridge. These three events resulted in dramatic changes in surface water movement and chemistry in Taylor Creek and adjacent regions of Florida Bay as well as increased mangrove litterfall and flood surge scouring of seagrass beds. Up to 5 cm of bay-derived mud was deposited along the ridge adjacent to the creek in this single pulsed event. These short-term events can account for a substantial proportion of the annual flux of freshwater and materials between the mangrove zone and Florida Bay. Our findings shed light on the capacity of these storm events, especially when in succession, to have far reaching and long lasting effects on coastal ecosystems such as the estuarine ecotone of the SE Everglades.

The Impacts of Pulsed Reintroduction of River Water on a Mississippi Delta Coastal Basin

Abstract During the twentieth century about 25% of the wetlands of the Mississippi delta was lost, partially a result of isolation of the river from the delta. River diversions are being implemented to reintroduce river water to the delta plain. We synthesize here the results of extensive studies on a river diversion at Caernarvon, Louisiana, one of the largest diversions in the delta.

Hydrological and nutrient budgets of freshwater and estuarine wetlands of Taylor Slough in Southern Everglades, Florida (U.S.A.)

Hydrological restoration of the Southern Everglades will result in increased freshwater flow to the freshwater and estuarine wetlands bordering Florida Bay. We evaluated the contribution of surface freshwater runoff versus atmospheric deposition and ground water on the water and nutrient budgets of these wetlands. These estimates were used to assess the importance of hydrologic inputs and losses relative to sediment burial, denitrification, and nitrogen fixation. We calculated seasonal inputs and outputs of water, total phosphorus (TP) and total nitrogen (TN) from surface water, precipitation, and evapotranspiration in the Taylor Slough/C-111 basin wetlands for 1.5 years. Atmospheric deposition was the dominant source of water and TP for these oligotrophic, phosphorus-limited wetlands. Surface water was the major TN source of during the wet season, but on an annual basis was equal to the atmospheric TN deposition. We calculated a net annual import of 31.4 mg m−2 yr−1 P and 694 mg m−2 yr−1N into the wetland from hydrologic sources. Hydrologic import of P was within range of estimates of sediment P burial (33–70 mg m−2 yr−1 P), while sediment burial of N (1890–4027 mg m−2 yr−1 N) greatly exceeded estimated hydrologic N import. High nitrogen fixation rates or an underestimation of groundwater N flux may explain the discrepancy between estimates of hydrologic N import and sediment N burial rates.

Radon Tracing of Submarine Groundwater Discharge in Coastal Environments

This chapter discusses the radon tracing of submarine groundwater discharge (SGD) in coastal environments. Direct discharge of groundwater into the coastal zone may be an important material flux pathway from land to sea in some areas. It has been largely ignored because of the difficulty in assessing its magnitude. While measurement problems persist, there is a growing recognition that groundwater flow into the sea is important. This chapter reviews an approach, using a simple one-dimensional model, to measure SGD via use of 222 Rn as a natural tracer. Radon has certain advantages over other potential geochemical tracers of groundwater discharge. Typically, it is greatly enriched in groundwater compared to seawater; it can be measured at very low concentrations, and is completely conservative. On the other hand, as a gas it is subject to losses at the air–sea interface which may limit its use in shallow water environments. The radon tracing is an excellent qualitative tool for identifying areas of spring or seepage inputs in most coastal environments. It is a good quantitative tool in shallow marine environments characterized by large amounts of SGD.

A multi-level pore-water sampler for permeable sediments

ABSTRACT The construction and operation of a multi-level piezometer (multisampler) designed to collect pore water from permeable sediments up to 230 cm below the sediment-water interface is described. Multisamplers are constructed from 1 inch schedule 80 PVC pipe. One-quarter-inch flexible PVC tubing leads from eight ports at variable depths to a 1 inch tee fitting at the top of the PVC pipe. Multisamplers are driven into the sediment using standard fence-post drivers. Water is pumped from the PVC tubing with a peristaltic pump. Field tests in Banana River Lagoon, Florida, demonstrate the utility of multisamplers. These tests include collection of multiple samples from the permeable sediments and reveal mixing between shallow pore water and overlying lagoon water.

A multi-proxy trophic state reconstruction for shallow Orange Lake, Florida, USA: Possible influence of macrophytes on limnetic nutrient concentrations

We retrieved four sediment cores from shallow, eutrophic, macrophyte-dominated Orange Lake (A = 51.4 km2, zmax <5 m, zmean < 2 m), north-central Florida, USA. The 210Pb-dated profiles were used to evaluate spatial and temporal patterns of bulk sediment and nutrient accumulation in the limnetic zone and to infer historical changes in lake trophic state. Bulk density, organic matter, total carbon, total nitrogen, total phosphorus and non-apatite inorganic phosphorus (NAIP) concentrations displayed stratigraphic similarities among three of four cores, as did accumulation rates of bulk sediment, organic matter and nutrients. Accumulation rates were slower at the fourth site. Nutrients showed generally increasing rates of accumulation since the turn of the century. Percentages of periphytic diatom taxa increased progressively in the cores after ~ 1930. Diatom-inferred limnetic total P trends were similar among profiles. Eutrophic conditions were inferred for the period prior to the turn of the century. The lake was hypereutrophic in the early decades of the 1900s, but inferred limnetic total P values declined after ~ 1930. Declining inferred limnetic total P trends for the last 60--70 years were accompanied by concomitant increases in accumulation rates of total P and NAIP on the lake bottom. Several lines of evidence suggest that after ~ 1930, phosphorus entering Orange Lake was increasingly utilized by submersed macrophytes. Paleolimnological records from Orange Lake highlight the importance of using multiple sediment variables to infer past trophic state and suggest that aquatic macrophytes can play a role in regulating water-column nutrient concentrations in shallow, warm-temperate lakes.

Isotope tracing of submarine groundwater discharge offshore Ubatuba, Brazil: results of the IAEA–UNESCO SGD project

Results of groundwater and seawater analyses for radioactive (3H, 222Rn, 223Ra, 224Ra, 226Ra, and 228Ra) and stable (D and 18O) isotopes are presented together with in situ spatial mapping and time series 222Rn measurements in seawater, direct seepage measurements using manual and automated seepage meters, pore water investigations using different tracers and piezometric techniques, and geoelectric surveys probing the coast. This study represents first time that such a new complex arsenal of radioactive and non-radioactive tracer techniques and geophysical methods have been used for simultaneous submarine groundwater discharge (SGD) investigations. Large fluctuations of SGD fluxes were observed at sites situated only a few meters apart (from 0 cm d(-1) to 360 cm d(-1); the unit represents cm3/cm2/day), as well as during a few hours (from 0 cm d(-1) to 110 cm d(-1)), strongly depending on the tidal fluctuations. The average SGD flux estimated from continuous 222Rn measurements is 17+/-10 cm d(-1). Integrated coastal SGD flux estimated for the Ubatuba coast using radium isotopes is about 7x10(3) m3 d(-1) per km of the coast. The isotopic composition (deltaD and delta18O) of submarine waters was characterised by significant variability and heavy isotope enrichment, indicating that the contribution of groundwater in submarine waters varied from a small percentage to 20%. However, this contribution with increasing offshore distance became negligible. Automated seepage meters and time series measurements of 222Rn activity concentration showed a negative correlation between the SGD rates and tidal stage. This is likely caused by sea level changes as tidal effects induce variations of hydraulic gradients. The geoelectric probing and piezometric measurements contributed to better understanding of the spatial distribution of different water masses present along the coast. The radium isotope data showed scattered distributions with offshore distance, which imply that seawater in a complex coast with many small bays and islands was influenced by local currents and groundwater/seawater mixing. This has also been confirmed by a relatively short residence time of 1-2 weeks for water within 25 km offshore, as obtained by short-lived radium isotopes. The irregular distribution of SGD seen at Ubatuba is a characteristic of fractured rock aquifers, fed by coastal groundwater and recirculated seawater with small admixtures of groundwater, which is of potential environmental concern and has implications on the management of freshwater resources in the region.

Subtidal Sea Level Variability in a Shallow Mississippi River Deltaic Estuary, Louisiana

The relative roles of river, atmospheric, and tidal forcings on estuarine sea level variability are examined in Breton Sound, a shallow (0.7 m) deltaic estuary situated in an interdistributary basin on the Mississippi River deltaic plain. The deltaic landscape contains vegetated marshes, tidal flats, circuitous channels, and other features that frictionally dissipate waves propagating through the system. Direct forcing by local wind stress over the surface of the estuary is minimal, owing to the lack of significant fetch due to landscape features of the estuary. Atmospheric forcing occurs almost entirely through remote forcing, where alongshore winds facilitate estuary-shelf exchange through coastal Ekman convergence. The highly frictional nature of the deltaic landscape causes the estuary to act as a low-pass filter to remote atmospheric forcing, where high-frequency, coastally-induced fluctuations are significantly damped, and the damping increases with distance from the estuary mouth. During spring, when substantial quantities of controlled Mississippi River inputs (−q = 62 m3 s-1) are discharged into the estuary, upper estuary subtidal sea levels are forced by a combination of river and remote atmospheric forcings, while river effects are less clear downestuary. During autumn (−q = 7 m3 s-1) sea level variability throughout the estuary is governed entirely by coastal variations at the marine boundary. A frequency-dependent analytical model, previously used to describe sea level dynamics forced by local wind stress and coastal forcing in deeper, less frictional systems, is applied in the shallow Breton Sound estuary. In contrast to deeper systems where coastally-induced fluctuations exhibit little or no frictional attenuation inside the estuary, these fluctuations in the shallow Breton Sound estuary show strong frequency dependent amplitude reductions that extend well into the subtidal frequency spectrum.