William C. Burnett

Groundwater and pore water inputs to the coastal zone

Both terrestrial and marine forces drive underground fluid flows in the coastal zone. Hydraulic gradients on land result in groundwater seepage near shore and may contribute to flows further out on the shelf from confined aquifers. Marine processes such as tidal pumping and current-induced pressure gradients may induce interfacial fluid flow anywhere on the shelf where permeable sediments are present. The terrestrial and oceanic forces overlap spatially so measured fluid advection through coastal sediments may be a result of composite forcing. We thus define “submarine groundwater discharge” (SGD) as any and all flow of water on continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force. SGD is typically characterized by low specific flow rates that make detection and quantification difficult. However, because such flows occur over very large areas, the total flux is significant. Discharging fluids, whether derived from land or composed of re-circulated seawater, will react with sediment components. These reactions may increase substantially the concentrations of nutrients, carbon, and metals in the fluids. These fluids are thus a source of biogeochemically important constituents to the coastal ocean. Terrestrially-derived fluids represent a pathway for new material fluxes to the coastal zone. This may result in diffuse pollution in areas where contaminated groundwaters occur. This paper presents an historical context of SGD studies, defines the process in a form that is consistent with our current understanding of the driving forces as well as our assessment techniques, and reviews the estimated global fluxes and biogeochemical implications. We conclude that to fully characterize marine geochemical budgets, one must give due consideration to SGD. New methodologies, technologies, and modeling approaches are required to discriminate among the various forces that drive SGD and to evaluate these fluxes more precisely.

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.

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.

Early diagenesis of organic matter in Peru continental margin sediments: Phosphorite precipitation

Abstract Pore water chemistry (total dissolved CO 2 , NH 4 , NO 3 , NO 2 , PO 4 , Si(OH) 4 , Ca, Mg, Fe, Mn, SO 4 , H 2 S and F, and titration alkalinity), solid phase chemistry (C org , P org , C TOT , N TOT , F, Si OPAL and S II ), and sediment characteristics (porosity, dry bulk density and formation factors) were determined on a centimeter-scale spacing in the upper 20–40 cm of sediments under intense upwelling areas on the Peru continental shelf. These data demonstrate that carbonate fluorapatite (CFA) is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content (up to 20% C org ), very high porosity (> 0.96 ml cm −3 ) and very low dry bulk density ( −3 ). Dissolved phosphate concentrations at the sediment-water interface range from 20 to 100 μ M , orders of magnitude higher than bottom-water concentrations, and much higher than predicted from regeneration of organic matter. The mechanism of this interfacial phosphate release is unclear, but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments. Fluoride is incorporated into CFA by diffusion from the overlying seawater, and carbonate ions are provided from pore-water alkalinity. Magnesium concentrations in this reaction zone are not significantly different from those of seawater, suggesting that magnesium depletion is not a necessary prerequisite for CFA precipitation. The environment of precipitation is interface-linked rather than driven by organic diagenesis of phosphorus deeper in the sediment. Most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region, but almost all show the precipitation signature near the interface. This interface-linked early diagenetic porewater environment for the precipitation of CFA explains many of the geochemical characteristics of phosphorites and provides a “testable” model to compare the modern phosphogenic analog with ancient phosphorite deposits. Two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites.

Measurement and significance of the direct discharge of groundwater into the coastal zone

Abstract While the major rivers of the world are reasonably well gauged and analysed, thus allowing comparatively precise estimates of riverine inputs to the ocean, it remains very difficult to evaluate the influence of direct groundwater discharge into the ocean. In spite of the recognition that many land–sea interfaces of the world are characterised by ‘leaky’ continental margins, it is unclear how important groundwater-derived springs and seeps are in terms of overall marine geochemical budgets. The principal reason that groundwater estimates have not caught up to the precision base typical of other oceanic inputs is that the direct discharge of groundwater into the coastal zone is inherently very difficult to measure. Concerted efforts are now being made to improve this situation by a variety of hydrological and oceanographic techniques. Standard hydrological approaches include measurement of the hydraulic gradient on land with flow being estimated either analytically or numerically. Groundwater flow is also assessed using standard water balance considerations. Unfortunately, the uncertainties in the data used for water balance calculations are often on the same scale as the flow being evaluated. Oceanographers have been using seepage meters, mini-piezometers and geochemical tracers to estimate submarine groundwater discharge (SGD) in the coastal zone. Hydrogeologists and oceanographers thus tend to approach the same problem from different ends. Rarely are methodologies combined or more than one approach applied in the same study. We suggest that intercalibration experiments be designed in order to provide a more complete recognition of the strengths and weaknesses of various approaches.

A continuous monitor for assessment of 222Rn in the coastal ocean

AbstractRadon-222 is a good natural tracer of groundwater flow into the coastalocean. Unfortunately, its usefulness is limited by the time consuming natureof collecting individual samples and traditional analysis schemes. We demonstratehere an automated system which can determine, on a “continuous”basis, the radon activity in coastal ocean waters. The system analyses 222Rn from a constant stream of water passing through an air-water exchangerthat distributes radon from the running flow of water to a closed air loop.The air stream is feed to a commercial radon-in-air monitor which determinesthe concentration of 222Rn by collection and measurement of theemitting daughters, 214Po and 218Po, via a charged semiconductordetector. Since the distribution of radon at equilibrium between the air andwater phases is governed by a well-known temperature dependence, the radonconcentration in the water is easily calculated.

The present day formation of apatite in Mexican continental margin sediments

Abstract Results of pore water and sediment analyses from the western Mexican continental margin strongly suggest the present day formation of apatite. The interstitial water phosphate and fluoride profiles indicate chemical removal at a depth which corresponds to a large maximum in the phosphorus content of the sediments. Apatite is identified within this maximum via X-ray diffraction but is elsewhere undetectable in the core. Radioisotopic thorium, uranium, and radium data support the conclusion that this deposit is modern. The present day depositional environment is consistent with those reported by other workers for phosphorite formation with the exception that pore water magnesium is not depleted below its seawater value.

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.

Radon tracing of groundwater input into Par Pond, Savannah River Site

Abstract The groundwater contribution into Par Pond, a former cooling reservoir for two nuclear reactors located on the Department of Energys Savannah River Site (South Carolina), was estimated using a standard hydrologic budget as well as one augmented by a natural tracer approach. We determined a geochemical budget for 222 Rn, normally found at much higher concentrations in groundwater than surface waters, to assist in constraining the hydrologic estimates. The radon budget accounted for all quantifiable surface sources and sinks including the flux across the sediment-water interface which was determined by application of an advection-diffusion model. All hydrologic parameters and radon concentrations were monitored seasonally from February 1994 to August 1995. Using the water balance approach alone, the average groundwater discharge entering the lake was estimated to have an upper limit of approximately 0.95 ± 0.13 m 3 s −1 . The groundwater contribution obtained using the combined hydrologic/ 222 Rn approach ranged from 0.17 to 0.76 m 3 s −1 with a best estimate of 0.35 ± 0.16 m 3 s −1 . Lake profiles show enhanced 222 Rn concentrations in some areas indicating that groundwater enters Par Pond mostly through a small region in the northern portion of the lake, probably via small seeps or springs. Estimates show that groundwater plays a significant role in the overall water budget of the lake, accounting for 10%–33% of the total estimated inflow from all measured sources. Our results show that supplementing a standard hydrological water balance with radon budget considerations helps to constrain estimated groundwater flow into surface reservoirs.

Air–Water Partitioning of 222Rn and its Dependence on Water Temperature and Salinity

Radon is useful as a tracer of certain geophysical processes in marine and aquatic environments. Recent applications include detection of groundwater discharges into surface waters and assessment of air/sea gas piston velocities. Much of the research performed in the past decade has relied on continuous measurements made in the field using a radon stripping unit connected to a radon-in-air detection system. This approach assumes that chemical equilibrium is attained between the water and gas phases and that the resulting air activity can be multiplied by a partition coefficient to obtain the corresponding radon-in-water activity. We report here the results of a series of laboratory experiments that describes the dependence of the partition coefficient upon both water temperature and salinity. Our results show that the temperature dependence for freshwater closely matches results that were previously available. The salinity effect, however, has largely been ignored and our results show that this can result in an overestimation of radon concentrations, especially in cooler, more saline waters. Related overestimates in typical situations range between 10 (warmer less saline waters) and 20% (cooler, more saline waters).