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SCOR/LOICZ Working Group 112

"MAGNITUDE OF SUBMARINE GROUNDWATER DISCHARGE AND ITS INFLUENCE ON COASTAL OCEANOGRAPHIC PROCESSES"

Co-chairs: William C. Burnett and Evgeny A. Kontar

Sponsored by
the Scientific Committee on Oceanic Research (SCOR) 

and 

the Land-Ocean Interactions in the Coastal Zone (LOICZ) 
programme element of the 
International Geosphere-Biosphere Programme (IGBP)

Table of Contents: 

1. Objectives and Terms of Reference
2. Members, Associate Members, Participating Scientists
3. Organizational Structure and Work Plan
4. Summary
5. Scientific Background and Issues
6. Bibliography
7. News Article on WG-112

Scientific Background and Issues
Groundwater flux to the ocean: Definitions, data, applications, uncertainties

Dr. Robert W. Buddemeier

Kansas Geological Survey

University of Kansas

Lawrence, KS 66047 USA

Buddemeier, R. W. 1996. Groundwater flux to the Ocean: Definitions, data, applications, uncertainties. in: Buddemeier, R. W. (ed.), Groundwater Discharge in Coastal Zone: Proceedings of an International Symposium. LOICZ Reports and Studies No. 8, LOICZ, Texel, The Netherlands, pp. 16-21.

Abstract

There are a wide range of reasons and applications for the study of groundwater relationships in the coastal zone. This diversity, in combination with the range of disciplines and of time and space scales involved, complicate the use of  data for purposes other than those envisioned by the original investigator. The challenge is particularly great in the case of local type or case studies designed for global or regional extrapolation, since errors or inappropriate assumptions will be greatly magnified. This paper examines the conceptual approaches, data needs, and limitations of various types of studies, and makes recommendations concerning precautions and presentation of results.

Introduction

The definitions and techniques used for the study of groundwater discharge to the coastal ocean are very scale- and problem-dependent, and the differences inherent in various applications undoubtedly contribute to the scientific fragmentation of the study of groundwater discharge to the coastal ocean (see Section 1, 1996 LOICZ Proceedings: Groundwater Discharge in the Coastal Zone). This paper focuses on the needs implicit in the global biogeochemical modeling task of the Land-Ocean Interactions in the Coastal Zone (LOICZ) core project of the International Geosphere-Biosphere Programme (Holligan 1990; Holligan and de Boois 1993; Pernetta and Milliman 1995). There are two reasons for this focus: organizationally, the requirements of this effort led LOICZ to co-sponsor the above Symposium and attendant review of coastal groundwater processes; scientifically, applications at this "meso-scale" (tens to hundreds of km) in the coastal ocean place the greatest demands on system conceptualization and understanding. 

Applications and Issues

The following incomplete list of applications permits a general overview of some of the typical approximations that are used in coastal groundwater studies-and that are generally not applicable to many of the problems of LOICZ applications (which are summarized below).

1. Global (or regional long-term) geochemical and water budget studies-there are many such efforts addressing (for example) the role of the ocean in natural global climate cycles. Some recent examples that specifically address questions involving groundwater flux and composition are publications of Milliman and co-workers (Milliman 1993; Milliman and Droxler 1996). Such studies, focused on time scales comparable to or longer than whole-ocean residence times and on budgets at the ocean basin or regional scale, can appropriately ignore the complexities of the coastal zone and consider the ocean as one or a few relatively simple homogeneous reservoirs. Estimates or measurements used for this purpose may serve to establish magnitudes or boundary conditions for more detailed studies, but almost invariably lack the spatial and temporal resolution needed for coastal ocean budget models.
2. Coastal water resource studies-although often too localized, these may address scales appropriate to coastal biogeochemical modeling. However, they tend to ignore the details of discharge pathways, and to treat the ocean as a simple uniform sink (in the case of fresh water loss or contaminant transport) or source (in the case of saltwater intrusion). Further, attention is usually focused on a single dominant aquifer (often but not always the shallowest) to the exclusion of other groundwater bodies and of surface water-groundwater interactions. If focused on the time scales of aquifers the temporal resolution will be inadequate for coastal processes, but studies that address interactions on seasonal or tidal time scales may be appropriate to marine biogeochemical budgets. In most cases boundary conditions or limiting estimates of flux are the most that can be extracted.
3. Marine geochemistry or biogeochemistry studies-such studies often focus on the scales and compartments of interest to LOICZ, but the marine research perspective can be difficult to couple to the concepts and data of terrestrial hydrology. For example, "groundwater" to a marine chemist may be operationally defined as any fluid that has interacted significantly with sediments or rocks-see, for example, Moore (1996)-whereas this grouping of circulating marine porewater with fresh and mineralized water of terrestrial origin complicates the budgeting process.

In pursuit of the goal of determining the role of the coastal oceans in the global cycle of carbon and other key substances (Smith and Hollibaugh 1993), LOICZ has adopted a hierarchical approach to globalization of the highly dispersed coastal data sets available (Gordon, Boudreau et al., 1995). The strategy is to use budgetary data to identify process controls and functional (e.g., source or sink) roles of generally identifiable types of coastal environments or habitats. Then, development of coastal typologies (Pernetta and Milliman, 1995; LOICZ 1996) is intended to provide a framework for extrapolation of data from well-characterized environments to the much larger global inventory of functionally similar but unstudied environments-and also to provide a methodological basis for incorporating social science components relating to the human dimensions of global change (Pernetta and Milliman, 1995).  Hydrologic issues enter this process at two key points, in quantifying inputs to specific coastal budgets, and in characterizing types of coastal environments in terms of behavior that will support extrapolation to a global "model." Both are important, and their combined requirements represent a significant challenge. Beyond the hydrologic questions are the key issues of hydrochemical fluxes, especially of nutrients, but also of carbon compounds and of other materials, natural or anthropogenic, that may affect biological and chemical cycles in the coastal zone. However, understanding of solvent behavior is typically a prerequisite for predicting the action of solutes, so an initial focus on hydrology is essential.

Coastal Hydrologic Processes and Pathways

Construction of accurate material budgets within the coastal zone requires careful attention to the definitions of pathways and fluxes. Figure 1 shows a simplified schematic illustration of the coastal zone. Materials may enter a defined volume of the coastal ocean by precipitation from or exchange with, the atmosphere, inflow of terrestrial surface water, discharge of groundwater, exchange or reaction with sediment porewater, and by ocean water fluxes. Loss terms typically include longshore and offshore transport, sedimentation or sediment reaction, and transfer to the atmosphere (by evaporation, suspension, or gas exchange). Fluxes from the marine environment into terrestrial water bodies (aquifers and estuaries) may be significant in terms of impact on the freshwater environment, but usually negligible compared to other loss terms in the coastal ocean budget.

>Figure 1. Stylized schematic view of the coastal zone, emphasizing the different pathways of groundwater discharge and the distinctions among the terrestrial compartment and the "estuarine zone" and the "open shelf zone" within the marine compartment>.
[Click on Figure for enlarged view]

Figure 1 indicates two distinct subsets of the coastal marine environment-one, the "estuarine zone," is intended to represent the typically narrow width of the marine environment that is most influenced by land, and which often has relatively high levels of variability as well as productivity. The estuarine zone includes true estuaries, but is further represented by a narrow band of nearshore water masses in locations where there are not streams adequate to form traditionally defined estuaries (as illustrated in Figure 2). It is, inevitably, the area of the ocean most subject to direct anthropogenic alteration. On a global average, this zone is not dominated by fluxes from the world's major rivers; advective processes are primarily oceanic, and terrestrial influences are often a mix of groundwater and runoff from local ungauged watersheds.

Figure 2. Plan view of the coastal zone, showing the relationships among the estuarine zone, open shelf zone, gauged and ungauged streams, and groundwater discharge to the ocean and to streams.
[Click on Figure for enlarged view]

The remainder of the coastal marine environment is referred to here as the "coastal shelf zone." This is the open-water part of the continental shelf that is dominated by oceanic advective processes and characterized by more nearly oceanic water characteristics. Figures 1 and 2, considered together, illustrate both a conceptual and an operational problem in considering hydrologic inputs to the coastal zone. Conceptually (and by definition), essentially all surface water flow enters the coastal ocean through the estuarine zone. Although this is a convenient assumption for shallow groundwater (Johannes 1980), it is not necessarily correct; confined aquifer systems, karst formations, etc., may result in either localized or distributed discharges of groundwater relatively far out on the continental shelf or slope. This situation is illustrated in Figure 3, along with indications of processes that must be considered "groundwater" in the larger geophysical sense, although not in the terms usually employed by water resource hydrologists. These offshore discharges are likely to be insignificant in terms of the water budget of the "coastal shelf zone," but because it is likely to be more highly mineralized it may be biogeochemically significant, and the flux of groundwater may also have an amplified influence on porewater environments and the benthic community (Tribble 1990; Tribble, Sansone et al. 1992). Large-scale physical phenomena with major geochemical implications may also be linked to offshore groundwater fluxes (Rona 1969).

Figure 3. Cross-sectional views of the coastal zone, illustrating the types and pathways of fluid movements that may be considered "groundwater" for various purposes.
[Click on Figure for enlarged view]

Problems of Measurement and Definition

One part of the operational problem alluded to above may be seen in Figure 2-the fact that the most nearshore river gauging stations are often well inland of the major estuary and delta systems. Where good monitoring records exist, gauging stations can provide data not only on total flow, but also dissolved and suspended solids loads and on runoff-baseflow (groundwater separations). However, the best (and often only, or last) data set available is typically above not only the estuarine zone, but also much or all of the coastal plain-the biogeochemically active terrestrial portion of the LOICZ region of interest. It is generally recognized that coastal runoff and groundwater discharge in between gauged streams is not included in river input data; somewhat more subtle is the fact that available river data often do not account for a substantial amount of distinctive flux in the lower reaches of the river.

Figure 4 illustrates in greater detail some of the uncertainties and potentials for error that arise when efforts are made to consider specific inputs to the estuarine zone with time constants suitable for comparison with marine data obtained on the scale of water residence times-typically quasi-synoptic data oriented toward times of hours to weeks. This level of detail is required by the "typology" approach to globalization of coastal zone data. Assumptions that may be valid or a source of minor error on the global scale may cause very large distortions in coastal process models at the local scale; if these errors are then amplified and propagated through the globalization process, the outcome could be prejudiced.

Figure 4. Detailed and expanded cross-section of the terrestrial coastal environment, depicting problems inherent in generalizing limited measurements to obtain system-level water and chemical fluxes.  >[Click on Figure for enlarged view]

The terrestrial portion of the coastal zone is an area of high human activity (agricultural, urban and industrial), with attendant perturbations and high gradients in water and contaminant fluxes. The problems of pathway identification and flux measurement discussed above become more critical at local scales; often aquifer characteristics and chemical sources are not well understood, and even where they are, there may not be adequate measurement points or data. Local authorities and researchers tend to focus on the best resource or the worst problem, rather than the widely distributed marginal-quality water that may carry most of the chemical load or system-relevant "signal." This is illustrated in Figure 4 by the chemical inhomogeneity of the shallow aquifer. Where salinity increases and surface contamination decreases with depth, darcian flow estimates based on a limited number of measuring points may be highly misleading. Additionally, there may be problems of estimating consumptive water use (and therefore effective gradient or flux) between the sea and the lowest inland measuring point (monitoring well or gauging station).

Requirements for Integration and Application of Data

It is not realistic to expect that all measurements, calculations, or publications of data relevant to coastal groundwater fluxes will suddenly conform to the needs (Gordon, Boudreau et al. 1995) or standards (Boudreau, Geerders et al. 1996) of the LOICZ project. However, one of the reasons for highlighting the disciplinary diversity in the origins and applications of the data is to encourage researchers both to seek out and to provide the data that will make their results more broadly useful, and in doing so, to encourage integration of the field of study. 

In publishing, compiling, or databasing groundwater flux results, a wide variety of information should be considered, included, or referenced. Examples include geographic coordinates of drainage basins or aquifer units, coastline segments, and well or study sites, as well as information on the sources of both water level and groundwater chemistry data. The groundwater data should include land and water level (or head) elevations, relevant times of measurement, and depth or elevation of the screened (sampled) interval. Measured or assumed hydrogeologic parameters and stratigraphic aquifer characteristics are also required for interpretation, which means that information on methods of measurement or derivation is needed.

The challenge of effectively integrating and comparing diverse hydrologic and oceanographic measurements and models, often at very different spatial and temporal scales, demands extensive documentation of both primary data and methods, as well as rigorous analysis of uncertainties. The difference between semiquantitative groundwater flux estimates and determinations useful for flux budgets will be very great in terms of the quality of documentation and analysis required. At least as important, however, is the fact that estimates can be quite useful, if the assumptions, methods, and uncertainties are specified. Without this evaluation, estimates produced for one application may be seriously misleading if used for others.

Summary

Measurements or estimates of groundwater and associated chemical fluxes, especially over substantial areas or time periods, are notoriously uncertain. "Groundwater discharge" may include the base flow component of stream and river discharge, direct seepage from phreatic aquifers though the intertidal and shallow subtidal zones into the coastal ocean, nearshore springs, deeper offshore discharge (as from confined aquifers), or any combination of these. Depending on the measurements made and definitions used, combining groundwater flux estimates with independent estimates of fluvial inputs and oceanic fluxes can result in over- or under-estimates (for example, double-counting river base flow as both river input and groundwater flux, or failure to account for riverine groundwater discharge between the lowest gauging station and the mouth). Additional complications arise when one considers issues such as short-term interactions between streams and alluvial aquifers, or lateral "interflow" of water within the normally unsaturated zone, processes that may be hydrologically, but not biogeochemically, insignificant.

Hydrologic calculations may overestimate fluxes by neglecting evapotranspiration losses in the coastal plain. Vertical stratification of both flow rates and water quality in coastal aquifers can lead to serious mismatches in calculation of chemical fluxes, as can ocean water intrusion into the aquifer. Assignment of chemical compositions to the hydrologic fluxes requires careful matching of data sets, and consideration of the correspondence between the two types of data, their sources, and their uncertainties.

Complete reporting of data and methods, and consideration of the wide range of potential applications for data relating to groundwater in the coastal zone are recommended. This will not only serve the needs of integrative projects such as LOICZ, but will also provide definition and cohesiveness to an important field of study that is now highly fragmented.

References

Boudreau, P. R., P. J. F. Geerders, et al. (1996). LOICZ Data and Information System Plan. LOICZ Reports and Studies No. 6. Texel, The Netherlands., LOICZ: ii + 62. 

Gordon, J., D. C. , P. R. Boudreau, et al. (1995). LOICZ Biogeochemical Modelling Guidelines. LOICZ Reports and Studies No. 5. Texel, The Netherlands, LOICZ: vi + 96. 

Holligan, P. M. and H. e. de Boois (1993). The LOICZ Science Plan. IGBP Report No. 25. Stockholm, IGBP: 50. 

Holligan, P. M. e. (1990). Coastal Ocean Fluxes and Resources. IGBP Report No. 14. Stackholm, IGBP: 53.

Johannes, R. E. (1980). "The ecological significance of the submarine discharge of groundwater." Marine Ecology Progress Series 3: 365-373. 

LOICZ (1996). LOICZ Workshop on Statistical Analysis of the Coastal Lowlands Database. LOICZ/WKSHP/96.14. Meeting Report No. 18. Texel, The Netherlands, LOICZ. 

Milliman, J. D. (1993). "Production and accumulation of calcium carbonate in the ocean: Budget of a nonsteady state." Global Biogeochemical Cycles 7(4): 927-957.

Milliman, J. D. and A. W. Droxler (1996). "Neritic and Pelagic Carbonate Sedimentation in the Marine Environment: Ignorance is not Bliss." Geologische Rundschau 85: 496-504.

Moore, W. S. (1996). "Large groundwater inputs to coastal waters revealed by ²²⁶Ra enrichments." Nature 380(April 18, 1996): 612-614. 

Pernetta, J. C. and J. D. e. Milliman (1995). Land-Ocean Interactions in the Coastal Zone Implementation Plan. IGBP Report No. 33. Stockholm, IGBP: 215.

Rona, P. A. (1969). "Middle Atlantic continental slope of United States: deposition and erosion." American Association of Petroleum Geologists Bulletin 53(7): 1453-1465.

Smith, S. V. and J. T. Hollibaugh (1993). "Coastal metabolism and the oceanic organic carbon balance." Reviews of Geophysics 31(1): 75-89.

Tribble, G. W. (1990). Early Diagenesis in a Coral Reef Framework. Oceanography. Honolulu, University of Hawaii: 228.

Tribble, G. W., F. J. Sansone, et al. (1992). "Hydraulic Exchange between a Coral Reef and Surface Seawater." Geological Society of America Bulletin 104: 1280-1291.

Last Updated 19 Oct 2000 by DPS

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