Land and Marine Hydrogeology

M. Taniguchi,K. Wang,T. Gamo

• Verlag: Elsevier Science
• Jahr: 2003
• ISBN:
  9780080535364
• Thema: Physical Sciences
• Thema: Hydrology

This volume represents an effort to bring together communities of land-based hydrogeology and marine hydrogeology. The issues of submarine groundwater discharge and its opposite phenomenon of seawater invasion are discussed in this book from the geophysical, geochemical, biological, and engineering perspectives. This is where land hydrogeology and marine hydrogeology overlap. Submarine groundwater discharge is a rapidly developing research field. The SCOR and LOICZ of the IGBP have recently established a working group for this research. IASPO and IAHS under IUGG also recently formed a new joint committee "Seawater/Groundwater Interactions" to collaborate with oceanographers and hydrologists.

The other articles introduce frontier research topics in more typical land and marine environments, such as fluid flow in karst aquifers, the biological aspects of fluids in sedimentary basins and submarine sedimentary formations, respectively, and vigorous fluid flow in subsea formations and their significance in global tectonics. Geochemical characteristics of hydrothermal activities at a number of active continental margins are also reviewed, and multidisciplinary geophysical constraints of the permeability of young igneous oceanic crust are summarized. A variety of driving mechanisms for fluid flow is discussed in land and subsea formations; terrestrial hydraulic gradient, buoyancy driven free convection, tidally induced flow, flow induced by tectonic strain, flow due to sediment compaction.

Assessment methodologies for submarine groundwater discharge

M. Taniguchi^a, W.C. Burnett^b, J.E. Cable^c and J.V. Turner^d, ^aDepartment of Earth Sciences, Nara University of Education, Nara 630-8528 Japan; ^bDepartment of Oceanography, Florida State University, Tallahassee, Florida 32306 USA; ^cDepartment of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana 70803 USA; ^dCentre for Groundwater Studies, CSIRO Division of Land and Water, Private Bag, PO Wembley, WA 6014 Perth, Australia. E-mail address: [email protected], [email protected], [email protected], [email protected]

Submarine groundwater discharge (SGD) in the coastal zone is now recognized as a potentially significant material pathway from the land to the ocean. This article provides an overview on several methodologies used to estimate SGDs. Measurements of SGD using “manual seepage meters” show that consistent and reliable results can be obtained if one is aware of and careful to prevent known artifacts. New “automated seepage meters” will help to understand the hydrological and coastal oceanographic processes with longer-term and higher-resolution measurements. Direct measurements of SGD by seepage meters and piezometers in local areas may be scaled up to a regional basis by use of natural geochemical and geophysical tracers. Water balance estimates, although useful for rough estimates, are usually not very precise because the uncertainties in the various terms used to construct the balance are often on the same order as the groundwater discharge being evaluated. Estimates of SGD via analytical and numerical methods depend mainly on the evaluations of the thickness of the aquifers and representative hydraulic conductivities, of which well-constrained values are usually difficult to obtain except in very well-studied areas.

1. INTRODUCTION

Submarine groundwater discharge (SGD), which occurs as springs and seeps in near-shore areas, is one of the pathways of water discharge from land to the ocean in the global water cycle. Submarine springs are found in many parts of the world and some of these are large enough to provide fresh water for human needs [Kohout, 1966]. The slow seepage of groundwater that flows out along most shorelines of the world may be even more important volumetrically than discrete springs. While it is difficult to detect groundwater seepage through sediments, this diffuse input may occur over broad areas and deliver potentially significant amounts of fresh water and dissolved components to the world’s coastal oceans.

The term “submarine groundwater discharge” has been used in different ways over the years [Taniguchi et al., 2002]. The definition of SGD with or without recirculated seawater has been ambiguous in the literature [Younger, 1996]. In this paper, we use the term SGD to represent all direct discharge of subsurface fluids across the land-ocean interface. The exact definition of SGD is described in detail by Taniguchi et al. [2002].

The direct discharge of groundwater into the coastal zone has received increased attention in the last few years as it is now recognized that this process may represent a potentially important pathway for material transport. One of the outcomes of this recent interest in SGD has been the establishment of SCOR (Scientific Committee on Oceanic Research)/LOICZ (Land – Ocean Interactions in the Coastal Zone) Working Group 112 [Burnett, 1999; Kontar and Zektser, 1999]. They recognized the need to define further and improve the methodologies of SGD assessment. As a consequence, SGD assessment intercomparison exercises have been organized in several “flagship” coastal sites. These experiments are being performed in order to compare directly several independent methodologies. The aim is to develop standardized approaches for the evaluation of SGD in coastal zones.

Several methods for direct measurements of SGD have been employed, including seepage meters [Carr and Winter, 1980], piezometers, and geochemical/geophysical tracers. Indirect methods, including water balance methods, hydrograph-separation techniques, theoretical analyses and numerical simulations, are used widely for basin-scale estimations of groundwater discharge into the ocean.

SGD has been reviewed by Fairbrige [1966], Stringfield and Legrand [1969, 1971] and Zektzer [1973], however, no overview work has been done since 1970′s. The objective of this paper is to review the most commonly used methodologies in estimating SGD: (1) seepage meters; (2) piezometers; (3) tracer methods; (4) water balance approaches; (5) hydrograph separation techniques; and (6) theoretical analysis and numerical simulations. Furthermore, we identify specific areas that require further analysis and study.

2. SEEPAGE METERS

Measurements of groundwater seepage rates into surface water bodies are often made using manual “seepage meters.” This device was first developed by Israelsen and Reeve [1944] to measure water loss from irrigation canals. Lee [1977] designed a seepage meter consisting of an end section of a 55-gallon (208 liters) steel drum fitted with a sample port and a plastic collection bag (Figure 1). The drum forms an open0bottom chamber that is inserted into the sediment. Water seeping through the sediment will displace water trapped in the chamber forcing it up through the port into the plastic bag. The change in volume of water in the bag over a measured time interval provides the flux measurement.

Figure 1 Lee-type manual seepage meter [Lee, 1977].

Studies involving seepage meters have reached the following general conclusions: (1) many seepage meters are needed because of the natural spatial and temporal variability of seepage rates [Shaw and Prepas, 1990a,b]; (2) the resistance of the tube [Fellows and Brezonik, 1980] and bag [Shaw and Prepas, 1989; Belanger and Montgomery, 1992] should be minimized to prevent artifacts; (3) use of a cover for the collection bag may reduce the effects of surface water movements due to waves, currents or streamflow activities [Libelo and MacIntyre, 1994]; and (4) caution should be applied when operating near the seepage meter detection limit [Cable et al., 1997].

The influence on seepage measurements of frictional resistance in the tube connecting the drum to the collector bag produces inaccurate seepage rates. These inaccuracies can be minimized by using larger (0.9 cm ID) diameter plastic tubes [Fellows and Brezonik, 1980]. Shaw and Prepas [1989] found an anomalous short-term influx of water occurred when water was entering the collection bags after the bags were attached to seepage meters. They also showed that pre-filling the bags with a measured amount of water (about 1000 mL) reduced this measurement artifact substantially. The effect of surface water flow over the seepage meter and collection bag was discussed by Libelo and MacIntyre [1994], because it altered the hydraulic head within the meter augmenting seepage flow. Surface-water flow due to waves, currents, or streamflow reduces the hydraulic head in the meter resulting in an anomalously high value of measured seepage flux. Covering the collection bag with a bucket to isolate it from overlying surface water flow significantly reduces this error.

Although the application of manual type seepage meters has had some problems as menti...