COASTAL AQUIFER MANAGEMENT: monitoring, modeling, and case studies - Chapter 5

CHAPTER 5 Leaky Coastal Margins: Examples of Enhanced Coastal Groundwater and Surface-Water Exchange from Tampa Bay and Crescent Beach Submarine Spring, Florida, USA P.W. Swarzenski, J.L. Kindinger 1. INTRODUCTION As populations and industry migrate toward sought-after coastal zone real estate, increased pressure on these fragile margins demands a realistic and comprehensive understanding of the underlying hydrogeological framework. One of the most threatened resources along these coastal corridors is groundwater, and coastal management agencies have developed complex strategies to protect these resources from overexploitation and contamination. Obvious consequences of coastal groundwater mismanagement may include accelerated saltwater intrusion into supply aquifers, inadequate groundwater supply versus demand, and infiltration of organic and inorganic contaminants into aquifers. Two examples of proactive management strategies in direct response to threatened coastal groundwater resources include the construction and maintenance of injection barrier wells [Johnson and Whitaker, this volume], and the construction of large-scale desalinization plants, such as in Tampa Bay, Florida [Beebe, 2000]. Leaky coastal margins, where exchange processes at the land–sea boundary are naturally enhanced, can include the following environments: i) carbonate platforms, ii) modern and paleo river channels, iii) geothermal aquifers, iv) shorelines that are mountainous or have large tidal amplitudes or potentiometric gradients, and v) lagoons, where evaporation can force density-driven exchange (Figure 1). In these coastal environments, facilitated fluid–solute exchange can play an important role not only for coastal groundwater/surface water management (i.e., water budgets), but also in the delivery of recently introduced contaminants to coastal bottom waters. This submarine input for nutrients and other waterborne constituents may contribute to coastal eutrophication and other deleterious estuarine impacts. ` 2004 by CRC Press LLC Figure 1: A cartoon depicting some leaky coastal margins. Such effects can exhibit a full range in scale from being highly localized, for example around a point discharge, to an eventual ecosystem wide shift. This chapter will discuss some hydrogeologic characteristics unique to leaky coastal margins, and will then illustrate these features by examining two examples from Florida: Tampa Bay and Crescent Beach submarine spring. At each of these sites coastal groundwater resource issues form a critical component in overall ecosystem health, which demands a vigorous interdisciplinary science curriculum. 1.1 Leaky Coastal Margins—Characteristics and Definitions Thomas [1952] reminded us that the principles of hydrology would be quite simple if the earth’s surface could be considered impervious. Components of the water budget would thus be a simple function of precipitation, runoff, and evaporation/transpiration without all the complications of hard to constrain rock–water interactions. We know, however, that water does indeed infiltrate the earth’s surface layer. Once a water parcel has been absorbed into subsurface strata, it can accumulate, flow through, be involved in chemical transformation reactions, and eventually discharged. The ability of these strata to hold and transport groundwater depends on the nature of the bedrock and sediments as well as any post-depositional alteration such as faults and dissolution features. The underlying hydrogeologic framework of leaky coastal margins exhibits such subsurface features that directly enhance groundwater transport across a land–sea boundary. This section describes some of the most prevalent coastal depositional environments where such exchange is facilitated. ` 2004 by CRC Press LLC 1.1.1 Carbonate Platforms Along land–sea margins, limestone, which consists largely of calcite produced by marine organisms, plays a fundamental role in the delicate balance of geologic and biologic cycles. Limestone is biogeochemically reactive as groundwater slowly percolates through interstitial pores and lattices. Dissolution of carbonate rock is caused principally by reactions with water undersaturated in calcium carbonate or acidic water, and will result in pore space enlargements, conduit formation, or large-scale cavities. Dissolution/collapse features such as sinkholes provide direct hydrologic communication between groundwater and surface water and can greatly facilitate water exchange within leaky coastal margins. Often, this facilitated exchange across the sediment–water interface makes it difficult to geochemically distinguish between groundwater and surface water. Along carbonate land–sea margins, the ubiquity of onshore and offshore springs further emphasizes the geologically enhanced water and solute exchange. 1.1.2 Modern and Paleo River Channels As rivers flow seaward, fluvial processes such as discharge and turbulence continuously sort particles in both the bed and suspended load. As a consequence, paleo and modern river channels are typically well sorted and consist of coarser grained particles such as sands and silts. When a stream or river extends into its adjacent bed or banks, this exchange is considered to occur in the hyporheic zone, and provides a mechanism for the dynamic mixing of groundwater and surface water. Fluctuations in sea level may play an important role in the historic delivery and trajectory of off-continent riverine materials. Coastal riverbeds are therefore an important potential hydrostratigraphic conduit for enhanced groundwater transport offshore. Modern as well as paleo river channels along the eastern seaboard of the United States offer examples of such enhanced exchange. 1.1.3 Geothermal Aquifers Most work on marine geothermal vents has focused on dramatic open ocean vent systems that are typically basaltic in origin, such as the Galapagos spreading center [Edmond et al., 1979] or the high temperature submarine springs off Baja, California [Vidal et al., 1978]. In Florida, Kohout and colleagues (cf. [Kohout, 1965]) have postulated a geothermally regulated process whereby cold, deep seawater can migrate into the highly permeable layers of the deep Floridan aquifer. Here this water is heated during upward transport and eventually discharged as warm, saline submarine spring water [Fanning et al., 1981]. Because coastal carbonate platforms are fairly common geologic features and as no intense magmatic ` 2004 by CRC Press LLC heat source is required to drive such submarine discharge, the flux of heated groundwater from limestone deposits is likely to be widespread and large enough to affect localized oceanic budgets. 1.1.4 Large Potentiometric Gradients For many decades, groundwater hydrologists have studied the dynamic transition zone that separates freshwater from saltwater along coastal margins to better predict saltwater intrusion as a potential groundwater contaminant and to more accurately assess the quantity of fresh coastal groundwater. A general observation from such studies is that the interface in coastal aquifers tends to dip landward due to the increased density of seawater over freshwater, and that the saltwater tongue often extends inland for considerable distances. Another characteristic inherent in any model of this interface, i.e., Badon-Ghijben-Herzberg, Glover [1959], Edelman [1972], Henry [1964], Mualem and Bear [1974], and Meisler et al. [1984], is the direct dependence of the extent of submarine groundwater discharge on elevated potentiometric heads measured at the coast. For example, on the northern Atlantic coastal margin, where shoreline potentiometric heads were estimated at 6 m, freshwater was modeled to extend about 60 km offshore [Meisler et al., 1984]. Indeed, further south off the coast of northern Florida, freshened groundwater masses were observed to discharge directly into Atlantic bottom waters [Swarzenski et al., 2001]. It is likely that many of these freshened submarine paleo-groundwater masses formed during the Pleistocene when sea levels were lower than at present. This suggests that trapped paleo-groundwaters beneath continental shelfs and shallow seas could provide a substantial groundwater resource, if these deposits could be tapped before processes of natural seawater infiltration contaminate them. 1.1.5 Lagoons Lagoons are shore-parallel river-ocean mixing zones that are typically developed by marine wave action as opposed to the more traditional river dominated processes that form a deltaic estuary. Lagoons are often shallow and poorly drained and as a result, water mass residence times are sufficiently long to cause significant increases in water column salinities that can extend considerably above marine values. Circulation in a lagoon is a composite of gravitational, tidal, and wind-driven components, which all contribute to a typically well-mixed water column, rather than the classic stratified two-layered estuarine regime. Tidal- (e.g., tidal pumping) and wind-driven circulation is particularly pronounced in shallow lagoons that most often occur along low-lying land–sea margins where gravitational circulation is negligible. The development of a hyper-saline water column ` 2004 by CRC Press LLC above freshened submarine groundwater masses can initiate density-driven upward flow. This buoyancy-driven advection/diffusion can enhance the transport of water and its solutes across the sediment-water interface of leaky coastal margins. 1.2 Submarine Groundwater Discharge The complex interaction of hydrogeologic processes coupled with anthropogenic perturbations within a coastal aquifer control the transport and delivery of subsurface materials as they are exchanged across leaky coastal margins. Recent developments in numerical and mathematical models on the dynamic freshwater–saltwater transition zone serve to better predict future coastal groundwater resources by more quantitatively assessing fresh coastal groundwater reserves as well as the extent and rate of coastal saltwater intrusion. These studies have largely focused on the onshore distribution or trends in groundwater salinities of supply and monitor wells. Attempts to realistically portray and predict the dynamic nature of the freshwater– saltwater transition zone have developed from a need to better constrain the onshore domain of such models by groundwater hydrologists, as well as the need to better understand coastal groundwater characteristics by oceanographers. The focus of this section is on the coastal discharge of groundwater and the implication of this flux to coastal aquifers and ecosystem health, rather than on saltwater intrusion. While not as evident as surface water runoff, groundwater also flows down gradient and discharges directly into the coastal ocean. The discharge of coastal groundwater has become increasingly important as industry and populations continue to migrate toward fragile coastal zones. The submarine groundwater delivery of certain dissolved constituents such as select radionuclides, trace metals, and nutrient species to coastal bottom waters has often been overlooked [Krest et al., 2000; Valiela et al., 1990; Reay et al., 1992; Simmons, 1992]. This omission from coastal hydrologic and mass balance budgets by both hydrologists and oceanographers alike is largely due to the difficulty in accurately identifying and quantifying submarine groundwater discharge [Burnett et al., 2001a, b; Burnett et al., 2002]. Unfortunately, hydrologists and coastal oceanographers still today sometimes use varied definitions to describe hydrogeologic terms and processes. This problem is clearly manifested in a recent response article by the hydrologist Young [1996] to oceanographer Moore’s [1996] very large coastal groundwater flux estimates derived for the mid-Atlantic Bight. There is consequently a real need to merge the disciplines of hydrology and oceanography to develop an integrated approach for studies of coastal ` 2004 by CRC Press LLC ... - tailieumienphi.vn