5.15 Groundwater Dating and Residence-Time Measurements F
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5.15 Groundwater Dating and Residence-time Measurements F. M. Phillips New Mexico Tech, Socorro, NM, USA and M. C. Castro University of Michigan, Ann Arbor, USA 5.15.1 INTRODUCTION 452 5.15.2 NATURE OF GROUNDWATER FLOW SYSTEMS 452 5.15.2.1 Driving Forces 452 5.15.2.2 Topographic Control on Flow 453 5.15.2.3 Hydraulic Conductivity and Its Variability 453 5.15.2.4 Scales of Flow Systems 454 5.15.2.4.1 Vadose-zone scale 454 5.15.2.4.2 Local scale 454 5.15.2.4.3 Aquifer scale 455 5.15.2.4.4 Regional scale 455 5.15.2.5 Sources of Solutes at Various Scales 455 5.15.2.5.1 Meteoric (recharge) 455 5.15.2.5.2 Weathering 456 5.15.2.5.3 Diagenetic 456 5.15.2.5.4 Connate 456 5.15.2.5.5 “Basement waters” 456 5.15.3 SOLUTE TRANSPORT IN SUBSURFACE WATER 456 5.15.3.1 Fundamental Transport Processes 457 5.15.3.1.1 Advection 457 5.15.3.1.2 Diffusion 457 5.15.3.1.3 Dispersion 457 5.15.3.2 Advection–Dispersion Equation 458 5.15.3.3 Interaction between Hydrogeological Heterogeneity and Transport 458 5.15.3.3.1 Small-scale transport and effective dispersion 458 5.15.3.3.2 Large-scale transport and mixing 458 5.15.3.4 Groundwater Dating and the Concept of “Groundwater Age” 459 5.15.4 SUMMARY OF GROUNDWATER AGE TRACERS 460 5.15.4.1 Introduction 460 5.15.4.2 Radionuclides for Age Tracing of Subsurface Water 460 5.15.4.2.1 Argon-37 460 5.15.4.2.2 Sulfur-35 460 5.15.4.2.3 Krypton-85 461 5.15.4.2.4 Tritium 461 5.15.4.2.5 Silicon-32 463 5.15.4.2.6 Argon-39 463 5.15.4.2.7 Carbon-14 463 5.15.4.2.8 Krypton-81 464 451 452 Groundwater Dating and Residence-time Measurements 5.15.4.2.9 Chlorine-36 465 5.15.4.2.10 Iodine-129 465 5.15.4.3 Stable, Transient Tracers 466 5.15.4.3.1 Chlorofluorocarbons and sulfur hexafluoride 466 5.15.4.3.2 Atmospheric noble gases and stable isotopes 466 5.15.4.3.3 Nonatmospheric noble gases 467 5.15.5 LESSONS FROM APPLYING GEOCHEMICAL AGE TRACERS TO SUBSURFACE FLOW AND TRANSPORT 468 5.15.5.1 Introduction 468 5.15.5.2 Approaches 469 5.15.5.2.1 Direct groundwater age estimation 469 5.15.5.2.2 Modeling techniques: analytical versus numerical 470 5.15.5.2.3 Identification of sources and sinks of a particular tracer 471 5.15.5.2.4 Defining boundary conditions (modeling approach) 471 5.15.6 TRACERS AT THE REGIONAL SCALE 471 5.15.6.1 Introduction 471 5.15.6.2 Examples of Applications 472 5.15.6.2.1 Noble-gas isotopes—Paris Basin 472 5.15.6.2.2 Great Artesian Basin—noble-gas isotopes, 36Cl, and 81Kr 474 5.15.6.2.3 Implications at the regional scale 477 5.15.7 TRACERS AT THE AQUIFER SCALE 478 5.15.7.1 Introduction 478 5.15.7.2 Carrizo Aquifer 478 5.15.7.3 Implications at the Aquifer Scale 482 5.15.8 TRACERS AT THE LOCAL SCALE 483 5.15.8.1 Introduction 483 5.15.8.2 3H/ 3He, CFC-11, CFC-12, 85Kr—Delmarva Peninsula 483 5.15.8.3 Implications at the Local Scale 485 5.15.9 TRACERS IN VADOSE ZONES 486 5.15.10 CONCLUSIONS 490 REFERENCES 490 5.15.1 INTRODUCTION fluxes through subsurface systems are very difficult to measure. To some extent, subsurface fluxes can Water is the key to the geochemistry of the upper be estimated using methods based on the physics of crust and the surface of the Earth. Water interacts water flow (e.g., numerical modeling of flow extensively with minerals through hydrolysis, systems), but the very large variability of per- dissolution, precipitation, interface solute layers, meability imparts a high uncertainty to the results. etc., and is necessary for life, which directly and Fortunately, measurement of certain geochemical indirectly affects the chemistry of Earth systems. constituents in water can define residence times The local characterization of aqueous geochemical and fluxes in subsurface systems. This chapter will reactions is a natural and straightforward extension explore the geochemical means that are available of classical chemistry. However, neither the to help quantify subsurface fluxes, and will composition nor the evolution of the upper layers evaluate the implications of their application to a of the Earth can be explained by local-scale variety of specific systems. geochemistry. It is the open-system nature of geochemistry in this environment that provides most of its explanatory power, as well as its 5.15.2 NATURE OF GROUNDWATER intellectual challenge. The fundamental aspects of FLOW SYSTEMS near-surface geochemistry such as weathering 5.15.2.1 Driving Forces reactions, precipitation of sedimentary minerals, diagenesis, and metamorphic evolution all depend Water moves from areas of higher total fluid on transport of mass in and out of these systems. In energy to areas of lower energy. Kinetic energy, most cases, moving water is the transport agent. mechanical energy (e.g., compression), gravita- Geochemical constituents are transported by the tional potential, interfacial potential, and chemical circulation of the Earth’s three major near-surface potential can drive fluid flow. In general, velo- dynamic systems: the oceans, the atmosphere, and cities in the subsurface are low enough, so that continental water. This chapter deals with the third kinetic energy can be neglected. Water, therefore, of these, specifically with subsurface water. Sur- moves from areas of higher to ones of lower fluid face water is an important medium for transporting potential (Hubbert, 1940). In systems that are only geochemical constituents on a global scale, and is partially saturated with water (e.g., the vadose relatively easily accessed and quantified. In con- zone and oil and gas reservoirs), the combination trast, subsurface water is not easy to access, and of gravitational potential and interfacial potential Nature of Groundwater Flow Systems 453 (capillary action) serves to drive flow. In relatively law, has an extremely wide range in natural shallow (less than a few kilometers) saturated materials, extending over ,16 orders of magnitude groundwater systems, gravitational potential is (Freeze and Cherry, 1979; Manning and usually dominant. In deeper settings, additional Ingebritsen, 1999). Although, in any particular forces—such as compression due to compaction subsurface setting, the range will probably be much or tectonic movement, pressure generated by less, variations of five or more orders of magnitude chemical reactions, or chemical potentials deve- are common. To compound the difficulties of loped across semipermeable shales—may also be quantifying the behavior of fluids in such hetero- important (Neuzil, 1995, 2000; see Chapter 5.16). geneous systems, access to the subsurface is generally very limited and actual measurements of hydraulic conductivity are generally sparse. 5.15.2.2 Topographic Control on Flow Thus, although the physics of groundwater flow is The fundamentals of the major subsurface water well understood and amenable to quantitative cycle are straightforward. Water precipitates out of analysis, the great variability of properties and the atmosphere onto areas of relatively high the lack of constraints on their values render such topography. Some of this water infiltrates into the analyses, subject to great uncertainty. It is in this subsurface. Because it has greater gravitational perspective that the utility of geochemical tracers potential energy than water at lower elevations, it for water residence time can be appreciated. A flows downward, eventually re-emerging at the sequence of residence-time measurements along a surface either at the lowest elevation within the flow path essentially integrates the velocity history region, or at higher elevations where low- of the groundwater and thus provides a very permeability barriers force it to the surface. valuable constraint on the permeability structure. Topographically driven groundwater flow is The structure of the heterogeneity in hydraulic generally more important for geochemical pro- conductivity is typically complex. The fundamen- cesses than flow produced by the alternative tals of the geological controls on conductivity have mechanisms mentioned above. This is because been recognized for many years. For example, the circulation of meteoric water, driven ulti- Meinzer (1923) codified long-standing ideas of mately by solar energy, is a process that provides a high- and low-conductivity units termed “aquifers” continuous source of subsurface water that can and “aquitards.” However, continued research has carry large fluxes of solutes. Although other shown that these are considerable oversimplifica- driving forces, such as sediment compaction or tions of a typically very complex subsurface tectonic compression, may, for periods of time, permeability structure. An alternative view is to provide water velocities comparable to topogra- characterize the permeability structure as essen- phically driven flow at similar depths, the total tially fractal (Neuman, 1990, 1995); in other water flux is limited to the pore volume of the rock words, as the spatial scale of measurement undergoing compression. increases, the variability of hydraulic conductivity, One frequently underappreciated aspect of and the size of units of differing conductivity, also topographically driven flow is the pervasive extent increases in some proportional fashion, without of water circulation through the shallow crust. any apparent upper limit. Further refinement of Fairly subdued topography can drive appreciable these ideas would conceptualize hydraulic con- water flow to depths of several kilometers. ductivity as being characterized by nested scales of Under favorable circumstances, more significant variability, such that the increase in variability relief can produce circulation to depth below might be large for certain ranges of spatial scale 5km(Person and Baumgartner, 1995; Person and and small for others (Gelhar, 1993).