Groundwater-lake systems

Use of Oxygen-18 and Deuterium To Assess the Hydrology of Groundwater-Lake Systems... 

Hydrogeologists take a variety of approaches in quantifying the rates of groundwater exchange with lakes. These approaches can be physically or chemically based isotopic approaches are one example of the chemical methods. The application of stable isotopes to the study of groundwater-lake systems is the primary focus of this chapter. 

This chapter demonstrates the usefulness of stable isotopes in investigating groundwater-lake systems. The discussion emphasizes isotopic applications to groundwater-lake systems characteristic of the temperate glaciated regions of the north-central and northeastern United States. Thus, it is also applicable to similar systems in other glaciated parts of the world, such as the Scandinavian peninsula and northern Asia. The applications stem from our experience with lake systems in the lake district of north-central Wisconsin. As such, we restrict our discussion to shallow groundwater systems that are hydraulically connected to freshwater lakes. 

Figure I. Schematic dratoing of the hydrological components of a groundwater-lake system and their respective isotopic fractionation characteristics.

Although lakes are more complex, many can be considered as surface expressions, or outcrops, of the water table. Depending on the distribution of hydraulic heads surrounding a lake, groundwater-lake systems can be described as one of the following types ... 

Stable-Isotope Mass-Balance Method. The equations presented in this section apply to groundwater-lake systems that are at hydrological and isotopic steady states. Equations that describe isotopic mass balances for non-steady-state systems and forms that pertain to the estimation of evaporation from lakes have been presented by other authors (13, 14). 

Application of Isotopic Methods to Non-Steady-State Groundwater-Lake Systems. Isotopic compositions of lakes whose hydraulic-residence times are relatively short (about 2 years or less) vary seasonally (32). Seasonal response occurs whenever a significant mass of water of a different isotopic composition is either added to or removed from the lake. Seasonal variations in P, E, G, 8P, 8a, and 8 are the principal driving forces behind observed variations in 8j for isotopically non-steady-state systems. In northern Wisconsin, many lakes are isotopically non-steady-state. 

Experience gained from studies in northern Wisconsin indicates that stable isotopes can be valuable tools for assessing the hydrology of groundwater-lake systems. Several conclusions are evident ... 

The purpose of this chapter is not to promote the replacement of traditional physically based methods of assessing groundwater-lake systems with isotopic methods, but rather to demonstrate the utility of isotopic techniques. Physically based methods can provide more detailed information on the spatial and temporal variability of a groundwater-lake system than isotopic approaches can provide. Regardless of the method chosen, however, an adequate number of piezometers is necessary to ensure that groundwater samples are collected from upgradient areas. 

Krabbenhoft D. P., Bowser C. J., Kendall C., and Gat J. R. (1994) Use of oxygen-18 and deuterium to assess the hydrology of groundwater-lake systems. In Environmental Chemistry of Lake and Reservoirs (ed. L. A. Baker). American Chemical Society, Washington, DC, pp. 67-90. 

Colloids are present in natural waters (i.e., surface and groundwaters). Surface systems receive terrestrial input as runoff, which carries solid-derived materials into streams, rivers, lakes, or estuaries. Groundwater receives leachates from land fills and percolation water and is frequently well connected with surface water bodies. Colloids may also be formed in situ by native processes of precipitation and dissolution, suspension, or biological activity [103,104]. 

Further classification of lakes relates to their position within the regional groundwater-flow system. Terminal-lake systems are defined as lakes that function as the discharge point of the regional groundwater-flow system. For terminal lakes, water is removed by evaporation and sometimes through surface outflow. These lakes typically evolve into saline lake systems characteristic of the semiarid or arid regions of the world (32). 

LRL is a groundwater recharge system (no surface inlets or outlets) and receives 98-100% of its water from precipitation directly onto the lake surface. Groundwater seepage accounts for about 35% of the water output from the lake, and evaporation accounts for the remaining 65%. Water residence time, tw, is about 9-11 years. Most of LRL is situated above the regional... 

The statements above have introduced several concepts that need to be defined and expanded upon. A system is a grouping of atoms, minerals, rocks, and/or gases and waters under consideration within a single volume of space, the boundaries of which can be defined as is convenient. A system could be one mineral grain, a drop of rain, a water-logged soil, a well-mixed lake, or a regional groundwater/rock system tens of kilometers in diameter. 

Lerman, A. 1979. Geochemical Processes, Water and Sediment Environments. Wiley, New York. 481 pp. Winter, T. C. 1999. Relation of streams, lakes, and wetlands to groundwater flow systems. Hydrogeol. J. 7 28-45. 

The statistical collection and representation of the weather conditions for a specified area during a specified time interval, usually decades, together with a description of the state of the external system or boundary conditions. The properties that characterize the climate are thermal (temperatures of the surface air, water, land, and ice), kinetic (wind and ocean currents, together with associated vertical motions and the motions of air masses, aqueous humidity, cloudiness and cloud water content, groundwater, lake lands, and water content of snow on land and sea ice), nd static (pressure and density of the atmosphere and ocean, composition of the dry ir, salinity of the oceans, and the geometric boundaries and physical constants of the system). These properties are interconnected by the various physical processes such as precipitation, evaporation, infrared radiation, convection, advection, and turbulence, climate change... 

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