Isotopic methods 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. 

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. 

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. 

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