We compare two models that have been used frequently for describing biodegradation during contaminant transport in groundwater. One is a “simple” model based upon macroscopic properties only, and the other is a “biofilm” model that accounts for contaminant diffusion and reaction in biofilms. Although the simple model has been used frequently, its mathematical formulation appears inadequate to describe the physics of the biodegradation process. Hence it is unclear when the simple model should be considered valid, and analyses predicated upon the simple model are called into question. We resolve this issue by arguing that the simple model should be considered valid when it is mathematically equivalent to the conceptually superior biofilm model. We demonstrate that the two models are exactly equivalent at the macroscopic scale when steady state conditions prevail. Under these conditions the equivalent macroscopic degradation rate coefficient k can be related to microscopic rate parameters that describe mass transfer across a boundary layer, diffusion within the biofilm, and reaction within the biofilm. Under transient (nonsteady state) conditions the two models are not strictly equivalent. However, the error between the two models is negligible in certain cases. In particular, when the rate‐limiting step for biodegradation is either mass transfer across the boundary layer or diffusion within the biofilm, there is no distinguishable difference between the predictions of the two models. Thus this paper can be considered a theoretical foundation for use of the commonly employed simple biodegradation model as well as an elucidation of the conditions for its validity.

Semi-arid regions with little surface water commonly experience rapid water table decline rates. To hedge against the falling water table, production wells in central Mexico are commonly installed to depths of several hundred meters below the present water table and constructed as open boreholes or perforated casings across their entire length. Such wells represent highly conductive pathways leading to non-negligible flow across chemically distinct layers of an aquifer—a phenomenon known as ambient flow. The objectives of this study were to estimate the rate of ambient flow in seven production wells utilizing an end-member mixing model that is constrained by the observed transient chemical composition of produced water. The end-member chemical composition of the upper and lower layers of an urban aquifer that overlies geothermal heat is estimated to anticipate the future quality of this sole source of water for a rapidly growing urban area. The comprehensive water chemistry produced by seven continuously perforated municipal production wells, spanning three geologically unique zones across the city of San Miguel de Allende in Guanajuato State, was monitored during one day of pumping. The concentration of conservative constituents gradually converged on steady-state values. The model indicates that, relative to the lower aquifer, the upper aquifer generally has higher specific conductance (SC), chloride (Cl), nitrate (NO₃), calcium (Ca), barium (Ba) and magnesium (Mg). The lower aquifer generally has a higher temperature, sodium (Na), boron (B), arsenic (As) and radon (Rn). Ambient flow ranged from 33.1 L/min to 225.7 L/min across the seven wells, but this rate for a given well varied depending on which tracer was used. This new 3D understanding of the chemical stratification of the aquifer suggests that as water tables continue to fall, concentrations of geothermally associated contaminants of concern will increase in the near future, potentially jeopardizing the safety of municipal drinking water.