Predicting Pesticide Attenuation in a Fractured Aquifer Using Lumped‐Parameter Models

Abstract

Neighboring springs draining fractured-rock aquifers can display large differences in water quality and flow regime, depending on local variations of the connectivity and the aperture size distribution of the fracture network. Consequently, because homogeneous equivalent parameters cannot be assumed a priori for the entire regional aquifer, the vulnerability to pollution of such springs has to be studied on a case by case basis. In this paper, a simple lumped-parameter model usually applied to estimate the mean transit time of water (or tracer) is presented. The original exponential piston-flow model was modified to take land-use distribution into account and applied to predict the evolution of atrazine concentration in a series of springs draining a fractured sandstone aquifer in Luxembourg, where despite a nationwide ban in 2005, atrazine concentrations still had not begun to decrease in 2009. This persistence could be explained by exponentially distributed residence times in the aquifer, demonstrating that in some real world cases, models based on the groundwater residence time distribution can be a powerful tool for trend reversal assessments as recommended for instance by current European Union guidelines.

On the use of spring baseflow recession for a more accurate parameterization of aquifer transit time distribution functions

“…The greater this proportion, the closer g (τ ) will be to g (τ ). Details can be found in Farlin et al (2013) and are omitted here for space reasons. Since the relationship between g (τ ) and g (τ ) is bijective and the parameters η and t em can be converted uniquely without ambiguity, we will refer in the rest of the text to the parameters of g (τ ) only.…”

“…where t epm = total mean transit time of the tracer in the system (in a double porous medium), t pf = mean transit time of the tracer in the unsaturated zone and t em = mean transit time of the tracer in the saturated zone The parameters of g (τ ) are then calculated from those of g (τ ) using the land-use distribution (Farlin et al, 2013). The second step is necessary to take into account the fact that atrazine is only applied on agricultural surfaces, whereas tritium infiltrates homogeneously over the entire recharge area.…”

“…The transit time distribution function used here is the exponential piston-flow model (EPM) proposed by Maloszewski and Zuber (1982), a combination of two simpler models: piston-flow and exponential. The piston-flow component simulates an unsaturated zone of near-constant thickness where all flow lines are approximately vertical and of equal length and transit time (Farlin et al, 2013), and the exponential component simulates the transit time distribution in the saturated zone. Transit time distributions in real aquifers, which are by nature heterogeneous, most probably differ from simple analytical transit time distributions (Etcheverry, 2001).…”

“…Transit time distributions in real aquifers, which are by nature heterogeneous, most probably differ from simple analytical transit time distributions (Etcheverry, 2001). However, since no data is available concerning the spatial heterogeneity of the fracture network and in the absence of empirical evidence of extreme heterogeneities on the study site (Farlin et al, 2013), the simplest model of a pseudo-homogeneous aquifer was adopted. As was shown by Haitjema (1995) and Etcheverry (2001), ground water transit times are exponentially distributed in a homogeneous aquifer with constant saturated thickness.…”

“…Atrazine concentration in spring water C out is predicted from the input leaching concentration C in by (Farlin et al, 2013)…”