Modeling subsurface transport in extensive glaciofluvial and littoral sediments to remediate a municipal drinking water aquifer

Abstract

Abstract. Few studies have been carried out that cover the entire transport process of pesticides, from application at the soil surface, through subsurface transport, to contamination of drinking water in esker aquifers. In formerly glaciated regions, such as Scandinavia, many of the most important groundwater resources are situated in glaciofluvial eskers. The purpose of the present study was to model and identify significant processes that govern subsurface transport of pesticides in extensive glaciofluvial and littoral sediments. To simulate the transport processes, we coupled a vadose zone model at soil profile scale to a regional groundwater flow model. The model was applied to a municipal drinking-water aquifer, contaminated with the pesticide-metabolite BAM (2,6-dichlorobenzoamide). At regional scale, with the combination of a ten-meter-deep vadose zone and coarse texture, the observed concentrations could be described by the model without assuming preferential flow. A sensitivity analysis revealed that hydraulic conductivity in the aquifer and infiltration rate accounted for almost half of the model uncertainty. The calibrated model was applied to optimize the location of extraction wells for remediation, which were used to validate the predictive modeling. Running a worst-case scenario, the model showed that the establishment of two remediation wells would clean the aquifer in four years, compared to nine years without them. Further development of the model would require additional field measurements in order to improve the description of macrodispersion in deep, sandy vadose zones. We also suggest that future research should focus on characterization of the variability of hydraulic conductivity and its effect on contaminant transport in eskers.

Modeling Subsurface Fate of S‐Metolachlor and Metolachlor Ethane Sulfonic Acid in the Westliches Leibnitzer Feld Aquifer

“…The resulting recharge and leaching mass time series can subsequently be coupled with a groundwater flow and pollutant transport model. Alternatively, a three-dimensional model that describes water flow and solute movement in variably saturated media could be applied (e.g., as an extension to the approach described by Bergvall et al, 2011). In the case of shallow groundwater tables, where plant roots reach into the groundwater, the coupling of models between the vadose and the saturated zone has to be implemented at an appropriate time interval (e.g., 1 d, depending on the temporal dynamics of the dominating processes); current results from the groundwater model (i.e., groundwater level and compound concentration) are then used as the lower boundary conditions for the following time step in the vadose zone model.…”

“…They concluded that although macropore processes have no dominating effect on groundwater recharge at the catchment scale, they will have significant effects on pesticide leaching to groundwater because some of the pesticides are transported rapidly downward in the soil to zones with less sorption and degradation. Bergvall et al (2011) coupled the vadose zone model HYDRUS‐1D to the groundwater flow model MODFLOW and the solute transport model MT3D to describe significant processes that govern the subsurface transport of the pesticide metabolite 2,6‐dichlorobenzoamide in a glaciofluvial esker aquifer. They reproduced the observed concentrations at the regional scale, attributed half of the model uncertainty to hydraulic conductivity in the aquifer and infiltration rate, and applied the model to optimize the location of extraction wells for remediation.…”

Degradation of trace concentrations of the persistent groundwater pollutant 2,6-dichlorobenzamide (BAM) in bioaugmented rapid sand filters

“…Dichlobenil was banned in 1997, but BAM is still detected in approximately 20% of the Danish wells, and in 5% of the wells it is above the 0.1 mg/L threshold value (Thorling et al, 2013). It is also widely present in the groundwater of many other countries (e.g., Bergvall et al, 2011 and references therein), although it is not monitored regularly in most countries outside Northern Europe.…”

A drive point application of the Guelph Permeameter method for coarse-textured soils

“…Of the two sets of K fs values, those provided by the drive point GP approach are considered more accurate because of the soil's medium-coarse sand texture and negligible silt and clay content. Supporting this is a modelling study by Bergvall et al (2011) who used calibration of the HYDRUS-1D water-solute transport model to estimate 2.89 × 10 − 4 m s − 1 ≤ K fs ≤ 4.63 × 10 − 4 m s − 1 for the top 0.9 m at the field site, which includes the range of drive point GP results (i.e. 2.87 × 10 − 4 m s − 1 ≤ K fs ≤ 3.74 × 10 − 4 m s − 1 , Table 4).…”