Tracer test modeling for characterizing heterogeneity and local-scale residence time distribution in an artificial recharge site

Abstract

Abstract. Artificial recharge of aquifers is a technique for improving water quality and increasing groundwater resources. Understanding the fate of a potential contaminant requires knowledge of the residence time distribution (RTD) of the recharged water in the aquifer beneath. A simple way to obtain the RTDs is to perform a tracer test. We performed a pulse injection tracer test in an artificial recharge system through an infiltration basin to obtain the breakthrough curves, which directly yield the RTDs. The RTDs turned out to be very broad and we used a numerical model to interpret them, to characterize heterogeneity, and to extend the model to other flow conditions. The model comprised nine layers at the site scaled to emulate the layering of aquifer deposits. Two types of hypotheses were considered: homogeneous (all flow and transport parameters identical for every layer) and heterogeneous (diverse parameters for each layer). The parameters were calibrated against the head and concentration data in both model types, which were validated quite satisfactorily against 1,1,2-Trichloroethane and electrical conductivity data collected over a long period of time with highly varying flow conditions. We found that the broad RTDs can be attributed to the complex flow structure generated under the basin due to three-dimensionality and time fluctuations (the homogeneous model produced broad RTDs) and the heterogeneity of the media (the heterogeneous model yielded much better fits). We conclude that heterogeneity must be acknowledged to properly assess mixing and broad RTDs, which are required to explain the water quality improvement of artificial recharge basins.

Assessing the Fate of Benzophenone-Type UV Filters and Transformation Products during Soil Aquifer Treatment: The Biofilm Compartment as Bioaccumulator and Biodegrader in Porous Media

“…Groundwater flow and conservative transport in the two SAT systems were comparable, showing similar arrival times of both EC and lithium at the different sampling points (Figure S3a,b and c,d respectively). In the two systems, preferential flow dominated transport through the recharge zone, with a fast-early arrival and a broad range of residence times at the first observation point of the two systems (O-piezometer), similarly to Valhondo et al On the other hand, transport in the aquifer was what might be expected from a relatively homogeneous aquifer with moderate dispersion for short transport length (up to A2), but more significant dispersion up to B2 (Figure S3).…”

Reactive Barriers for Renaturalization of Reclaimed Water during Soil Aquifer Treatment

“…We estimated first-order degradation rates (λ) and retardation coefficient (R) for 10 CECs at Sant Vicenç dels Horts site [41,49] and compared them with those reported in the literature from other experiments. While λs can be highly uncertain, they can be considered "relative measures for comparison" of results [50].…”

“…We also performed a pulse injection tracer test to obtain the residence time distribution of the recharged water at six monitoring points. The heads and breakthrough curves were used to estimate the flow and conservative transport parameters of the aquifer using a quasi-3D numerical model [49]. The model was built using the finite element code Transdens [51][52][53].…”

“…The model was built using the finite element code Transdens [51][52][53]. Secondly, CEC concentrations measured in the source water and nine monitoring points were used to estimate λ and R (in three and two defined subdomains, respectively) for 10 CECs [41,49]. The subdomains for λ estimation were BARR (reactive barrier), UZ (unsaturated zone), and AQF (aquifer).…”

Impacts of Streambed Heterogeneity and Anisotropy on Residence Time of Hyporheic Zone

“…The influence of heterogeneity on the groundwater RT or RTD in unconfined aquifers has generally been deduced by comparison between a homogeneous and a heterogeneous case (e.g., Luther and Haitjema ; Valhondo et al ). Also, as discussed in the previous section, the mean and median RTs are shorter and the RTR is larger in a heterogeneous HZ.…”