Tracer test modeling for local scale residence time distribution characterization 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 times distribution (RTD) of the water beneath the artificial recharge infrastructure. 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 yield directly the RTDs. These were very broad and we used a numerical model to interpret them, and to extend the characterization 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 satisfactory 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 were caused by both the complex flow structure generated under the basin (the homogeneous model produced broad RTDs) and the heterogeneity of the media (the heterogeneous model yielded much better fits). We conclude that acknowledging heterogeneity is required to properly assess mixing and broad RTDs, which are required to explain the water quality improvement of artificial recharge basins.

Enhanced groundwater recharge rates and altered recharge sensitivity to climate variability through subsurface heterogeneity

“…For instance, groundwater-pumping rates could be adapted to the temporally variable water availability (34). Additionally, temporal variability could be compensated for by artificially recharging aquifers with longer residence times using water discharged from the more heterogeneous regions (35,36). Regardless, the requirements to sustain environmental flow (1) and the increased vulnerability to contamination due to preferential recharge (33) have to be accounted for in any water management plan.…”

Monitoring induced denitrification during managed aquifer recharge in an infiltration pond

“…The MAR pond of Sant Vicenç undergoes two main operational periods: (1) recharge periods (RPs), when the pond is full of the infiltration water and total saturation conditions are not obtained under the infiltration pond [Valhondo et al, 2015[Valhondo et al, , 2016 and (2) nonrecharge periods (NRPs), when the pond is emptied for operational redevelopment and/or when the quality of the river water is low. NRPs are implemented when the control parameters of the infiltration water are exceeded, such as when NH 4 + concentrations are higher than 1.5 mg L -1 , electrical conductivity (EC) is higher than 2000 μS cm -1 , river turbidity is greater than 100 NTU and input water turbidity exceeds 25 NTU.…”

“…Travel time of the recharge water to the different piezometers was determined from fluctuations in EC during the infiltration tests [Valhondo et al, 2014]. The results of the infiltration tests were validated by Valhondo et al [2016] using a numerical model. The results indicated a travel time from the pond to the piezometer of around 18 to 24 hours for P8.3, nearly days for P2 and P5, 10 days for P10 and P8.1, and more than 20 days for P9.…”

“…Massmann et al [2006], investigating changes in redox conditions below an artificial recharge pond in Berlin, found that the level of PhACs in groundwater was controlled by the hydrochemical conditions of the system. Thus, to increase the quality of recharge water and groundwater, the infiltration pond can be coupled to a permeable reactive layer (PRL), an organic reactive layer at the bottom of the pond [Valhondo et al, 2014[Valhondo et al, , 2015[Valhondo et al, , 2016] that promotes diverse redox conditions along the recharge path to enhance the degradation of pollutants.…”

“…In the present work, we monitored denitrification processes in a MAR-PRL system located at Sant Vicenç dels Horts, Barcelona, Spain [Valhondo et al, 2014[Valhondo et al, , 2015[Valhondo et al, , 2016, which has a layer of vegetal compost at the bottom of the infiltration pond. The aim of the present study is to test the usefulness of a combined isotope analysis and depth specific hydrochemical data to: (i) assess the long-term effectiveness of the reactive layer in promoting denitrification 5 years after installation; (ii) identify the denitrification processes occurring at different aquifer depths and locations along the saturated zone below the infiltration pond (including the mixing zone between recharge and native groundwater).…”