A hillslope-scale aquifer-model to determine past agricultural legacy and future nitrate concentrations in rivers

Guillaumot, Luca; Marçais, Jean; Vautier, Camille; Guillou, Aurélie; Vergnaud, Virginie; Bouchez, Camille; Dupas, Rémi; Durand, Patrick; Dreuzy, Jean‐Raynald de; Aquilina, Luc

doi:10.1016/j.scitotenv.2021.149216

Cited by 14 publications

(11 citation statements)

References 108 publications

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“…However, aquifer length, constraining characteristic time, is much smaller here. Such estimates are in line with aquifer characteristic times typically inferred from streamflow and water quality data in the region (Guillaumot et al, 2021).…”

Section: The Unsaturated Zone and Recharge Fluxessupporting

confidence: 87%

Frequency domain water table fluctuations reveal impacts of intense rainfall and vadose zone thickness on groundwater recharge

Guillaumot

Longuevergne

Marçais

et al. 2022

Hydrol. Earth Syst. Sci.

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Abstract. Groundwater recharge is difficult to estimate, especially in fractured aquifers, because of the spatial variability of the soil properties and because of the lack of data at basin scale. A relevant method, known as the water table fluctuation (WTF) method, consists in inferring recharge directly from the WTFs observed in boreholes. However, the WTF method neglects the impact of lateral groundwater redistribution in the aquifer; i.e., it assumes that all the WTFs are attributable to recharge. In this study, we developed the WTF approach in the frequency domain to better consider groundwater lateral flow, which quickly redistributes the impulse of recharge and mitigates the link between WTFs and recharge. First, we calibrated a 1D analytical groundwater model to estimate hydrodynamic parameters at each borehole. These parameters were defined from the WTFs recorded for several years, independently of prescribed potential recharge. Second, calibrated models are reversed analytically in the frequency domain to estimate recharge fluctuations (RFs) at weekly to monthly scales from the observed WTFs. Models were tested on two twin sites with a similar climate, fractured aquifer and land use but different hydrogeologic settings: one has been operated as a pumping site for the last 25 years (Ploemeur, France), while the second has not been perturbed by pumping (Guidel). Results confirm the important role of rainfall temporal distribution in generating recharge. While all rainfall contributes to recharge, the ratio of recharge to rainfall minus potential evapotranspiration is frequency-dependent, varying between 20 %–30 % at periods <10 d and 30 %–50 % at monthly scale and reaching 75 % at seasonal timescales. We further show that the unsaturated zone thickness controls the intensity and timing of RFs. Overall, this approach contributes to a better assessment of recharge and helps to improve the representation of groundwater systems within hydrological models. In spite of the heterogeneous nature of aquifers, parameters controlling WTFs can be inferred from WTF time series, providing confidence that the method can be deployed in different geological contexts where long-term water table records are available.

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Section: The Unsaturated Zone and Recharge Fluxessupporting

confidence: 87%

Frequency domain water table fluctuations reveal impacts of intense rainfall and vadose zone thickness on groundwater recharge

Guillaumot

Longuevergne

Marçais

et al. 2022

Hydrol. Earth Syst. Sci.

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“…We only relied on one sampling campaign on three different sampling sites to date the groundwater and its contribution to the river. A solution would be to sample more sites, for example, by systematically sampling springs (Rademacher et al., 2001, 2005) or to sample CFCs at different seasons (Guillaumot et al., 2021). More intensive sampling indeed enables to link discharge (Morgenstern et al., 2010) or dissolved solutes concentrations to groundwater ages (Marçais et al., 2018), which offers opportunities to strengthen the spatial and temporal resolution of transit times in the river (Peters et al., 2014).…”

Section: Discussionmentioning

confidence: 99%

“…Aerobic conditions prevent CFC degradation (Dunkle et al., 1993; Kolbe et al., 2020). In any case, exchanges between the samples and the atmosphere have been minimized, by sampling far from the river re‐aeration zones and at the very bottom of the water column (Guillaumot et al., 2021; Vautier et al., 2020). The two wells were chosen because they are known to be located just upstream of well‐identified springs.…”

Section: Flow Model Transport Model and Study Sitementioning

confidence: 99%

Dynamic Contributions of Stratified Groundwater to Streams Controls Seasonal Variations of Streamwater Transit Times

Marçais

Derry

Guillaumot

et al. 2022

Water Resources Research

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Streamwater transit time distributions display a variable proportion of old waters (≥1 year). We hypothesize that the corresponding long transit times result from groundwater contributions to the stream and that seasonal streamwater transit time variations result from (a) the variable contributions of different flowpaths (overland flow, seepage flow and baseflow) and (b) the stratification of groundwater residence times. We develop a parsimonious model to capture the groundwater contribution to the stream discharge and its effect on transient transit times. Infiltration is partitioned according to the aquifer saturation between Boussinesq groundwater flow and overland flow. Time‐variable transit time distributions are obtained with a new 2D particle tracking algorithm. Hydraulic conductivity, total and drainable porosities are calibrated by using discharge and CFC tracer data on a crystalline catchment located in Brittany (France). The calibrated models succeed in reproducing CFCs concentrations and discharge dynamics. The groundwater flow contribution to the stream is controlled by the aquifer hydraulic conductivity, while its age is controlled by the drainable and total porosities. Old groundwater (≥1 year) is the source for approximately 75% of the streamflow with strong seasonal variations (between 40% and 95%). Mean transit times are approximately 13 years, varying between 6 and 20 years, proportional to the groundwater contribution. These seasonal variations are driven by the groundwater versus overland flow partitioning. The stratification of groundwater residence times in the aquifer plays a minor role in the streamwater transit times but is key for the transit time dynamics of the groundwater contribution to the stream.

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“…(2000) used a response time of 10–30 yr when applying the MONERIS model to the river basins of Germany. In Brittany, studies that used different approaches converged on estimated mean response times of 5–15 yr (Aquilina et al., 2012; Durand et al., 2015; Dupas et al., 2020; Guillaumot et al., 2021).…”

Section: Discussionmentioning

confidence: 99%

Nitrogen mitigation scenarios to reduce coastal eutrophication

Malik

Oehler²,

Sgro

et al. 2022

Agrosystems Geosci & Env

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Macroalgal blooms in coastal areas, which pose unique risks to the environment, citizens, stakeholders, and the economy, mainly are due to nitrate pollution in rivers. The Saint‐Brieuc Bay has some of the largest algal blooms in Brittany (western France). It is fed by three catchments—Gouessant (420 km2), Gouet (256 km2), and Anse d'Yffiniac (130 km2)—characterized by an oceanic climate and mixed‐farming systems with high livestock density. The main objective of this study was to assess the ability of nitrogen (N) mitigation scenarios to reduce N emissions to coastal water using the spatially distributed agrohydrological model TNT2. Once TNT2 was calibrated and validated for the catchments, scenarios were simulated for them from 2008 through 2035: (a) reference (REF) scenario, (b) agricultural management practices (AMP) scenario (i.e., changes to mineral and organic N fertilization, catch crops, and crop rotations), (c) increasing percentages of agricultural land conversion (ALC) into unmanaged grassland from downhill to uphill, and (d) scenarios that combined AMP and ALC. Results showed that the AMP scenario could reduce N load in the bay by 31% vs. the REF scenario. The ALC scenario was much more effective when located downhill, with >50% of maximum effectiveness achieved with 10% of agricultural land converted. These results varied strongly among catchments. Because reducing coastal eutrophication requires a drastic decrease in N loads, we recommend combining AMP and ALC scenarios to achieve good environmental status. The study illustrates the TNT2 model's ability to simulate complex scenarios and guide mitigation policy.

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A hillslope-scale aquifer-model to determine past agricultural legacy and future nitrate concentrations in rivers

Cited by 14 publications

References 108 publications

Frequency domain water table fluctuations reveal impacts of intense rainfall and vadose zone thickness on groundwater recharge

Frequency domain water table fluctuations reveal impacts of intense rainfall and vadose zone thickness on groundwater recharge

Dynamic Contributions of Stratified Groundwater to Streams Controls Seasonal Variations of Streamwater Transit Times

Nitrogen mitigation scenarios to reduce coastal eutrophication

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