Regional and local scale pesticide transport in the alluvial plain of the Swiss Rhône River Valley

Kirsten Meiwirth
EPFL, 2003
Thèse EPFL

Résumé

The herbicide transport and the groundwater vulnerability to pesticide contamination were studied in the alluvial aquifer of the Rhône River Valley near Martigny (Southwest Switzerland). The low clay and organic matter content of the soil, the intensive agricultural use, and the low depth of the groundwater table, lead to an alleged high vulnerability of the groundwater to contamination. Groundwater is used for irrigation, drinking water supply, and industrial purposes; it is of great economic and ecological importance which makes it particularly interesting to study. The work consists of local scale transport experiments and a regional monitoring of the groundwater over a two year period. At the local scale, two herbicides (atrazine and isoproturon) and a tracer were applied to instrumented field plots (4 m2) in two consecutive years and the transport through the vadose zone was studied in detail. Water flow and solute transport are closely linked to climatic factors. After the application, the pesticides remain at the soil surface as long as no precipitation occurs. Following to the first heavy rainfall, the chemicals are quickly transported through the vadose zone and part of them reaches the groundwater in a short time. During dry periods, the concentrations decrease steadily in the soil profile and the groundwater. After further rainfall, additional concentration peaks are observed in the groundwater, while only small peaks appear near the soil surface. Approximately 2.5 months after the application, the chemical concentration in the soil and in the groundwater has decreased considerably. The experimental results have been used to evaluate a mechanistic deterministic root zone model (HYDRUS-1D). The aim of the simulations was to define the processes involved in pesticide transport and, if possible, to predict the fate of chemicals applied at the soil surface. The MIM (mobile-immobile water) concept was used in order to account for the rapidity of the observed transport. In the study area, the shallow groundwater influences considerably the water conditions in the unsaturated zone; apparently, in such cases the use of a one-dimensional model to simulate the water flow and the chemical transport in the vadose zone is hindered due to difficulties in defining the lower boundary condition. Groundwater flow is typically three-dimensional and therefore, a global (saturated - unsaturated) 3-D model or the coupling of an unsaturated 1-D model to a 3-D saturated model would be more appropriate. Nevertheless, HYDRUS-1D allowed to describe qualitatively the observed results and to confirm the assumption that accelerated flow occurs on the experimental plots. At the regional scale, 13 piezometers were installed in an observation area (400 ha), and the groundwater was regularly analysed for the concentration of 6 herbicides (atrazine, terbutylazine, simazine, deethylatrazine, isoproturon, and diuron). Herbicides were detected in 12 piezometers, sometimes at high concentrations, but during a short period of time (mid May to mid July). Throughout the rest of the year, the concentrations remained below the drinking water limit. The observations are in good agreement with the conclusions of the local experiments (rapid transport, important influence of the climatic conditions, quick decrease of concentration peaks). Furthermore, the groundwater characteristics play an important role for the observation of herbicides. Near the Rhône river, high hydraulic gradients quickly dilute chemicals leaching from the fields above and herbicides coming from more distant fields may also be observed. Near the slope, an inflow of contaminated water from the hill side (vineyards) contributes to the groundwater contamination. Within the plain, high concentrations of herbicides applied to an above located field can temporarily be observed. Contaminations are confined in time and space and different substances are detected from one piezometer to another. This lack of continuity in pesticide concentrations will complicate the prediction of the solutes' fate in the groundwater, and a very precise knowledge of the pesticides use, both in space and time, is required.

Official source

https://infoscience.epfl.ch/record/33344?ln=fr

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Concepts associés

Chargement

Publications associées

Chargement

Kirsten Meiwirth
EPFL, 2003
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/33344?ln=fr

À propos de ce résultat

Concepts associés (29)

Concepts associés

Chargement

Publications associées (84)

Publications associées

Chargement

The effect of seasonal soil frost on the alpine groundwater recharge including climate change aspects

Daniel Bayard

In alpine areas, the snow cover plays an important role as a water reservoir. Water is stored as snow over the winter and released in spring, recharging mountain aquifers through infiltration. These aquifers are essential, especially for supplying water for human activities during dry seasons. Numerous studies have shown that locally soil frost can drastically reduce the water infiltration. However, we know much less about the hydrological impact of soil frost at a larger scale, in particular with regard to groundwater recharge. This was our motivation for initiating an extensive field experiment in the southern Swiss Alps. Several methods at different spatial scales were adapted to explore (a) the local effect of a partially frozen ground on the snowmelt discharge in an alpine area, (b) the key processes influencing groundwater recharge during snowmelt periods, such as snow cover and soil frost evolution, snowmelt water runoff types, (c) the large-scale effect of soil frost on the aquifer recharge, and (d) winter situations that are critical with respect to flooding. In order to take into account spatial, altitudinal and climatic differences, the field study was run for two winter seasons at Gd St Bernard (2500 m), a site with very high winter precipitation and strong winds, and at Hannigalp above Grächen (2100 m), a ski resort with a rather dry winter climate. The different components of the soil water balance (lateral runoff, deep percolation, liquid soil water content) were measured on delimited plots of 5 m2. Additionally, a dye tracer experiment visualized the snowmelt infiltration patterns into the frozen, respectively non-frozen ground. At Gd St Bernard, investigations of the liquid water content and the soil temperature were additionally conducted at two secondary sites, differing in their orientation, so as to gain a better insight of the spatial soil frost expansion. Finally, an analysis of the water-table variation at the village of Grächen (some 500 m below Hannigalp) during the snowmelt was carried out for a 10-year time period, including our experimental seasons. We complemented our field investigations by using a numerical soil-snow-atmosphere transfer model that simulates the seasonal development of the soil frost and the snow cover, as well as lateral and vertical water flow in the top soil. This model was used as an upper boundary condition to a 2-D groundwater model, to calculate the outlet flow at the base of a simplified alpine aquifer. The two winters investigated had contrasting meteorological conditions, which resulted in intriguing differences and similarities with respect to the soil physical conditions: the first one having a thick snowpack and hardly any soil frost, and the second one with hardly any snow until January and a deep and persistent soil frost. In spite of a very thick snowpack, the water balance measurements from winter 2000/2001 showed for both sites zero to low surface and subsurface flow due to the fact that soil frost was local and the soil permeability high. During the next winter, approximately 25% of the total meltwater run off laterally. The soil infiltration capacity was mainly reduced by the presence of a basal ice sheet, caused by the freezing of meltwater after a mid-winter snowmelt event. The field data were used to calibrate and validate a physically-based one-dimensional model. To allow an accurate parameter setting for the water repellent soil of Hannigalp, laboratory infiltration experiments on soil columns were additionally carried out. Using the resultant information, the model predicted correctly most measured soil parameters, in particular good matching was observed between measured and simulated snowmelt discharge. At Gd St Bernard, the simulated results were less satisfactory, especially during the first winter, when large discrepancies were noted between measured and simulated lateral runoff. Apart from the shortage in the soil parameter characterization, the deviation was a result of the simplified description of the experimental field. The model was, for example, not able to simulate surface runoff generated by a steep slope under unfrozen conditions, as the soil surface is considered by the model to be flat. Nevertheless, results were accurate enough to reproduce the occurrences of snowmelt events and the accumulated values of snow-melt discharge pathways under frozen conditions. Long-term weather data were used to simulate the soil frost depth at both sites. The simulation results showed that the probability of the occurrence of frozen soils was lowest on slopes with south or north exposure. On southerly exposed plots, the heat stored in the soil during the summer inhibited the development of deep and persistent soil frost, while most pore ice thawed due to underneath heating. On north plots, the building of soil frost was rare, as early snowfall insulated the ground from the atmosphere. However, during snow-poor winters, the frost penetrated deep into the ground, and the low underneath heat flux at that location hardly affected the frost depth until snowmelt. The snow depth and average soil temperature were hence the two components affecting the frost depth extent. The altitude soil frost variation was related to the spatial extent of these two components. The higher the altitude, the higher the precipitation and the lower the mean air/surface temperature. Simulating the whole Grächen recharge area, seasonal soil frost was rare between 1800 m and 2000 m, as the snowpack was thick enough to insulate the ground, and the soil was warm enough to inhibit deep soil frost. In lower areas, the shallow snowpack enabled the soil to freeze during most winters, whereas at higher areas, deep and persistent soil frost was simulated due to the low underneath heating. A 10-year water-table depth record was used to examine possible large-scale effects of seasonal soil frost. We noted that the water-table rise at snowmelt was lowest at the end of winters with extensive soil frost. However, such winters were characterized by low precipitation, and reduced snow cover. In contrast, winters with little soil frost were mostly snow rich. Consequently, only a reduced number of winters were directly comparable. Considering these winters, the reduction in the water-table rise at snowmelt due to seasonal soil frost varied only between 10 and 30%, as, during snowmelt, most meltwater was able to re-infiltrate into the very permeable ground in lower unfrozen areas. We may assume that a soil with a smaller infiltration capacity can increase the influence of the soil frost on the aquifer recharge. Finally, we investigated the effect of a changing climate on the hydraulic and thermic regimes at different altitudes in the area of Grächen. We assume an increase in the air temperature of 2°C generates deeper soil frost on lower areas, as the resultant smaller snow cover does not insulate the soil from the atmosphere any more. However, it does reduce the soil frost on higher, snow rich areas, due to the warmer air temperature. As a whole, minor changes were simulated on the mean groundwater recharge, as more/less meltwater was able to infiltrate in higher/lower areas. By increasing additionally the precipitation by 15%, the soil frost diminished slightly over the whole catchment. By combining both climate change scenarios, we may expect an increase in extreme events, as, on the one hand, the rainfall intensity increases, and, on the other hand, rain on snow events over frozen soils occurs more often in lower areas. In practice, this work shows the importance of better investigating the hydrothermal behaviour of alpine regions. Such a knowledge is necessary, as more and more efforts are put into managing efficiently, durably and globally the available water resources. This work stresses also the necessity to consider seasonally soil frost in alpine risk management. As an example, flooding is mostly a consequence of strong rain precipitation at high altitudes combined with heavy snowmelt. Obviously, the risk of extreme runoff events increases during frozen winters, when the soil infiltration capacity is further reduced by the soil ice. Also, it has been shown that unstable slopes are mostly related to specific hydrogeological situations, like strong precipitation, which induces a sharp increase in the underneath water circulation. The melt water in spring is hence a potential activating factor, and a frozen soil may reduce the risk of sliding, as less water infiltrates into the ground.

EPFL2003

Chargement

A new concept of road runoff management, based on diffuse infiltration over the shoulder, was tested in a real-scale experimentation site located near Grandson, Switzerland. This new concept consists in diffusely infiltrating the road runoff in the infiltration slope adjoining the road shoulder; where road contaminants are retained. This presupposed that the road shoulder was as tightened as possible to effectively drive the road runoff to the slope, and that the infiltration slope effectively retains all road pollutants. Those two main postulates needed strong scientific verification. For this purpose, five different shoulder designs were tested in Grandson to assess which one presented the best hydraulic performance, i.e. which shoulder was the most impervious. Those designs were made of gravel mixed with humus (SGL), gravel mixed with clay (SGC), gravel seeded with lawn (SGL), a prolonged HMF road base (SH), and a bentonitic geotextile tightening (SB). Also, to verify the retention of pollutant, two infiltration slopes adjoining SGL (LW) and SH (LH) were set as lysimeter (basal geomembrane collecting road effluents); the third infiltration slope (LB) had a direct connection with the aquifer which was closely monitored (6 piezometers located up- and downstream from the road; influence of the road runoff was thus emphasized). Finally, to ensure that this new concept do not lower the road bearing capacity, deflectometers were placed in the road structure; 3 campaigns of leveling assessed the road settlement. Hydraulic assessment of the shoulders, based on 112 natural precipitation as well as 3 artificial watering tests, showed strong discrepancies between all shoulders behaviours. They had to deal with potentially high amount of water. Lag times ranged from a few minutes to several hours. Results showed that lag times were only function of the precipitation mean intensity. The infiltration process could be assessed by a modified Green & Ampt equation. The moisturizing front was frank and linearly progressed downward. Shoulders vertical hydraulic conductivities ranged from 4.6·10-6 (SGC) to 2.2·10-5 m·s-1 (SGL). The exfiltration volume is function of the water volume available at the shoulder surface. The drought preceding the rain event has no influence on the stock variation or the exfiltration volume. Road runoff used two different infiltration paths: the macroporosity drives the water efficiently and rapidly, while the microporosity is less efficient and retains the moisture longer. Tracer tests (Brine) proved that the contamination first flush passed over the shoulders. This is also confirmed by the flux calculation. Runoff coefficients CR ranged from 0.3 (SGL) to 0.9 (SB). This coefficient decreased rapidly once the shoulder is wet. Only SB really fulfilled its task, letting water through only in harsh hydraulic conditions (100 mm; 40mm/h). It is highly recommended for further road development: for new roads as well as old roads being refurbished. The shoulder SH proved to be inefficient for structural and material reasons. Other shoulders are comparatively ineffective. SGC must be enhanced; in that case it could offer a valuable alternative. The bearing capacity of the road was very good. The road lifetime was evaluated to more than 20 years (1·109 residual standard axle load). The Grandson experimental site could not conclusively demonstrate a worsening of the geotechnical behaviour of the road. This is due to the particularly good material and road pavement thickness used for this road. Infiltrations in shoulders are high enough and would have doubtless caused a loss of bearing capacity in case the road would have been more modestly dimensioned. It is therefore clear that the shoulder must be tightened. Geochemical results concerned six families of contaminant: MTE, PAH, Cx, BTEX, MTBE, and PCB. Batch and column tests allowed calculating MTE and PAH distribution coefficient Kd in the Grandson soils, as well as in other typical Swiss soils. Results showed that mobile elements are B, Br, Mo, Ba, Sb, Zn, and V (0 < Kd < 90). Less mobile elements are Ni, Cd, Cr, and Rb (200 < Kd < 300). Elements with very low mobility are Pb, Mn, and Ti (1500 < Kd < 3500). PAH mobility was high for the light PAH (naphthalene, acenaphtene, fluorene, anthracene; Kd < 75). Heavy PAH had lower mobility (Kd > 175). Comparisons with other typical soils enlightened the predominant influence of the pH. Batch test performed with acidic soils (pH = 5.5) confirmed lower MTE Kd. MTE are thus more mobile in acidic soils. Other physicochemical parameters have a smaller influence. The column test performed with a synthetically loaded solution (MTE and PAH) infiltrating the Grandson soils showed that MTE are more mobile in dynamic conditions. This is easily explained by the reduced contact time between the contaminant and the soil aggregates. PAH were not detected at the column outlet, thus showing a very good retention in the column soil. Soil layers analyses demonstrated that PAH were mostly retained in the surface layers. Also, MTE with low mobility show the same behaviour. Mobile elements showed no preferential retention layer. Concerning the infiltration slope, lag times were usually greater than those encountered for the shoulders. Lag time were function of the rain mean intensity. The influence of the preceding drought was significant. Exfiltration volumes EL were high in case of large event; it could be approximated by a simple equation function of the API and surface available water. Moisture redistribution processes occurred in the soils as long as there is a volumetric water content θ gradient. Fluxes strongly varied in magnitude and direction during a precipitation event: usually from about 1·10-8 to 1·10-5 m·s-1 and from up- to downward (evaporation to infiltration). The first flush effect was poor to medium. NaCl signal was highly buffered. The pH was also completely buffered by the soil high carbonate content. It thus shifted from 6.5 in the rainwater to 8 in the LW exfiltration water. To especially emphasize the geochemical behaviour, an artificial precipitation test was created to control every aspect of the rainfall. It was judged very representative of a natural storm. MTE and PAH contaminants had two different behaviours: mobile elements moved mainly in solute and have concentration correlated with EC. Elements with low mobility had higher concentrations correlated with the turbidity peaks. They were transported sorbed to particles. Comparison between the particular and solute form concentrations confirmed that mobile elements are measured in similar concentrations whether the sample was filtered at 0.45 µm, at 1 µm, or not filtered at all prior to acidification. On the contrary, less mobile elements are easily sorbed to particles. The MTE concentrations under sorbed form range from 5% (Rb, Sb) to 98% (Pb) of the total concentration. They had strong first flush effects. B, V, Cr, Mn, Ni, CU, Zn, Br, Mo, Cd, Sb, and Pb were identified as road tracers. Fe and Al represented 80% of the road runoff. Overall, the total MTE concentration in the lysimeter first water collected decreased to 1/30th of the road runoff first flush concentration. Mobile elements were less retained. The PAH total concentration in the lysimeter first collected water was about 1'000 times lower than in the road runoff first flush. The PAH concentration was mainly constituted by mobile PAH naphthalene and phenanthrene. Cx were well represented in the road runoff. However, only heavy species were commonly found. C32 was the most common specie. The triplet C16-C18-C20 was always present in high concentration in the lysimeter exfiltration water. This triple is a great road tracer. BTEX, while present in the road runoff, were not detected in the lysimeter exfiltration water. The alluvial aquifer was also monitored. It is confined between the Arnon (north) and a basement till which shallows up southward. Eastern and western limit are less known but might coincide with the river. The aquifer is clearly linked to the river. This is proved by the piezometric level, EC and T follow-up. EC and temperature also emphasized the higher activity in downstream piezometer. The road runoff infiltration could not be emphasized by the NaCl tracer test. The infiltrated water concentration was possibly buffered by the aquifer volume. Also, the brine was heavier than the aquifer water: it could have plunged to the bottom of the aquifer. All families of contaminant are presented in the aquifer. However, the provenance of those contaminants remains uncertain. The groundwater was indeed as concentrated upstream as downstream. Moreover, the river concentrations had similar, or even higher, values. The contamination provenance is thus either the aerial deposition (equally distributed up- and downstream), either the Arnon River infiltrating the aquifer. The contaminant behaviours in the aquifer could be described. It confirmed the contaminant mobility previously noted during the batch, column and test precipitation experiments. Mobile substances are seen in higher concentrations, whereas substances with low mobility are sparsely found. This project thus demonstrated that the new "over the shoulder diffuse infiltration" concept is clearly implementable. Aquifer concentrations were compared to Swiss legal concentration limits. Except for Cr, the aquifer water was of drinkable quality. This proved that the concept is robust, environmental-friendly and conclusive.

EPFL2007

Chargement

Carbon, oxygen and nitrogen dynamics in a soil profile: Model development and application

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli

In order to meet demands for crops, pasture and firewood, the rate of land use change from forested to agricultural uses steadily increased over several decades, resulting in an increased release of nutrients towards groundwater and surface water bodies. In parallel, the degradation of riparian zones has diminished their capacity to provide critical ecosystem functions, such as the ability to control and buffer nutrient cycles. In recent years, however, the key environmental importance of natural, healthy ecosystems has been progressively recognized and restoration of degraded lands towards their former natural state has become an area of active research worldwide. Land use changes and restoration practices are known to affect both soil nutrient dynamics and their transport to neighboring areas. To this end, in order to interpret field experiments and elucidate the different mechanisms taking place, numerical tools are beneficial. Microbial decomposition is the main driving force on biogeochemical transformations of soil organic matter and soil nutrients. The activity of the soil biota is primarily controlled by water availability and by the pore-solution oxygen concentrations, which ultimately depend to a large extent on meteorological conditions, e.g., precipitation. In this work a model is presented that simulates carbon and nitrogen turnover and transport in a 1D profile under variably-saturated conditions. The model is based on the mechanistic batch model of Porporato et al. (Adv. Water Res., 26: 45-58, 2003), but extends its capabilities to simulate the vertical transport of the mobile components. Furthermore, oxygen dynamics are included such that the pore-water concentration is dependent on microbial degradation rates and soil moisture level. The model was applied to simulate the effect of land use change from forested to agricultural soils on pedo-fauna activity and nutrient distribution and abundance across the vertical profile. External forcing, i.e., precipitation time series, were generated stochastically. Modeling results showed that soil tillage practices, which modify soil structure and thus soil aeration, were responsible for higher decomposition and mineralization rates in agricultural soils, thus for the lower soil carbon and nitrogen concentrations. Furthermore, higher plant uptake rates and leaching of agricultural soils contributed also to lower nitrate concentrations in such soils.

2009

Chargement

The effect of seasonal soil frost on the alpine groundwater recharge including climate change aspects

Daniel Bayard

EPFL2003

EPFL2007

Carbon, oxygen and nitrogen dynamics in a soil profile: Model development and application

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli

2009