Road runoff over the shoulder diffuse infiltration

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.

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

The diffuse infiltration of road runoff: an environmental improvement

Michaël Bensimon, Aurèle Parriaux, Pascal Piguet

The Laboratory of Engineering and Environmental Geology (GEOLEP) has been mandated by Swiss authorities (Swiss Federal Road Office FedRO) to test a new road runoff management concept. This concept promotes the diffuse infiltration of road runoff into infiltration slopes designed for this purpose. Soils retain particles and contaminants; this lowers the road impact on the environment and simultaneously improves aquifer recharge. This concept has to be adapted to aquifer vulnerability and traffic conditions. Thus, a real-scale experimental station was designed and built in Switzerland to assess the feasibility of this new concept. This station allowed the testing of two lysimeters composed of 80cm of A and B- horizons. Water and chemical fluxes were measured at the lysimeter bases. Road runoff was sampled in a distinct collector. Infiltration of road runoff into the local aquifer was monitored thanks to six piezometers. Water quality and quantity were therefore measured at each step of the infiltration process. Results provided by 112 natural events showed that soil horizons accommodated road runoff flows. The least favourable conditions for contaminant retention are encountered during thunderstorms, when high concentrations of substances deposited on the road are remobilised within a short time and rapidly percolates through soil horizons. Thus, three artificial events were designed to mimic heavy thunderstorms. Concentrations measured in road runoff notably decreased after soil filtration. Substances with high distribution coefficients Kd (low mobility) had concentrations reduced to lower values (1/1000 to 1/10,000), while those with high mobility retained similar concentrations even after soil filtration. However, these mobile substances exhibited low concentrations in the underlying aquifer due to dilution. This innovative road runoff management concept can thus be readily implemented outside groundwater protection zones where aquifers are slightly vulnerable; it undoubtedly lowers the environmental impact of roads, does not endanger road integrity, and locally increases aquifer recharge.

2008

Typologie des eaux souterraines de la molasse entre Chambéry et Linz (France, Suisse, Allemagne, Autriche)

The geochemical typology of natural subsurface waters of the Molasse basin is part of the AQUITYP project, initiated by the Geology Laboratory (GEOLEP) of the Swiss Federal Institute of Technology in Lausanne (EPFL). This study provides an overview of the variation in space of the hydrochemical parameters, especially of trace elements, in order to determine the geological origin of their aquifers. Our data base is mainly made up of two observation networks: a subsurface water network and a deep groundwater network. Complete analyses of all samples have been executed using standard methods for anions and silicium, and high resolution inductively coupled mass spectrometry (HR-ICP-MS) for major cations and trace elements down to the detection limit of about 0.2 µg/l. Subsurface water network It is the main observation network of this investigation and comprises 112 water samples from different springs, collected all over the Molasse basin between Chambéry (France) and Linz (Austria), but mainly in the Swiss Molasse basin. For this region, four groups are generally distinguished, according to the sediment deposition in a continental or marine environment, in ascending stratigraphic order: (1) the Lower Marine Molasse (UMM and North Helvetic "flysch" p.p.), (2) the Lower Freshwater Molasse (USM), (3) the Upper Marine Molasse (OMM) and (4) the Upper Freshwater Molasse (OSM). Our water samples originate from typical sedimentary deposits which we divided in about 30 different sub-type aquifers according to their geological origin and their depositional environment. They are defined on the basis of lithological and structural homogeneity. Therefore, only catchments that completely drain a single, lithologically homogenous, Molasse terrain unit are considered here. Spring catchments selected were generally not affected by runoff from towns, or paved roads. Agricultural activities have been avoided as well as possible. Such spring water should be free of major human influences and only in few cases contain a certain amount of fertilizer products (p.ex: nitrates). Leachate tests of crushed rocks and comparison with snow and soil water compositions allowed to decipher the geogene origin of the dissolved ions in natural groundwaters. The water withdrawn can be generally classified as a calcium-magnesium-hydrogenocarbonate (Ca-Mg-HCO3)-type water. The trace analyses allowed to distinguish several subtypes within this homogenous water type group. The general spacial distribution of the trace elements show, that spring waters in the Bavarian Molasse basin are caracterized by elements as lithium and uranium linked to acid rocks, while those of the Swiss Molasse basin are preferentially influenced by elements derived from mafic or ultramafic rocks, such as chromium and cobalt. The following elements show specific links between aquifer rock type and groundwater. Sulfates: Total mineralisation of spring waters issued from the gypse-bearing Molasse (USM) and the "Glimmersande" (OSM) are highly influenced by high sulfate contents. It can be demonstrated that sulfate ions are produced from two different sources within this two sub-type aquifers: from the dissolution of gypsum in water, producing high calcium contents as well, and from the oxydation of sulfurous ore minerals, especially pyrite, which are abundant in the "Glimmersande". The "Glimmersand" aquifers in NE Switzerland are furtheron caracterized by high lithium, molybdenum and uranium concentrations, reflecting the granitic composition of this sandstones. High barium contents (>150 µg/l) have been found in spring waters issued from aquifers with a dominant fissure permeability ("Grès de la Cornalle", "Gibloux-Delta" and "Rigi-Schüttung"). Its origin is supposed to be the mineral barite, which tends to precipitate in fractures. Therefore, high barium concentrations may indicate the presence of fracture flow within the aquifer. Considerably high silicon contents are linked to the quartzitic and fine grained aquifers of the brackish marine molasse in Bavaria. Both, petrographical composition and slow flow rate are favouring the water-rock interaction. Caracteristic lithium footprints are found in several waters, as in the aquifers composed of the "Glimmersande", the gypse-bearing molasse, the Rigi-conglomerates, the brackish and the estuarine marine molasse. Chromium strongly marks the waters originating from the Swiss marine Molasse aquifers. It is probably related to ophiolithic detritus derived from the flysch of the Prealpes. A couple of waters show weak anomalies of cobalt, which are nevertheless intimately linked to the presence of ophiolithic components in the sediments. Cobalt might be used as a hydrostratigraphical tool to identify greenstone inputs. Analysed colloidal aluminum may serve as a hydrodynamical parameter, indicating microturbidity within the aquifer. High amounts of aluminum varying in time, have been found in estuarin deposits of the OMM. Generally speaking, we propose three main origins of trace elements in the spring water of the Molasse basin: presence of certain petrographical components within the sediments such as basic and ultrabasic rocks, heavy minerals and evaporites. a small flow rate through interstitial porosity of fine grained sediments, favouring water-rock interaction. the abundance of colloidal material in the waters which depends on the petrographical composition of the aquifer and on sample treatment, e.g. filtration techniques. Deep groundwater network A second observation network composed of 21 borehole waters has been added for comparison. Compared to the subsurface waters, deep water chemistry might differ. The following reasons seem to be responsible: Long residence times of water in the underground favours slow reaction kinetics. Chemistry of very deep water can be influenced by mixing with older formation waters. Most of the deep boreholes in the Swiss Molasse basin are situated in the OMM and therefore represent a likely homogenous lithology. Borehole waters, because of different aquifer rock types and unknown surface and borehole influences, are least likely to represent typical Molasse water composition.

EPFL1995