Depuration of highway runoff water into grass-covered embankments

The management of polluted road runoff water is an important issue in environmental protection. A strategy could be to perform local depuration by infiltration into the soils of the embankment, but knowledge for designing such systems is lacking. This study aims at discussing the relevant soil properties, by estimating the long-term. depuration of road runoff water infiltrating into the sandy soil embankment of the A9 highway in Wallis, Switzerland. This was done by estimating the heavy metals (HM) mass balance of two sites 23 and 12 years old, respectively. The accumulated HM were estimated by soil and GB analyses. The HM input was estimated by average water quality and traffic. The results were discussed using two-dimensional simulation of infiltration and a 14 months in situ monitoring of the runoff from the pavement to the embankment and at the bottom of the embankment. The soil properties were appropriate for both small particle adsorption and filtration. A good match between input and stored pollutant charges was found, and the HM profiles accorded well with infiltration simulation and monitoring results, which showed that 80-100% of the runoff water infiltrated into the embankment. Replacement of the cracked concrete gutters by an infiltration channel made of similar soil is recommended. These results oppose the Swiss guidelines for road-polluted water infiltration, as much more clayey soils are recommended. These later soils are difficult to find in Switzerland, and may allow for preferential flow through macro pores, in contrast to the studied site.

Road runoff over the shoulder diffuse infiltration

Pascal Piguet

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

Assessment of microbial community changes and limiting factors during bioremediation of hydrocarbon-polluted soil with new miniaturized physiological methods

Karin Kaufmann

Due to human activities, organic pollutants are spilled to the environment where they threaten public health, often as contaminants of soil or groundwater. Living organisms are able to transform or mineralize many organic pollutants, and bioremediation techniques have been developed to remove pollutants from a contaminated site. However, fast and easy methods to document both the efficacy of bioremediation and the changes in soil microbial communities during bioremediation are not well developed. The major aim of this thesis was to develop miniaturized methods targeting the physiology of microbial communities during pollutant degradation in soil and to assess pollution-induced community changes. Furthermore, the methods should identify factors limiting efficient pollutant biodegradation. Petroleum hydrocarbons have been chosen as pollutants because these compounds are frequently spilled, readily water-soluble, partly toxic and carcinogenic, and therefore undesirable in soils and drinking water supplies. The influence of petroleum hydrocarbons on a microbial community in the vadose zone was assessed under field conditions. An artificial hydrocarbon mixture consisting of volatile and semi-volatile compounds similar to jet-fuel was emplaced in a previously uncontaminated vadose zone in nutrient-poor glacial melt water sand. The experiment included monitoring of microbial parameters and CO2 concentrations in soil gas over 3 months, in- and outside the hydrocarbon vapor plume that formed around the buried petroleum. Microbial and chemical analyses of vadose zone samples were performed on material from 9 cores drilled on 3 days at 3 distances from the buried petroleum mass to 3.3 m depth. Significantly elevated CO2 concentrations were observed after contamination. Total cell numbers as determined by fluorescence microscopy were strongly correlated with soil organic carbon and nitrogen content but varied little with contamination. Redundancy analysis (RDA) allowed direct analysis of effects of selected environmental variables or the artificial contamination on microbiological parameters. Variation in biomass and CO2 production was to 46 % explained by soil parameters and to 39.8 % by the duration of contamination. The microbial community structure was assessed by community-level physiological profile analysis using BiologTM EcoPlates. Only 35.9 % of variation in BiologTM data could be attributed to soil parameters and contamination, however, the samples with greatest exposure to hydrocarbons grouped together on RDA plots. It is concluded that at this nutrient-poor site, the microbial community was dominated by the natural heterogeneity and that the influence of petroleum hydrocarbon vapor was weak. BiologTM Eco-Plates combined with RDA were able to distinguish between samples with high and low hydrocarbon exposure. However, the method was not sensitive enough to produce consistent patterns in three replicates when extracts from subsurface soils with low cell numbers were inoculated. A more sensitive, MPN-based system was therefore developed and its ability to provide information about specific pollutant degraders was tested in alluvial sand. Eight dilutions of sand extracts were inoculated on medium- and substrate-containing polypropylene deep-well plates. To keep the concentrations of the volatile substrates constant over several days of incubation without intoxicating the soil community, a substrate-containing organic carrier phase was applied to each vial. The biochemically inert 2,2,4,4,6,8,8-heptamethylnonane (HMN) was appropriate for this purpose. Thus, the volatile substrates partition from HMN into the microorganism-containing soil water where they achieve relatively low concentrations. The sensitive watersoluble tetrazolium dye (WST-1) was added to each well after 7 days incubation to detect dehydrogenase activity. A dark yellow signal was developed for the substrates toluene, n-octane, n-dodecane, 1,2,4-trimethylbenzene and methylcyclohexane but not for isooctane, being strongest in n-alkane amended wells. With this method, more hydrocarbon degrading bacteria were detected in an alluvial sand exposed to kerosene for 72 hours compared to the community in the pristine sand. Both communities were mainly composed of n-alkane degraders. Although encouraging results were achieved, we noticed that the tetrazolium reduction was inhibited in some cases. Wells with high cell numbers, in which obvious growth occurred, did not produce a signal. In a next step, the two-liquid phase system with HMN was applied to modify a miniaturized respiration detection system using whole soil samples instead of soil extracts (Campbell et al., AEM 69, p. 3593). Soil was incubated in deep-well plates in presence of single volatile organic hydrocarbons. The carrier phase was added to the interstitial space in a bead layer on the bottom of the well (below the soil sample), and the hydrocarbons diffused into the soil-water. The soil activity was determined by means of an agar plate containing a pH indicator, which changes color as a function of the produced CO2. Physiological profiles specific to petroleum hydrocarbons in pristine and contaminated soils were assessed. The substrate concentrations, which induced highest microbial activity, as well as concentrations causing inhibition, were determined with effect-concentration curves. Community-level physiological profiles based on hydrocarbon degradation were obtained by applying multivariate analysis on the CO2 yields. The first measurement period of 0-6 hours after addition of relatively high hydrocarbon concentrations to the system yields values, which best separated soil types as well as pristine and contaminated soils. If low concentrations were applied, we obtained only significant separations after 24 hours incubation, probably after a growth step. The same micro-respiration system equipped with a 14CO2 detection plate was applied to study N and P limitation of aerobic mineralization of 14C-toluene in a soil previously exposed to petroleum. Significant nutrient limitation during short-term tests (24-48 hours) was identified and the optimized combination of nutrient addition was determined. Conclusively, miniaturized physiological methods have been proven to detect soil microbial community changes in petroleum hydrocarbon-contaminated soils. Adding an organic carrier phase to the miniaturized systems allowed assuring a volatile organic hydrocarbon supply in non-toxic concentrations in the soil water or the medium for several days. Thus simple, cost-effective systems to investigate the soil community during bioremediation and to address questions of nutrient shortage in soils have been provided with this thesis.

EPFL2004

Fonctionnement hydrique de différents types de placages sableux dans le sahel burkinabè

Dial Niang

The study was performed in the Sahelian part of Burkina Faso. The country is subject to difficult climatic conditions, a strong demographic growth and a continuous decrease in soil fertility. The resulting degradation affects the processes which govern ecological systems and leads, on the long run, to a modification of the ecosystems. This is particularly the case of the eolian formations, the only ecological units susceptible of being useful in the area, since they present good infiltration capacity and support the main part of the vegetation. Field measurements (rainfall, runoff, saturated hydraulic conductivity, bulk density, hydraulic conductivity function, soil water content and pressure head) have been done to asses the impacts of the environmental degradation on the hydrodynamic behavior of these formations. Within this context, an experimental device made of seven measurement sites has been installed in three contrasting zones. The results show significant differences in the hydrodynamic behavior of the soils, according to their surface properties. On one hand, the sites located on an erosion crust as well as on a drying crust in transition towards an erosion crust, are characterized by a low infiltration capacity which favors runoff. The quantity of infiltrated water is very low when compared to total precipitation. Water is stored near the soil surface (in the first 30 cm) favoring evaporation in the days following rainfall events. Thus, little water is available for plants. The resulting water stress causes a weak density, or even complete absence of plants which have difficulties to colonize such surfaces. Surface runoff is favored with runoff coefficients ranging from 50 to 80 %. On the other hand, the sites situated on drying crusts are characterized by good hydraulic conductivity and infiltration capacity. The storage of water in the root zone is more important. Water flows deeper into the soil and, occasionally, drains at the reference depth of 50 cm. The runoff coefficients were found to be lower than in the other sites (30 to 40 %). This differentiated behavior is one of the main factors affecting the formation of different landscapes in the area through the existence of a relationship between soil surface properties, the type of landscape and the ratio runoff/infiltration. The sites located on an erosion crust and those in transition towards an erosion crust behave like impluviums, while those situated on drying crusts are more favorable to infiltration. Observation of the evolution of the hydraulic properties of the soil surface shows a progressive alteration that frequently leads to strongly degraded mediums unfavorable to plants. Control and rehabilitation methods considered in the study, namely "restoration" and "mise en défens" have only partial and time-limited effects; they do not make it possible to stop durably degradation or to favor rehabilitation. Among the other measures assessed on the INERA plots, only the restitution to the soil of the beforehand mown plants causes an improvement of the soil surface properties; the restitution causes the formation of an important macroporal system favoring water infiltration and circulation in the soil. The effects of those different measures occur mainly in the first years after their introduction and concern especially vegetation regeneration, soil protection against degradation factors, improvement of hydrodynamic soil properties, reduction of runoff and erosion. They make it possible to slow down soil degradation, but not to stop it definitively. Within this context, proposals of further studies focussed on other measures to control and rehabilitate degraded areas have been suggested, taking into account the specificity of the sahelian zone, the endogenous knowledge of the agro-pastors, as well as socio-economic and ecological conditions.

EPFL2006