Thermal Effects on Flow and Salinity Distributions in Coastal Confined Aquifers

Coastal aquifers provide an important hydrologic connection between terrestrial freshwater and oceanic seawater. Understanding the water flow and salinity distributions in those aquifers is essential to manage seawater intrusion and regional groundwater resources. For the last 50 yr, these processes have been extensively examined; however, previous studies typically assumed isothermal conditions and overlooked groundwater-seawater temperature contrasts, as commonly found along the global coastline. Here, we validated a solute and heat transport model against laboratory experiments and then revisited the classical Henry problem for coastal confined aquifers. We found that as the seawater temperature increases or freshwater temperature decreases, the freshwater-seawater interface retreats seaward and thus the freshwater storage increases. This occurs as a result of temperature-induced changes to the fluid density and aquifer hydraulic conductivity. Furthermore, double diffusion of salt and heat can induce two circulation cells in opposing directions in the saltwater wedge once the seawater temperature exceeds that of groundwater by a certain extent. Consequently, the circulating seawater flux and associated travel time change dramatically compared with isotheral conditions. Overall, the effect of thermal forcing can be significant, as demonstrated by a sensitivity analysis considering a large range of groundwater-seawater temperature contrasts. These results highlight the impact of changing thermal regimes on the flow and salinity distributions in coastal confined aquifers and provide guidance for better assessment of water resources in coastal zones.

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

Groundwater flow and salt transport in a subterranean estuary driven by intensified wave conditions

David Andrew Barry, Clare Robinson

A numerical study, based on a density-dependent variably saturated groundwater flow model, was conducted to investigate flow and salt transport in a near-shore aquifer under intensified wave conditions caused by offshore storms. Temporally varying onshore hydraulic gradients due to wave set-up were determined as the seaward boundary condition for the simulated aquifer. The results showed a rapid increase in influxes across the aquifer-ocean interface in response to the wave event followed by a more gradual increase in effluxes. The upper saline plume first widened horizontally as the wave set-up point moved landward. It then expanded vertically with recirculating seawater pushed downwards by the wave-induced hydraulic gradient. The time for the salt distribution to return to the pre-storm condition was up to a hundred days, and correlated strongly with the time for seawater to recirculate through the aquifer. The pathways of recirculating seawater and fresh groundwater were largely modified by the wave event. These pathways crossed through the same spatial locations at similar times, indicating significant salt/freshwater mixing. The flow and salt transport dynamics were more responsive to wave events of longer duration and higher intensity, especially in more permeable aquifers with lower fresh groundwater discharge. Despite their larger response, aquifers with higher permeability and beach slope recovered more rapidly post-event. The rapid recovery of the flows compared with the salinity distribution should be considered in field data interpretation. Due to their long-lasting impact, wave events may significantly influence the geochemical conditions and the fate of chemicals in a subterranean estuary.

American Geophysical Union2014

Typology of recent groundwaters from different aquifer environments based on geogenic tracer elements

A key problem in environmental science and in engineering geology is the often incomplete understanding of the origin of dissolved components in groundwater. The dissolved contents of trace elements in groundwater are of special importance for groundwater quality control. The AQUITYP project aims to establish a detailed typology of recent groundwaters based on their geogenic trace element compositions, and to derive a so-called "geo-reference" for groundwaters from five principal aquifer lithologies in the Alpine belt. This geo-reference provides a database for investigations related to groundwater contamination, groundwater resources management and engineering geology. Groundwaters from crystalline, carbonate, and evaporite rocks, as well as molasse and flysch sediments in Switzerland and in neighbouring countries were sampled and documented by previous researchers (Dubois, 1993; Dematteis, 1995; Mandia, 1993; Hesske, 1995; Basabe, 1993). Based on a statistical analysis of the data and examination of the relationship between aquifer lithology and chemical groundwater composition these researchers identified a number of characteristic tracer elements (geogenic tracers). The present study includes (1) a synthesis of the hydrogeology and the hydrochemistry of recent groundwaters in these five aquifer types based on groundwater data acquired within the AQUITYP project, and (2) a hydro-geochemical investigation of the origin and chemical behaviour of the geogenic tracer chromium, based on a comprehensive field and laboratory study. To enable a comparison of the chemical groundwater data gathered and analysed over a time span of 20 years, a rigorous quality control of the entire database was made. An assessment was made of the sampling techniques, the sample treatment and the analytical methods applied in the AQUITYP project since 1981. Different tests were carried out to evaluate the quality and comparability of the data, including geochemical model calculations, comparison of different analytical techniques, and tests to estimate the influence of the filtration procedure. In order to make this large number of quality controlled data accessible also for future investigations, a groundwater data storage system was developed (AQUITYP-DataBase). 1.) Typology of recent groundwaters In the synthesis of groundwater hydrochemistry, the emphasis was laid on the hydro-geochemical evolution leading to the characteristic groundwater composition in each of the five aquifer types. Chemical characteristics and differences between the groundwaters from the different aquifer types were identified and natural concentration ranges for each aquifer type derived. The proposed geogenic tracers were evaluated and the potential sources of these tracers identified. The dominant processes leading to the typical mineralisation of recent groundwaters were investigated using geochemical modelling strategies. Finally, the concentrations of chemical elements in the groundwaters from the different aquifer types were compared to the Swiss drinking water standards in order to assess the quality of the investigated groundwaters. It has been found that each rock type contributes in a characteristic way to the major and trace element composition of the corresponding groundwater: The groundwaters derived from the crystalline Mont-Blanc and Aiguilles-Rouges massifs are characterised by a low total mineralisation (TDS 22 to 158 mg/L) dominated by Ca2+, Na+, Mg2+, alkalinity, SO42-, and F- (Ca-Na-HCO3,-SO4, waters). Elevated amounts of Mo, U, W, and As occur. These groundwaters derive their mineralisation mainly from the interaction with hydrothermal minerals present along fractures. Fractures act as major groundwater flow paths. Minerals relevant for groundwater mineralisation include carbonates (Ca2+, Mg2+, HCO3-), clay minerals (Ca-Na ion exchange), fluorite (F-, Ca2+), Fe-, As-, and Mo-sulphides (SO42-, As, Mo), and U- and W-minerals (U, W). In these crystalline groundwaters the natural concentrations of F- (23% of the investigated springs) and As (7%) exceed the Swiss limits for drinking water. In addition, the WHO guideline values for U are exceeded in 65% of the cases and for Mo in 15% of the cases. The carbonate karst groundwaters obtain their low to intermediate mineralisation (TDS 161 to 547 mg/L) from the dissolution of calcite (Ca-HCO3, waters), as well as in certain regions dolomite (Ca-Mg-HCO3, waters) and gypsum (Ca-Mg-HCO3,-SO4, waters). Together with their very short residence times, the carbonate karst groundwaters generally contain very low trace element contents. Nevertheless, geogenic trace elements occur in specific regions in relation with fossil organic matter (I, V) and accessory minerals such as barite in deep sea limestones (Ba), evaporite minerals (gypsum, celestite: Sr, Li), clay- and Fe-minerals (V), and Mo-sulphides and U-minerals in dolomitic limestones (Mo, U). In 18% of the carbonate karst springs atmospheric derived Pb exceeds the Swiss drinking water quality target value. The groundwaters from Triassic evaporites in the Swiss Rhone basin are characterised by a high total mineralisation (TDS 760 to 2788 mg/L) expressed by elevated amounts of Ca2+, Mg2+, Sr2+, SO42-, and alkalinity (Ca-Mg-SO4-HCO3 waters). Elevated amounts of the trace elements Mn, Ni, Cu, Li, Rb, Y, and Cd occur. The hydrochemical evolution of these groundwaters is governed by incipient dedolomitisation, involving dissolution of gypsum, celestite and dolomite and simultaneous precipitation of calcite. The Na+ and K+ contents probably controlled by ion-exchange reactions on clay minerals. Characteristic trace elements originate mainly from the dissolution of dolomite (Mn, Ni) and small amounts of apatite (Y, Cd), and from the oxidation of sulphide minerals (Cu, Ni, Cd). The elevated concentrations of the highly soluble Li and Rb may be related to brine inclusions in evaporite minerals and eventually to clay minerals. These evaporite groundwaters contain SO42- concentrations exceeding the Swiss quality target for drinking water in all springs, and the concentrations of U and Ni exceed the WHO guideline values in respectively 58% and 2% of the cases. In addition, Mn, Cd, and As concentrations exceed the Swiss quality targets in respectively 11%, 10%, and 7% of the investigated springs. Recent groundwaters circulating in the porous and fissured molasse sandstones and conglomerates acquire their intermediate mineralisation (TDS 48 to 714 mg/L) primarily by dissolution of calcite and minor dolomite (Ca-Mg-HCO3 waters). The particular mineralogy of certain Molasse formations is reflected in specific trace element compositions of corresponding groundwaters: ophiolite detritus in OMM sandstones in western Switzerland (Cr), barite fracture mineralisations in subalpine and folded Molasse units (Ba), granitic detritus containing sulphides (Mo), U-minerals (U) and abundant mica (Li) in the "Glimmersand" (OSM, Ca-Mg-HCO3-SO4 waters), and evaporite minerals (Li, Sr), sulphides (Mo), and U-minerals (U) in the "Gypsum-bearing Molasse" (USM, highly mineralised Ca-Mg-SO4-HCO3 waters, TDS 984 to 1346 mg/L) . In these molasse groundwaters the Swiss quality target values for drinking water of Cr and Pb are exceeded in 36% and 6% of the springs, respectively. The U concentrations of 14% of the molasse groundwaters ("Gypsum-bearing Molasse" and "Glimmersand"), exceed the WHO guideline value. The groundwaters from the "Gypsum-bearing Molasse" display similar quality problems as the evaporite groundwaters. Groundwaters derived from the shallow flysch aquifers in the Niesen and Gurnigel nappes are poorly evolved Ca-(Mg)-HCO3 waters. Their low to intermediate total mineralisation (TDS 160 to 459 mg/L) is acquired primarily by dissolution of calcite and to a lesser degree dolomite. The low trace element content is dominated by Ba originating from barite fracture mineralisations. The poor chemical evolution of recent flysch groundwaters results from (1) their short residence time in the fractured flysch rocks, and (2) the absence of readily dissolving minerals except carbonates and barite. In 32% of the flysch springs Pb derived from atmospheric sources exceeds the Swiss quality target value. 2.) Case study on the chemical weathering of molasse sandstone: Sources and chemical behaviour of Cr For the characterisation of the potentially toxic tracer element Cr a comprehensive field study was carried out on a selected catchment (Lutry spring catchment near Lausanne) situated in a molasse sandstone (OMM, Burdigalian). The investigation of the processes controlling the dissolved Cr content in these groundwaters was based on the groundwater chemical data, as well as on mineralogical, geochemical, and hydrological data. Relic Cr-bearing spinel and pyroxene in the sandstone were identified to be the primary sources of Cr. An electron microscope study showed that in the Burdigalian sandstone and overlying soil these minerals are strongly weathered. The slow weathering of these minerals is the major Cr releasing process. Under the oxidising conditions reigning in the investigated groundwaters, Cr prevails in solution in its highly soluble and toxic hexavalent state (CrO42-). In this case, retention by secondary Cr-hydroxide phases does not occur, as can be shown by geochemical model calculations. Laboratory leaching experiments were carried out with Burdigalian molasse sandstone from the field site, in order to support the field study findings and to quantify the processes responsible for the Cr mineralisation observed in the Lutry groundwater. Two experiments with mountain-wet oxidised and reduced sandstone were carried out over a time span of 2 months each, to obtain information about the influence of the oxidation state of the substratum on the mobilisation of Cr. The experiments clearly showed that the Cr-releasing processes are fast enough to explain the Cr contents found in the groundwater, and that the release of Cr into the groundwater depends on the weathering state of the sandstone. In the oxidised, Fe-hydroxide-coated sandstone, Cr is faster released into solution than in the less altered reduced sandstone. This indicates that the Cr contained in the reduced sandstone is in a more stable state, i.e. mainly incorporated in detrital minerals, while in the oxidised sandstone, the Cr is partly in an unstable state, i.e. adsorbed on the surface of secondary Fe-hydroxides, from where it is more easily leached.

EPFL2001

Typology of recent groundwaters from different aquifer environments based on geogenic tracer elements

EPFL2001

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

Daniel Bayard

EPFL2003