J. Novotny ¹, M. Kohut ², M. Mozny ², V. Herbstova ¹

¹ Faculty of Science, Charles University of Prague, Czech Republic, SG-Geotechnika,a.s., Czech Republic.

(e-mail: novotny@geotechnika.cz,Vladislava.Herbstova@seznam.cz)

² Czech Hydrometeorological Institute, Czech Republic. (e-mail: mojmir.kohut@chmi.cz)

Abstract:  On the example of a model locality built by Cretaceous mudstones of low permeability, specific properties of the hydrogeological conditions of the saturated and unsaturated zone of these rocks are presented in relation to climate. The assessment of mutual relations between the climate and behaviour of the saturated and unsaturated zone is based on the results of field measurements. The field data obtained during several years of monitoring of the water conditions in the saturated and unsaturated zone are confronted with the measured and calculated climatic characteristics. The results of several years of intense observations are confronted with long-term conditions.

Key words: mudstone, precipitation, evapotranspiration, suction, piezometers, wells , monitoring, water stor

 

Introduction

To understand  the climatic impact to the water conditions of the rock environment is of basic importance for various fields of human activity. It is of use, for example, in agriculture in relation to the solution of irrigation problems, in hydrology when determining the run-off volume during the assessment of risks of the climatic impact on the evolution of floods in a given terrain, in hydrogeology to assess conditions leading to enrichment of groundwater aquifer in given rocks and in engineering geology and geotechnics with respect to stability conditions of water saturated slopes. 

The model locality of a landslide area is at Trebenice in the north-western region of the Czech Republic; it is one of model localities in the country, where the water conditions of the saturated and unsaturated zone have been monitored in relation to climate. This locality was monitored due to vast landslide motions in Cretaceous mudstones that repeatedly interrupted the railway traffic leading through this territory. The purpose of this long-term monitoring was to study the relation between the climate, hydrogeological conditions and the activity of landslides. In the framework of this long-term programme, the relation of water conditions between the shallow aquifer and unsaturated zone has been studied in this model locality of the landslide area at Trebenice in the context of climatic conditions. Some results of this monitoring are given in this article for the 1982 - 2005 period.

Conditions of the Model Locality

The rock massif of the model locality of the landslide area at Trebenice is formed by Cretaceous calcareous mudstones. Permeability of the shallow aquifer in the subsurface zone of the rock massif was determined in this locality by slug tests, carried out on the tips of the installed piezometers of the Casagrande type at an interval of depths 2.5-10 m. The hydraulic conductivity values, determined on the basis of hydrodynamic tests according to Hvorslev (1951), are generally in the interval between nx10^-10 to nx10^-8 m/s (Novotny 2005). The results of the performed tests (Fig. 1) indicate vertical anisotropy of the shallow aquifer.

In depth 2.5 to 6 m below ground level, where highly weathered up to decomposed mudstones were found with a limited occurrence of cracks, hydraulic conductivity was generally found in the interval between 4×10^-10 to 6×10^-9 m/s. In weathered mudstones containing a system of subhorizontal and subvertical cracks (Fig. 2),  in a depth of 6 to 10 m below ground level, increased permeability was determined by the field tests, in comparison with the overlying decomposed or highly weathered zone, with hydraulic conductivity reaching 4 x10^-8 m/s.

From the depths of more than 10 m below ground level, where weathered up to sound rock was found by drilling, the frequency of the system of subhorizontal and subvertical cracks decreases significantly, which also decreases the hydraulic conductivity. The permeability in the weathered and sound base is related to the local occurrence of tectonic failures along the more significant tectonic lines. The above-indicated anisotropic distribution of the permeability of the shallow aquifer can be related to the frequency of incidence of the system of subhorizontal and subvertical cracks that were formed during quaternary denudation as the result of unloading the overconsolidated massif, when up to several hundred meters of overburden rocks were removed at the site of the model locality. The determined vertical anisotropy of the hydraulic conductivity is typical for sedimentary rocks and was documented exceptionally also in other formations (Marechal et al. 2003, Cho et al. 2003). On the surface of the terrain of the model locality, the rate of infiltration was determined by the field and laboratory experiments. Depending on the degree of the soil saturation with water, its value is in the interval between n×10^-6 and n ×10^-4m/s.

Instrumentation of the model locality for the field measurement of the water and climate conditions:

1. The water conditions in the saturated zone

The water conditions in the saturated zone of the model locality of the landslide area have been monitored since 1982 by monitoring of the fluctuations of the groundwater level in the network of hydrogeologic observation wells of a depth 8-15 m. Positive pore pressures have been monitored in the saturated zone since 1994 by measuring piezometric levels in the installed piezometers of the Casagrande type in the periodical or quasicontinual readings. The tips of the piezometers are located in various depths from 2.7 to 10 m below ground level.

2.The water conditions in the unsaturated zone

The water conditions in the unsaturated zone have been monitored since 2003 by means of tensiometers, measuring instruments using the direct contact method for measuring matric suction (see for example Fredlund & Rahardjo 1993). Matric suction in relation to the mechanical behaviour of problematic soils has also been measured in stock-piles (Herbstova & Bohac 2005). The tensiometer tips are installed in the unsaturated zone in a depth of 1 m and 2 m below ground level. When suction is measured, the groundwater level  is approximately 4-5 m below ground level at the site of the installation of tensiometers.

3.The climatic conditions

The climatic conditions have been monitored directly in the Trebenice model locality since April 2005, by means of an automatic climatic station that serves for measuring meteorological elements necessary for the calculations of potential, reference and actual evapotranspiration. All the climatic data that are necessary for the climate long-term analysis within the submitted study, were taken over from the nearby Doksany agrometeorological station.

4.Climatic data

The daily values of the evapotranspiration of a standard grass surface were calculated in a daily step for the 1982-2005 period by using the climatic data from the Doksany station. These values were used together with the total precipitation for the determination basic water balance of the pertinent area.

Under the basic water balance, we understand the mutual difference between the total precipitation water and the evapotranspiration of the grass surface or a hypothetical surface that is close to the grass surface (Roznovsky & Kohut 2004). This balance is calculated as a reference balance in accordance with the worldwide recommended FAO methodology (Allen et al. 1998) or a potential balance (Thompson et al., 1981, Hough et al. 1997). In both cases, this is the application of the modified Penman-Monteith method. In this approach, potential water balance is involved because the potential, but also the reference evapotranspiration represents the maximally or almost maximally possible evapotranspiration. However, such water balance does not express the real volume of water in the area, but serves as a suitable indicator for mutual comparison of monitored locations or observation years. The positive values of the water balance mean the excess, the negative values mean the deficit of precipitation.

Results

Saturated zone

Time interval 1982-2005

The course of the fluctuation of the groundwater level in the model locality is indicated in Fig. 3B as an average from 10 monitored hydrogeological wells in the 1982-2005 period. Pronounced oscillations are obvious in the groundwater level that fluctuates not only seasonally, but also in a longer interval of several years. Periods of several years of rising groundwater level appear in general, when the enrichment of the shallow aquifer prevails over the groundwater run-off or conversely, several years of groundwater lowering prevails in general, when the groundwater run-off exceeds the enrichment of the groundwater aquifer. These oscillations are in accordance with the general climate changes (Fig. 3AB). The last period of the significant culmination of groundwater levels was 2002.

Time interval 2003-2005

The period from 2003 until 2005 was evaluated in more detail (Fig. 4). In this period, gradual groundwater lowering is obvious. Significant lowering was recorded primarily in the course of 2003. The lowering was smaller in 2004, 2005 is typical by the year-round stagnation of the groundwater level at approximately the same level. It is obvious from the comparison of Fig. 3 with Fig. 4 that the groundwater aquifer lost water from the condition of the long-term maximum to the long-term minimum.

The maximum-recorded groundwater lowering during 2003-2005 was 6 m in some monitored hydrogeological wells; on the other hand, part of the wells exhibited mild lowering of about 1 m. In general, higher oscillations of positive pore pressures were obvious in piezometers with tips installed in depths 3-5 m below ground level, when the pore pressure decreased in a difference up to 40 kPa. Deeper piezometers with tips 7-10 m below ground level generally recorded a pressure difference of 10 kPa during general lowering.

Fig. 4 The course of the water balance (A), suction (B,C), average groundwater level and pore pressures (C) in the 2003-2005 period

Unsaturated zone

    Tensiometers with tips 1 m below ground level exhibit significant oscillations of the matric suction (Fig. 4B). The largest suctions were measured in a depth of 1 m below ground level from the summer to autumn seasons, when they reach  80 to 90 kPa. The value of the suction pressure of -90 kPa is the limit of the measuring capacity of tensiometers. At higher suctions, there is the effect of cavitation of the water column in the measuring instrument (see for example Fredlund & Rahardjo 1993, Herbstova & Bohac 2005). The real matric suction may therefore be higher in the summer and autumn season, however, it is not measurable with this type of tensiometers. A sudden drop of suction to zero value occurred in January to February 2004 and 2005 within 1-2 months. In April (May), the suction started to increased from zero value.

In comparison with tensiometers with the tips at a level of 1 m below ground level, lower  suction pressures were recorded with the tips at a level of 2 m below ground level (Fig.4C). The maximum suction was recorded in the winter months. At the turn of 2003/2004, matric suction pressures of -25 to -45 kPa were measured, at the turn of 2004/2005, suction pressures of -55 to -70 kPa.

The suction does not decrease suddenly as in the zone 2 m under the terrain. The suction decreases measured in May. In this period, the suction pressures gradually from a maximum to minimum during a period of 4-5 months. The minimum suction was  decrease to -10 up to -20 kPa; contrary to the 1 m zone below ground level, they do not decrease to zero. In a depth of 2 m below ground level, there is a trend of suction growth in the 2003-2005 period that is related to the deficit trend in the water balance in this period (Fig. 3A). In the records of tensiometers with the tips 2 m below ground level, the time delay of the suction minima is 2-3 months behind the same suction minima in depths of 1 m below ground level. The delay in the transfer of humidity in the soil profile is also caused by a low unsaturated hydraulic conductivity that is distinctively dependent on the degree of soil saturation. With the growing matrix suction and decreasing degree of saturation, the unsaturated hydraulic conductivity decreases and the response of deeper tensiometers to precipitation events is therefore delayed.

Climate

Time interval 1982-2005

The climatic situation was assessed for the 1982-2005 period (Fig. 3A and Fig. 5AB). In the 1982-2005 period, the values of the monthly averages of the basic humidity balance have a very distinct course during the year with high negative values in the summer half of the year (Fig. 5B). The monthly averages of the data from April to August are balanced. The situation is more favourable in the winter or autumn period, with the positive water balance.

          In the course of the monitored 1982-2005 period, the potential (reference) evapotranspiration prevailed regularly over precipitation with one exception in 2001, when a positive water balance was achieved (Fig. 3A). In comparison with the monitored 1961-2000 period, the analyzed 1982-2005 period has less precipitation; on the contrary, the values of the potential and reference evapotranspiration are more distinct. It follows from these facts that the basic water balance of the 1982-2005 period is less favourable from the long-term point of view with a mild deficit of precipitation and larger evaporation by about -44 mm. The groundwater storage follows this climatic situation; in general, there was a mild trend in groundwater lowering in the monitored 1982-2005 period (Fig. 3B).

Time interval 2003-2005

The following facts were obtained from the detailed evaluation of the course of the monthly total precipitation and potential evapotranspiration of the standard grass surface during 2003 – 2005. In the vegetation period, primarily from May to September, potential evapotranspiration prevails over precipitation in the absolute majority of cases. The largest differences were recorded primarily in 2003-2004. Here it is necessary to draw attention primarily to the very high monthly values of the potential evapotranspiration in June, July and August in the very dry period of 2003. The current data of the evapotranspiration are not so extreme in view of the fact that nothing was left for evaporation (precipitation deficit and the low humidity of the soil mantle). Conversely, in the cold half of the year (primarily winter months), both meteorological elements are more or less balanced and in the absolute majority of cases, precipitation in form of rain or snow prevails over evaporation. This fact is also obvious during the evaluation of the basic water balance (Fig. 4A). The trend of the course of the water balance corresponds well to the change of suction at a depth of  1 m below ground level. In greater depths, the time delay of the humidity progress behind the climatic impulse becomes obvious.

Conclusion

The state of the groundwater storage in the shallow aquifer in Cretaceous mudstones with low permeability changes with time in dependence on climatic conditions. In the mild climate, oscillations of the groundwater level appear in various hierarchic levels: Interval – day, season, several years, several tens years. In the evaluated period 1982-2005, there is in general mild groundwater lowering, the maxima of several years are related to 1987, 1995-1997, 1999 and 2002.

In the mild climate, primarily precipitation of the very late autumn and the snow precipitation in winter months are decisive for increasing the groundwater storage of Cretaceous mudstones; this becomes obvious after defrosting in the period of the spring snow-melt. Total precipitation in warm summer months do not contribute significantly to the growth of the groundwater storage with respect to their high loss due to evapotranspiration and in part also by surface run-off.

        The water transfer from the surface to the groundwater level is affected by the gradually growing delay of the order of magnitude of several months, caused by the low permeability of the subsurface zone that decreases significantly with the decreasing degree of the water saturation above the groundwater level.

Acknowledgement

This contribution was supported from the grant of the Grant Agency of the Czech Republic No. 205/05/2188.

References

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