Role of the aeration soil zone on the groundwater flow contamination

1. Role of the aeration soil zone on the groundwater flow contamination Zdravko Diankov, Olga Nitcheva Institute of water problems to the Bulgarian academy of sciences Sofia, BULGARIA

2. Introduction • The investigation consists in a registration of the soil water distribution in the unsaturated soil zone • under development of • grass vegetation and • in applying numerical simulation of the processes for the same conditions as in the field. • Soil samples were collected periodically by hand drilling at each 10 cm to the groundwater level from two points (EAST and WEST) of the investigated area (situated in Sofia kettle).

3. Field investigations The soil water content  [cm3/g] ( man=0,24 cm3/g) was obtained by gravimetric method. The measured soil moisture data for a given date (example for 20 March 2003) could be present in graphical way – it is the profiles of the moisture distribution. The volumetric soil moisture θ (cm3/ cm3) in the soil layers was obtained according to the relation θ= .ρ where ρ (g/cm3) is the soil dry bulk density (ρ~1,5 g/cm3, θmax=0,36)

4. During the investigation 08.05.2002 – 28.08.2004 were created profiles of the gravimetric soil moisture for 12 dates from the test plots WEST and EAST. The data have been analyzed together with the having influence factors on soil moisture status.

5. The groundwater level changes observed in EAST and WEST plots are presented on this figure. Comparing the two lines it is remarked that the depths of the groundwater levels are in synchronous and away each other with 0.75 m..

7. The profiles of the soil moisture θ for observations in 2003 are seen in the following figuresSimilar are the graphs for the observations in 2002 and 2004 • Two essential factors are pointed out • The capillary zone (capillary fringe - CF) phenomenon is very clearly manifested in the soil layers over the zone of saturation and • 2. The volumetric soil moisture θ on the “depth of groundwater” exhibits lower values than the maximal ones (=max). This indicates that under the zone of the measured groundwater level a zone of incomplete saturation is formed too.

8. On the direction of soil water movement • Let consider the soil moisture status in two layers (1) and (2), with the difference in the ordinates Δz=z 2-z 1, they have different soil water content (θ2) and(θ1). The velocity of water flow from point (1) to point (2) related to equation of Richards is derived from the differential expression: • where ψ(θ) is soil water retention values. • On the scheme are presented tree possible values of the hydraulic potential in point (2) related to point (1). It is shown: • a) for Ψ2 = Ψ1+Δz, v=0; the velocity is zero • b) for Ψ2> Ψ1+Δz, v>0, the velocity has the direction of increasing elevation of soil layers. • c) for Ψ2 < Ψ1+Δz, v<0, the velocity has the direction of decreasing elevation of soil layers). The conclusion may be drawn that the direction of soil water flow in the vadoze zone can be determined from the type of the graphs θ(z), or θ(H-t).

9. They were registered time periods when the moving of the water in aeration zone is in ascending direction. In such case the feeding of the capillary fringe comes from the saturated zone. They were registered time periods, after rainfalls, when the soil moisture reached to the maximal values, then the water motion went down and leaded to a water recharge of the saturated zone Summarily about the soil moisture regime in the observed area and period

10. Numerical modeling The well-known WAVE mathematical model and software product was used for representing the processes of water exchange in the vadoze zone A part of the input parameters were those from the field investigations – precipitation, temperature of the air, groundwater level kinetics, data for soil mechanical composition and , moisture retentionψ(θ) -ig.8. The other part of the input data were mainly: the daily development of evapotranspiration (kind of plant), filtration coefficients of schematized soil layers, conductivity[k(θ)]depending on volumetric moisture content (fig. 9).

11. The graphs in the followingtwo figures show the volumetric water content θ obtained by the simulation procedure with the WAVE software for the dates 16.05.2003 and 10.06.2003. The data for the determined soil moisture profiles by the field investigation, for the same days, are shown as well. The analogy between the experimentally obtained and calculated profiles gives a good reason for further iterative procedures by varying the input parameter values to satisfactory correspondence between the experimental (tested) and computation results

12. The graph in the following figure shows the water exchange (recharge) between the unsaturated and the saturated zone with the simulated intervals of the upward and downward water flow formation

13. Conclusion The exhibited investigations show that the soil, hydraulic, atmospheric and other conditions have a marked influence on the processes of the water and salt (fertilizer) moving in the aeration soil zone. The intensity and the direction of the water and fertilizer exchange between the unsaturated soil zone and the groundwater flow can be different in the different time periods of the year. The prognostication for a groundwater pollution as a result of fertilizing of intensive land use calls for methodical experimental (field) and simulation investigations. In this area of problems are in developing now investigations in the Institute of Water Problems to the Bulgarian Academy of Sciences - Sofia