S.L. Witczak,  A. Karpinska-Rzepa,  K. Rozanski,  J. Czana

AGH University of Science and Technology, Krakow, Poland

The common phenomenon in many parts of the world is nowadays a migration of anthropogenic pollutants through the unsaturated zone, originating from both point and diffuse sources. This process is of particular importance in heavily populated and industrialized regions, such as the European continent, where anthropogenic impact is persisting since many decades. Adequate assessment of the migration parameters, such as transport velocity and rates of geochemical interactions within the matrix, is of utmost importance for assessing the vulnerability of shallow groundwater systems against pollution. Such systems often serve as important source of potable water.

The presented study was focused on assessing the migration parameters of pollutants of industrial origin (SO₄, heavy metals) to the shallow groundwater system located in the vicinity of a large metallurgical plant. The plant, which was established in 1950s, is located on the outskirts of Krakow, a large town in the south of Poland, with the population of ca. 800,000 inhabitants. Since that time, the unsaturated zone of this system is receiving heavy loads of pollutants mainly as airborne particulates (heavy metals) and as sulfur compounds (wet and dry deposition). The unsaturated zone of the shallow groundwater system consists mainly of loess. Its depth varies between 8 and 15 meters. The aquifer is continuously exploited since the 1960s by a system of boreholes supplying potable water to the factory. Some wells are already affected by the pollutants.

Water and pollutant transport through the unsaturated zone was studied using a multi-level lysimeter system and by analysis of soil cores drilled in different parts of the aquifer. The infiltration velocity of water was determined using bomb-tritium, as well as chloride and bromide ions. Measurements of tritium content in soil moisture profiles were carried out for the first time in 1993. The soil moisture was extracted using vacuum distillation at 150^oC.The maximum of bomb-tritium concentration in the unsaturated zone was detected at ca. 10 m depth. The average infiltration velocity derived from the displacement of the tritium peak and the measured water content was equal around 30 cm per year. If applied to entire profile, it would suggest that the tritium peak should leave the unsaturated zone ca. ten years later. In fact, this was confirmed by the bomb-tritium measurements carried out again in 2003.

The infiltration velocity was also studied using Cl and Br ions. Chloride was applied in 1987 on several plots located in different parts of the system. Measurements were carried out on soil cores every year until 1992. Bromide was injected in 2000, in immediate vicinity of the multi-level lysimeter system. The measurements were carried out in the lysimeters every 3-4 months until 2005. Tracer experiments using Cl and Br ions yield infiltration velocities between 30 and 90 cm per year, depending on the location within the system.

Sulfates were measured both in the multi-level lysimeter system and in the soil cores. The sulfate concentrations were recorded in lysimeters between 1995 and 2003. The measured concentrations varied from 50 to 750 mg per liter, exceeding in major part of the profile the maximum permissible level of SO₄ in drinking water (250 mg per liter). Assuming that sulfates are transported in the unsaturated zone without significant delay with respect to the water flux, the arrival time to groundwater table of this pollutant in the studied groundwater system was estimated to be between 10 and 50 years. Heavy metals are confined within the first 50 cm of the soil profile.

The movement of Br and Cl ions in the unsaturated zone was modeled using 1D flow and transport model CXTFIT. General agreement between measurement results and model predictions could be reached, although the measured distribution of tracers along the profile suggests that the flow has a more complex character than the simple steady-state model allows.