Hydrological characteristics of degraded soils of a landfill in Northern Italy: a comparison between instrumental data and predictive models

The coverage soils of a landfill in Piacenza (Italy) are actually involved in a Life + project (10 ENV/IT/0400 Life New Life), which has, among their objectives, the restoration of degraded soils by treatment for their improvement with an innovative reconstitution method consisting in an incorporation of organic matter by mechanical and chemical processes that originate new aggregates of soil. The aim of this study is to describe some aspects of soil’s degradation; the degradation is described by the results of chemical and physical analyzes and those relating to the vegetation ecological study. For this purpose, we focus on the determination of the hydrological characteristics. Through the tensiometric cassette and the Richard’s plates the amount of soils water content at -0.10 KPa,-10kPa and -31.62 KPa and -1496.24 KPa pressures was measured; the available water content for plants was subsequently calculated. The water volumetric content at the different pressure values and the resulting water retention curve were compared with the predictive models, which are mathematical models that correlate the water retention and hydraulic conductivity with some easily measurable chemical and physical parameters of the soil such as: texture, density, porosity and organic carbon content. The 18 PTFs used in this study are based on two models: van-Genutchen and Brooks-Corey; it was also used the SPAW program. Through the calculation of RMSE, the best curve in describing the behavior of the investigated soils was identified. To describe the soils degradation the real water retention curves were compared, also, with an ideal curve derived from the average of the PTFs curves developed for a hypothetical soil. The inputs for this hypothetical soil, which was assumed non-degraded, are: silty loam texture, bulk density 1.4 g cm-3 and particle density 2.6 g cm-3, (such as landfill soils), depth 1 m (deeper than the landfill soils), organic carbon content 1% (lower than the average of 2.6% of the landfill soils), well structured. This comparison shows that the investigated soils have less available water content than the hypothetical soil, this combined with the low depth and compacted structure represents a state of degradation.