Il trasporto solido nei corsi d'acqua minori dell'Oltrepo Pavese: alcuni caratteri peculiari nel contenuto delle torbide del T. Versa

Abstract

Ciancetti G., Dolza G. & Pilla G.: The solid transport of the Oltrepo Pavese minor streams: some peculiar characteristics in
the bottom currents content of the T. Versa. (IT ISSN 0394-3356, 2008).
The studies about superficial waters of Pavia's area, managed by the ICG Laboratory of the Earth Science Department, University of
Pavia, led us to detect some characteristics that are common to all the little streams that flow in the mountain territory of the Oltrepo
Pavese, with the exception of T. Staffora.
During this study we analyzed geolithological aspects, climatic conditions, the extent and morphology of basins, soil use and stability.
The Versa valley, located in the Oltrepo area, is devoted to vine growing since the beginning of the last century. Agricultural practises
and clay-rich soils favour the accumulation of heavy metals such as copper and zinc which are present in applied pesticides.
The Versa valley may be divided in two different geological sectors, also reflected by the morphological characteristics of the hydrographic basin. The northern part of the valley is hilly and constituted by marls and clay-rich formations, with the sporadic presence of pebblestones, sandstones and limestones. The southern sector is instead dominated by the presence of limestones. The low permeability of the geological formations enhances rainwater runoff rather than infiltration, causing an accelerated soil erosion and a conspicuous transport of suspended matter in rivers and streams.
The precipitation pattern in Oltrepo shows maximum amounts in November and minimum in August, while throughout the year precipitation is evenly distributed. The Versa river discharge is closely correlated to meteoric events and is characterised by short and intense floods, generally occurring in late fall and occasionally in spring. When precipitation is scarce, discharge may decrease and completely disappear, due to a limited groundwater contribution to base flow.
A monitoring station for the control of hydrology and turbid sediments has been installed near Stradella. Fall and spring floods differ
not only for the amount, but also for the heavy metals concentration of suspended matter. Peculiar links with the mineral phases of the sediment have been evidenced. During a flood, not only continuous changes in the mineralogical content of the solid transport were observed, but also an enrichment in the finest grain sizes during the last phase of the flood. The most abundant mineral phases are carbonates (mean content close to 30%), smectite (about 22%), other clay minerals (adding up to about 30%), feldspars (11%) and
quartz (about 7%). Comparing this composition with that of the Versa valley soils, the identified phases clearly reflect the mineralogical
composition of the northern sector, although carbonates are more abundant in the suspended sediment and silicates.