Sustainable use of groundwater resources in Alpine wetlands: multi-depth hydraulic head monitoring relationship to direct recharge and spring exploitation

Demographic pressure combined with agricultural and industrial development have incremented the demand for groundwater employment so far as to make the preservation of the water quantity and quality difficult. The problem consists of maintaining high qualitative standards with an increasing demand on the one hand, on the other to balance groundwater use with environmental issues for the protection of natural ecosystems. A conflict so arises and, at the same time, the need to quantify the effects on the ecosystems of different exploitation schemes, even in apparently highly protected environments. Wetlands in the Alpine highlands are fragile ecosystems of high environmental values and, in some instance, may be subjected, in a world with increasing pressures on water resources, to potentially adverse impacts due to groundwater exploitation. This is the case of a couple of wetland systems located in the Eastern Alps (Veneto Region, Northern Italy), respectively at the eastern edge of the Asiago plateau (Marcesina site, 1300 m a.s.l.) and near the border with Austria at the northern edge of the Piave river watershed (Colotrondo site, 1800 m a.s.l.). Both the sites have a high naturalistic importance for the occurrence of some topic habitat declared as deserving protection priority in Europe; at the same time near Marcesina site a well has been drilled to pump groundwater for municipal water supply whereas near Coltrondo springs are used for farming. Understanding groundwater flow systems inside the wetland porous medium and quantifying the components of the wetland hydrological budget is the main way to forecast the adverse effects of anthropic impact and to manage them. The paper deals with the results of a hydrological continuous monitoring system set-up to reconstruct the hydraulic head field internal to the wetlands and to verify and quantify the components of the recharge-discharge system (direct recharge, groundwater flow from surrounding aquifers and moraines, discharge from springs, discharge from evapotranspiration through the unsaturated zone, discharge from surface creeks). A total of 39 open-stand pipe piezometers (diameter 60 mm) have been drilled in triplets at 3 distinct depths below ground surface (respectively, on the average, 5 m, 2,5 m, 1 m). The total thickness of the wetland porous medium is, on the average, varying between 0,8 and 7 m. During the vegetative period of 2005 (from August to October) a discontinuous weekly monitoring of hydraulic heads have been performed; during all the 2006 summer a total of 12 piezometers record continuously the hydraulic head via mini pressure transducers connected to data-loggers. At the same time main springs and wells discharge, creeks surface flow and rainfall are monitored. Depending on the ombrotrophic or minerotrophic type of the wetland we have monitored different patterns of groundwater flow (upward or downward oriented). The final goal is the implementation of a numerical flow model to simulate the outcome of the possible modifications on the wetland groundwater flow systems induced by anthropic pressures.