Pumping response of volcanic aquifers: examples from the volcanic areas of Naples and Viterbo

Abstract

Determining the sustainable well yield in heterogeneous aquifers is an arduous task, because of the variability of the time-drawdown relation. The present work deals with the pumping response of wells that penetrate in the volcanic rocks of Naples and Viterbo; in these areas the average aquifer yield is lower than 10 l/s per km2. Four wells have been tested at a constant discharge: P1, 20 m deep, penetrates a leaky aquifer in pyroclastic soils and Neapolitan Yellow Tuff and has been tested at 2.5 l/s for 3200 min; P2, 70 m deep, penetrates a perched aquifer in Peperino (a quartz-latitic ignimbrite) and has been tested at 0.3 l/s for 30 min; P3, 170 m deep, penetrates an unconfined aquifer in Peperino and has been tested at 0.9 l/s for 90 min; P4, initially 250 then 290 m deep, penetrates an unconfined aquifer in lavas and tuffs of Vico vulcano and has been tested at 25 l/s for 1410 min and 15 l/s for 1620 min. Drawdown has been measured during the pumping and residual drawdown during the recovery in the same pumped well. For all wells, the relation time-drawdown has a similar trend both during pumping and recovery without reaching the steady-state. A faster drawdown is instead observed during the pumping period. Transmissivity values, calculated with recovery data, ranges from 7.21 x 10-5 to 1.47 x 10-3 m2/s and are higher than those estimated through pumping data. Results were interpreted according to the heterogeneity of the aquifer, dual porosity of tuffs, well-loss, and aquifer recharge. These results highlighted a first hydraulic characterization of volcanic rocks and, at the same time, led to some suggestions for the determination of the sustainable well yield.