Cost- and time-effective methodology to evaluate aquifer parameters in coastal aquifers

The increasing request of freshwater in coastal areas due to intense anthropization has intensified the research on saltwater intrusion. A sustainable groundwater management requires the estimation of some fundamental aquifer parameters (such as hydraulic conductivity) and their variability in order to quantify the magnitude and direction of groundwater fluxes. In case of saltwater intrusion in coastal aquifers also the vertical variability of groundwater quality is fundamental since seawater is characterized by elevated density and viscosity with respect to freshwater and this feature induces density driven flow within the aquifers. For these reasons there is the need of cost- and time-effective techniques to define real physical parameters and dynamics of the aquifer. In this study we present the methodology developed via straddle low pressure packers to characterize simultaneously vertical hydraulic gradients, permeability, and groundwater quality in standard fully penetrating piezometers. The methodology uses two inflatable packers to isolate a window of 0.2 m. The original instrument has been modified by positioning a Levelogger LTC Solinst within the sampling window. The Levelogger monitors head, temperature and electrical conductivity every 1 second. In line with the packers is connected a centrifuge pump with a flow controller. The system is set at the desired depth within the piezometer and the packers are inflated by a portable air compressor. Before to start the pumping test, the inflated packers system is left to stand until the piezometric equilibrium, monitored by phreatimeter, is reached. Once the piezometer equilibrium is restored and the point head measurement is recorded, a constant rate pumping test is performed using a flowmeter; or whenever the flux is very low, the outflow rate can be monitored by a litre counter and chronometer. The methodology has been applied on several piezometers of the regional monitoring network of the Emilia-Romagna Regio. The 2” piezometers are fully screened with a geotextile sock instead of gravel pack, to prevent short-circuits during the pumping tests. The Levelogger LTC records simultaneously the piezometric drawdown induced by pumping and any variations in temperature and electrical conductivity within the aquifer window isolated by the packers. From these data, the equivalent freshwater head and intrinsic permeability values can be derived. During the pumping test the piezometric head can also be monitored by Leveloggers placed above and below the packers; this allows to verify if there is any pressure drop within the piezometer due to a defective isolation of the packers. Finally, groundwater sample can be collected as soon as the pumping test has completed without removing the straddle packer. The use of a Hydrolab flow cell connected to the Hydrolab MS-5 probe also permits to acquire in situ the other hydrochemical parameters (i.e. dissolved oxygen, pH, and redox potential), which are not recorded by the Levelogger. The results of pumping test obtained using the packers system have been compared with the intrinsic permeability calculated by pedotransfer formulas. The comparison shows a general agreement along each piezometer profile. Although, the major discrepancies have been found in peat layers, where the grain size characterization is particularly challenging due to the organic nature of these sediments. The proposed methodology is relevant for a detailed characterization of coastal aquifer parameters and flow dynamics. Application of this integrated method permits to simultaneously perform groundwater sampling, groundwater quality control and aquifer testing. Intrinsic permeability and multilevel equivalent freshwater heads can be easily calculated from acquired data and vertical gradient in the aquifer can be highlighted. Moreover this technique makes the data available during the monitoring phases and it can be integrated with other techniques of vertical investigations. It is a cost- and time-effective technique that is able to optimize the phase of aquifer characterization with the phase of groundwater monitoring and sampling. The major issue associated with the presented technique is the hydraulic tightness of the packers that has to be verified before each measure; once that is done, the methodology allows multilevel groundwater sampling in a single monitoring campaign.