Effects of Decaying Hydraulic Conductivity on the Groundwater Flow Processes in a Managed Aquifer Recharge Area in an Alluvial Fan

Groundwater artificial recharge and medium characteristics represent the major factors in controlling the groundwater flow processes in managed aquifer recharge areas. According to the depositional features of alluvial fans, an analogous homogeneous phreatic sand tank aquifer and the corresponding inhomogeneous scale numerical models were established to investigate the groundwater flow under the combined influence of artificial recharge (human activities) and decaying hydraulic conductivity (medium characteristics). In this study, groundwater flow through a managed aquifer recharge area in an alluvial fan was analyzed under the conditions of decaying hydraulic conductivity (K) with depth or length from apex to apron. The results showed that groundwater flow processes induced by artificial recharge were significantly controlled by the increasing decay exponents of K. The decaying K with depth or length in alluvial fan areas expanded the degree of influence of artificial recharge on groundwater flow. With the increase of decay exponents, the flow directions gradually changed from a horizontal to vertical direction. Groundwater age and spatial variability could also be increased by the increasing decay exponents. The residence time distributions (RTDs) of ambient groundwater and artificially recharged water exhibited logarithmic, exponential, and power law behavior. Penetration depth and travel times of ambient groundwater flow could be affected by artificial recharge and decay exponents. Furthermore, with the increase of decay exponents, the thickness of the artificially recharged water lens and travel times of artificially recharged water were increased. These findings have important implications for the performance of managed aquifer recharge in alluvial fan areas as well as the importance of considering the gradual decrease of K with depth and length.

Importance of the Induced Recharge Term in Riverbank Filtration: Hydrodynamics, Hydrochemical, and Numerical Modelling Investigations

While ensuring adequate drinking water supply is increasingly being a worldwide challenging need, managed aquifer recharge (MAR) schemes may provide reliable solutions in order to guarantee safe and continuous supply of water. This is particularly true in riverbank filtration (RBF) schemes. Several studies aimed at addressing the treatment capabilities of such schemes, but induced aquifer recharge hydrodynamics from surface water bodies caused by pumping wells is seldom analysed and quantified. In this study, after presenting a detailed description of the Serchio River RBF site, we used a multidisciplinary approach entailing hydrodynamics, hydrochemical, and numerical modelling methods in order to evaluate the change in recharge from the Serchio river to the aquifer due to the building of the RBF infrastructures along the Serchio river (Lucca, Italy). In this way, we estimated the increase in aquifer recharge and the ratio of bank filtrate to ambient groundwater abstracted at such RBF scheme. Results highlight that in present conditions the main source of the RBF pumping wells is the Serchio River water and that the groundwater at the Sant’Alessio plain is mainly characterized by mixing between precipitation occurring in the higher part of the plain and the River water. Based on chemical mixing, a precautionary amount of abstracted Serchio River water is estimated to be on average 13.6 Mm3/year, which is 85% of the total amount of water abstracted in a year (~16 Mm3). RBF is a worldwide recognized MAR technique for supplying good quality and reliable amount of water. As in several cases and countries the induced recharge component is not duly acknowledged, the authors suggest including the term “induced” in the definition of this type of MAR technique (to become then IRBF). Thus, clear reference may be made to the fact that the bank filtration is not completely due to natural recharge, as in many cases of surface water/groundwater interactions, but it may be partly/almost all human-made.

New Insights on Scale-dependent Surface-Groundwater Exchange from a Floating Self-potential Dipole

In south-central Texas the lower Guadalupe River has incised into the outcrop of the Carrizo-Wilcox aquifer. The river and the aquifer are hydraulically connected across the outcrop, although the connectivity is obscured at the surface by alluvium and surface-water and groundwater exchange dynamics are currently poorly understood. To investigate surface-water and groundwater exchange dynamics between the lower Guadalupe River and the Carrizo-Wilcox aquifer, a geophysical study was completed along a 14.86 km reach of the river by using water-borne gradient self-potential (SP) profiling and two-dimensional direct-current electric resistivity tomography. This paper explores the applicability of these water-borne geoelectric methods in delineating gaining and losing channel reaches, and demonstrates that geoelectric signals in the form of total electric field strength can be logged with an electric dipole and decomposed into component SP signals depicting regional and local groundwater flow patterns attributable to regional and localized hydraulic gradients. Localized SP anomalies of several tens of millivolts, indicative of hyporheic exchange flows, are observed and superimposed upon a 124 mV regional SP anomaly indicative of ambient groundwater exchange flows between the river and the aquifer. The observed SP signals are interpreted through two-dimensional finite-element modeling of streaming potentials attributable to ambient groundwater exchange and hyporheic exchange flow patterns. Variables of the channel environment such as temperature and concentration gradients, depth, and velocity are considered and subsequently eliminated as alternative sources of the SP signals that are presented.