Modelo conceitual hidrogeológico dos aquíferos Alter do Chão e Aluvionar na região de Anavilhanas, município de Novo Airão, AM

Abstract

The Anavilhanas Archipelago, in the Negro River (AM), is part of a complex hydrogeological system, and during the 2023–2024 El Niño the record low river levels affected both this dynamic and the access of Amazonian populations to essential services. This study aimed to develop a conceptual hydrogeological model of the area by evaluating the river–aquifer–precipitation relationship, recharge processes, the hydraulic properties of the local aquifers, and the hydrochemical and isotopic characteristics of the waters. For this purpose, a monitoring site with four piezometers was installed, two located on Anavilhanas Island and two on the right bank of the Negro River. Automatic and manual monitoring of groundwater levels was conducted between July 2024 and October 2025, associated with Negro River stage data and local precipitation. The sedimentological characterization was based on descriptions and granulometric analyses of sediments collected during the installation of the piezometers. Hydraulic tests and granulometric methods allowed the estimation of hydraulic conductivity, and recharge was quantified using the water balance method, complemented by electrical resistivity sections to analyze subsurface infiltration. Water samples were collected for physicochemical and isotopic analyses. The integration of these methods enabled the development of a preliminary conceptual model showing that, to the west, silty-clayey and sandy layers at depth are in contact with fluvial sandy sediments deposited by the rivers along the channel and the margins. To the east, the islands are composed of a 5 m silty-clayey layer overlying progressively coarser sandy sediments down to 24 m, a depth interpreted as the transition to riverbed deposits. Hydraulic conductivity ranged between 2.8 × 10⁻⁵ and 3.6 × 10⁻⁴ in the 14 m and 30 m sections in Anavilhanas, and between 2.0 × 10⁻⁶ and 1.3 × 10⁻⁶ m/s on the right bank. This set forms an aquifer system with three flow systems: a shallow one, controlled by the local topography of the island and the riverbank; an intermediate one, which follows the sediments deposited at the base of the islands and the Negro River channel in the NW–SE direction; and a deep one, in the Alter do Chão Aquifer, with a W–E direction. From mid-September 2024 to early January 2025, the shallow groundwater flow on the island received sufficient rainfall recharge to generate baseflow to the Negro River. During the remainder of the monitoring period, the Negro River lost water to the shallow portion of the groundwater flow system. The hydraulic head of the intermediate flow system was always lower than that of the Negro River, causing the river to continuously lose water. Recharge from precipitation was 11% (2022), 24% (2023), and 8% (2025), with no recharge in 2024, reflecting reduced rainfall, accumulated water deficit, and decreased storage in the unsaturated zone. Hydrochemically, the aquifer waters showed higher mineralization compared to the river, with notably high iron concentrations at shallow levels. The isotopic signatures indicated meteoric origin, with seasonal variations associated with flood and low-water periods.