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Hydrogeological, hydrochemical, and natural isotopes evaluation of groundwater and surface water interactions in the alluvial plain, tropical region of southeastern Brazil

Groundwater-Surface water interactions in alluvial plains facing
morphological features are the subject of the study. Considered
transitions zones, alluvial plains have different morphological features
interfering with groundwater flow and hydrochemistry. The alluvial plain
of Mogi Guaçu river (southeastern Brazil) presented
topography-controlled groundwater flow, nevertheless, natural levees,
wet fields, oxbow lakes, and abandoned meanders can control local flow
and interfere in discharges points of the main river. Two sampling water
campaigns were done in the dry and wet season for physicochemical and
natural isotopes analysis, collecting in total 44 groundwaters samples
from monitoring wells and eight water samples from the river, creek, and
lake. The groundwaters in wet fields and terraces, and surface waters
from creek and lake presented low mineralization (EC from 8 to 37
μS.cm), pH acidic (4.98 to 5.8), and essentially Ca
and Na-HCO composition. River waters samples presented
pH between 5.92 e 7.69 (acidic in the rainy season and basic in the dry
season), and EC from 24.2 and 181.1 μS/cm (lower values in the wet
season), Na-HCO and
Na-HCO-SO (dry season) and
Ca-HCO and Na-HCO (rainy season)
compositions. In dry season groundwaters composition showed evolution
from sodium mixed (SO – HCO) to
bicarbonate waters and higher mineralization; in wet season waters
varied from Ca to Na-HCO composition and low
mineralization, denoting dilution due to rainwater infiltration. Closer
to the river margins, in abandoned meanders and oxbows, the groundwaters
have increased values of EC and major ions indicating GW-SW mixtures,
and effluent-influent changes (descendent and ascendent flux) in wet and
dry seasons, respectively. Natural isotopes in groundwaters imply
meteoric origin, without evaporation during recharge and high d-excess
can be influenced by continental air masses and Amazonia Basin low-level
jet. Shallow water table, permeable silty-sand material of vadose zone,
flat terrain, and pristine conditions can contribute to direct
infiltration of rainwaters, recharging the shallow aquifer.