Abstract

GOMEZ, María Laura; MORIENA, Rubén; FELIZZIA, Juan  and  SCHIAVO, Hugo. Hydrogeochemical characterization and identification of mixing processes in an aquifer affected by solid and liquid urban inflow. Bol. Soc. Geol. Mex [online]. 2009, vol.61, n.3, pp.437-450. ISSN 1405-3322.

The unconfined aquifer of Coronel Moldes is constituted mainly by aeolian fine sand/silt (loess) of Holocene age, where rural activities rely for the most part on the exploitation of groundwater. Environmental problems and concerns regarding this aquifer are related to mismanagement of urban solid and liquid waste, which has accumulated in an area of sand dunes since 1982. At this site, nearly 84 tons of different types of solid waste are dumped directly on the soil every week without any waste management or control. Also, liquid wastes originating from Coronel Moldes processed at an on-site-sanitation system have formed a lagoon inside the municipal waste disposal site. This paper is focused on the study of the hydrogeochemistry and geochemical modeling (with NETPATH) of a municipal disposal site of a typical phreatic aquifer of the Chacopampean plain. The groundwater directly below the municipal waste disposal site shows an increase in concentrations of Cl^-, SO₄^-2, Na⁺, HCO₃^-, K⁺, Mg⁺², NO₃^-, NO₂^-, Si, As, F^-, Cu, Pb, COD, BOD₅, as well as aerobic and anaerobic bacteria, and a decrease in dissolved oxygen. These values decrease 30 m downgradient from the waste disposal site but do not reach the background values found upstream of the disposal site. The geochemical modeling contributes to estimate the mixing proportions between the leachate and non-contaminated groundwater. The mixing models indicate that CE, Cl^-, Na⁺ y SO₄-² in the aquifer immediately under and downstream of the municipal waste disposal site came principally from the lagoon leachates and possibly the disposal site, resulting in local geochemical conditions that favor certain reactions (exchange, precipitation, oxidation-reduction, and dissolution), which in turn control mobility and concentrations of Ca+2, As, F^- and Si in the aquifer. It is probable that adsorption and precipitation processes of trace elements control the concentration of Fe and other metals in the contaminated aquifer. The principal attenuation processes (retardation, reduction, and chemical and biological transformation of contaminants) occur in the unsaturated zone, mostly in the soil, where biological activity is greater. The alkaline conditions and oxidants in the aquifer, along with redox reactions, ion exchange, precipitation/dissolution and adsorption, are the main processes that attenuate contamination through the unsaturated and saturated zone.

Keywords : Groundwater; loess; waste disposal site; Chacopampean plain; geochemical modeling; NETPATH.