Spatial modeling of physical and chemical parameters for the evaluation of the Río Puerta Grande stream in Mexico City

The Río Puerta Grande stream is in the Tarango micro-basin ravine in Mexico City and is considered environmental value area (EVA) due to its ecological importance. However, despite this, urban sewage is discharged into the stream. A physical, chemical, and biological characterization of its waters was realized in 2020 during the dry season and after the rainy season. As part of the analytical results, in situ and ex-situ parameter spatial distribution maps of seven sampling sites were obtained using ArcGIS software’s inverse distance weighted (IDW) interpolation tool. The variation in concentration for each parameter during the two seasons is shown on the maps, where red indicates the highest concentration and blue is the lowest. At the residual discharge sites, the concentration increased during the dry season; specific parameters in the period after the rainy season decreased due to a dilution effect, whereas others increased, such as fecal coliform, suspended solids, and biochemical oxygen demand (BOD). BOD revealed high concentrations during the dry season when it was influenced by urban discharge but decreased following the rainy season. Despite the low dissolved oxygen value between 4.39 and 3.80 mg/L, the stream does not currently show anoxia. Regarding the overall water quality index (WQI) during both periods, the stream’s water quality at all sites was “Highly contaminated,” according to Montoya et al. According to León-Dinius, the WQI was “Bad” in both seasons and in the period after the rainy season, it was deemed “Bad” and “Very bad”. The contaminants that should be considered as producing the most risk are organic matter (as reflected by BOD) and bacteriological pollutants.