Chemical Quality of Waters of the Atoyac-Verde River As It Passes Through Forest Ecosystems of Oaxaca, Mexico

In granitic regions, water salinity typically ranges from 30 to 40 mg L −1 at the surface and from 300 to 500 mg L −1 for groundwater. Technogenic activity in Oaxaca has altered the concentration and chemical composition, which explains the occurrence of salinization processes and contamination of natural waters. To determine the physical–chemical composition of the Atoyac-Verde river, fifty-two water samples were collected from tributaries and semi-deep wells in the basin. The pH, electrical conductivity (EC), major cations and anions, total dissolved solids (TDS), residual sodium carbonate (RSC), and sodium adsorption ratio (SAR) were identified. Likewise, osmotic potential (Ψπ) was evaluated, and the effective salinity (ES) and potential salinity (PS) indexes. Results indicate that the water of the basin is bicarbonate-magnesium-calcium. Of the 52 samples analyzed, only 2 identified in the high relief points presented high levels of alkalinity and salinity with a pH of 8.1, EC of 2320 µS cm −1 , Ψπ of − 0.08 MPa, SAR of 3.74 (mmol c L −1 ) 1/2 , RSC of 0.53, ES of 9.88, and PS of 7.79 mmol c L −1 , indicating restriction for its use in agricultural and domestic activities. In the meso relief, the water did not present restrictive levels for anthropic use, while in the low relief, only two points presented high salinity levels and restriction of anthropic use. In conclusion, the greatest chemical alteration of the water was detected in areas close to the city of Oaxaca and to a lesser extent in areas adjacent to riverside populations of more than 2500 inhabitants.