Chemical response to groundwater extraction southeast of Mexico City

An alternative procedure of pumping test data interpretation is used through a joint analysis of the standard time-drawdown curve and simultaneous field measurements of total dissolved solids (TDS); additional support is also provided by the temperature of extracted groundwater and the chemical composition of extracted water. The overall information was applied to characterise the groundwater flow system and its sources, the hydraulic conditions of the aquifer and hydraulic response of extraction boreholes. The analysis of this information suggests the presence of: (i) a local flow system that circulates at shallow depth through basalt units interstratified with fine grained sediments and pyroclastics; these materials contain water with TDS of 127–600 mg/L and Na of 24–178 mg/L, and temperature of 18–19.5 °C; (ii) intermediate flow in granular material under reducing conditions by the oxidation of organic matter in aquitard sediments; this water has TDS and Na values of 203–940 and 30–370 mg/L, respectively, and temperatures of about 20–22 °C; (iii) regional flow through volcanic rocks and limestone, with TDS content of 300–700 mg/L, Na of 80–230 mg/L and temperature of 23.0–24.8 °C. The hydraulic response and the chemical composition of the water produced by some boreholes are affected by the seepage inflow from sewage effluents, the input from an overlying aquitard unit and the inducement of regional flow. The conception of the flow regime thus obtained allowed the recognition of hydraulic conditions which were more consistent with the hydrogeological setting, than if only a time vs. drawdown test analysis would have been carried out.