Water quality index in the lower watershed of the San Pedro river in Chihuahua, Mexico

Abstract

A Water Quality Index (WQI) represents a unified model to detect potential tendencies of pollution in water bodies. The objective was to determine a WQI for the San Pedro River in Chihuahua, Mexico using the quantitative variables of metals, physical-chemical properties, and microbiological characteristics. A total of 165 water samples were collected at five locations, three repetitions and monthly samples from October 2005 to August 2006. The locations were: close to the Francisco I. Madero dam (LP); between Rosales and Delicias (RD); Meoqui (M); El Torreon (ET); and Julimes (EG). The levels of arsenic (As), cadmium (Cd), and lead (Pb) were determined using an Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) Perkin Elmer 2100. In addition, the following variables were determined: physical (water temperature, electrical conductivity (EC), total solids (TS), suspended solids (SS) and dissolved solids (DS)); chemical (pH, Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD), dissolved oxygen (DO) and hardness) as well as microbiological (total coliforms and fecal coliforms). The ICA model for this study was calculated with the equation WQI=Σ[ Log Nat P *(Q P/p * Wi)]/100, where Σ=Summation; Log Nat=Natural Logarithm; P=Parameter; Q P/p=Obtained value of dividing the Log Nat of each variable for the maximum value of the permitted concentration; Wi=Pondered value of each variable in the equation; and 100=Value to normalize the indices to a base of 0 to 100. The maximum value of WQI was detected for EG with 76.1 as an average of all monthly samples in comparison with locations LP with 47.0, RD with 67.3, M with 66.7 and ET with 69.1. Considering monthly samples, the maximum WQI value was detected in November and July both with 73.0. With respect to the other variables the general means were: pH of 7.69±0.405 SD, temperature of 24° C±23.85 SD, TS of 1,266.73 mg l-1±0.60 SD, BOD of 4.51 mg l-1±0.06 SD, OD of 5.98 mg l-1±1.80 SD, EC of 1.337 mS cm-2±0.68 SD, hardness of 116.06 mg l-1±39.93 SD, total coliforms of 1,729 NMP±322.12 SD and fecal coliforms of 814.18 NMP±800.13 DS. The analysis of metals detected a range for As of 0.0110 mg l-1 to 0.587 mg l-1while the general mean for Pb was 0.023 mg l-1±0.01 SD, and Cd was 0.003 mg l-1±0.0025 SD. It was possible to establish a WQI for the specific conditions of the San Pedro River in Chihuahua, Mexico.