Seasonal variations in fecal contamination of shallow groundwater in Altamira, Amazon, Brazil

Altamira city, in the north of Brazil, is known to be a support region since the construction of the Belo Monte Hydroelectric Complex. The Xingu River permeates the city, being extremely important as a tributary of the Amazon River. Only a portion of the population is provided with basic sanitation, and alternatively use water wells and septic tanks, which can be sources of contamination. The study sought to understand the seasonal variation of fecal contamination in groundwater resources. The households were divided into six zones, from which 30 wells were sampled in the dry and wet seasons. For the Escherichia coli detection, a Compartment Bag Test (CBT) (Aquagenx®) was used, and for the Bacteroides thetaiotaomicron (B. theta) detection, the microbial source tracking (MST) approach, and quantitative polymerase chain reaction assays (qPCR) were used. The presence of E. coli and B. theta was verified in all seasons. In the dry season, 63.3% of sampled wells showed E. coli contamination, and in the wet 76.6%. B. theta was detected in 43.3% of the wells in the dry season and 36.6% in the wet season. In the dry season, 26.6% of the wells presented E. coli and B. theta simultaneously, and in the wet season, 33.3%. The quantification of B. theta was lower in wet seasons, indicating that the source of contamination for both, E. coli and B. theta, may be different, also, the mechanism of reach and maintenance for E. coli in the aquifer appears to be related to runoff and percolation of contaminants from the soil surface not just associated with human fecal contamination.