Soil and water changes after sewage irrigation practice in semi-arid region of Northeast Brazil

Abstract

Due to scarcity and uneven water distribution in many regions of the world, irrigation practices are carried using water with poor chemical and microbiological quality favouring salt accumulation in soil, groundwater contamination and health risks. These impacts can be easily evaluated with small soil columns, considered an equivalent experimental plots, given sufficient information for reuse practice such as plant water needs, water losses by evaporation and percolation, groundwater quality, culture productivity and microbiological contamination of soil and culture. This work describes chemical and microbiological changes in two soils of Paraíba State - Northeast Brazil, before and after irrigation with clean water and polluted superficial water. Also investigated are the changes in water quality before and after percolating these soil columns and lettuce (Lactuca sativa, L.) contamination. The analysed parameters in water were: pH, electrical conductivity, ions (calcium, magnesium, potassium, chloride and sodium), ammonia, total phosphorus, soluble orthophosphate, BOD, fecal coliforms (FC) and fecal streptococci (FS). The pH values for both irrigation waters were slightly neutral to alkaline. Electrical conductivity values were high in irrigation and percolating waters (up to 1,753 and 2,367 mmho/cm) due to waters and soil features, not affecting plant growth. The concentrations of calcium, magnesium, potassium were considered adequate for irrigation water but not for chloride and sodium, although indirect effects on lettuce growth were observed. The BOD5 of polluted water ranged from 8 to 15 mgO2/L and was reduced to 85% after the percolating soil columns. FCs in polluted water were well above the recommended WHO values of 1,000 CFU/100 mL with soils reducing these values in 99.8% for FC (2 × 106 - 4 × 103 CFU/100 mL) and 98% for FS (2.5 × 104 - 4 × 102 CFU/100 mL). Soils were highly contaminated with both FC (2.2 × 104 MPN/100 g) and E. coli (8.3 × 102 MPN/100 g) and when polluted water was used these numbers either increased or did not change. There was an increase of soil nitrogen and organic matter percentages after polluted water was added to the soils. Lettuces were also contaminated when both waters were used (with polluted water FC up to 2.0 × 105 MPN/100 g) and associated with aerosol formation during manual irrigation. Although some inconveniences are shown, water reuse must be considered as an alternative for food production in semi arid regions and will be successful when adequate and continuously technical support is given.