Effects of irrigation water quality on vegetables Part 2: Chemical and nutritional content

Abstract

The internal qualities of edible vegetable crops have not been widely researched in South Africa. The aim of this study was to determine possible accumulation of nutrients and heavy metals within these crops after being irrigated with greywater (generated from bathing and dishwashing activities), potable water (from a tap stand) and diluted greywater (1:1 ratio). Trials were conducted at the Umtata Dam Research Station, in a completely randomised design testing combinations of cabbage and onion, Swiss chard (here referred to as spinach) and beetroot, and carrot and lettuce across four sequential planting seasons. Greywater for gravity-fed drip irrigation was collected at a central point from a nearby small informal settlement. Chemical and nutritional content analysis on edible portions of vegetables was carried out. Crude protein (CP) was significantly elevated by 20% (4.5 mg kg⁻¹) 25% (57 mg kg⁻¹) on cabbage (during the first season) and lettuce, respectively, when irrigated with diluted greywater compared with the control, whereas the CP of onion (during first season) and carrot was significantly higher by 40% (2.4 mg kg⁻¹) and 31% (6.4 mg kg⁻¹), respectively, due to greywater irrigation compared with potable water. Significant increases in nutritional elements, such as iron (Fe) and zinc (Zn), were observed on cabbage (in both seasons), onion (first season) and spinach when irrigated with diluted greywater, whereas in lettuce the Fe content was significantly elevated by greywater irrigation. There were no significant differences in cadmium (Cd) in spinach and lettuce due to greywater irrigation. The study concluded that the insignificant content of heavy metals observed posed no obvious human health risk from consumption of vegetables irrigated with greywater.

Keywords: diluted greywater, heavy metals, irrigation, nutritional value, vegetables