Biofilters for urban agriculture: metal uptake of vegetables irrigated with stormwater

In an era where increasing urbanization is resulting in issues such as urban poverty, malnutrition and unemployment, urban agriculture is increasingly regarded as a multi-functional approach to addressing these issues. However, increasing water scarcity limits the feasibility of urban agriculture and hence alternative water sources for irrigation are required. While stormwater has the potential to be used for urban agricultural irrigation, stormwater contaminants can pose potential health risks. Thus, a column study was conducted to (1) determine whether biofilters planted with vegetable crops are capable of treating urban stormwater, and (2) identify the level of heavy metal uptake into various vegetable crops when irrigated with stormwater. The column study was conducted over nine weeks with nine vegetable species (broad beans (Vicia faba), kohlrabi (Brassica oleracea Gongylodes Group), kale (Brassica oleracea Acephala Group), lettuce (Lactuca sativa), mint (Mentha spicata), mustard spinach (Brassica juncea), radish (Raphanus sativus), spinach (Spinacia oleracea) and sweet corn (Zea mays)) irrigated with stormwater. The treatment function of the system was not compromised by the use of vegetable crops. 70% concentration reduction was achieved for Cu, Pb, Zn, Mn and Ni. The concentration of total nitrogen and total phosphorus in the effluent was reduce by up to 47% and 69%, respectively. Heavy metal accumulation was limited in the edible portions but the levels of Cd and Pb concentration exceeded the Food Standards for Australia and New Zealand and World Health Organization guideline values, deeming it unsafe for consumption. Cultivating vegetable crops in biofilters did not affect plant growth and the biofilter's stormwater treatment functions. However, heavy metal concentrations within plants does raise potential health concerns, requiring further studies to improve crop safety.