Effect of untreated or reclaimed wastewater drip-irrigation for lettuces and leeks on yield, soil and fecal indicators

Sanitary and environmental impacts, as well as effects on agricultural yields constitute important parameters to be taken into account in wastewater reuse projects, and depend on the treatment applied to the wastewater. Extensive treatments or irrigation with untreated wastewater are rarely studied, although they represent a significant share of unplanned or indirect wastewater reuse in the world. The aim of this study was to contribute to the evaluation of chemical properties of soil, crop yields, and the fate of fecal indicators in soil and plants when vegetable crops (lettuces and leeks) are irrigated by drip-irrigation using untreated wastewater (UW), water reuse or treated wastewater by stabilization ponds (TW) and fresh water (FW). Over two years (2018 and 2019), three lettuces (Lactuca sativa) and two leeks (Allium porrum) growth cycles were performed in a greenhouse, and soil analyses show increases in electrical conductivity (EC), chlorine (Cl), sodium (Na) and nitrate nitrogen concentrations (NO₃) when TW and UW irrigation methods were compared to FW. As a result, FW irrigation and conventional fertilization lead to lower lettuce yields compared to TW (between 43 to 85% lower yields depending on the cultivation period) ($p$-value < 0.05). However, trends are not clear for UW. Leeks irrigated with FW have also a smaller fresh mass compared to TW and UW (around 35 % and 45 % differences in the median for UW and TW, respectively, $p$-value < 0.05), which shows a possible positive effect of continuous nutrient supply against the risks associated with higher water conductivity. In addition, in the soil, the irrigation with TW and with UW increases some concentrations, especially EC, mainly due to Cl and Na values. Fecal indicators increased from 1 to 2 log with the used of UW but the decay of fecal indicators in topsoils was slow (less than 1 log for enterococcus load after 6h), as shown by culture-based and molecular techniques (qPCR, droplet digital PCR). Fecal indicators were found in plants irrigated with untreated wastewater.