Monitoring of water quality inflow and outflow of a farm in Italian Padana plain for rice cultivation : a case study of two years

Rice cultivation requires a large use of pesticides and nutrients to control weed proliferation and improve production. The water quality of four neighboring rice fields located in the Lomellina area (Italian Padana plain) was monitored in this study along with the cultivation period (before, during, and after the period of planting), for two successive agricultural seasons (2015 and 2016). Two paddy fields were traditionally cultivated with wet-seeding and the other fields with dry-seeding. Eighteen sampling points were considered: eight points for surface water, two points for underground water, and eight points for porous cups with two different depths. In order to evaluate the goodness of the paddy field system to maintain unchanged the quality of the inflow with respect to the outflow water, three of the most used herbicides in Italian rice cultivation (imazamox, oxadiazon, and profoxydim) and other physical-chemical parameters were determined, namely biological oxygen demand after 5 days; chemical oxygen demand; total suspended solids; anionic surfactants; total hardness; total amount of phosphorus, nitrogen, and potassium; and heavy metal concentrations. In general, all the collected data confirmed that paddy fields did not contribute to worsen the environmental pollution. The different flooding techniques adopted in the fields did not highlight significant differences in concentrations of pesticides or metals. The pesticides reached their maximum concentration (of the magnitude order of few ng mL(-1)) on the day after the administration and on the day after the application in the adjacent field. A slight reduction of total As in grain was obtained adopting a dry period from steam elongation up to booting. From the collected data, it was possible to identify a general water flow direction in the paddy fields from north-west to south-east: this prevailing flow direction was useful to understand not only the diffusion of the pesticides and their degradation products in the fields but also that of the nutrients. Concerning nutrients, it was important not to activate a recirculation of the water in the field during the first 10 days from the administration, in order to avoid loss of nitrogen in the water vents or for percolation. Moreover, the monitoring of potassium concentration allowed to avoid the use of unnecessary potassic fertilization when there was already a high amount of this element in the paddy field derived from irrigation. However, all the investigated water quality parameters were under the limits fixed by the European regulation. In addition, the presence of seven unexpected compounds was identified by the nontarget approach in both campaigns in samples collected in the early summer period. Four of these emerging contaminants were identified as N, N-diethyl-meta-toluamide, tricyclazole, amidosulfuron, and one of the imazamox photodegradation products. Although the obtained low concentrations of oxadiazion, tricyclazole, and arsenic, in particular, justified a preexisting contamination of the water inflow or of the investigated paddy area, the obtained results supported the good quality of the paddy water outflow, confirming the rational use of the water resource and the correct use of agronomic practices.