Minimising off-site movement of contaminants in furrow irrigation using polyacrylamide (PAM). II. Phosphorus, nitrogen, carbon, and sediment

Abstract

Off-site movement of nutrients and sediment from furrow-irrigated agriculture has been a concern in the Ord River Irrigation Area, Western Australia. After consultation with growers, a range of management strategies were tested to assess the effectiveness of various practices to minimise off-site movement of nutrients during irrigation. This paper reports on the effectiveness of the additions of high molecular weight, anionic, polyacrylamide (PAM) to irrigation water to minimise off-site movement of phosphorus, nitrogen, carbon, and sediment. Surface runoff water quantity and quality from 4 separate irrigation bays, which contained 25 furrows per irrigation bay, was monitored over time for a single irrigation 35 days after sowing. Addition of PAM as a puck (cylindrical disc 55 mm diameter by 23 mm height) to the head of each irrigation furrow significantly (P < 0.001) decreased the average volume of surface runoff water leaving the irrigation bays by 54%, from 599 kL for the control irrigation bays to 277 kL for the PAM-treated irrigation bays. The addition of PAM also significantly (P < 0.001) decreased the average total suspended sediment load for the duration of the irrigation from 94.9 kg/ha for the control bays to 13.4 kg/ha for the PAM-treated irrigation bays. The concentrations of the different forms of N, P, and C measured in the runoff water were not significantly different between the 2 treatments. The amounts (g) of particulate (>0.45 µm) P and dissolved organic C were significantly (P < 0.01) less from the PAM-treated bays than from the control bays. There was a consistent trend for the addition of PAM to decrease the cumulative mass loss of all nutrients (N, P, and C) measured. However, significant decreases were only seen for particulate (>0.45 µm) P (by 94%), unfiltered (or total) N (by 56%), and unfiltered (or total) C (by 60%). This experiment demonstrated that the addition of PAM to irrigation waters has the potential to decrease the off-site movement of nutrients bound to colloidal material. However, in this study off-site movement of contaminants present in the ‘soluble’ (<0.45 µm) fraction is unlikely to be mitigated by the addition of PAM to irrigation water. The mode of application of PAM, however, may affect water infiltration and hence vertical movement of ‘soluble’ contaminants and requires further investigation to ensure that while off-site surface transport is being minimised, contamination of groundwater is not being increased. Other strategies to minimise off-site movement for contaminants in the dissolved phase also need investigation.