Estimating polyacrylamide concentration in irrigation water

One practice used to control irrigation-induced erosion amends irrigation furrow inflows with water-soluble, anionic polyacrylamide (PAM) at low concentrations (0.25-10 mg L -1 ). Researchers wish to determine the fate of PAM, once added to furrow water streams. We developed and tested an instrumented flocculation test for quantifying PAM concentration in irrigation water. A kaolinite mineral standard is mixed with a PAM-amended water sample, agitated, then placed in a spectrophotometer. The PAM concentration in the suspension was correlated with settling-related transmittance changes. One highly correlated (r = 0.91-0.98) parameter, the time needed to initiate suspension clearing (clarity-shift inflection, CSI), was used as the procedure endpoint. The procedure was sensitive to variations in the amount of kaolinite added, and sample volume, water salinity, and original sediment content. A 10% change in these factors altered measured CSIs by 10 to 50%. The sediment affected CSI by increasing the sample's dissolved organic C concentration. The procedure detected as little as 0.1 mg L -1 PAM dissolved in irrigation water ; in samples containing >4 mL settled sediment per liter, the PAM detection limit was approximately 0.25 mg L -1 . Precision ranged from ±0.06 to 0.11 mg L -1 for 0 to 2.5 mg L -1 PAM and ±0.39 to 0.86 mg L -1 for 2.5 to 10.0 mg L -1 PAM. The PAM concentration in runoff from irrigated furrows equaled that of the inflow stream after 3 h continuous treatment at 10 mg L -1 . The CSI test provides a simple and accurate method of determining polyacrylamide in surface waters.