2.3.1. Donnan Membrane Technique Setup

The Donnan membrane technique (DMT), which identifies water-soluble Cd and Pb throughout the rice growing season, was used in-field for every month from August to November 2016, 2017, and 2018. This in-situ technique utilizes an acceptor cell (i.e., 10 mL of 0.01 mol L−<sup>1</sup> CaCl2, with Ca moving out of the cell when heavy metal cations move into the cell), separated by a positively charged membrane (BDH, No. 55165 2U) held by two O-rings [26] (Figure S1). The BDH membrane has a matrix of polystyrene/divinylbenzene, with sulphonic acid groups which are fully deprotonated above pH = 2. The solution was changed every month over the study period. Solution Cd and Pb concentrations were determined by graphite furnace atomic absorption spectrometry (GFAAS, Zeenit 700p, Jena, Germany).
