Samples were collected in the lower reaches of Von Guerard stream in January 2019 along a transect P2 as reported by Heindel et al. 2021. Sediment sampling methods, metadata, and information regarding total extractable ammonium and additional ancillary data are also reported by Heindel et al. 2021. We utilized subsamples from the P2 (pool morphology) transect. Wet sediment subsamples were weighed in triplicate for each sampling location (1-5) into sterile 250 mL HDPE wide mouth bottles and kept frozen (-20 ºC) prior to running the assay.

For the assay, we prepared five solutions of varying conductivities using potassium chloride. The specific conductivity of each treatment solution was measured using a benchtop probe and a three or four point calibration. Sediment samples were removed from the freezer and treatment solution was added to each bottle to achieve a 2:5 sediment:solution ratio by mass. Bottles were inverted three times and placed on a shaker table at 300 rpm for 1 hour. Slurries were centrifuged at 3000 rpm for 5 minutes and then the supernatant was filtered through glass fiber filters (GF/F) and stored frozen prior to analysis.

Supernatant aliquots were analyzed by the Arikaree Environmental Laboratory at the University of Colorado Boulder. Potassium was measured by Method EPA 7000B on a Perkin Elmer Model AAnalyst 200 Atomic Absorption Spectrometer with a limit of detection of 0.004 mg K⁺ per liter. Chloride was measured by Method EPA 300.1 on a Metrohm 930 Compact IC Flex with a limit of detection of 0.03 mg Cl^- per liter. Nitrogen as ammonium measured by Method 10-107-06-1-A on a Lachat Quickchem Flow Injection Autoanalyzer with a limit of detection of 0.005 mg N per liter. Nitrogen as nitrate plus nitrite measured by Methods 10-107-04-1-A and 10-117-07-1-B on a Lachat Quickchem Flow Injection Autoanalyzer with a limit of detection of 0.002 mg N per liter.