Assessing nutrient loading from reclaimed water irrigation using the chemical marker iohexol

Reclaimed water irrigation is a beneficial practice that could worsen nitrogen impairment of surrounding waterbodies. Estimating this contribution requires development of a suitable chemical marker. Toward that end, reuse effluents throughout Florida analyzed by liquid chromatography tandem mass spectrometry or high-resolution atomic mass identified the radiographic contrast medium iohexol as a marker unique to reclaimed water. Because iohexol may be subject to biodegradation during subsurface transport or photolability during surficial flow, this study measured iohexol degradation from solar insolation and its stability relative to nitrate during soil transport for inclusion within a mass balance calculation for a nitrogen impaired surface water in Naples, Florida. In this application, the reuse irrigation fractional volumetric flow contribution was ≤7% of the flow to the impaired waterbody. Subsurface flow was not considered because column experiments with local soil demonstrated preferential denitrification over iohexol biodegradation. Additional studies are needed to further demonstrate the potential merit of this approach in different regions.