Long-term Monitoring of Irrigation Ditch Sediment Metal(loid)s in the Animas and San Juan River Watershed, New Mexico (USA) after the Gold King Mine Spill

The 2015 Gold King Mine spill released 11.3 million liters of metal-laden water into the Animas River. To determine its impact downstream, we monitored Al, As, Cr, Fe, Mn, Pb, Zn, Ca, and Cu concentrations in irrigation ditch sediments fed by the Animas River during the 2017-2020 growing seasons. The elements, chosen for their presence in the original contaminant plume, were compared to regulatory risk assessment guidelines for soil. Sediments were collected from the mid-point and one side (at the high-water level mark) of fifteen irrigation ditch transects. Mixed models assessed for sampling time, sampling point (center or side), and their interaction for repeated measures in time and location in the ditch. Total concentrations collected from ditch sides were higher than the center concentrations for all elements except Mn. Only As exceeded the New Mexico Environment Department soil screening level of 7.07 mg kg⁻¹, increasing from 6.38 ± 0.47 mg kg⁻¹ (2017 pre-growing season) to 12.24 ± 2.67 mg kg⁻¹ (2019 post-growing season), a net increase of 5.86 ± 2.72 mg kg⁻¹. The Pb concentrations in ditch center sediments also increased over time, rising from 30.29 ± 6.90 mg kg⁻¹ to 49.87 ± 10.63 mg kg⁻¹, but remained below the U.S. EPA Residential Soil Screening Level of 400 mg kg⁻¹. In contrast, Mn and Zn fluctuated over the sampling period, while the other metal(loid) concentrations did not vary significantly. Monitoring sediment metal(loid) concentrations in irrigation ditches is essential not only for tracking the legacy impacts of upstream mining activities, but also for protecting downstream agricultural systems that depend on diverted river water, particularly in regions where irrigation infrastructure directly influences soil health, crop safety, and community resilience.