Spatial and campaign-scale dynamics of trace elements in lakes and ponds of the Iowa and Nebraska - Missouri River floodplain.

Trace-element contamination in small floodplain water bodies can vary sharply among neighboring catchments, and regional averages may conceal individual hotspots. Water from six lakes and ponds in western Iowa and eastern Nebraska were examined during three site-wide surveys: Summer 2024, Fall 2025, and Spring 2026, yielding 18 site-campaign observations. Eleven trace elements-aluminum (Al), arsenic (As), cadmium (Cd), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), selenium (Se), and zinc (Zn) were retained after data verification, and concentrations were standardized to µg L-1. Site-specific exceedance ratios (ER = C/S) and Heavy Metal Pollution Index (HPI) values were calculated using the study screening benchmarks. Lake Manawa had the highest mean HPI (327.13) and the highest overall mean ER (1.85), driven principally by Co (mean ER = 7.19), Pb (5.16), Mn (1.99), Al (1.94), and Cd (1.49). Among the remaining sites, Pinky's Glen had the highest mean ER (1.57), dominated by Mn (5.31), Al (3.24), and Co (3.15), whereas Steinhart Park showed elevated Mn (5.61), Co (3.05), and Al (2.59). An additive two-factor variance partition indicated that water-body identity accounted for more than 90% of variation in Al, Co, Cu, Fe, Mn, Pb, and Zn during the sampled campaigns. The campaign accounted for 95.8% of As variation and 54.2% of Cd variation. An equal-weight score based on min-max-normalized mean HPI, exceedance frequency, and mean ER ranked Lake Manawa, Pinky's Glen, and Steinhart Park as the three highest priorities for confirmatory monitoring. A recovered physicochemical survey showed the highest mean TDS and EC at Carter Lake (264.67 mg L-1 and 529.33 µS cm-1), water temperatures of 3.03-6.57 °C, and circumneutral mean pH (6.95-7.62). Because the three surveys occurred in different years, temporal comparisons and benchmark exceedances are interpreted as screening-level evidence. The findings support site- and analyte-specific confirmatory monitoring rather than pooled regional assessment.