BaseEMap: BASELINE CONCENTRATIONS OF ELEMENTS OF ENVIRONMENTAL INTEREST IN IOWA'S STREAM SEDIMENTS - YEAR THREE REPORT

This is the third year report for BaseEMap, a long-term study to chart background concentrations of elements of environmental interest in Iowa stream sediments. The project allows some students in the University of Northern Iowa Liberal Arts Core Capstone class Environment, Technology and Society to participate in ongoing research related to the environment. The samples were collected during six semesters from Fall 2008 to Spring 2011 by students in eighteen sections of the class.

Students were encouraged to collect samples from their home towns to promote interest in the project. Students collected as individuals, or in teams following chain-of-custody protocol. Samples consisted of sand- or silt/clay-dominant material from the stream bottom or bank within the channel. This update describes the addition of new samples, including replicates, to a total of 465 grab samples of stream sediment. The bulk of samples (~96%) come from east of the Mississippi-Missouri divide, and sample density is finally sufficient to display concentrations on maps for that part of the state.

The samples were minimally prepared and were analyzed as loose powders using low-power energy-dispersive XRF. Trace elements included Ba, Cr, Cu, Mn, Ni, Pb, Rb, S, Sr, V, Zn, and Zr (as parts per million, ppm) and P, and other elements and oxides as needed to characterize the samples or to improve spectrum deconvolution. Results for As, Cd, Co and Hg continue to be elusive, owing to low concentration, lack of calibration standards, or peak interferences. Treatment with H₂O₂ to remove organic material leads to lower values for Cr, Mn, S, and V, but not other elements.

Concentrations typically show log-normal distributions where either the mean or the bulk of the distribution is less than published estimates for average crustal values. Preliminary results indicate ranges (min – max ppm) as follows: Ba (<120 – 700), Cr (<10 – 62), Cu (<10 – 52), Mn (<20 – 2900), Ni (<10 – 40), Pb (<10 – 45), S (<25 – 2100), Zn (<10 – 150), Zr (<15 – 625), and P₂O₅ (<0.13 – 0.86 %). Sand-dominant samples yield trace element concentrations that were lower, or below limits of analysis, whereas silt/clay-rich samples had higher concentrations, especially for metals in organic-rich samples. At this time, no “hot spots” have been identified.