Forest species within the Mississippi River floodplain include northern red oak (Quercus rubra), eastern cottonwood (Populus deltoides), American elm (Ulmus americana) white oak (Quercus alba), black walnut (Juglans nigra), silver maple (Acer saccharinum), yellow poplar (Liriodendron tulipifera), pin oak (Quercus palustris), American sycamore (Platanus occidentalis), green ash (Fraxinus pennsylvanica), sweetgum (Liquidambar styraciflua), black willow (Salix nigra), red maple (Acer rubrum), nuttall oak (Quercus texana), water oak (Quercus nigra), common hackberry (Celtis occidentalis), river birch (Betula nigra), boxelder (Acer negundo), and bald cypress (Taxodium distichum) (* Aide et al., 2021 ). In many locations, the forest vegetation has been cleared for urbanization and agriculture.

Two pedons of the Commerce series (Fine-silty, mixed, superactive, nonacid, thermic Fluvaquentic Endoaquepts) and two pedons of the Caruthersville series (Coarse-silty, mixed, superactive, calcareous, thermic Typic Udifluvents) were morphologically described and sampled. The Commerce pedons presented A-Bw-Bg-Bssg horizon sequences and displayed ochric and cambic horizons, whereas the Caruthersville pedons presented A-C-Cg horizon sequences and displayed ochric epipedons.

Nine surface horizons of the Wilbur series (Coarse-silty, mixed, superactive, mesic Fluvaquentic Eutrudepts) were sampled using aqua regia digestion to specify the Geochemical Background concentrations. The Wilbur pedons have no history of environmental impact and these soils drain into the Mississippi River floodplain study area. The Wilbur pedons have A – Bw – Cg silt loam horizon sequences and reside on silty floodplains. The ochric and cambic soil horizons do not have sediment deposition from the modern Mississippi River, rather they receive silty sediment arising from erosion of the surrounding loess uplands. Additionally, one pedon of the Commerce soil series, which is not currently experiencing annual flooding, was also sampled to provide alternative lead, zinc, and cadmium geochemical background concentrations.

The climate is humid continental. The historical weather data base for Cape Girardeau County provides that the annual rainfall is 1.07 m (49.3 inch), the average daily maximum annual air temperature of 20˚C (68˚F), average daily minimum annual air temperature of 8.6˚C (48˚F), average daily annual air temperature of 14˚C (57˚F), and the average annual total solar radiation of 14 MJ∙m⁻² ( Festervand, 1981 ). Annual flooding durations are generally 1 to 7 days; however, long flood intervals may occur during spring snowmelt in the upper Mississippi River watershed. The local floodplain geomorphology features broad to narrow flood prone areas with incised variably sized stream channels that supply sediment from the overlying hinterland. Mississippi River floodplain soils are Holocene age, which include the soil orders Entisol, Inceptisol, Mollisol, and Vertisol ( Festervand, 1981 ).

Soil pH in water, exchangeable cations, total neutralizable acidity, soil organic matter content by loss on ignition, Bray 1 phosphorus, and 2M potassium chloride extractable SO₄-S were determined by the University Missouri-Columbia Soil Testing Laboratory.

An aqua-regia digestion was performed to estimate elemental concentrations associated with whole soil soluble, exchangeable, organically-complexed, adsorbed/co-precipitated with oxyhydroxide environments and the lattice degradation of phyllosilicates ( Aide & Fasnacht, 2010 ). In this procedure, 0.25 g of finely ground fine earth fraction was digested in 0.01 liter of aqua regia (1 HCl:3HNO₃) for one hour, followed by 0.45 µm filtering with an aliquot analyzed using inductively coupled plasma—atomic emission spectrometry. Quality assurance and analytical accuracy involved four certified reference materials for mining and exploration and duplicate samples.

For the Na-acetate leach, a 0.75 g sample of passing a 60-mesh soil horizon material is leached with a sodium acetate matrix at 30˚C for 1 hour. Two controls and duplicate samples were also Na-acetate leached. The solutions are analyzed using inductively coupled plasma-mass spectrometry. Soil analysis for the aqua regia digestion and Na-acetate extraction were performed by Activation Laboratories (Ancaster, Ontario).

The aqua regia digestion concentrations in the Caruthersville pedons are typical and are within the ranges for soils without evidence of heavy metal impact ( Kabata-Pendias, 2020 ). The iron concentrations range from 8,300 to 11,800 mg∙kg⁻¹, whereas the manganese concentrations range from 194 to 456 mg∙kg⁻¹ ( Table 2(a) ). We selected zinc, cadmium, and lead for additional scrutiny because the literature suggests that some Mississippi River floodplain soils are impacted by these elements. The elemental aqua regia digestion concentration ranges are: (i) 24.2 < zinc < 37.4 mg∙kg⁻¹, (ii) 0.08 < cadmium < 0.26 mg∙kg⁻¹, and (iii) 5.9 < lead < 8.8 mg∙kg⁻¹.

For the Caruthersville pedons, the mean and standard deviation values for other selected aqua regia digestion elemental concentrations (mg∙kg⁻¹, except Au which is µg∙kg⁻¹) are: Metal (mean, standard deviation), Co (4.6, 0.5), Ni (13.3, 3.7), Cu (5.0, 1.6), V (20.0, 2.6), Cr (10.7, 1.4), Mo (0.22, 0.07), Ag (0.032, 0.006), Sn (0.38, 0.11), Sb (0.18, 0.03), Ti (0.04, 0.005), Nb (0.36, 0.05), Sc (1.38, 0.27), Au (1.44 ppb, 0.72 ppb), Zr (2.56, 0.28), Y (6.64, 0.55), Hg (0.03, 0.01) and Tl (0.09, 0.03). For both soil series, the concentrations for some elements were below detection: (i) tungsten and hafnium are less than 0.1 mg∙kg⁻¹, (ii) Tantalum is less than 0.05 mg∙kg⁻¹, and (iii) rhenium is less than 0.001 mg∙kg⁻¹.

The aqua regia digestion concentrations in the Commerce pedons are typical and are within the ranges for soils without evidence of environmental impact ( Kabata-Pendias, 2020 ). The selected elemental values for the Commerce pedons are significantly greater than those of the Caruthersville pedons, features attributed to the greater clay contents. The iron concentrations range from 18,300 to 25,200 mg∙kg⁻¹, whereas the manganese concentrations range from 650 to 1,120 mg∙kg⁻¹ ( Table 2(b) ). The measured elemental concentrations range from (i) 56.4 < zinc <85.7 mg∙kg⁻¹, (ii) 0.45 < cadmium < 0.81 mg∙kg⁻¹, and (iii) 17.4 < lead < 27.8 mg∙kg⁻¹. Normal soil metal concentrations range for selected elements are: (i) 60 < Zinc < 89 mg∙kg⁻¹, (ii) 0.2 < cadmium < 1.1 mg∙kg⁻¹, and (iii) 3 < lead < 90 mg∙kg⁻¹.

For the Commerce pedons, the aqua regia digestion mean and standard deviation concentrations (mg∙kg⁻¹, except Au which is µg∙kg⁻¹) for other selected elements include: Co (9.8, 1.1), Ni (25.3, 3.2), Cu (18.7, 2.3), V (47, 5), Cr (25, 3), Mo (0.76., 0.52), Ag (0.11, 0.01), Sn (0.93, 0.07), Sb (0.39, 0.09), Ti (0.03, 0.01), Nb (0.49, 0.08), Y (12.3, 1.0), Zr (2.44, 0.30), Sc (3.58, 0.40), In (0,028, 0.006), Te (0.05, 0.01), Hg (0.06, 0.01) and Au (2.28 ppb, 1.15 ppb). The concentration values are within the geogenic concentrations documented in Kabata-Pendias (2020) .

Sodium acetate extractions estimate metal concentrations that readily participate in cation exchange with ion-bearing solutions. As an example, [clay =] Cd²⁺ + 2Na⁺ = [clay=]2Na⁺ + Cd²⁺. For the two pedons of the Caruthersville series the mean and standard deviation values (µg∙kg⁻¹) for iron, manganese, cadmium, and lead concentrations are: iron (19,000, 12,000), manganese (100,000, 50,000), cadmium (81, 31), and lead (463, 274). Iron, manganese and cadmium show little soil depth variation, whereas lead shows somewhat greater concentrations in the deeper soil horizons and zinc is generally less concentrations than the detection limit. For the Caruthersville pedons the mean and standard deviation values for other selected Na-acetate extraction concentrations (µg∙kg⁻¹) include: Co (156, 76), Ni (1,411, 865), Cu (417, 119), V (90, 32), Mo (43, 14), Nd (Sb (17, 7), Nd (754, 147), Ti (2350, 261), Tl (4.4, 0.6), and Y (879, 165). For the Na-acetate extraction, undetected concentration values are: (i) Cr is less than 100 µg∙kg⁻¹, (ii) Ag is less than 5 µg∙kg⁻¹, (iii) Sn is less than 10 µg∙kg⁻¹ and (iv) Zn is less than 2,000 µg∙kg⁻¹. The zinc concentration was less than the detection limit (2,000 µg∙kg⁻¹).

The mean and standard deviation values (µg∙kg⁻¹) for iron, manganese, cadmium, and lead concentrations for the two pedons of the Commerce series are: iron (8,000, 3,000), manganese (52,000, 20,000), cadmium (242, 36), lead (386, 39), and Zinc (3,700, 675). The mean lead value of the Commerce series is smaller than that of the Caruthersville; however, the t-test value (P = 0.18) is not significant. The mean cadmium value of the Commerce series is significantly greater than that of the Caruthersville series.

For the Commerce pedons, the mean and standard deviation values (µg∙kg⁻¹) for other elemental Na-acetate extractions include: Co (37, 19), Ni (785, 159), Cu (440, 52), V (121, 28), Mo (23, 5), Sb (24, 3), Nd (865, 64), Ti (2550, 673), Tl (6, 0.5), Y (1017, 97). Soil depth variations for Mn are slightly greater for the near surface horizons; however, the soil depth variations for zinc, cadmium and lead are rather uniform. Undetected Na-acetate extraction concentration values include (i) Cr less than 100 µg∙kg⁻¹, (ii) Ag less than 5 µg∙kg⁻¹, and (iii) Sn less than 10 µg∙kg⁻¹.

For the Caruthersville pedons, the comparison of the Na-acetate manganese extraction concentration (100 mg∙kg⁻¹) with the manganese aqua regia digestion concentration (306 mg∙kg⁻¹) presents a ratio percentage of 33%. The corresponding values for cadmium (41%) and lead (6.3%) vary greatly, testifying to their intrinsically different soil chemistries, including adsorption intensities. The cadmium value of 41% suggests that a near majority of the cadmium cations are biologically available, whereas the lead is only incidentally biologically available at 6.3%. For the Commerce pedons, the comparison of the Na-acetate manganese extraction (52 mg∙kg⁻¹) with the manganese aqua regia digestion (928 mg∙kg⁻¹) is a ratio percent of 5.6%, The corresponding values for cadmium (40%), zinc (4.7%) and lead (2%) also vary with their respective elemental chemistries. Interestingly, the cadmium and lead values are similar between the soil series.

The PI pollution index for aqua regia digestion concentrations (PI) is described as:

PI = [Metal Concentration]/[Geochemical Background Concentration]. The calculated PI values may be interpreted as: (i) PI < 1 is absent, (ii) 1 < PI < 2 is low, (iii) 2 < PI < 3 is moderate, (iv) 3 < PI < 5 is strong, (v) PI > 5 is very strong ( Kowalska et al., 2018 ; Gong et al., 2008 ).

The EF Enrichment factor pollution index for aqua regia digestion concentrations is described as: EF = [Metal Concentration/Ti Concentration]/[Background Metal Concentration/Ti background Concentration]. The calculated EF values may be interpretated as: (i) EF < 2 is minimal enrichment, (ii) 2 < EF < 5 is moderate enrichment, (iii) 5 < EF < 20 is significant enrichment, (iv) 20 < EF < 40 is very high enrichment, and (v) EF > 40 is extremely high enrichment ( Kowalska et al., 2018 ; Gong et al., 2008 ).

Using the Geochemical background concentrations from the aqua regia digestion the Wilbur series surface samples (mg∙kg⁻¹) and standard deviation (mg∙kg⁻¹) for the following elements are: (i) Ti (344/53), (ii) Fe (15400/2500), (iii) Mn (894/266), (iv) Zn (55/10), (v) Cd (0.19/0.07), (vi) Pb (14.8/3.5). We also sampled a Commerce pedon. The mean and standard deviation concentrations of the Commerce pedon are: (i) Ti (700, 200), (ii) Zn (53, 25), (iii) Cd (0.13, 0.12) and (iv) Pb (15, 8).

The Pollution Index and the Enrichment Factor were calculated using the Wilbur surface horizons for geochemical background initiation. The average pollution index (PI) for the Caruthersville pedons indicate “no pollution” for Zn, Cd or Pb, whereas these values for the Commerce pedons indicate “low pollution” for Zn and Pb and “moderate pollution” for Cd ( Table 3 ). Some horizons in the Commerce pedons have PI values indicating “strong pollution” involving cadmium. The Enrichment Factor values suggest “absent” or “minimal enrichment” for Zn, Cd and Pb in the Caruthersville pedons and for Zn and Pb in the Commerce pedons. The Commerce pedon’s enrichment factors for Cd indicate “moderate enrichment” and a few horizons indicate “strong” enrichment.

PI is Pollution Index, EF is Enrichment Factor.

The Pollution Index calculation using the Commerce pedon geochemical background data provided pollution index values indicating “absent” or “low” pollution impacts for zinc and lead and “moderate” pollution impact for cadmium for the Caruthersville series. For the Commerce series the pollution impacts are considered “low” for zinc and lead and “strong” for cadmium. One horizon indicated very strong “pollution impact” for cadmium.

We exercise caution with respect to the pollution index values inferring that these soils are heavy metal impacted, especially for cadmium. The aqua regia digestion concentrations do not support a significant heavy metal accumulation. The upper range for normal concentrations for (i) zinc is 89 mg∙kg⁻¹, (ii) cadmium is 0.60 mg∙kg⁻¹, and (iii) lead is 90 mg∙kg⁻¹. The mean concentrations for the Commerce pedons are (i) zinc is 78 mg∙kg⁻¹, (ii) cadmium is 0.60 mg∙kg⁻¹, and lead is 21.5 mg∙kg⁻¹. The mean cadmium level of the Commerce pedons is precisely equivalent to the upper level for cadmium’s typical soil distribution, whereas the Commerce pedon’s zinc and lead mean concentrations are below the upper limits for a normal soil presentation. The geochemical background values derived from the Wilbur surface samples exhibited small concentrations: (i) zinc at 55 mg∙kg⁻¹, (ii) cadmium at 0.15 mg∙kg⁻¹, and (iii) lead at 14.8 mg∙kg⁻¹, which lead to implication that annual Mississippi River flood prone soils are more cadmium impacted than the Wilbur pedons. The geochemical background values derived from the Commerce reference pedon demonstrate similar pollution index and enrichment factor values. It remains entirely possible that these Mississippi River floodplain soils are receiving cadmium; however, that amount of cadmium in these soils is not necessarily a significant indication for remediation. Future research involving additional samples and continued monitoring of existing sites is warranted.