Kenneth F. Steele

Dew and frost chemistry at a midcontinent site, United States

From July 1989 to July 1990 a total of 98 dew and 9 frost samples were collected at the University of Arkansas Agricultural Experiment Station, Fayetteville. The total water flux from dews and frosts per year was less than 2% of that from rains. Acid and nutrient fluxes were also much lower in dew. In the following series of ions the number in parentheses gives the percent of the yearly flux of the ion in dew compared to rain for the same time period: H+ (0.06), Ca2+(25), Mg2 + (11), K+ (21), Na+ (4), NH4+ (10), Cl− (4), NO3− (5), and SO2−4 (5). In contrast, the concentration of the various ions in dew, except H+, is generally several times that in rain. Dew is primarily a Ca(HCO3)2 and (NH4)2SO4 solution; whereas rain is primarily a NH4HSO4 solution. The mean pH of dew is 6.37 compared to 4.99 for rain. The average acetate-formate concentration of dew in equivalents was (1) equal to about 53% of the nitrate-sulfate concentration in rain for April-June and (2) 10 times more concentrated than in rain for the year. The steps governing dew composition are indicated to be (1) formation on dry deposition solids (2) dissolution of the soluble portion of the dry deposition by dew water, and (3) sorption of gaseous NH3, acetic, and formic acids into the dew solution.

Distribution and variability of redox zones controlling spatial variability of arsenic in the Mississippi River Valley alluvial aquifer, southeastern Arkansas

Twenty one of 118 irrigation water wells in the shallow (25-30 m thick) Mississippi River Valley alluvial aquifer in the Bayou Bartholomew watershed, southeastern Arkansas had arsenic (As) concentrations (<0.5 to 77 microg/L) exceeding 10 microg/L. Sediment and groundwater samples were collected and analyzed from the sites of the highest, median, and lowest concentrations of As in groundwater in the alluvial aquifers located at Jefferson County, Arkansas. A traditional five-step sequential extraction was performed to differentiate the exchangeable, carbonate, amorphous Fe and Mn oxide, organic, and hot HNO(3)-leachable fraction of As and other compounds in sediments. The Chao reagent (0.25 M hydroxylamine hydrochloride in 0.25 M HCl) removes amorphous Fe and Mn oxides and oxyhydroxides (present as coatings on grains and amorphous minerals) by reductive dissolution and is a measure of reducible Fe and Mn in sediments. The hot HNO(3) extraction removes mostly crystalline metal oxides and all other labile forms of As. Significant total As (20%) is complexed with amorphous Fe and Mn oxides in sediments. Arsenic abundance is not significant in carbonates or organic matter. Significant (40-70 microg/kg) exchangeable As is only present at shallow depth (0-1 m below ground surface). Arsenic is positively correlated to Fe extracted by Chao reagent (r=0.83) and hot HNO(3) (r=0.85). Arsenic extracted by Chao reagent decreases significantly with depth as compared to As extracted by hot HNO(3). Fe (II)/Fe (the ratio of Fe concentration in the extracts of Chao reagent and hot HNO(3)) is positively correlated (r=0.76) to As extracted from Chao reagent. Although Fe (II)/Fe increases with depth, the relative abundance of reducible Fe decreases noticeably with depth. The amount of reducible Fe, as well as As complexed to amorphous Fe and Mn oxides and oxyhydroxides decreases with depth. Possible explanations for the decrease in reducible Fe and its complexed As with depth include historic flushing of As and Fe from hydrous ferric oxides (HFO) by microbially-mediated reductive dissolution and aging of HFO to crystalline phases. Hydrogeochemical data suggests that the groundwater in the area falls in the mildly reducing (suboxic) to relatively highly reducing (anoxic) zone, and points to reductive dissolution of HFO as the dominant As release mechanism. Spatial variability of gypsum solubility and simultaneous SO(4)(2-) reduction with co-precipitation of As and sulfide is an important limiting process controlling the concentration of As in groundwater in the area.

Geochemistry of Carboniferous limestone units in northwest Arkansas

Abstract Three Mississippian limestone units, the St. Joe, Boone and Pitkin Formations, and two Pennsylvanian limestone units, the Brentwood and Kessler Members of the Bloyd Formation, were sampled over a seven-county area in northwest Arkansas. The HCl-soluble portions of the limestones were analyzed for Ca, Mg, Sr, Ba, Al, Si, Na, K, Li, Fe, Mn, Zn, Ni, Cu, Co and Cr. Using histograms, the background concentrations of these elements and the percentage HCl-soluble material were determined for each unit. All units averaged more than 92% HCl-soluble material and more than 91% calcite for the soluble portion. Mississippian limestones had less insoluble residue and less cation impurities than Pennsylvanian limestones. Ratios of Sr/Ca and Mg/Ca for the five units decreased with increasing geological age. The concentrations of cation impurities were comparable to a “world” average limestone for all units except for Ba, Al, Si, Cu, Na and K. Compared to limestone of comparable age, the Ba and Cu contents were normal. The concentrations of Na, K, Al and Si are low because they are contained mainly in the acid-insoluble residues. All five limestone units showed a regular increase in the following elements with increasing Mg content: Na, Sr, Li, Mn and Fe. It is postulated that this may be due to disorder in the calcite lattice and proxy sites caused by Mg 2+ ions which facilitate substitution of cation impurities. A graphical method is developed for determining the concentration of an element in the completely soluble (carbonate) and partially soluble (non-carbonate) portions of a limestone. The concentrations of the various impurity elements in the carbonate portions were generally less than the concentrations in the partially soluble fractions. The percentage of the total impurity in the carbonate fraction ranged as follows for several elements among four limestone units: Na (24–75%), K (0–33%), Li (76–89%), Mg (39–83%), Sr (35–91%), Al (5–47%), Fe (10–68%), Mn (44–85%). The two ions, K + and Al 3+ , least partitioned into the carbonate fraction, are misfits both in terms of charge and size for the calcite lattice.

Rain chemistry at a midcontinent site, U.S.A., 1980–1984

Abstract Rains were collected weekly at Fayetteville, Arkansas and classed as normal or anomalous by the cation/anion equivalents per equivalent ratios. For normal rains (83 % of the precipitation) the ratios were 0.85–1.15. The deviation of anomalous rains from this range was primarily because of HCO3− for which no analyses were made. A close association of certain cations with certain anions in normal rains is indicated by a constant ratio of their fluxes in equivalents per equivalent. These associations and the flux ratios for normal rains during 1980–1984 are (with ratios for anomalous rains in parentheses): ( NH 4 + + H + ) SO 4 2− = 1.01 (1.13) and ( Ca 2+ + Mg 2+ + K + ) NO 3 − = 0.96 (2.26) . Weekly flux ratios for these ions for normal rains are also near unity for all seasons. Correcting for sea salt contributions, the weekly fluxes have the following relationships based on the flux of ions in equivalents ha−1 week−1: NH4+ + H+ − SO42− = ΔSO42−Ca2+ + Mg2+ + K+ + Na+ − NO3 = ΔNO3−ΔSO42− + ΔNO3− =0 and ∑ΔSO42− = 0 and ∑ΔNO3− = 0 within experimental error, when the summations are over a year or a season. In normal rains ΔSO42− and ΔNO3− averaged 18 % and 31 %, respectively of their base values due to scavenging of NH4NO3, HNO3 or mineral carbonate. In anomalous rains, the Δ values may be 100 % of the base values due to scavenging of large amounts of mineral carbonate. Most notable seasonal variations were: 1. (1) greater flux of all ions in spring and summer. 2. (2) SO 4 2− NO 3 − flux ratios in equivalents are 2.3 in winter, 1.8 in summer, and 2.0 in spring and fall. The following composition, in wt %, represents the average rain composition for the 1980–1984 collection period: 1.37 (NH4)2SO4 · H2SO4(51%), Ca(NO3)2 (27%), Mg(NO3)2(3%), NaNO3(3%), NH4NO3(2%). KNO3(4.5%), NaCl (6.8%), and several sea-salts of less than 2%.

Stoichiometry of rain across the U.S.A.: Evidence of independent neutralization of sulfate and nitrate acidities

Annual wet-deposition fluxes of ions in the rains for 1982–1984 at the NADP/NTN sites across the U.S.A. were corrected for contributions from sea salt and anomalous rains (∫cations/∫ anions ratios outside 0.85 to 1.15). The resulting fluxes of normal rains in gram equivalents/ha/yr were used to determine the flux ratios of (NH4+ + H+)/SO42− and (Ca2+ + Mgt+ + K+ + Na+)/N03 . The first ratio is 1 ± 0.15 over a large area including most sites west of the Mississippi River, except California. A smaller core of this area has the second ratio equal to 1 ± 0.15. These results are attributed to mineral carbonate aerosol from the Western Plains being distributed by the prevailing westerly winds. This aerosol acts as a sink for NOx and forms mineral nitrates. When the supply of carbonate is limited, free HN03 occurs. The aerosol NH4HSO4 is the main sink for NH3 and SO2. Thus, a major conclusion is that sulfate and nitrate acidities are neutralized independently.

Metal concentrations in the ground water of the Ouachita Mountains, Arkansas, U.S.A.

Ninety-one ground water samples (predominantly from springs) in two mineralized areas of the Ouachita Mountains in west-central Arkansas, were analyzed for Fe, Mn, Zn, Cu, Co, Ni, Pb, Hg, Sb, Sr, Ba, Ca, and Li. These areas contain Mn, barite, strontianite, cinnabar, stibnite and scattered Pb-Zn mineralization, Cumulative frequency curves were used to determine the threshold and anomalous concentrations for each element in the two areas. These values were, in general, higher in the ground water from the more mineralized area for several of the base metals, but most notably for Mn and Fe, the principal metals in the Mn oxide minerals.The United States Environmental Protection Agency (EPA, 1976) criteria for Fe (300 μg L −1) and Mn (50 μg L−1) in drinking water were exceeded, respectively, in 34% and 30% of the springs in Area I, and 13% and 23% in Area II. One spring exceeded the EPA Hg criterion (2 μg L−1) and 3 springs exceeded the 50 μg L−1 criterion for Pb. In spite of the large number of anomalous Ba concentrations, the highest concentration of Ba was 930 μg L−1 (EPA criterion 1000 μg L −1).

Trace metal relationships in bottom sediments of a fresh water stream; the Buffalo River, Arkansas

ABSTRACT Sediments were sampled over 130 mi of the Buffalo River in northern Arkansas. Aqua regia extracts of the minus 95 mesh fraction of the sediments were analyzed by atomic absorption for Na, K, Mg, Ca, Zn, Cd, Cu, Pb, Fe, Co, Cr, Ni, and Mn. Zirconium was determined by X-ray fluorescence. There was a general decrease downstream in Fe, Cu, Cr, Ni, Mn, Pb, K, and Na as the drainage area increased in carbonate rock and decreased in shale. The elements Mg, Ca, Zn, and Cd increased in sediments downstream. The sediments are mainly quartz and chert grains. These grains are apparently coated with hydrous iron oxide which acts as a sorbent for most of the metals and is a dominant transport mechanism for acid extractable Co, Cr, Ni, Cu, Mn, and K. Other acid extractable metals, particularly Mg, Ca, Zn, Cd, and Pb are mostly in clastic grains. Cadmium to zinc ratios in the sediments and in ore minerals of the area are similar. The ore metals of the drainage area are Zn, Pb, and Cu. The acid extractable amounts of these metals in the sediments rise and fall with the extractable amounts of Ca plus Mg in the sediments, indicative of the favorite host rock, dolomite. Tributaries are sites of rapid rise and fall of metal values within a few miles from background to anomalously high values and return to background.

Use of stream sediment elemental enrichment factors in geochemical exploration for carbonatite and uranium, Arkansas, U.S.A.

Three hundred and thirty-five stream sediment samples (<75×10−3 mm diameter) from 3275 km2 in central Arkansas, U.S.A. were utilized to evaluate geochemistry as an exploration method for carbonatite and U in this geologically diverse area. Approximately one-half of the area is located in the Ouachita Mountains and the other half is located in the Gulf Coastal Plain. Alkalic igneous intrusions are located in both regions; however, shale, sandstone, novaculite and unconsolidated sediments are the major lithologies. The areas was divided into four geochemical regions based on similarities in geology and geochemistry. Neutron activation analysis was used to analyze for rare earth elements (REE), Ti, F, U, and 17 other elements. A REE+Ti+ F map based on an enrichment index and a U concentration map delineate two anomalous areas. One of these anomalous areas is near the Magnet Cove intrusion which contains carbonatite, and the other is near lignite deposits that contain U and unusually high concentrations of REE. R-mode cluster analysis indicates that these two areas are distinctly different geochemical environments. Less anomalous samples are associated with syenite intrusions and shale. Anomalous samples associated with the carbonatite are enriched in light REE relative to anomalies associated with syenite intrusions and are enriched in light REE and Ti compared to anomalies associated with sedimentary rocks and lignite.

Stream sediment geochemical investigations in Arkansas - comparisons of manganese, zinc and lead-zinc districts with an unmineralized area

Abstract Aqua regia extracts of the minus 95-mesh fraction of bottom stream sediments from four areas in the State of Arkansas, U.S.A., all with contrasting mineralogy and/or lithology, were analyzed for Fe, Mn, Co, Ni, Cu, Zn, and Ba. Each of these elements increased uniformly with Mn content in Area I, a Mn district, whereas in the other areas only some of the elements followed this pattern. Background metal values of the stream sediments are similar for the manganese area and two other areas, all three of which have appreciable amounts of black shale outcrops. A fourth area containing little shale and mineralized in Zn is characterized by uniformly low background metal values, except for Zn. The largest anomalous values for Co, Cu, Ni and Ba are associated with the area enriched in Mn (Area I). Apparently Mn coatings on clasts and clasts of Mn ore are enriched in Co, Cu, Ni, and Ba and are responsible for these anomalous values. These high values, rather than base metal/Mn ratios, are the best geochemical indicators in stream sediments for this type of strata-bound Mn mineralization.

Hydrogeochemical exploration for Mississippi valley-type deposits, Arkansas, U.S.A.

Abstract Three areas located in northern Arkansas, U.S.A., representing differing host rock and mineralization, were selected to investigate the usefulness of hydrogeochemical exploration for Mississippi Valley-type Pb-Zn mineralization. Despite the geologic differences among the areas, there were no great differences in groundwater chemistry and threshold values. Anomalous Pb concentrations, and also anomalous Zn concentrations to a lesser extent, are useful in detecting the Pb-Zn mineralized areas; however, specific deposits could not be located. Because of the low threshold values (about 20 μg/1) for Pb and Zn, spring water must be utilized in order to avoid plumbing contamination.