Relationship of arsenic and chromium availability with carbon functional groups, aluminum and iron in Little Washita River experimental watershed reservoirs, Oklahoma, USA

Authors: NGATIA, LUCY - Florida A & M University; OLIVEIRA, L - Florida A & M University; BETIKU, O - Florida A & M University; FU, R - Florida State University; Moriasi, Daniel; STEINER, JEAN - Kansas State University; Verser, Jerry - Alan; TAYLOR, R - Florida A & M University

Submitted to: Ecotoxicology and Environmental Safety
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/6/2020
Publication Date: 10/29/2020
Citation: Ngatia, L.W., Oliveira, L.M., Betiku, O.O., Fu, R., Moriasi, D.N., Steiner, J.L., Verser, J.A., Taylor, R.W. 2020. Relationship of arsenic and chromium availability with carbon functional groups, aluminum and iron in Little Washita River experimental watershed reservoirs, Oklahoma, USA. Ecotoxicology and Environmental Safety. 207. https://doi.org/10.1016/j.ecoenv.2020.111468.
DOI: https://doi.org/10.1016/j.ecoenv.2020.111468

Interpretive Summary: As a result of the Watershed Protection and Flood Prevention Act 1954, USDA installed 45 reservoirs between 1969 and 1982 in the Little Washita River Experimental Watershed (LWREW), located in central Oklahoma. In this study, sediment from three of the 45 reservoirs located in cropland, forest and grassland areas were analyzed to determine total and extractable or soluble concentrations of arsenic (As) and chromium (Cr) heavy metals, their availability drivers and the potential ecological risks to organisms living in these reservoirs. Results showed that total As concentrations ranged from 16.2 mg kg-1 (parts per million – ppm) for the reservoir located in the forest area to 141 ppm in cropland while total Cr ranged from 5.06 ppm in forest to 40.10 ppm in cropland area. Soluble As concentrations ranged from 0.24 ppm in forest to 1.21 ppm in grazing area while soluble Cr concentrations ranged from 0.13 ppm in cropland to 0.58 ppm in forest area. The resultant soluble As and Cr concentrations were above the Environmental Protection Agency (EPA) limit levels for water, which is 10 parts per billion (ppb) for As and 50 ppb for Cr. However, only approximately <1.5% of total As and <4% of total Cr were soluble. The sediments exhibited alkaline pH ranging from 8.0 to 8.7 resulting in high sorption of both As and Cr to aluminum, iron. The O-alkyl carbon a form of carbon that is easily degradable highly favored As and Cr sorption in the reservoirs. The sediment concentration analysis results showed that As concentrations exceeded their respective either TEL (threshold effect level) and/or PEL (probable effect level) in all reservoirs, indicating that existing concentrations of As in these sediments were suf'ciently high to cause adverse ecological effects. Results also indicated that Cr concentrations were lower than both TEL and PEL in all three reservoirs. However, changes in water and soil chemistry that would lead to increased sediment acidity and increased decomposition of easily changeable C (O-alkyl C) has the potential to increase As and Cr availability, potentially enhancing their toxicity.

Technical Abstract: Reservoir sediment contaminated with heavy metals act as a secondary source to aquatic ecosystems. Once released to the water column, these metals could be toxic to biota and humans. Sediment from three reservoirs located in the Little Washita River Experimental Watershed (LWREW) in Oklahoma, USA with contrasting dominant land uses were analyzed for total and extractable concentrations of arsenic (As) and chromium (Cr), determined their availability drivers, and the potential ecologic risk to benthic organisms. The results indicate that both total and extractable metal concentrations were highly variable between cropland, grazing, and forest dominant land uses. Extractable As ranged from 0.24 to 1.21 mg kg-1, in the order grazing>cropland>forest and 0.13 to 0.58 mg kg-1 for extractable Cr, in the order of forest>grazing>cropland. The results indicate that both extractable As and Cr were above EPA limit level for water which is 10 µg L-1 for As and 50 µg L-1 for Cr indicating potential for exchanged between sediment and the water column. However, only approximately <1.5% of total As and <4% of total Cr were extractable. Total As ranged from 16.2 to 141 mg kg-1 and total Cr ranged from 5.06 to 40.1 mg kg-1 both in the order of cropland>grazing>forest. The sediment exhibited an alkaline pH ranging from 8.0-8.7, with As sorption exhibiting a positive relationship with Al (r = 0.9995; P= 0.0001), Fe (r = 0.9829; P= 0.0001), and C (r = 0.4090 ; P= 0.0017) and Cr correlated positively with Al (r = 0.9676 P= 0.0001 ), Fe (r = 0.9818 ; P= 0.0001), and C (r = 0.3368; P= 0.0111). In addition, both As and Cr exhibited positive relationships with carbon (C) functional groups in the order of O-alkyl C > methoxyl C> alkyl C> aromatic C > carboxyl C> phenolic C. The sediment concentration analysis results illustrated that As in all reservoirs exceeded their respective either TEL (threshold effect level) and/or PEL (probable effect level), indicating that existing concentrations of metals in these sediments were suf'ciently high to cause adverse effects. However, Cr concentrations in all reservoirs evaluated was found lower when compared to the TEL and PEL. In conclusion, changes in water and soil chemistry that would lead to increased sediment acidity and increased decomposition of the labile O-alkyl C has the potential to increase As and Cr availability, potentially enhancing their toxicity.