2012 Annual Report
1a.Objectives (from AD-416):
Conduct laboratory soil column studies, to determine the arsenic adsorption properties of sediments collected by the USGS from Antelope Valley CA.
1b.Approach (from AD-416):
We will examine arsenic adsorption and transport for the different sediment texture and 2 pH values (7.5 and 8.5) and two arsenic concentrations (10 and 100 µg/L) of applied water. We will also evaluate computer model predictions based on comparison with experimental data.
The project is related to Objective 2 of the parent project 5310-61000-016-00D, objective 2: Improve our ability to predict the impact of degraded waters on infiltration into soils and plant response to irrigation with these waters. In consultation with USGS scientists we adjusted the conditions of the leaching experiment to pH 6.5 and 8.0 and the arsenate concentrations of the leaching solutions to 30 and 300 ug/L. We completed the leaching study of the clay loam material at the two pH values and two arsenate concentrations, and initiated and completed the leaching with the other two materials, a loamy sand, and a gravelly sand. We characterized the arsenate adsorption envelope (adsorption as a function of pH) and a Langmuir adsorption isotherm for the three materials. For each material we ran 12 columns, two pH values, two arsenate concentrations and three replicates. Solutions were prepared to simulate the composition of the water used at the recharge site. Our results indicate that for two of the materials no arsenate above the background 2 ug/L level was detected in the column effluent after the 50 pore volumes of leaching, at either pH 6.5 or pH 8.0 and 30 and 300 ug/L arsenate. The effluent from the gravelly sand, the coarsest material increased in arsenate above the background concentration after about 47 pore volumes, for both pH values and both arsenate concentrations. Columns are being partitioned and the sediment arsenate analyzed.
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