Arsenic in Hawaii SoilsN. V. Hue and W. G. Cutler, Univ. of Hawaii at Manoa

Introduction:Chronic exposure to arsenic (As) is known to cause adverse health effects in humans, including skin lesions and hardening (keratosis), cancers and neurological disorders.

In Hawaii, sodium arsenite was used as a weed control in sugarcane fields for many years from 1930’s to 1960’s. Such usage appears to have resulted in elevated As in some soils. Total As levels, ranging from 40 to over 100 mg/kg, are not uncommon. For comparison, “background” levels of As in Hawaii soils are between 4 and 20 mg/kg, and the remediation action level is being set at 22 mg/kg for unrestricted residential land use by the State.

Much of the former sugarcane lands in Hawaii are being redeveloped for residential, community (schools, civic centers), small-business farming and commercial uses. Elevated As in these soils would pose serious risk to those residing or working on these lands. As the first step in containing potential As problems, we decided to conduct a study on As status and chemical behavior in certain Hawaii soils.

Materials & Methods:Sampling of Soils, Sediments and Water

Surface (2 – 10 cm) soil samples were collected, based on soil Series distribution through out the islands. Most samples belong to 3 soil Orders: Andisols (Hawaii), Oxisols and Ultisols (Kauai, Maui and Oahu). Detailed samplings were performed in Anahola, Kauai, where a Brown Fields redevelopment project is planned, and in Kea’au, Hawaii, where soil As at some localities can reach several hundreds mg/kg. A few sediment and water samples were also collected on the island of Oahu.

Chemical Analysis of Arsenic.

* Total As – extractant: 30% conc. HNO3 + 70% conc. HClO4; 1.0g/50 ml; 24-hour shaking.

* Bioaccessible As - extractant: HCl, pH 1.5; 1.0g/100 ml; 1-hour shaking, 37 oC.

* Exchangeable As – extractant: 0.01 M CaCl2; 2.0 g/20 ml; 1-hour shaking (also used in desorption).

* Mehlich-3 As – Mehlich-3 solution (dilute acetic and nitric acids, NH4F, EDTA); 2.0g/20 ml,

5-minute shaking.

*Fractional As - extracted sequentially with 4 extractants: (1) 1.0 M CaCl2 (total exchangeable),

(2) 0.10 M Na-pyrophosphate (As associated with organic matter), (3) 0.30 M NH4-oxalate in the

dark (As associated with amorphous Al & Fe oxides), and (4) 8 M HNO3 (residual); 2.0g/20 ml;

24-hour shaking, each step.

As levels in filtered solutions were measured with an inductively coupled plasma spectrometer (ICP)

set at 189 nm.

Phytoremediation – Brake fern (Pteris vittata L.) was used as a test plant.

Results and Discussion:Arsenic Levels (mg/kg) in Selected Soils and Sediments/water

of Hawaii

Our preliminary data suggest the followings: (1) As levels in Hawaii soils are quite variable, ranging from 15 to 450 mg/kg . Observed elevated As levels in some stream waters are likely related to As-impacted soils. (2) Andisols seem to contain more total As than the other soil Orders, perhaps because of high As sorption capacity of amorphous Al/Fe oxides that are predominant in the Andisols. (3) Mehlich-3 solution extracts only about 2% of the total As in most Hawaii soils.

Arsenic Fractionation & Chemical Behavior in Hawaii Soils

The Olaa Andisol has 10% org.carbon (OC), and is X-ray amorphous, whereas the Lihue Oxisol has 1.5% OC and over 50% kaolinite-crystalline Fe oxides. These contrasting properties in part may explain for the 448 mg/kg total As in the Andisol vs. 15.6 mg/kg in the Oxisol. The distribution patterns of As fractions in the 2 soils may have serious implications: The Andisol has only 0.5% total As as exchangeable, implying most of its As is not easily released to the solution nor become available to living organisms. On the other hand, nearly 25% of its As is associated with organic matter (OM), suggesting that some As may be gradually released upon the oxidation of soil OM.

Sorption & Desorption

Consistent with the different As fractions, As is desorbed (released) faster in the Oxisol (maximum desorption at cycles 3 -4) than in the Andisol (max. at cycle 5). In general, it took at least 10 cycles to desorb soil As to levels below our detection (< 0.005 mg/l in solution or 0.05 mg/kg in soil). In contrast, the 2 soils can still sorb considerable amounts of As. To maintain the 0.20 mg As/l in solution, it requires an addition of 220 mg As/kg to the Andisol and 150 mg/kg to the Oxisol (The action level of As in stream water in Hawaii is set at 0.19 mg/l.) The sorption is much stronger (steeper slope) for the Andisol when equilibrium solution As is below 0.10 mg/l (or 100 ppb).

Soil Amendments Affecting As Bioaccessibility

Adding P fertilizer to the high-As Andisol releases significant amounts of As. For example, bio-accessible As increased from 18 to 31 mg/kg when 200 mg P/kg as Ca(H2PO4)2 was added. On the other hand, adding colloidal Fe(OH)3 decreases bio-accessible As markedly. For example, 1% Fe(OH)3 reduced bioaccessible As from 9% of total As to 5%.

Phytoremediation with brake fern (Pteris vittata L.)

Our on-going work on Kauai (Anahola site) has shown that brake ferns can take up considerable amounts of soil As. Preliminary data show that leaf tissue As levels were 30 mg/kg before transplanting vs. 1050 mg/kg after 3 months of growth on the Lihue Oxisol. Nearby vegetations contained only 2 – 5 mg/kg As in their leaves.

In summary, past usage of arsenical herbicides has resulted in high levels of As in many Hawaii soils. Soil mineralogy and chemistry, especially organic matter and amorphous Al-Fe materials, play important roles in retaining As. Soil As can be made more available by adding P fertilizers or less available by adding colloidal Fe(OH)3. Brake ferns show promise in extracting As from soils.