Draft Identification and investigation of acid sulfate soils and groundwater
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Draft Identification and investigation of acid sulfate soils and groundwater
Appendix 3: Interpreting soil field pH tests
Field pH tests should be conducted on the soil profile at regular intervals (0.25 metres) using a field pH meter calibrated according to the manufacturer's instructions. All results (pHF and pHFOX values, peroxide reaction) should be tabulated and reported.
Other semi-field tests such as examination under a microscope for pyrite and its reaction with peroxide on the slide may be useful tools to identify pyrite presence, but they require experience and training.
1. Field pH test (pHF) i.e. pH of soil and water paste
The pHF test measures the existing acidity of a 'soil:water' paste, and is therefore used to help identify if ASS are present. If the measured pH of the soil paste is pHF ≤ 4, oxidation of sulfides has probably occurred in the past, indicating the presence of AASS. Highly organic soils or heavily fertilised soils may also return a pHF close to 4. A pHF >4 but ≤5 indicates an acid soil, but the cause of the acidity will need to be further investigated by laboratory analysis. The pHF test does not detect any unoxidised sulfides (i.e. PASS). For this reason, this test must be used in conjunction with the pHFOX test.2. Field pH peroxide test (pHFOX) i.e. pH of soil and peroxide mix and reaction with peroxide
The pHFOX test is used to indicate the presence of iron sulfides or PASS. This test involves adding 30% hydrogen peroxide (pH adjusted to 4.5-5.5) to a sample of soil. If sulfides are present a reaction will occur. The reaction can be influenced by the amount of sulfides present in the sample, the presence of organic matter or the presence of manganese. Once the reaction has occurred, the pH is measured.A combination of three factors is considered in arriving at a 'positive field sulfide identification':
A reaction with hydrogen peroxide. The strength of the reaction with peroxide is a useful indicator but cannot be used alone. Organic matter, coffee rock and other soil constituents such as manganese oxides can also cause a reaction. Care should be exercised in interpreting a reaction on surface soils and high organic matter soils such as peats and coffee rock, and some mangrove/estuarine muds and marine clays. This reaction should be rated, e.g. L = Low reaction, M = Medium reaction, H = High reaction, X = Extreme reaction.
The actual value of pHFOX. If the pHFOX <3, and a significant reaction occurred, then it strongly indicates a PASS. The more the pHFOX drops below 3 the more positive the presence of inorganic sulfides.
A much lower pHFOX than field pHF. The lower the final pHFOX value and the greater the difference between the pHFOX compared to the pHF, the more indicative the presence of PASS. This difference may not be as great if starting with an already very acid pHF (close to 4), but if the starting pH is neutral or alkaline then a larger change in pH should be expected. Where fine shell, coral or carbonate, is present the change in pH may not be as large due to buffering. The 'fizz test' (effervescence with 1 M HCl) should be used to test for carbonates and shell.
NOTE: Field techniques are useful exploratory tools, but are indicative only and definitely not quantitative. They are not a replacement for quantitative laboratory analyses. Although it is commonly assumed that a low %S value means a low risk, the situation is more complex: [%S] x [volume disturbed] = risk assessment. Lowering of groundwater is another level of complication that needs to be addressed.
Field and laboratory tests, sampling intensity and action levels (based on oxidisable sulfur results) relevant to the investigation of ASS are outlined in the Guidelines for Sampling and Analysis of Lowland Acid Sulfate Soils in Queensland 1998. These guidelines, also available at www.environ.wa.gov.au, should be used to guide Western Australian investigations in combination with the Draft DEWCP and EPA Guidance on Acid Sulfate Soils (2002).
