Summary - City of Gosnells

Methods 23To determine the depth to the water table and the stratigraphic sequence at each of these 16 study sites, holeswere augered manually down to the water table (where possible). In some sites, the low moisture levels ofshallow materials such as laterite or mud prevented augering down to the water table. Surface sediments andthe presence of waterlogging or inundation of surface sediments were recorded opportunistically elsewherethroughout the survey area. The observations of the surface perching of rainwater (early in the wet season) inareas where shallow, relatively impermeable layers were encountered later during the augering werevaluable.Due to the time constraints of the survey it was not practical to install piezometers at the study sites tomonitor watertable change over time. Thus the depth to the watertable was measured only once (immediatelyafter augering) in the holes where the watertable was reached. This method, when augering in sediments thathave low hydraulic conductivity, sometimes does not allow enough time for the groundwater level to rise toits full equilibrium height before measurement. Thus all of the ‘one-off’ watertable levels recorded in thisstudy in situations where there were peaty or muddy sediments should be viewed as being an underestimateof the true height of the water table at each location on the date of measurement.The sediments from the auger holes were sampled at 10 cm intervals, labelled and then examined via a lowpower stereoscopic microscope to assess grain size and sediment fabric. The presence of calcareous materialsin the samples was determined after application of several drops of 10% hydrochloric acid to each sample.The sediments obtained from the augered holes were classified and described using terms consistent withSemeniuk and Semeniuk (2004) (Tables 3.3 and 3.4).Table 3.3: Wetland Sediment Classification (adapted from Semeniuk and Semeniuk, 2004)Descriptorgravelcoarse to very coarse sandfine to medium sandvery fine sandmud or peatDiameter of Grains>2 mm0.5-2 mm0.125-0.5 mm0.063-0.125 mm