ADG Dewatering Management Plan · OPW21/0315

                                                                                         ADG Consulting Pty Ltd


3    Previous investigations
     This DMP is based on site specific information sourced from the following, provided to ADG by Chris Clout
     Designs:
      −       Preliminary Geotechnical Investigation, Proposed Residence 33 Wyuna Drive, Noosaville. 2020.
              Douglas Partners Pty Ltd. Ref: 95120.00.R.001.
     A brief discussion of relevant soil and groundwater conditions is provided below.
3.1 Douglas Partners Pty Ltd 2020
     A preliminary Geotechnical Investigation was undertaken at the site in September 2020 by Douglas
     Partners Pty Ltd (Douglas).
3.1.1 Soils
     A total of one (1) borehole (BH1) was drilled on the proposed development footprint to 6.0 m below
     ground level (bgl).

     The subsurface profile encountered is generally described as loose to medium dense brown, orange-
     brown grey sand fill to 2.35 m depth. Underlaying the fill layer were natural sands starting with a relative
     density of medium dense grey silty sand to 3.3 m depth, then a layer of medium dense, dense to very
     dense grey sands to 4.9 m depth. A layer of very dense (indurated) dark brown silty sand was encountered
     until the termination depth of 6.0 m.

     As part of the Preliminary Geotechnical Investigation (2020), Acid Sulfate Soil (ASS) samples were
     collected at 0.25m intervals in BH1 to 4.0 m depth. A total of sixteen (16) samples were collected for ASS
     screening and four (4) samples were selected for further analysis by the Chromium Reducible Sulfur suite
     (CRS).

     ADG notes that the testing was limited to one borehole and with ASS testing to 4.0 m depth and DP
     reports that within this depth profile both the existing sand fill and natural silty sand are considered to
     be acidic. However only the natural sands are considered to contain PASS. The PASS is not likely attributed
     to sulfides, but rather naturally occurring acidity commonly found in non-ASS soils.
3.1.2 Groundwater
     During drilling activities for the Preliminary Geotechnical Investigation, groundwater seepage/inflow was
     encountered at 2.35 m depth below the ground surface (unknown elevation relative to m AHD) (Douglas
     Partners, 2020).


3.2 Groundwater quality
     ADG is of the opinion that localised monitoring well data can often be misleading and does not always
     accurately represent the properties of the groundwater that will be extracted and treated from the entire
     site and local surrounds during dewatering. Therefore, based on numerous groundwater investigations
     and dewatering monitoring projects that have been undertaken by ADG throughout the Sunshine/Noosa
     Coast Region in recent years, our understanding of the groundwater quality in the region is provided
     below:
          −      Groundwater is generally mildly acidic to neutral, with pH levels ranging from approximately 5.5
                 to 6.5 and electrical conductivity ranging from brackish to saline between 2,000 µS/cm to
                 48,000 µS/cm. Groundwater pH has the potential to reduce (become more acidic) following
                 oxidisation of PASS as dewatering progresses, particularly if re-wetting of the soils occurs.
          −      Dissolved oxygen levels in static groundwater typically range from approximately 35 – 40 %
                 saturation.




ADG1290.21 Dewatering Management Plan_Rev0_12.10.21                                                        Page 4