Pr Dewatering - Acid Sulfate Soil Work Instruction

Transcription

1 Document Owner References SHEQ Service Delivery Manager Dear SE, Moore NG, Dobos SK, Watling KM and Ahern CR (2002). Soil Management Guidelines. In Queensland Acid Sulfate Soil Technical Manual. Department of Natural Resources and Mines, Indooroopilly, Queensland, Australia. Ahern, C.R., Ahern, M.R. and Powell, B. (1998) Guidelines for Sampling and Analysis of Lowland Acid Sulfate Soils (ASS) in Queensland QASSIT, Department of Natural Resources, Resource Sciences Centre, Indooroopilly. Treatment and Management of soils and water in acid sulphate soil landscapes, Department of Environment and Conservation Western Australia (July 2011). Identification and Investigation of Acid Sulfate Soils and acidic landscapes, Department of Environment and Conservation Western Australia (May 2009). Erosion and Sediment Control Manual, Sunshine Coast Regional Council (November 2008). Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC and ARMCANZ, 2004) 1. Purpose To provide guidance on appropriate dewatering practices to ensure minimal harm to the receiving environment. 2. Scope This work instruction applies to construction dewatering activities within either Acid Sulfate Soil (ASS) areas or Potential Acid Sulfate Soil Areas (PASS). Note that an ASS investigation should have been carried out during the project planning phase. Site specific environmental management of ASS should be outlined with the ASSMP. This work instruction is complementary to the CEMP. 3. Responsibilities and Authorities The detail of responsibilities and authorities is provided within section Definitions Page 1 of 6

2 Term PICOW CEMP NATA ASS PASS ASSMP EC Meaning in Charge of Workplace Construction Environmental Management Plan National Association of Testing Authorities Acid Sulfate Soil Potential Acid Sulfate Soil Acid Sulfate Soil Management Plan Electrical Conductivity 5. Instruction 1. Undertake assessment/inspection of construction area and wastewater (ASS affected) to be discharged. The following environmental aspects must be assessed: Chosen treatment option, considering: Chemical storage area(s), if required, Sufficient area to allow preferred treatment option to occur, Environmental controls applicable to treatment activities (from CEMP); Water quality : Visual assessment (blue-green or extremely clear water indicate increased aluminium levels), Field measurements to confirm previous ASS/water investigations or need for further in-depth analyses (request assistance from Environmental Affairs), Minimising or eliminating ASS surfaces that become exposed to air as water is being removed from excavations. All trenches (where possible) should be backfilled within 6hours to decrease oxidation) Can wastewater be discharged to sewer? Liaise with relevant STP. ECO Page 2 of 6

3 2. Undertake on-site water treatment as required. NOTE: Re-testing may be required when: a significant period of time has elapsed since the original investigation; or there have been significant weather influences since the original investigation (e.g. significant rain fall or prolonged dry periods), and when field analysis indicates water is acidic (ph < 6) or has high electrical conductivity (EC) or has low dissolved oxygen (< 6 mg/l) - request assistance from Environmental Affairs. Treatment options that are generally utilised are: In-line treatment system Aerator tank/spray Impermeable sediment basin Refer to figure 1 below for possible treatment options. ECO When using impermeable sediment basin: ensure it is not located in subsurface affected by ASS or lined with an impermeable layer. Other options may include an above ground structure (drum, bin, tank), and ensure impermeable layer (or walls of structure) is maintained throughout treatment process undertake regular visual inspections. For all types of treatment options: Implement environmental controls as per ASSMP or CEMP; Conduct daily visual assessment to ensure environmental controls remain effective and treatment activities are achieving desired outcome; Implement wastewater quality monitoring program as per CEMP: o For in-line treatment and aerator/spray systems, the monitoring frequency and parameters must correspond with Page 3 of 6

4 3. Discharge treated wastewater to environment. 4. Regulate and monitor flow and pump conditions. 5. Dispose of accumulated sediment, if necessary 6. Undertake post dewatering assessment. o the type and expected water quality of the receiving environment. For sediment basins/tanks, a wastewater quality test must be undertaken and approval for discharge must be confirmed by Environmental Affairs prior to release. The discharge location must: Be located in an area that is undisturbed, slope <5% and densely vegetated (or other suitable area consider consultation with Environmental Affairs), Not to be located close to a waterway or stormwater drain (ensure a sufficient buffer exists to prevent transfer of sediment into nearby water bodies), Have erosion and sediment controls installed to prevent scour and to catch grit and sediment (controls could include rock check dams, baffles, sediment fence and/or silt socks), and Be via a sediment settling basin, if required. Dewatering activities must be monitored and supervised at all times to ensure that: Pump intake is held at the surface to prevent pumping excess sediments and creating turbid water, Water is discharged at a velocity that causes minimal harm to the receiving environment, The flow of water must be continually monitored and discharge point moved as required. If excavation of in-ground pit is required, transport sediment to appropriate waste disposal facility. Sediment collected within tanks etc. to be disposed to appropriate waste facility. A post dewatering inspection must be conducted to ensure that the site has been returned to pre-dewatering conditions. As part of this, assessment must be made ECO Page 4 of 6

5 for signs of: Scour, Vegetation die back, Changes in soil structure (cracking, etc.). Figure 1: Schematic view of possible ASS treatment system. 6. Water Quality Indicator Table The below table can be used as an indicator guide. Consultation with Environmental Affairs may be required where laboratory results require data interpretation. Table 1 - Water Quality Indicator Levels for Dewatering Activities Water Quality Parameter Parameter Measurement Unit(s) Receiving Environment Lowland River Freshwater Lakes and Reservoirs Dissolved oxygen (field) % saturation 85 to to 120 Dissolved oxygen (field ) mg/l 6 to 9 6 to 9 ph (field) ph U 6.5 to to 8.5 Electrical Conductivity (field) μs/cm 125 to to 30 Turbidity (field) NTU 6 to 50 1 to 20 Temperature (field) o C 15 to to 35 Ammonium (NH4 + ) μg NH3/L Total Phosphorus (TP) μg P/L Chlorine (total residual) N/A* N/A* N/A* Dissolved Aluminium (at ph > μg/l Page 5 of 6

6 6.5) Dissolved Arsenic (3+) μg/l Dissolved Chromium (6+) μg/l Dissolved Cadmium μg/l Dissolved Manganese μg/l Dissolved Nickel μg/l Dissolved Selenium (total) μg/l Dissolved Zinc μg/l Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC and ARMCANZ, 2004) *NOTE: No regulatory guideline limit is provided for total residual chlorine. A guidance level of 5mg/L from the discharge point can be adopted but the following controls (at a minimum) must be in place. Discharge point must be located 150m (at a minimum ) from a waterway/drainage channel; Water to be run through dense vegetation and/or controls (i.e. straw bales); Liaison with Environmental Affairs is required before discharge; and No water is to be discharged directly into a waterway/drainage channel. Page 6 of 6