Draft Identification and investigation of acid sulfate soils and groundwater
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Draft Identification and investigation of acid sulfate soils and groundwater
5.0 Groundwater investigation prior to disturbance
At an early stage in the ASS investigation, it is important to consider any potential groundwater issues, particularly if groundwater is likely to be disturbed beyond normal seasonal fluctuations.The hydrological regimes that operate in an area affect the behaviour of ASS and the transport of oxidation products. ASS which are permanently waterlogged remain benign, but if the groundwater level is lowered (either temporarily or permanently) then oxidation may occur resulting in acid formation and metal mobility. Vertical and horizontal groundwater flows transport oxidation products (acid, toxic by-products) to other areas and adjacent subsoil.
It is also vital to keep in mind that groundwater treatment in situ is usually not feasible. A suitably qualified professional person experienced in assessing and managing ASS and groundwater issues should undertake all investigations.
Groundwater should be investigated prior to soil disturbance and should demonstrate:
- the presence or absence of acidic groundwaters prior to works;
- that the proposed development will not result in previously saturated ASS (both
onsite or offsite) being aerated through groundwater extraction or excavation;
- that if previously-saturated ASS will become aerated (as a result of the proposed
development), this information will be used to assist in the production of effective
treatment and management plans; and
- that any acidic groundwater will not migrate from the site and impact adjacent properties and sensitive environments.
5.1 Required information
Prior to on-site works, the groundwater investigations should include a description of water quality, including seasonal variations where applicable. Essential information that should be collected and documented as part of the groundwater investigation includes but is not limited to:
- field measurements of pH, electric conductivity, dissolved oxygen and redox. If field
measurement of groundwater pH is less than 5, additional investigations for calcium,
magnesium, total iron, dissolved iron, dissolved manganese, filtered aluminium, arsenic,
chromium, cadmium, bicarbonate, carbonate, chloride and sulfate should be conducted;
- determine the depth to the watertable with an indication of the seasonal variation. The
greater the groundwater depth, the risk of impacting the groundwater or changes to the
watertable levels are lower;
- identify adjoining on and off-site groundwater related environments (e.g. wetlands,
springs, rivers and creeks) and any likely recharge areas (e.g. areas of waterlogging). Sites
that contain surface water linkages to the groundwater increase the likelihood of
groundwater being affected; and
- identify any adjoining existing groundwater users, density of water extraction bores and uses of groundwater extraction.
A soluble chloride: soluble sulfate (Cl-:SO42-) ratio should also be calculated to determine if sulfide oxidation has occurred in the past. Where the analysis indicates that there is an elevated level of sulfate ions relative to the chloride ions, these results may indicate the presence of ASS in the landscape. A Cl-:SO42- ratio by mass of less than four, and certainly a ratio less than two, is a strong indication of an extra source of sulfate from previous sulfide oxidation (Mulvey 1993).
5.2 Using the information
If groundwater investigations indicate that existing groundwaters do not conform to the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC and ARMCANZ 2000) water quality criteria, then any extracted groundwater must be contained and treated before release.If groundwater investigations indicate that existing groundwaters are acceptable by the ANZECC/ARMCANZ 2000 guidelines water quality criteria:
- daily monitoring for pH is still required prior to any release to receiving waters to
ensure that there is no deterioration of water quality standards since previous
measurements; and
- weekly monitoring for pH is still required if an on-site water storage (greater than 100 m3 or 0.1 megalitre) interacts with groundwater to ensure that there is no deterioration of water quality standards since previous measurements. If the above monitoring indicates a pH result outside the acceptable range of the ANZECC guidelines, waters must be treated in accordance with the recommended liming rates.
While the treatment of relatively small quantities of water may appear to be quite straightforward, applicants should seek qualified professional assistance, as the chemistry of water quality can be a complex environmental issue.
In cases where excessive iron, aluminium and other salts are present, particularly in large volumes, sophisticated treatments may be required in consultation with the Land and Water Quality Branch,DEWCP.
Where iron is precipitating from the acidic water, very low dissolved oxygen levels may result. Wherever possible, dissolved oxygen should be measured over the full diurnal cycle for a period of a few days to establish the diurnal range in concentration.
