Note: As, Cd, Cr, Pb and Ni are shaded if they exceed the NHMRC 1996 Drinking Water Guidelines.
Al and Fe are shaded if they exceed the ANZECC 1992 Aquatic Freshwater Ecosystem Protection
pH is shaded if it is <5.5.
pH is commonly used as an indicator of acidity or alkalinity in water or soil. Most natural fresh
waters have a pH close to 7.0 (neutral), and marine waters close to 8.2. In groundwater the pH is
usually controlled by the presence of carbonate-bicarbonate buffer system. It is generally
accepted that a pH range of 5-9 is not considered acutely lethal to most aquatic life, however pH
less than 4.5 can be extremely toxic to plants and gilled organisms including harmful effects on
eggs and fry of sensitive fish3. Although pH is generally used as an indicative measurement of
soluble acidity in water for ASS, it does not account for the total soluble acidity in ASS because
of the presence of soluble iron and aluminium species. However, the reaction between acid and
soil constituents, mainly peat clay soils, releases aluminium, iron, manganese and other metals
such as arsenic and copper4.
3Fromm, P.O., 1980 - A review of some physiological and toxicological responses of freshwater fish to acid stress.
4 Fitzpatrick et al, 1996 - Acid sulfate soil assessment coastal , inland and minesite conditions
The pH screening test was conducted to determine the potential risk of acidic groundwater
contamination in Stirling. More than 800 domestic bores and 13 shallow monitoring piezometer
were installed at strategic locations to determine the extent of groundwater acidity. Figure 2
shows the extent of acidity which affected 49 domestic bores with pH less than 5.5. The
impacted areas are primarily close to the development sites on Jones Street, Landrail Road,
Sittela Street and the northern intersection of Telford Crescent and Karrinyup Road. An isolated
groundwater acidic condition was detected close to Spoonbill lake system including the lake.
Generally the pH varied between 2.6 and 4.5.
Table 6 shows the results of the shallow monitoring bores and metals analysis. Down gradient
monitoring bores SLA#2 and SLA#3 show that the acidic groundwater conditions extend to
6.6m below ground level (BGL) with the exception of SLA#4 located on the south-western
extremity of Spoonbill lake (north) showing the acidity extends deeper than 14m below the
ground level. The upgradient bore located north-east of Spoonbill lake shows a similar pattern of
acidity but limited to less than 4m below the ground level. The results indicate that the local
groundwater in the immediate vicinity of the two lakes has been impacted by acidity.
Monitoring bore SLA#5 located adjacent to Roselea development recorded acidic condition
down to 16m BLG. The local acidic groundwater has impacted a number of domestic bores
along Jones Street within the immediate vicinity. The upgradient bore from the Roselea
development on Graham Burkett Reserve recorded close to neutral pH values, indicating that the
area east of the development site is not affected by groundwater acidity.
Monitoring bore SLA#6 located adjacent to the Stirling Lakes development indicated that acidic
groundwater extended to 10m below the ground level, and this again is confirmed by a number
of impacted domestic bore along Telford Crescent downgradient of SLA#6.
An assessment of the groundwater quality adjacent to the peat stockpile at the Stirling Lakes
development found bore SLA#9 has acidic groundwater at 3m BGL. This indicates that the
shallow acidic groundwater may be moving offsite in a south westerly direction. The upgradient
bore SLA#12 recorded close to neutral pH values indicating that the area east of the development
along Hamilton Street is not impacted by the acidic groundwater.
Arsenic is released into the environment naturally by weathering of arsenic containing rocks and
from human activities. Several forms of arsenic occur in natural waters and the arsenic solubility
is extensively influenced by redox and pH within the soil environment. In well aerated peat
soils, the inorganic forms of arsenic are expected to predominate.
The World Health Organisation set the guideline concentration of arsenic at 0.01 mg/L (WHO,
1994). In Australia, the National Health and Medical Research Council has recommended a
threshold of 0.007 mg/L (NHMRC, 1996).
Of the 49 domestic bores tested for arsenic, 22 exceeded the Australian Drinking Water
Guidelines for arsenic as shown in Table 5. The highest arsenic concentration was about 114
times above the health guideline value. The arsenic hotspots, as shown in Figure 3, are generally
located in those areas having low pH with the exception of one at pH 6.4.
The presence of oxidation of ASS generates significant amounts of acidity and mineral activity
from the concentration of metals undergoing hydrolysis and dissociation in water. This is
demonstrated in Charts 1 and 2 showing that higher concentration of arsenic is related to low pH
Chart 1: SLA#3 - Downgradient and at south-western extremity of southern Spoonbill Lake.
Chart 2: SLA#4 - Downgradient and at south-western extremity of northern Spoonbill Lake.
Arsenic contamination above the health criteria was detected in the monitoring bores
downgradient of Spoonbill lakes and the development sites. Although SLA#1 located
upgradient of Spoonbill north lake has shown high arsenic concentrations it could be attributed
by the radial groundwater influence from the lake system. The highest arsenic detection was at
SLA#5 on Jones Street that exceeded the health guidelines by more than 1000 times. The
arsenic concentration ranges between 7.3 - 0.009 mg/L.
Lead contamination in the environment is likely from precipitation, fall out from dust in urban
areas, street runoff, agricultural and industrial applications. The use of lead-rich herbicides and
insecticides was a significant source of lead in the past, with some orchard soils receiving up to
10kg Pb/Ha/yr. This has caused lead concentrations in some of soils to exceed 5000 mg/kg5.
Lead is generally present in very low concentration in the natural waters. In fresh waters, the
main species of lead are those in the lead organic complexes, with very small amount of free lead
ions. The acute toxicity of lead to several species of freshwater animals was greater in soft water
than hard water.6 Both NHMRC and ANZECC guidelines are used to assess the lead
contamination because the aquatic ecosystem is less tolerant to higher lead concentrations.
5Merry et al 1983 - Accumulation of copper, lead arsenic in Australian orchard soils.
6 Hart, B.T. 1982 - Water quality management: monitoring and diffuse runoff.
Figure 4 shows the spread of lead contamination in the sampled bores that exceeded the health
guidelines ranging between 0.18 and 0.02 mg/L. In all 6 domestic bores and 6 groundwater
monitoring bores exceeded the health guidelines and 4 groundwater samples exceeded the
environmental guidelines for aquatic ecosystem protection.
The bioavailability and toxicity of aluminium is generally greatest in acid solutions7.
Aluminium is generally more toxic over the pH range 4.4-5.4, with a maximum toxicity
occurring around pH 5.0-5.2.8. Under very acidic conditions, the inorganic monomeric
aluminium concentrations of only 0.1 mg/L can be toxic to some fish species9
7Campbell & Stokes, 1985 - Acidification and toxicity of metals to aquatic biota
8 Schofield & Trojnar, 1980 - Aluminium toxicity to brook trout in acidified waters.
9 Driscoll et al, 1980 - Effect of aluminium speciation on fish in dilute acidified waters.
Although there is no health limit for aluminium toxicity, the high aluminium concentrations can
impact both flora and fauna and therefore the ANZECC 1992 guidelines for aquatic ecosystems
protection were used to assess environmental risk. Figure 5 shows the extent of aluminium
contamination in all the sampled bores that exceeded the ANZECC 1992 guidelines by orders of
magnitude. The highest concentration is 58,000 times above the ANZECC guidelines for acidic
5.1.5 Chloride Sulfate Ratio
The oxidation of sulfides to sulfate in ASS generates sulfate that will narrow the chloride (Cl) to
sulfate (SO4) ratio. The Cl:SO4 ratio in seawater and in rainfall is 7. A Cl:SO4 ratio less than 2
is a strong indication of an extra source of sulfate from previous sulfide oxidation. Table 5
shows that most Cl:SO4 ratio in the monitored bores are less than 2 suggesting that they are
affected by ASS, and that there is high potential of continuing acidification unless active
management measures are taken.
5.1.6 Other metals
The groundwater sampling detected the presence of other toxic metals including copper, zinc,
cadmium and nickel, but contamination by these metals is not considered wide spread within the
Copper was detected in six domestic bores at concentrations that exceeded the ANZECC 1992
guidelines but were within the health limits. The exceedance is not considered high when
compared to other urban catchments.
Zinc contamination was detected in 13 domestic bores at concentrations that exceeded the
ANZECC 1992 guidelines. The zinc contamination is not considered excessive when compared
to other urban catchments.
Cadmium was detected in five monitored bores and in one domestic bore above the health limit.
Compared to lead, cadmium is readily taken up by plants, but unlike the other metals, cadmium
is not phytotoxic at low concentrations that pose concern from the human health viewpoint10.
The highest cadmium concentration in groundwater is in Jones Street. It is interesting to note
that the upgradient bore off Hamilton Street also recorded high cadmium concentration. It is
likely caused by previous market garden practices in fertiliser and pesticide application on soils.
10McLaughlin et al, 1998 - Metlas and micronutrients - food safety issues
11Wakao et al 1984 - Bacterial pyrite oxidation III, Adsorption of Thiobacillus ferooxidants on solid surfaces and
its effect on iron release from pyrite
Total chromium was detected in three monitored bores and in one domestic bore above the
ANZECC 1992 and the health guidelines. The concentration ranges from 0.07 - 0.31 mg/L.
Minor nickel contamination was found in six sampled bores with concentration ranging between
0.29 - 0.04 mg/L exceeding the health guidelines.
High iron and sulfate concentrations were found in most domestic bores. This is expected in
areas affected by ASS where the oxidation of the pyritic sulfur produces dissolved ferrous iron
sulfate, and further oxidation to ferric iron is mediated by iron oxidising bacteria, particularly
Thiobacillus ferooxidans11. At low pH the ferric iron remains in solution and diffuses to the
pyrite surface where it is reduced to ferrous generating acidity. The soluble iron may migrate
several kilometres offsite in acid solution before precipitating as ochre in a more oxidising
environment. The residents in the affected area have reported increased iron contamination in
their bores causing considerable staining and crop damage
None of the plant samples tested exceeded the ANZFA Food Standards Code, 1998 value for
arsenic of 1 mg/kg. Fourteen samples exceeded the ANZFA Food Standards Code, value for
lead of 0.1 mg/kg. No samples exceeded the cadmium value of 0.1 mg./kg.