Results of toxicity monitoring
Since their inception in 1991–92, toxicity monitoring tests have been performed approximately every other week (ie once a fortnight) during the wet season. Tests usually commence in December and cease in early April, the period of significant flow in Magela Creek. The results of the toxicity monitoring tests are plotted as part of a continuous time series of actual response and paired-site (upstream-downstream) 'difference' data. Figure 1 displays toxicity monitoring data for freshwater snail egg production acquired using the in situ testing procedure that has been deployed since the 2006–07 wet season. In the 2009–10 wet season, Gulungul Creek was added to the toxicity monitoring program and testing is now conducted fortnightly, alternating with Magela Creek. Data collected from 1991–92 to 2007–08 using, primarily, the creekside monitoring procedure, are available in the archived results.
Each year's results are reported and discussed in detail on an annual basis (see respective Supervising Scientist Annual Reports). Results for 2010–11 are preliminary and will be analysed and discussed in further detail at the end of the wet season.
For a more detailed discussion about the history of the toxicity monitoring program and a detailed explanation of the testing methodology please see the Explanatory notes: history of toxicity monitoring.
2011–12 wet season
Continuous surface flow commenced in Magela on 22nd November 2011, with the first Magela test deployed on 1st December 2011. In total, nine four-day tests were completed at fortnightly intervals, thus: 1–5 December, 15–19 December, 12–16 January (2012), 30–3 February, 9–13 February 23–27 February, 8–12 March, 22–26 March and 5–9 April. The results from this wet season are displayed in Figure 1a.
The 2011–12 wet season results showed that, on average, slightly greater egg production occurred at the downstream site compared to the upstream site. This is reflected by the 2011–12 mean upstream-downstream difference value of -8.2 (Figure 1a). This result continues the trend of greater egg production downstream and is within previously recorded ranges. This has resulted in a new "all years" mean difference value of -8.0 (-5.8 pre-2009–10). The larger than expected difference values of 2009–10 and 2010–11 wet seasons (-22.3 and -12.8 respectively) are discussed in the respective wet season sections below.
Analysis Of Variance (ANOVA) testing of the 2011–12 wet season data found no significant difference between the responses from this wet season (upstream-downstream difference values) and those from previous years (P = 0.907).
As a consequence of the significantly different results reported below in both 2009–10 and 2010–11, analyses have been undertaken to better understand potential causative factors, and provide an improved understanding, of environmental conditions affecting the production of snail eggs during the toxicity monitoring tests. These analyses included:
- Seeking correlates of variability in snail egg difference values using water quality variables recorded via the Surface Water Chemistry Monitoring Program;
- The effects of electrical conductivity (EC) and water temperature upon snail egg production; and
- Seeking relationships between snail egg responses and suspended inorganic and organic matter – as highlighted in previous website reports for 2009–10.
These results are discussed in detail and will be available in the 2011–12 Supervising Scientist Annual Report (chapter 3).
2010–11 wet season
For the 2010–11 wet season, continuous surface flow in Magela Creek commenced on November 15th 2010. However, creek flow remained very low for several weeks thereafter and water levels were too shallow for the deployment of continuous (water quality) and toxicity monitoring equipment for this period. The first toxicity monitoring test commenced in Magela Creek on 17 December, once moderate creek flows were established. In total, ten four-day toxicity monitoring tests were completed at fortnightly intervals, thus: 17–21 December 2012, 6–10 January 2011, 20–24 January, 3–7 February, 18–22 February, 4–8 March 17–21 March, 31 March–4 April and 15–19 April and 29 April–03 May.
The results for the 2010–11 wet season showed that, on average, egg numbers at the downstream site were greater than those measured at the upstream control site (Figure 1a), with a mean upstream-downstream difference value of -12.8, a value intermediate between the running mean of the previous wet seasons and the value reported in 2009–10 (Figure 1a).
ANOVA testing found that when compared to previous years, the 2010–11 results were non-significant. However, when the results of 2009–10 and 2010–11 were grouped together and compared to previous years a significant difference was observed. These results were investigated in further detail in the Supervising Scientist Annual Report 2010–11 (pp 70-74, chapter 3.4) and Scientist Annual Report (chapter 3).
2009–10 wet season
Toxicity monitoring results for the 2009-10 wet season, where consistently higher egg production at the downstream site was observed (see top panel of Figure 1), represented the first instance in which the difference values for the wet season were significantly different to results from all previous wet seasons, as determined by a Analysis Of Variance (ANOVA) testing (p=0.046). A number of factors have the potential to cause the different behaviour in snail egg production response observed for the 2009–10 wet season: methodological or systematic operator problems during the wet season; an unusual suppression in egg number upstream over the wet season; or enhancement of egg number downstream that may be associated with inputs of water (as measured by electrical conductivity or turbidity data) from the Ranger site.
Following the 2009–10 wet season, each of the above potential causative factors was assessed in detail, including an examination of the extensive available historical grab sampling and continuous water quality monitoring datasets. No correlation was found between any of these factors and the positive downstream effect on egg production. Other lines of evidence, including water physico-chemistry and results of the stream macroinvertebrate community studies that are conducted by SSD in the late wet season recessional flow period each year (reported below) also supported the conclusion of no mine-related effects upon freshwater snail egg production.
At this time it appears that the most probable explanation for the higher downstream egg production is an increase in food supply at this site as a result of increased settling-out of particulate matter. Field monitoring staff have noted that in recent times there has been a deepening of the channel at the downstream site. This deepening would result in a relative reduction in water velocity across the stream profile and hence an increased likelihood for deposition of suspended material. A visible increase, compared with previous years, in the amount of particulate material trapped inside the toxicity monitoring containers at the downstream site was noted during the 2009–10 wet season. In light of this, field monitoring staff are developing a method to quantify the nature and amount of particulate matter that is settling in the testing containers, with samples being collected after each of the test periods. These samples will be used to assess if there is any positive correlation between the amount and nature of the deposited material and egg production.
Results for 2011–12 wet season
Continuous uninterrupted flow between the upstream and downstream sites commenced on 31st November 2011. In total, 9 four-day tests were completed in the 2011–12 wet season, thus: 8–12 December, 16–20 December, 5–9 January (2012), 19–23 January, 2–6 February, 16–20 February, 1–5 March, 15–19 March and 29–2 April. The second test deployed on 16th December was deemed invalid due to an excess amount of debris that had collected on the equipment, restricting water flow to the snails during the test period (Figure 1b).
Last year's (2010–11 wet season) consistently higher downstream egg production was also evident in the results of this wet season, with a mean upstream-downstream difference value of -16.49. The results of 2011–12 show a marked decrease in the variability of egg production and water quality, confirming last year's observation of higher variation in egg production being attributed to higher variability in water quality (Supervising Scientist Annual Report 2010–2011, pp32-33 Chapter 22.214.171.124).
These results are discussed in detail and will be available in the 2011–12 Supervising Scientist Annual Report.
2010–11 wet season
Nine four-day tests were completed in Gulungul Creek, thus: 20–24 December 2010, 13–17 January, 27–31 January 2011, 10–14 February, 24–28 February, 10–14 March, 24–28 March, 7–11 April and 5–9 April.
Results for Gulungul Creek show snail egg production at the downstream site was consistently higher than at the upstream site, with eight of the nine tests producing a negative difference value (Figure 1b). These results are in contrast to those observed during the previous (2009–10) wet season when four out of the five tests conducted in Gulungul Creek resulted in positive difference values (indicating higher upstream egg production). Confirming this observation, ANOVA testing found a significant difference between the upstream-downstream difference data for 2010–11 compared with difference data for 2009–10 (P <0.05). This reflects interannual variability evident in a small dataset of only two years.
2009–10 wet season
Five tests were conducted through the 2009–10 wet season, over a range of flow conditions, and in alternate weeks to the routine Magela Creek testing. Tests were conducted in the periods 25–29 January, 22–26 February, 22–26 March, 9–13 April and 19–23 April 2010. The results are shown in the bottom panel of Figure 1. The range of egg numbers produced over the wet season was similar to that recorded in Magela Creek.
Four out of the five tests resulted in positive difference values with more eggs produced upstream than downstream. This pattern was opposite to that observed in Magela Creek during the same period, where eight of the nine tests produced a negative difference value (Figure 1a).
Figure 1. Toxicity (in situ) monitoring results for freshwater snail (Amerianna cumingi) egg production for wet seasons between 2006/07 and 2011/12. Magela Creek (a); Gulungul Creek (b). In situ testing for Gulungul Creek started in the 2009-10 wet season. The 96 hour tests are conducted in each creek at fortnightly intervals, with 9 tests completed in both Magela Creek from 1st December 2011 until 9th April 2012 and in Gulungul Creek from the 8th December 2011 until 2nd April 2012. Chart last updated April 11th 2012.