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4.3     Predicted ecological impact

Once water quality has been predicted, the ecological impact associated with that water quality can also be estimated. Coal mines with untreated AMD will have the most severe impact on streams. High acid production from mining areas leads to water characterised by low pH and elevated concentrations of metals.

The impact of acidic conditions on stream biota may be influenced by the natural water chemistry. For example, on the West Coast there are numerous brown-water streams where dissolved humic and fulvic acids have been known to reduce pH to less than 4.0. These naturally acidic streams contain diverse macroinvertebrate and fish communities, indicating some taxa pre-adapted to tolerate low pH. Moreover, toxicological experiments indicated mayfly populations from naturally acidic streams were more tolerant of low pH than those from circum-neutral streams. However, pre-adaptation to low pH does not necessarily preclude impact from mine drainage as streams impacted by AMD typically contain high concentrations of metals (See Appendix D for further discussion). The abundance of naturally acidic streams on the West Coast, and the consequent adaptation of endemic species, means that international water quality guidelines such as ANZECC and ARMCANZ (2000) may not be appropriate when identifying and setting water quality targets for mining impacts for such regions. We have used a combination of biological survey data (from the West Coast) and toxicity experiments using West Coast invertebrates to establish the potential impacts outlined below (Figure 12). Six general outcomes are illustrated and described briefly below. The pH limits described are reasonably well defined, while the metal limits are based on Fe and Al concentrations only and are qualitative. Further details about these outcomes, and the toxicological effects of selected metals, are provided in Appendix D.

We know relatively little about the responses of Southland stream invertebrates to acidification. In Southland, naturally occurring acidic streams are rare, so invertebrates are unlikely to exhibit the pre-adaptation to low pH observed in West Coast streams. Consequently, AMD impacts are likely to be more severe for a given water chemistry than those predicted for West Coast stream invertebrates. While the ANZECC water quality guidelines (ANZECC and ARMCANZ 2000) may be more relevant to Southland streams because of the lack of naturally acidic streams, guideline values for some relevant metals (e.g. Fe) are missing. More information on the toxicological effects of selected metals is provided in Appendix D.2.

Outcome 1

The most severe impact on stream ecosystems occurs when water is highly acidic (pH < 4) and has a high concentration of metals. No New Zealand fish can survive for long in such water. Few macroinvertebrates, of very limited diversity, will be found. Algae and microbes, however, may be present, and even in high abundance in some cases. These communities tend to be dominated by a few taxa that are able to tolerate the stressful conditions. Remedial and treatment strategies will be essential.

Outcome 2

Streams with very low pH usually have high metal concentrations as well, but there can be cases where acidic streams have low metal levels. These streams would also be highly impacted, although not as severe as Outcome 1. More algal and invertebrate taxa can be found, although fish would still be rare or not found. Some moss can be found in such streams as well. Remedial and treatment strategies will be essential.

Outcome 3

Streams of moderate acidity and high metal concentrations will usually have a moderate impact on aquatic life. Although some fish can tolerate the acidic conditions, dissolved metals may be toxic to them depending on concentrations. Similarly, invertebrates in these streams can suffer from either the low pH or the elevated dissolved metals. At pH above 4, Fe can be present as Fe precipitates, which, along with other metal precipitates, can impose stress on fish and invertebrates, either through a reduction in habitat quality or food supply. Algal and plant (moss, macrophyte) diversity can be moderately high, but their biomass and activity can vary based on deposition of metal precipitates. Remedial strategies are likely to be required.

Outcome 4

Many streams on the West Coast have pH in the range 4–6 because of natural humic and fulvic acids (often called tannins). Streams affected by mines in this pH range can still support high diversity and abundance of aquatic life if the metal concentrations are low such that no toxicity occurs. Fish, invertebrate, algal, plant and microbial diversity can all be high and comparable with those in pristine streams. Exceptions may occur at the low pH range (close to 4) or in cases where metal precipitates or sediments impair habitat in the stream. Remedial strategies are unlikely to be required.

Outcome 5

Streams with water around neutral pH clearly are not affected by acidity. These waters, if downstream from mines, may have high concentrations of metals such as Zn or copper (Cu) that are toxic to fish and invertebrates. Furthermore, metal precipitates, if present in high amounts, may limit the habitat or food supply of fish and invertebrates. Remedial strategies may be required.

Outcome 6

Waters of neutral pH and very low metal content should support a full diversity and abundance of aquatic life for the area. Natural features of the catchments could affect some biota, such as waterfalls blocking migratory fish species. Mining still could affect stream habitat if turbidity or sedimentation (from mining operations) were present. Otherwise, species and food webs should be comparable with pristine streams in the area.

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